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ABSTRACT Seafood is an important resource for global nutrition and food security, with both land and marine aquaculture playing pivotal roles. High visual acuity is key for health and survival of farmed, cultured, and wild fish. Cleaner fish technology to control parasite infestation has become important in marine aquaculture and highlights the importance of visual acuity in the efficacy of cleaner fish species. New clinical diagnostic approaches towards understanding and optimising fish visual health could benefit both aquacultured and wild fish populations. Opportunities for developing and using advanced non‐invasive clinical assessment and diagnosis of ocular health in wild, cultured, and experimental fish are key to more rapidly realising how threats to eye health in these animals might be better understood and mitigated. Ophthalmoscopy can rapidly and non‐invasively image anatomical aspects of retinal and anterior ocular tissues and has been used in mammalian biomedicine since the turn of the 20th century. More now than ever, labour‐intensive post‐mortem approaches for ocular analysis such as histology are increasingly being replaced or supplemented by application of various forms of optical coherence tomography (OCT) imaging of ocular tissues in mammalian biomedicine. Advances and availability of other methodological approaches such as three‐dimensional printing and computer science make instrument customisation affordable and adaptable. This review article will outline how ophthalmoscopy, OCT, and other methodologies are being applied towards understanding ocular health in teleost fish species and will describe some of the future opportunities that technological advances might afford in advancing ocular imaging in fish health and disease in general. [ABSTRACT FROM AUTHOR]

The karyotype, which is the number and shape of chromosomes, is a fundamental characteristic of all eukaryotes. Karyotypic changes play an important role in many aspects of evolutionary processes, including speciation. In organisms with monocentric chromosomes, it was previously thought that chromosome number changes were mainly caused by centric fusions and fissions, whereas chromosome shape changes, that is, changes in arm numbers, were mainly due to pericentric inversions. However, recent genomic and cytogenetic studies have revealed examples of alternative cases, such as tandem fusions and centromere repositioning, found in the karyotypic changes within and between species. Here, we employed comparative genomic approaches to investigate whether centromere repositioning occurred during karyotype evolution in medaka fishes. In the medaka family (Adrianichthyidae), the three phylogenetic groups differed substantially in their karyotypes. The Oryzias latipes species group has larger numbers of chromosome arms than the other groups, with most chromosomes being metacentric. The O. javanicus species group has similar numbers of chromosomes to the O. latipes species group, but smaller arm numbers, with most chromosomes being acrocentric. The O. celebensis species group has fewer chromosomes than the other two groups and several large metacentric chromosomes that were likely formed by chromosomal fusions. By comparing the genome assemblies of O. latipes, O. javanicus, and O. celebensis, we found that repositioning of centromere‐associated repeats might be more common than simple pericentric inversion. Our results demonstrated that centromere repositioning may play a more important role in karyotype evolution than previously appreciated. [ABSTRACT FROM AUTHOR]

Aim: To identify the physiological role of the acid‐base sensing enzyme, soluble adenylyl cyclase (sAC), in red blood cells (RBC) of the model teleost fish, rainbow trout. Methods: We used: (i) super‐resolution microscopy to determine the subcellular location of sAC protein; (ii) live‐cell imaging of RBC intracellular pH (pHi) with specific sAC inhibition (KH7 or LRE1) to determine its role in cellular acid‐base regulation; (iii) spectrophotometric measurements of haemoglobin–oxygen (Hb‐O2) binding in steady‐state conditions; and (iv) during simulated arterial‐venous transit, to determine the role of sAC in systemic O2 transport. Results: Distinct pools of sAC protein were detected in the RBC cytoplasm, at the plasma membrane and within the nucleus. Inhibition of sAC decreased the setpoint for RBC pHi regulation by ~0.25 pH units compared to controls, and slowed the rates of RBC pHi recovery after an acid‐base disturbance. RBC pHi recovery was entirely through the anion exchanger (AE) that was in part regulated by HCO3−‐dependent sAC signaling. Inhibition of sAC decreased Hb‐O2 affinity during a respiratory acidosis compared to controls and reduced the cooperativity of O2 binding. During in vitro simulations of arterial‐venous transit, sAC inhibition decreased the amount of O2 that is unloaded by ~11%. Conclusion: sAC represents a novel acid‐base sensor in the RBCs of rainbow trout, where it participates in the modulation of RBC pHi and blood O2 transport though the regulation of AE activity. If substantiated in other species, these findings may have broad implications for our understanding of cardiovascular physiology in vertebrates. [ABSTRACT FROM AUTHOR]

Histopathological studies of parasitic infections in fish from the natural environment of Brazilian Amazon, are quite scarce. In this study, we investigated the histopathological changes of the proximal intestine of specimens of the Amazonian fish Hoplias malabaricus infected by the hematophagous nematode Procamallanus (Spirocamallanus) paraensis. The prevalence of the infection was 60%, with an average abundance of 1.46 and an average intensity of 2.43 parasites/fish. No significant difference was observed in the prevalence of infection and fish size or sex, but larger fish showed greater infection intensity, which was also significantly higher in male hosts. Histological sections of the proximal intestine showed reduction and loss of the epithelial lining, exposure of the lamina propria where the nematode interacts with the intestine wall, through the insertion of the buccal capsule and fish cellular debris in the intestinal lumen. In addition, areas with bleeding and inflammatory infiltrate were observed, but no changes or presence of parasite structures were observed in the other tunics of the intestinal wall. [ABSTRACT FROM AUTHOR]

Skin microbiomes provide vital functions, yet knowledge about the drivers and processes structuring their species assemblages is limited—especially for non‐model organisms. In this study, fish skin microbiome was assessed by high throughput sequencing of amplicon sequence variants from metabarcoding of V3–V4 regions in the 16S rRNA gene on fish hosts subjected to the following experimental manipulations: (i) translocation between fresh and brackish water habitats to investigate the role of environment; (ii) treatment with an antibacterial disinfectant to reboot the microbiome and investigate community assembly and priority effects; and (iii) maintained alone or in pairs to study the role of social environment and inter‐host dispersal of microbes. The results revealed that fish skin microbiomes harbour a highly dynamic microbial composition that was distinct from bacterioplankton communities in the ambient water. Microbiome composition first diverged as an effect of translocation to either the brackish or freshwater habitat. When the freshwater individuals were translocated back to brackish water, their microbiome composition converged towards the fish microbiomes in the brackish habitat. In summary, external environmental conditions and individual‐specific factors jointly determined the community composition dynamics, whereas inter‐host dispersal had negligible effects. The dynamics of the microbiome composition was seemingly non‐affected by reboot treatment, pointing towards high resilience to disturbance. The results emphasised the role of inter‐individual variability for the unexplained variation found in many host‐microbiome systems, although the mechanistic underpinnings remain to be identified. [ABSTRACT FROM AUTHOR]

Fish, first jawed vertebrate group appearing in evolution, have immune system similar to those of mammals. These early animals' innate and adaptive immune systems are completely developed, having head kidney and thymus as primary, spleen and mucosa associated lymphoid tissue as secondary immune organs. The adaptive immunity performs key role in defence against recurring infections and elimination of fish pathogens with the help of its memory cells, T‐cell‐receptors and immunoglobulins. Epidermal secretions including antibacterial peptides, lysozymes, lectins, complement and C‐reactive proteins which function in disruption of antigens, phagocytosis, inflammatory response and mending of tissue injury play a significant role as elements of fish immune system. Fish, however, have greater innate immunity than humans do. Estrogens play crucial role in immunomodulation through nuclear oestrogen receptors, which are found in majority of immune cells and lymphoid organs in fish. There is mounting proof that xenoestrogens, that can attach as agonists to oestrogen receptors, pose significant ecotoxicological risk by disrupting the defence mechanism in fish. Though less potent than natural oestrogen, they can bioaccumulate to finally reach a substantial dose. This review's objective is to give an overview of the fish defence system, its estrogenic regulation and estrogenic‐endocrine disruption. [ABSTRACT FROM AUTHOR]

We studied the topography of retinal ganglion cells (GCs) and estimated spatial resolving power (SRP) in the pajama cardinalfish Sphaeramia nematoptera (Bleeker, 1856), a relatively small brightly colored fish inhabiting coral reefs and lagoons in the Western Pacific. S. nematoptera is an active night predator feeding on near‐bottom animal plankton and benthos. DAPI staining was used to label nuclei of GCs and non‐GCs in the inner plexiform and ganglion cell layers. Non‐GCs were distinguished from GCs in Nissl‐stained retinal wholemounts based on cell size, shape, and staining intensity. The proportion of displaced amacrine cells (DACs) varied from 15.46 ± 1.12 (visual streak [VS]) to 17.99 ± 1.06% (dorsal periphery) (mean ± S.E.M., N = 5); the respective proportions of glial cells were 6.61 ± 0.84 and 5.89 ± 0.76%. Thus, 76%–78% of cells in the ganglion cell layer and inner plexiform layer were GCs. The minimum spatial coverage of GCs (3600–4600 cells/mm2) was detected in the dorsal and ventral periphery. It gradually increased toward the central retina to form a moderate VS. The maximum GC density (11,400–12,400 cells/mm2) was registered in the central portion of the VS. No pronounced concentric retinal specializations were found. The total number of GCs ranged within 595.2–635.9 × 103. The anatomical spatial resolving power was minimum in the ventral periphery (4.91–5.53 cpd) and maximum in the central portion of the VS (8.47–9.07 cpd). The respective minimum separable angles were 0.18–0.20° and 0.11–0.12°. The relatively high spatial resolving power and presence of the VS in the pajama cardinalfish are in line with its highly visual behavior. [ABSTRACT FROM AUTHOR]

Mormyroidea is a superfamily of weakly electric African fishes with great potential as a model in a variety of biomedical research areas including systems neuroscience, muscle cell and craniofacial development, ion channel biophysics, and flagellar/ciliary biology. However, they are currently difficult to breed in the laboratory setting, which is essential for any tractable model organism. As such, there is a need to better understand the reproductive biology of mormyroids to breed them more reliably in the laboratory to effectively use them as a biomedical research model. This review seeks to (1) briefly highlight the biomedically relevant phenotypes of mormyroids and (2) compile information about mormyroid reproduction including sex differences, breeding season, sexual maturity, gonads, gametes, and courtship/spawning behaviors. We also highlight areas of mormyroid reproductive biology that are currently unexplored and/or have the potential for further investigation that may provide insights into more successful mormyroid laboratory breeding methods. Research Highlights: Mormyroids have great potential as a model in biomedical research, but are difficult to breed in the lab.Reviewing literature on mormyroid reproductive biology reveals common themes throughout the group and highlights areas where further research may inform lab breeding efforts. [ABSTRACT FROM AUTHOR]

The secondary general visceral sensory nucleus (SVN) receives ascending fibers from the commissural nucleus of Cajal (NCC), or the primary general visceral sensoru in the medulla oblongata of teleosts. However, the full set of fiber connections of the SVN have been studied only in the Nile tilapia. We have investigated the connections of the SVN in goldfish by tracer injection experiments to the nucleus. We paid special attention to the possible presence of spinal afferents, since the spinal cord projects to the lateral parabrachial nucleus, or the presumed homologue of SVN, in mammals. We found that the SVN indeed receives spinal projections. Spinal terminals were restricted to a region ventrolaterally adjacent to the terminal zone of NCC fibers, suggesting that the SVN can be subdivided into two subnuclei: the commissural nucleus‐recipient (SVNc) and spinal‐recipient (SVNsp) subnuclei. Tracer injections to the SVNc and SVNsp as well as reciprocal injections to the diencephalon revealed that both subnuclei project directly to diencephalic structures, such as the posterior thalamic nucleus and nucleus of lateral recess, although diencephalic projections of the SVNsp were rather sparse. The SVNsp appears to send fibers to more wide‐spread targets in the preoptic area than the SVNc does. The SVNc projects to the telencephalon, while the SVNsp sends scarce or possibly no fibers to the telencephalon. Another notable difference was that the SVNsp gives rise to massive projections to the dorsal diencephalon (ventromedial thalamic, central posterior thalamic, and periventricular posterior tubercular nuclei). These differential connections of the subnuclei may reflect discrete functional significances of the general visceral sensory information mediated by the medulla oblongata and spinal cord. [ABSTRACT FROM AUTHOR]

The torus semicircularis (TS) of teleosts is a key midbrain center of the lateral line and acoustic sensory systems. To characterize the TS in adult zebrafish, we studied their connections using the carbocyanine tracers applied to the TS and to other related nuclei and tracts. Two main TS nuclei, central and ventrolateral, were differentiable by their afferent connections. From central TS, (TSc) numerous toropetal cells were labeled bilaterally in several primary octaval nuclei (anterior, magnocellular, descending, and posterior octaval nuclei), in the secondary octaval nucleus, in the caudal octavolateralis nucleus, and in the perilemniscular region. In the midbrain, numerous toropetal cells were labeled in the contralateral TSc. In the diencephalon, toropetal cells labeled from the TSc were observed ipsilaterally in the medial prethalamic nucleus and the periventricular posterior tubercle nucleus. TSc toropetal neurons were also labeled bilaterally in the hypothalamic anterior tuberal nucleus (ATN) and ipsilaterally in the parvicellular preoptic nucleus but not in the telencephalon. Tracer application to the medial octavolateralis nucleus revealed contralateral projections to the ventrolateral TS (TSvl), whereas tracer application to the secondary octaval nucleus labeled fibers bilaterally in TSc and neurons in rostral TSc. The TSc sends ascending fibers to the ipsilateral lateral preglomerular region that, in turn, projects to the pallium. Application of DiI to the optic tectum labeled cells and fibers in the TSvl, whereas application of DiI to the ATN labeled cells and fibers in the TSc. These results reveal that the TSvl and TSc are mainly related with the mechanosensory lateral line and acoustic centers, respectively, and that they show different higher order connections. [ABSTRACT FROM AUTHOR]

Sex steroids are known to modulate immune responses and as a result many of the immune parameters in seasonally breeding organisms show reproductive‐phase dependent variation. Androgens, the male sex steroids, are largely reported to be immunosuppressive. Together with other pattern recognition receptors, the nucleotide‐binding and oligomerization domain‐like receptors (NLRs) serve as intracellular sentinels and are essential to defense mechanisms. Interestingly, to date the transcriptional modulation of NLRs by androgens has not been explored. In the present study, we investigated the reproductive‐phase dependent expression of NLRs in the male spotted snakehead Channa punctata. Furthermore, the effect of dihydrotestosterone (DHT) on NLR expression was studied. The expression of NLRs was observed to be most pronounced during the spawning phase of the fish, which is marked by the highest testosterone level. In vivo as well as in vitro studies showed the diverse effect of DHT on NLR expression depending on the duration and mode of treatment, as well as the immune tissue studied. [ABSTRACT FROM AUTHOR]

In mammals, T‐cell development depends on the activity of the Foxn1 transcription factor in the thymic epithelium; mutations in the vertebrate‐specific Foxn1 gene are associated with profound T‐cell lymphopenia and fatal immunodeficiency. Here, we examined the extent of T‐cell development in teleosts lacking a functional foxn1 gene. In zebrafish carrying a deleterious internal deletion of foxn1, reduced but robust lymphopoietic activity is maintained in the mutant thymus. Moreover, pseudogenization or loss of foxn1 in the genomes of deep‐sea anglerfishes is independent of the presence or absence of the canonical signatures of the T‐cell lineage. Thus, in contrast to the situation in mammals, the teleost thymus can support foxn1‐independent lymphopoiesis, most likely through the activity of the Foxn4, an ancient metazoan paralog of Foxn1. Our results imply that during the early stages of vertebrate evolution, genetic control of thymopoiesis was functionally redundant and thus robust; in mammals, the genetic network was reorganized to become uniquely dependent on the FOXN1 transcription factor. [ABSTRACT FROM AUTHOR]

Mutations are the primary source of all genetic variation. Knowledge about their rates is critical for any evolutionary genetic analyses, but for a long time, that knowledge has remained elusive and indirectly inferred. In recent years, parent–offspring comparisons have yielded the first direct mutation rate estimates. The analyses are, however, challenging due to high rate of false positives and no consensus regarding standardized filtering of candidate de novo mutations. Here, we validate the application of a machine learning approach for such a task and estimate the mutation rate for the guppy (Poecilia reticulata), a model species in eco‐evolutionary studies. We sequenced 4 parents and 20 offspring, followed by screening their genomes for de novo mutations. The initial large number of candidate de novo mutations was hard‐filtered to remove false‐positive results. These results were compared with mutation rate estimated with a supervised machine learning approach. Both approaches were followed by molecular validation of all candidate de novo mutations and yielded similar results. The ML method uniquely identified three mutations, but overall required more hands‐on curation and had higher rates of false positives and false negatives. Both methods concordantly showed no difference in mutation rates between families. Estimated here the guppy mutation rate is among the lowest directly estimated mutation rates in vertebrates; however, previous research has also found low estimated rates in other teleost fishes. We discuss potential explanations for such a pattern, as well as future utility and limitations of machine learning approaches. [ABSTRACT FROM AUTHOR]

The increasing microplastic pollution in the marine environment has raised global concern. The main risk of microplastics in aquatic ecosystem is their bioaccumulation in aquatic organisms. A few studies have reported microplastic pollution in the digestive system of Caspian Sea fish species, but there is no research on sturgeon species, nor on fish gills. We investigated the occurrence of microplastics in the gastrointestinal tract (GIT) and gills of 62 specimens belonging to four species including three teleosts (Cyprinus carpio, Rutilus kutum, and Chelon aurata) and one sturgeon (Acipenser persicus, a valuable endangered species) from the Caspian Sea between January and March 2022. Fish tissues were removed, exposed for 24 h to 10% KOH, and then dried on filter paper. Particles were observed under a stereomicroscope and analyzed by Raman microspectrometry, scanning electron microscopy, and energy‐dispersive spectroscopy. A total of 91 microplastics were detected in the GIT (average of 1.46 ± 1.17 items/individual) and 63 microplastics in the gills (average of 1.01 ± 0.62 items/individual). A significant correlation was not found between the number of microplastics found in both tissues and fish body length, body weight, GIT weight, and gill weight (p > 0.05), except between microplastics isolated from gills and gill weight in C. carpio (rs = 0.707, p = 0.022). The abundance of microplastics in fish followed the order of A. persicus > C. aurata > R. kutum > C. carpio. The microplastics were in the size range of 45 to 5000 µm, with particles of 300 to 1000 µm being the most prevalent; 74.68% of the particles were shaped like fibers, 30.53% were red, and 70.6% were composed of nylon polymer. Environ Toxicol Chem 2023;42:2453–2465. © 2023 SETAC [ABSTRACT FROM AUTHOR]

The mapará, Hypophthalmus marginatus, is a rheophilic and planktophagous catfish and it is one of the main fishing resources in the Amazon region. To understand its nutritional characteristics, this work described the morphology and histochemistry of the digestive tract of H. marginatus. The oropharyngeal cavity has abundant, long and thin gill rakers, responsible for retaining plankton, while the short and muscular oesophagus promotes the transport of these items to the stomach and prevents water ingestion. The stratified oesophageal epithelium has goblet cells with mucins predominantly neutral that could facilitate the food passage the food passage. The U‐shaped siphonal stomach shows a columnar epithelium with neutral mucins to protect against autodigestion. The cardiac and fundic regions have gastric glands, while the pyloric region has a thick muscular layer with a sphincter. The coiled intestine with intestinal quotient of 2.14 ± 0.5 shows long longitudinal folds in the anterior region, decreasing in height in the aboral direction, and highlighting its function in digestion and nutrient absorption. Goblet cells are abundant in posterior intestine and rectum, and rectum shows epithelial cells with mucins in the apical cytoplasm, for protection and defecation. Intraepithelial lymphocytes also are abundant in posterior intestine and rectum, and it acts in immunological defence. [ABSTRACT FROM AUTHOR]

Background: Syngnathids are a highly derived and diverse fish clade comprising the pipefishes, pipe‐horses, and seahorses. They are characterized by a plethora of iconic traits that increasingly capture the attention of biologists, including geneticists, ecologists, and developmental biologists. The current understanding of the origins of their derived body plan is, however, hampered by incomplete and limited descriptions of the early syngnathid ontogeny. Results: We provide a comprehensive description of the development of Nerophis ophidion, Syngnathus typhle, and Hippocampus erectus from early cleavage stages to release from the male brooding organ and beyond, including juvenile development. We comparatively describe skeletogenesis with a particular focus on dermal bony plates, the snout‐like jaw morphology, and appendages. Conclusions: This most comprehensive and detailed account of syngnathid development to date suggests that convergent phenotypes (e.g., reduction and loss of the caudal fins), likely arose by distinct ontogenetic means in pipefishes and seahorses. Comparison of the ontogenetic trajectories of S. typhle and H. erectus provides indications that characteristic features of the seahorse body plan result from developmental truncation. Altogether, this work provides a valuable resource and framework for future research to understand the evolution of the outlandish syngnathid morphology from a developmental perspective. Key Findings: This study provides a comprehensive description of syngnathid (pipefishes & seahorses) development and skeletogenesis, highlighting how and when iconic features arise during the early ontogeny and across representatives of three main clades of this fish family.By comparatively analyzing the development of skeletal elements contributing to many of these fishes' most iconic features, such as their snout‐like facial morphology, the absence of certain fins and the bony plate armor, we identify key stages, structures, and suggest evolutionary mechanisms driving their unique ontogenetic trajectories.We find that several iconic syngnathid traits are the result of developmental truncation, leading to neotenic features or even feature loss, and we conclude that developmental heterochrony likely played a central role in establishing the diversity found among many morphological traits across and within syngnathid species. [ABSTRACT FROM AUTHOR]

Nascent pairs of ecologically differentiated species offer an opportunity to get a better glimpse at the genetic architecture of speciation. Of particular interest is our recent ability to consider a wider range of genomic variants, not only single‐nucleotide polymorphisms (SNPs), thanks to long‐read sequencing technology. We can now identify structural variants (SVs) such as insertions, deletions and other rearrangements, allowing further insights into the genetic architecture of speciation and how different types of variants are involved in species differentiation. Here, we investigated genomic patterns of differentiation between sympatric species pairs (Dwarf and Normal) belonging to the lake whitefish (Coregonus clupeaformis) species complex. We assembled the first reference genomes for both C. clupeaformis sp. Normal and C. clupeaformis sp. Dwarf, annotated the transposable elements and analysed the genomes in the light of related coregonid species. Next, we used a combination of long‐ and short‐read sequencing to characterize SVs and genotype them at the population scale using genome‐graph approaches, showing that SVs cover five times more of the genome than SNPs. We then integrated both SNPs and SVs to investigate the genetic architecture of species differentiation in two different lakes and highlighted an excess of shared outliers of differentiation. In particular, a large fraction of SVs differentiating the two species correspond to insertions or deletions of transposable elements (TEs), suggesting that TE accumulation may represent a key component of genetic divergence between the Dwarf and Normal species. Together, our results suggest that SVs may play an important role in speciation and that, by combining second‐ and third‐generation sequencing, we now have the ability to integrate SVs into speciation genomics. [ABSTRACT FROM AUTHOR]

The bidirectional interaction between the hypothalamic–pituitary‐gonadal (HPG) axis and the immune system plays a crucial role in the adaptation of an organism to its environment, its survival and the continuance of a species. Nonetheless, very little is known about this interaction among teleost, the largest group of extant vertebrates. Fishes being seasonal breeders, their immune system is exposed to seasonally changing levels of HPG hormones. On the contrary, the presence and infiltration of leukocytes, the expression of pattern recognition receptors as well as cytokines in gonads suggest their key role in teleostean gametogenesis as in the case of mammals. Moreover, the modulation of gametogenesis and steroidogenesis by lipopolysaccharide implicates the pathological significance of inflammation on reproduction. Thus, it is important to engage in the understanding of the interaction between these two important physiological systems, not only from a phylogenetic perspective but also due to the importance of fish as an important economic resource. In view of this, the authors have reviewed the crosstalk between the reproductive and immune systems in teleosts and tried to explore the importance of this interaction in their survival and reproductive fitness. [ABSTRACT FROM AUTHOR]

Microglia are macrophage‐like cells exerting determinant roles in neuroinflammatory and oxidative stress processes during brain regeneration. We used zebrafish as a model of brain plasticity and repair. First, by performing L‐plastin (Lcp1) immunohistochemistry and using transgenic Tg(mpeg1.1:GFP) or Tg(mpeg1.1:mCherry) fish, we analyzed the distribution of microglia/immune cells in the whole brain. Specific regional differences were evidenced in terms of microglia/immune cell density and morphology (elongated, branched, highly branched, and amoeboid). Taking advantage of Tg(fli:GFP) and Tg(GFAP::GFP) enabling the detection of endothelial cells and neural stem cells (NSCs), we highlighted the association of elongated microglia/immune cells with blood vessels and rounded/amoeboid microglia with NSCs. Second, after telencephalic injury, we showed that L‐plastin cells were still abundantly present at 5 days post‐lesion (dpl) and were associated with regenerative neurogenesis. Finally, RNA‐sequencing analysis from injured telencephalon (5 dpl) confirmed the upregulation of microglia/immune cell markers and highlighted a significant increase of genes involved in oxidative stress (nox2, nrf2a, and gsr). The analysis of antioxidant activities at 5 dpl also revealed an upregulation of superoxide dismutase and persistent H2O2 generation in the injured telencephalon. Also, microglia/immune cells were shown to be a source of oxidative stress at 5 dpl. Overall, our data provide a better characterization of microglia/immune cell distribution in the healthy zebrafish brain, highlighting some evolutionarily conserved features with mammals. They also emphasize that 5 days after injury, microglia/immune cells are still activated and are associated to a persistent redox imbalance. Together, these data raise the question of the role of oxidative stress in regenerative neurogenesis in zebrafish. [ABSTRACT FROM AUTHOR]

Advances in aquatic animal medicine and continued growth of the fish hobbyist and aquaculture communities have led to a developing interest in antemortem diagnostic imaging of aquatic species. The aims of this prospective, pilot study were to determine whether advanced neuroimaging can be safely achieved in live fish using clinically available equipment, to optimize imaging parameters, and to develop a comparative MRI atlas of a few fish species of economic or research value. Two each of channel catfish (Ictalurus punctatus), koi (Cyprinus rubrofuscus), and grass carp (Ctenopharyngodon idella) of at least 30 cm in length were individually anesthetized for 3 Tesla (3T) magnetic resonance imaging (MRI) of the brain. All fish achieved an adequate anesthetic level for prolonged immobilization during imaging. Diagnostic quality images were obtained for all subjects; however, the spatial resolution was maximized with larger fish. Imaging protocols were optimized for standard neuroimaging sequences. Additionally, inversion times for fluid‐attenuation inversion recovery (FLAIR) sequences were adapted to the naturally high protein content of fish pericerebral fluid. Following imaging, the fish successfully recovered from anesthesia, were humanely euthanized, and were immediately processed to assess brain histopathology. Necropsy confirmed the sex and health status of each fish. A limited comparative MRI atlas was created of the brains of these species for clinical reference. Findings from the current study supported the use of 3T MRI as an adjunct diagnostic test for fish with suspected neurologic disease and provided a limited anatomic atlas of the teleost brain for use as a reference. [ABSTRACT FROM AUTHOR]

The global aquaculture boom, fuelled by a reduction in wild population and detection of novel viruses, has created a demanding market, hence, there is a pressing need to investigate the immune system of fish, further. As the most diverse community of vertebrates and a central contributor to the progressing global aquaculture market, teleost continues to draw vast scientific interest. Recent breakthroughs in multi‐omics technologies have provided a platform to understand the role of long non‐coding RNA (lncRNA) in the host immune system during infection. Emerging evidence shows that teleost lncRNA might have a regulatory role in immune responses, mostly through lncRNA–microRNA (miRNA) sponging. Teleost lncRNA shares a functionally active short sequence complement to target the miRNA which is conserved among the several fish species. Recent report suggests that rhabdovirus exploits a lncRNA in teleost and, to dodge the host immune mechanism and negatively regulate the immune system. This observation reveals the essentiality of lncRNA in pathogen‐driven immunity in teleost. Reports available on the function of teleost lncRNA are still in early stages and experimental verifications are a limiting factor. Unravelling the lncRNA‐mediated immune regulation in fishes could be used against the invading pathogens to strengthen the aquaculture production. This review elaborates on the experimentally identified and functionally characterized lncRNA and its regulatory role in the teleost immune response during infection and pathogen‐driven host immune evolution, which could eventually lead to achieving high standards in aquaculture productivity. [ABSTRACT FROM AUTHOR]

The ability of an organism to cope with environmental stressors varies across the life span because of developmental stage–specific responses and age‐related functional declines. In the present study, we examined the effect of age on acute copper toxicity in Japanese medaka (Oryzias latipes). We first determined the median lethal concentration (LC50) at 96 h for embryos, 7‐day‐old fry, and 6‐month‐old medaka. Embryos were exposed to 0, 15, 30, 60, 125, 250, and 500 ppb CuSO4 through hatching. Fry were exposed to 0, 20, 50, 75, 100, 150, 250, and 500 ppb CuSO4 for 96 h. Adult fish were exposed to 0, 100, 150, 200, 250, and 300 ppb CuSO4 for 96 h. The 96‐h LC50 was 804 ppb for embryos, 262 ppb for embryonically exposed larvae, 60.3 ppb for 7‐day‐old fry, and 226 ppb for adults. We then challenged cohorts of fish aged 2, 3, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, and 16 months with a 225‐ppb CuSO4 exposure to determine the acute toxicity across the life span. The fish exhibited a bimodal tolerance to copper, with tolerance peaking in 2‐ and 3‐month‐old fish and again at 10 and 11 months of age. Our data demonstrate that copper sensitivity is dynamic throughout the medaka life span and may be influenced by trade‐offs with reproduction. Environ Toxicol Chem 2022;41:2999–3006. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

Quantifying discards is essential for assessing the impact of fisheries on non‐target species and the ecosystems in which these fisheries operate. In Western Australia (WA), fishers are required only to report catches of retained species. For the currently operating shark fisheries of WA, the authors quantified catch time series of discarded teleosts using data from at‐sea observers collected since 1993. Sixty‐two teleost species were observed in the catch of which 20 were routinely discarded. The most commonly discarded teleosts were western buffalo bream/silver drummer, Kyphosus cornelii/K. sydneyanus, and dusky morwong, Dactylophora nigricans. Annual discards peaked in the 1990s; nonetheless, current discard levels, 36.9 ± 2.1 (S.E.) t year−1, are much lower than the overall annual retained catches (c. 1000 t year−1). The catch time series reconstructed in this study are important inputs for assessing the ecosystem‐based approach used for managing WA's shark fisheries. [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) produced by the enzyme neuronal nitric oxide synthase serves as an important neurotransmitter in the central nervous system that is involved in reproductive regulation, learning, sensory processing, and other forms of neural plasticity. Here, we map the distribution of nnos‐expressing cells in the brain and retina of the cichlid fish Astatotilapia burtoni using in situ hybridization. In the brain, nnos‐expressing cells are found from the olfactory bulbs to the hindbrain, including within specific nuclei involved in decision‐making, sensory processing, neuroendocrine regulation, and the expression of social behaviors. In the retina, nnos‐expressing cells are found in the inner nuclear layer, presumably in amacrine cells. We also used quantitative PCR to test for differences in nnos expression within the eye and olfactory bulbs of males and females of different reproductive states and social statuses. In the eye, males express more nnos than females, and socially dominant males express more nnos than subordinate males, but expression did not differ among female reproductive states. In the olfactory bulbs, dominant males had greater nnos expression than subordinate males. These results suggest a status‐specific function for NO signaling in the visual and olfactory systems that may be important for sensory perception related to mating or territorial interactions to maintain the social hierarchy. The widespread distribution of nnos‐expressing cells throughout the cichlid brain is similar to that in other teleosts, with some conserved localization patterns across vertebrates, suggesting diverse functions for this important neurotransmitter system. [ABSTRACT FROM AUTHOR]

Temperature is an important environmental modulator of teleost immune activity. Susceptibility of teleosts to temperature variation depends on the species‐specific adaptive temperature range, and the activity of the teleost immune system is generally temperature‐dependent. Similar to many physiological and metabolic traits of ectotherms, temperature modulates the activity of immune traits. At low temperatures, acquired immunity of many teleost species is down‐modulated, and their immuno‐competence mainly depends on innate immunity. At intermediate temperatures, both innate and acquired immunity are fully active and provide optimal protection, including long‐lasting immunological memory. When temperatures increase and reach the upper permissive range, teleost immunity is compromised. Moreover, temperature shifts may have negative effects on teleost immune functions, in particular if shifts occur rapidly with high amplitudes. On the contrary, short‐term temperature increase may help teleost immunity to fight against pathogens transiently. A major challenge to teleosts therefore is to maintain immuno‐competence throughout the temperature range they are exposed to. Climate change coincides with rising temperatures, and more frequent and more extreme temperature shifts. Both are likely to influence the immuno‐competence of teleosts. Nonetheless, teleosts exist in habitats that differ substantially in temperature, ranging from below zero in the Arctic's to above 40°C in warm springs, illustrating their enormous potential to adapt to different temperature regimes. The present review seeks to discuss how changes in temperature variation, induced by climate change, might influence teleost immunity. [ABSTRACT FROM AUTHOR]

The chemical senses of olfaction and taste are well developed in fish and play a vital role in its various activities such as navigation, mate recognition, and food detection. The small teleost fish Astyanax mexicanus consists of interfertile river‐dwelling and cave‐dwelling populations, referred to as "surface fish" and "cavefish" respectively. An important anatomical feature of cavefish is the lack of eyes leading them to be referred to as blind fish and suggesting an enhanced functional role for other senses such as taste. In this study, we characterize the expression of bitter taste receptors (T2Rs or Tas2Rs) in A. mexicanus and investigate their functionality in a heterologous expression system. The genome database of A. mexicanus (ensemble and NCBI) showed 7 Tas2Rs, among these Tas2R1, Tas2R3, Tas2R4, and Tas2R114 are well characterized in humans and mice but not in A. mexicanus. Therefore, the 4 Tas2Rs were selected for further analysis and their expression in A. mexicanus was confirmed by in situ hybridization and RT‐PCR in early developmental stages. These Tas2Rs are expressed in various oral and extraoral organs (liver, fins, jaws, and gills) in A. mexicanus, and Tas2R1 has maximum expression and is localized throughout the fish body. Using the heterologous expression of A. mexicanus T2Rs in HEK293T cells coupled with cell‐based calcium mobilization assays, we show that A. mexicanus T2Rs are activated by commonly used fish food and known bitter agonists, including quinine. This study provides novel insights into the extraoral expression of T2Rs in A. mexicanus and suggests their importance in extraoral food detection. [ABSTRACT FROM AUTHOR]

Parkinson's disease (PD) is characterized by phosphorylation and aggregation of the protein α‐Synuclein and ensuing neuronal death progressing from the noradrenergic locus coeruleus to midbrain dopaminergic neurons. In 2019, Matsui and colleagues reported a spontaneous age‐dependent degeneration of dopaminergic neurons and an even greater neurodegeneration of the noradrenergic neurons in the short‐lived killifish Nothobranchius furzeri. Given the great possible relevance of a spontaneous model for PD, we assessed neurodegeneration of noradrenergic and dopaminergic neurons in two further laboratory strains of N. furzeri. We implemented, for the first time in N. furzeri, a whole‐brain clarification technique and proceeded to entire 3D nuclei reconstruction to quantify total cell numbers in two different stains of N. furzeri. In both strains, we observed that age‐dependent neurodegeneration is limited to the locus coeruleus and does not involve the posterior tuberculum. We also applied 3D counting to the optic tectum, an area of active adult neurogenesis, and detected an increase of neurons with age. Our results confirm age‐dependent neurodegeneration of noradrenergic neurons, a condition reminiscent of the presymptomatic stage of PD indicating that N. furzeri could be used in the future to identify modifying factors for age‐dependent neurodegeneration and open the intriguing possibility that natural genetic variation may influence the susceptibility of dopaminergic neurons. [ABSTRACT FROM AUTHOR]

We evaluated the effect of the light–dark (LD) cycle on energy expenditure indicators for two species of Amazonian fish of commercial interest, oscar (Astronotus ocellatus) and matrinxã (Brycon amazonicus). Fish were exposed to a 12:12 h LD cycle, with lights on at 'zeitgeber time' (ZT) = 0 h. Six animals were used for blood collection every 4 h for 24 h (six time points, three in the light period and three in the dark period). Most haematological parameters exhibited daily rhythms with the acrophase at the end of the dark phase or the beginning of the light period for both species, which may be related to the greater energy demand of these species during the day. For A. ocellatus, triglycerides and total protein showed acrophases at ZT = 13:32 ± 2:45 h and ZT = 23:42 ± 1:55 h, respectively, while the other plasma parameters showed no significant daily differences. In B. amazonicus, significant rhythms were observed for glucose levels (acrophase at ZT = 04:16 ± 2:47 h), triglycerides (ZT = 6:51 ± 4:06 h) and total cholesterol (ZT = 23:22 ± 3:45 h). However, total protein and cortisol levels did not show rhythmicity in this species. Our results highlight the importance of the evaluation of biological rhythms for plasma physiological parameters related to energy expenditure in species with commercial interest and susceptible to stressful situations in farming conditions. [ABSTRACT FROM AUTHOR]

Secretagogin (scgn), is a novel hexa EF‐hand, phylogenetically conserved calcium‐binding protein. It serves as Ca2+ sensor and participates in Ca2+‐signaling and neuroendocrine regulation in mammals. However, its relevance in the brain of non‐mammalian vertebrates has largely remained unexplored. To address this issue, we studied the cDNA encoding scgn, scgn mRNA expression, and distribution of scgn‐equipped elements in the brain and pituitary of a teleost, Clarias batrachus (cb). The cbscgn cDNA consists of three transcripts (T) variants: T1 (2185 bp), T2 (2151 bp) and T3 (2060 bp). While 816 bp ORF in T1 and T2 encodes highly conserved six EF‐hand 272 aa protein fully capable of Ca2+‐binding, 726‐bp ORF in T3 encodes 242 aa protein. The T1 showed >90% and >70% identity with scgn of catfishes, and other teleosts and mammals, respectively. The T1‐mRNA was widely expressed in the brain and pituitary, while the expression of T3 was restricted to the telencephalon. Application of the anti‐scgn antiserum revealed a ∼32 kDa scgn‐immunoreactive (scgn‐i) band (known molecular weight of scgn) in the forebrain tissue, and immunohistochemically labeled neurons in the olfactory epithelium and bulb, telencephalon, preoptic area, hypothalamus, thalamus, and hindbrain. In the pituitary, scgn‐i cells were seen in the pars distalis and intermedia. Insulin is reported to regulate scgn mRNA in the mammalian hippocampus, and feeding‐related neuropeptides in the telencephalon of teleost. Intracranial injection of insulin significantly increased T1‐mRNA expression and scgn‐immunoreactivity in the telencephalon. We suggest that scgn may be an important player in the regulation of olfactory, neuroendocrine system, and energy balance functions in C. batrachus. [ABSTRACT FROM AUTHOR]

We studied the expression of neurogranin in the brain and some sensory organs (barbel taste buds, olfactory organs, and retina) of adult zebrafish. Database analysis shows zebrafish has two paralog neurogranin genes (nrgna and nrgnb) that translate into three peptides with a conserved IQ domain, as in mammals. Western blots of zebrafish brain extracts using an anti‐neurogranin antiserum revealed three separate bands, confirming the presence of three neurogranin peptides. Immunohistochemistry shows neurogranin‐like expression in the brain and sensory organs (taste buds, neuromasts and olfactory epithelium), not being able to discern its three different peptides. In the retina, the most conspicuous positive cells were bipolar neurons. In the brain, immunopositive neurons were observed in all major regions (pallium, subpallium, preoptic area, hypothalamus, diencephalon, mesencephalon and rhombencephalon, including the cerebellum), a more extended distribution than in mammals. Interestingly, dendrites, cell bodies and axon terminals of some neurons were immunopositive, thus zebrafish neurogranins may play presynaptic and postsynaptic roles. Most positive neurons were found in primary sensory centers (viscerosensory column and medial octavolateral nucleus) and integrative centers (pallium, subpallium, optic tectum and cerebellum), which have complex synaptic circuitry. However, we also observed expression in areas not related to sensory or integrative functions, such as in cerebrospinal fluid‐contacting cells associated with the hypothalamic recesses, which exhibited high neurogranin‐like immunoreactivity. Together, these results reveal important differences with the patterns reported in mammals, suggesting divergent evolution from the common ancestor. [ABSTRACT FROM AUTHOR]

Phenotypic plasticity is an important driver of species resilience. Often mediated by epigenetic changes, phenotypic plasticity enables individual genotypes to express variable phenotypes in response to environmental change. Barramundi (Lates calcarifer) are a protandrous (male‐first) sequential hermaphrodite that exhibits plasticity in length‐at‐sex change between geographic regions. This plasticity is likely to be mediated by changes in DNA methylation (DNAm), a well‐studied epigenetic modification. To investigate the relationships between length, sex, and DNAm in a sequential hermaphrodite, here, we compare DNAm in four conserved vertebrate sex‐determining genes in male and female barramundi of differing lengths from three geographic regions of northern Australia. Barramundi first mature as male and later sex change to female upon the attainment of a larger body size; however, a general pattern of increasing female‐specific DNAm markers with increasing length was not observed. Significant differences in DNAm between males and females of similar lengths suggest that female‐specific DNAm arises rapidly during sex change, rather than gradually with fish growth. The findings also reveal that region‐specific differences in length‐at‐sex change are accompanied by differences in DNAm and are consistent with variability in remotely sensed sea temperature and salinity. Together, these findings provide the first in situ evidence for epigenetically and environmentally mediated sex change in a protandrous hermaphrodite and offer significant insight into the molecular and ecological processes governing the marked and unique plasticity of sex in fish. [ABSTRACT FROM AUTHOR]

Transposable elements (TEs) are significant genomic components which can be detected either through sequence homology against existing databases or de novo, with the latter potentially reducing the risk of underestimating TE abundance. Here, we describe the semi‐automated generation of a de novo TE library using the newly developed EDTA pipeline and DeepTE classifier in a non‐model teleost (Corydoras fulleri). Using both genomic and transcriptomic data, we assess this de novo pipeline's performance across four TE based metrics: (i) abundance, (ii) composition, (iii) fragmentation, and (iv) age distributions. We then compare the results to those found when using a curated teleost library (Danio rerio). We identify quantitative differences in these metrics and highlight how TE library choice can have major impacts on TE‐based estimates in non‐model species. [ABSTRACT FROM AUTHOR]

While the influence of Pleistocene climatic changes on divergence and speciation has been well‐documented across the globe, complex spatial interactions between hydrology and eustatics over longer timeframes may also determine species evolutionary trajectories. Within the Australian continent, glacial cycles were not associated with changes in ice cover and instead largely resulted in fluctuations from moist to arid conditions across the landscape. We investigated the role of hydrological and coastal topographic changes brought about by Plio‐Pleistocene climatic changes on the biogeographic history of a small Australian freshwater fish, the southern pygmy perch Nannoperca australis. Using 7958 ddRAD‐seq (double digest restriction‐site associated DNA) loci and 45,104 filtered SNPs, we combined phylogenetic, coalescent and species distribution analyses to assess the various roles of aridification, sea level and tectonics and associated biogeographic changes across southeast Australia. Sea‐level changes since the Pliocene and reduction or disappearance of large waterbodies throughout the Pleistocene were determining factors in strong divergence across the clade, including the initial formation and maintenance of a cryptic species, N. 'flindersi'. Isolated climatic refugia and fragmentation due to lack of connected waterways maintained the identity and divergence of inter‐ and intraspecific lineages. Our historical findings suggest that predicted increases in aridification and sea level due to anthropogenic climate change might result in markedly different demographic impacts, both spatially and across different landscape types. [ABSTRACT FROM AUTHOR]

Aim: Environmental DNA (eDNA) metabarcoding has demonstrated its applicability as a highly sensitive biomonitoring tool across small spatial and temporal scales in marine ecosystems. However, it has rarely been tested across large spatial scales or biogeographical barriers. Here, we scale up marine eDNA metabarcoding, test its ability to detect a major marine biogeographic break and evaluate its use as a regional biomonitoring tool in Australia. Location: North‐western Australia (NWA). Methods: We applied metabarcoding assays targeting the mitochondrial 16S rRNA and CO1 genes to 284 surface seawater eDNA samples collected from 71 mid‐shelf, inshore, coastal and nearshore estuarine sites over 700 km of the NWA coastline. Results: Metabarcoding detected a wide range of bony fish (404 taxa), elasmobranchs (44) and aquatic reptiles (5). We detected bioregional and depth differentiation within inshore bony fish communities. These findings support the presence of a marine biogeographic break, which is purported to occur in the vicinity of Cape Leveque, demarcating the border between the Kimberley and Canning bioregions. Inshore bony fish and elasmobranch communities, as well as coastal bony fish assemblages, were additionally found to differ between the South and North Kimberley regions suggesting previously unrecognized subregional differentiation amongst these taxa. The overall compositional data have been used to update distribution information for a number of endangered, elusive and data‐deficient taxa, including sawfish (family: Pristidae), northern river shark (Glyphis garricki) and wedgefish (genus: Rhynchobatus). Main conclusions: eDNA metabarcoding demonstrated a high level of sensitivity that was able to discern fine‐scale patterns across the large‐scale, remote and oceanographically complex region of North‐western Australia. Importantly, this study highlights the potential of integrating broad‐scale eDNA metabarcoding alongside other baseline surveys and long‐term monitoring approaches, which are crucial for the sustainable management and conservation of marine biodiversity in this unique marine region. [ABSTRACT FROM AUTHOR]

Interactions of organisms with their environment are complex and environmental regulation at different levels of biological organization is often nonlinear. Therefore, the genotype to phenotype continuum requires study at multiple levels of organization. While studies of transcriptome regulation are now common for many species, quantitative studies of environmental effects on proteomes are needed. Here we report the generation of a data‐independent acquisition (DIA) assay library that enables simultaneous targeted proteomics of thousands of Oreochromis niloticus kidney proteins using a label‐ and gel‐free workflow that is well suited for ecologically relevant field samples. We demonstrate the usefulness of this DIA assay library by discerning environmental effects on the kidney proteome of O. niloticus. Moreover, we demonstrate that the DIA assay library approach generates data that are complimentary rather than redundant to transcriptomic data. Transcript and protein abundance differences in kidneys of tilapia acclimated to freshwater and brackish water (25 g/kg) were correlated for 2114 unique genes. A high degree of non‐linearity in salinity‐dependent regulation of transcriptomes and proteomes was revealed suggesting that the regulation of O. niloticus renal function by environmental salinity relies heavily on post‐transcriptional mechanisms. The application of functional enrichment analyses using STRING and KEGG to DIA assay data sets is demonstrated by identifying myo‐inositol metabolism, antioxidant and xenobiotic functions, and signalling mechanisms as key elements controlled by salinity in tilapia kidneys. The DIA assay library resource presented here can be adopted for other tissues and other organisms to study proteome dynamics during changing ecological contexts. [ABSTRACT FROM AUTHOR]

The aging central nervous system (CNS) of mammals displays progressive limited regenerative abilities. Recovery after loss of neurons is extremely restricted in the aged brain. Many research models fall short in recapitulating mammalian aging hallmarks or have an impractically long lifespan. We established a traumatic brain injury model in the African turquoise killifish (Nothobranchius furzeri), a regeneration‐competent vertebrate that evolved to naturally age extremely fast. Stab‐wound injury of the aged killifish dorsal telencephalon unveils an impaired and incomplete regeneration response when compared to young individuals. In the young adult killifish, brain regeneration is mainly supported by atypical non‐glial progenitors, yet their proliferation capacity clearly declines with age. We identified a high inflammatory response and glial scarring to also underlie the hampered generation of new neurons in aged fish. These primary results will pave the way to unravel the factor age in relation to neurorepair, and to improve therapeutic strategies to restore the injured and/or diseased aged mammalian CNS. [ABSTRACT FROM AUTHOR]

Deep‐sea fishes have long captured our imagination with striking adaptations to life in the mysterious abyss, raising the possibility that this cold, dark ocean region may be a key hub for physiological and functional diversification. We explore this idea through an analysis of body shape evolution across ocean depth zones in over 3000 species of marine teleost fishes. We find that the deep ocean contains twice the body shape disparity of shallow waters, driven by elevated rates of evolution in traits associated with locomotion. Deep‐sea fishes display more frequent adoption of forms suited to slow and periodic swimming, whereas shallow living species are concentrated around shapes conferring strong, sustained swimming capacity and manoeuvrability. Our results support long‐standing impressions of the deep sea as an evolutionary hotspot for fish body shape evolution and highlight that factors like habitat complexity and ecological interactions are potential drivers of this adaptive diversification. [ABSTRACT FROM AUTHOR]

The definition of scavenger endothelial cells (SEC) is exclusively based on functional and structural characteristics. The following characteristics are common hallmarks for the vertebrate SEC: (a) All vertebrates examined are furnished with a population of special SEC that plays a role in the catabolism of physiologic and non‐physiologic soluble waste macromolecules. (b) From the ligands that are endocytosed, SEC in all seven vertebrate classes appear to express the collagen α‐chain receptor and the scavenger receptors. In addition, the hyaluronan and the mannose receptors are present on SEC of mammalia (several species) and osteichthyes (e.g., salmon and cod). It is likely that all four receptor types are present in all vertebrate classes. (c) Like liver endothelial cells (LEC) in mammals, SEC in all vertebrate classes are geared to endocytosis of soluble macromolecules, but phagocytic uptake of particles is taken care of mainly by macrophages. (d) The most primitive vertebrates (hagfish, lamprey and ray) carry their SEC in gill vessels, whereas phylogenetically younger fishes (salmon, carp, cod and plaice) carry their SEC in either kidney or heart and in all terrestrial vertebrates—SEC are found exclusively in the liver. (e) SEC of all vertebrates are localized in blood sinusoids or trabeculae that carry large amounts of slowly flowing and O2 poor blood. (f) SEC differs functionally and structurally from what is normally associated with "conventional vascular endothelium." [ABSTRACT FROM AUTHOR]

Fish harbour complex and dynamic microbiota in their mucosal surfaces. Due to the vital role of commensal microbiota in many biological functions including growth enhancement, nutrition, development, metabolism and immune response, many scholars have begun to refer to the microbiome as an 'extended self'. Several studies have indicated that a normal microbiota contributes to fish health, and disturbances in microbial communities may lead to secondary infections and disease. Under evolutionary selective forces, fish have developed excellent innate immune mechanisms and an adaptive immune system composed of B cells and T cells to discriminate between beneficial and opportunistic bacteria for microbiome homeostasis. Many functional studies have demonstrated the involvement of immune cells, cytokines and Igs in maintaining microbiome homeostasis, and many beneficial microbes contribute to improving the immune barrier against external threats. Additionally, many host‐derived probiotics have been empirically used in the aquaculture sector and have been demonstrated to provide effective protection against fish diseases. Although little is known about the interactions between microbiota and the mucosal immune system in fish compared to mammals, managing aquaculture system microbiota may represent a promising alternative to antibiotics for disease prevention and control. This review thus provides an updated and comprehensive summary of currently characterized microbe compositions in fish mucosal tissues. Thereafter, the current knowledge on the crosstalk between microbiota and the mucosal immune system and probiotics in application is also discussed herein, thereby providing a theoretical basis for the development of microbe‐based disease control technologies in aquaculture. [ABSTRACT FROM AUTHOR]

The present study investigated erythrocyte morphometry of 10 teleost species, Clarias gariepinus, Coregonus marena, Cyprinus carpio, Onchorynchus mykiss, Perca fluviatilis, Thymallus thymallus, Tinca tinca, Salmo trutta f. lacustris, Salvelinus fontinalis and Salvelinus umbla. Significant inter‐individual differences in erythrocyte morphometric parameters were found in all investigated species. Also significant species‐specific differences were observed, which were not related to their systematic position. The genetically fixed temperature optimum of the species correlated significantly and negatively with erythrocyte cell volume, cytoplasm volume, and cell surface and positively with erythrocyte surface to volume ratio indicating that erythrocyte morphometry is influenced by environmental parameters. In all investigated species, erythrocyte morphometry changed with the age of the fish as their size (cell volume and surface, cytoplasm volume) increased significantly in older ones. Changes in the surface to volume ratios were less evident as a significant decrease was only found in S. umbla and T. tinca. [ABSTRACT FROM AUTHOR]

The capacity of fishes to cope with environmental variation is considered to be a main determinant of their fitness and is partly determined by their stress physiology. By 2100, global ocean temperature is expected to rise by 1–4°C, with potential consequences for stress physiology. Global warming is affecting animal populations worldwide through chronic temperature increases and an increase in the frequency of extreme heatwave events. As ectotherms, fishes are expected to be particularly vulnerable to global warming. Although little information is available about the effects of global warming on stress physiology in nature, multiple studies describe the consequences of temperature increases on stress physiology in controlled laboratory conditions, providing insight into what can be expected in the wild. Chronic temperature increase constitutes a physiological load that can alter the ability of fishes to cope with additional stressors, which might compromise their fitness. In addition, rapid temperature increases are known to induce acute stress responses in fishes and might be of ecological relevance in particular situations. This review summarizes knowledge about effects of temperature increases on the stress physiology of fishes and discusses these in the context of global warming. [ABSTRACT FROM AUTHOR]

Global overfishing of higher‐level predators has caused cascading effects to lower trophic levels in many marine ecosystems. On coral reefs, which support highly diverse food webs, the degree to which top‐down trophic cascades can occur remains equivocal. Using extensive survey data from coral reefs across the relatively unfished northern Great Barrier Reef (nGBR), we quantified the role of reef sharks in structuring coral reef fish assemblages. Using a structural equation modeling (SEM) approach, we explored the interactions between shark abundance and teleost mesopredator and prey functional group density and biomass, while explicitly accounting for the potentially confounding influence of environmental variation across sites. Although a fourfold difference in reef shark density was observed across our survey sites, this had no impact on either the density or biomass of teleost mesopredators or prey, providing evidence for a lack of trophic cascading across nGBR systems. Instead, many functional groups, including sharks, responded positively to environmental drivers. We found reef sharks to be positively associated with habitat complexity. In turn, physical processes such as wave exposure and current velocity were both correlated well with multiple functional groups, reflecting how changes to energetic conditions and food availability, or modification of habitat affect fish distribution. The diversity of species within coral reef food webs and their associations with bottom‐up drivers likely buffers against trophic cascading across GBR functional guilds when reef shark assemblages are depleted, as has been demonstrated in other complex ecosystems. [ABSTRACT FROM AUTHOR]

The occurrence of neuroactive chemicals in the aquatic environment is on the rise and poses a potential threat to aquatic biota of currently unpredictable outcome. In particular, subtle changes caused by these chemicals to an organism's sensation or behavior are difficult to tackle with current test systems that focus on rodents or with in vitro test systems that omit whole‐animal responses. In recent years, the zebrafish (Danio rerio) has become a popular model organism for toxicological studies and testing strategies, such as the standardized use of zebrafish early life stages in the Organisation for Economic Co‐operation and Development's guideline 236. In terms of neurotoxicity, the zebrafish provides a powerful model to investigate changes to the nervous system from several different angles, offering the ability to tackle the mechanisms of action of chemicals in detail. The mechanistic understanding gained through the analysis of this model species provides a good basic knowledge of how neuroactive chemicals might interact with a teleost nervous system. Such information can help infer potential effects occurring to other species exposed to neuroactive chemicals in their aquatic environment and predicting potential risks of a chemical for the aquatic ecosystem. In the present article, we highlight approaches ranging from behavioral to structural, functional, and molecular analysis of the larval zebrafish nervous system, providing a holistic view of potential neurotoxic outcomes. Environ Toxicol Chem 2021;40:989–1006. © 2020 SETAC [ABSTRACT FROM AUTHOR]

Nonapeptides play a crucial role in mediating reproduction, aggression, and parental care across taxa. In fishes, arginine vasotocin (AVT) expression is related to social and/or reproductive status in most male fishes studied to date, and is linked to territorial defense, paternal care, and courtship. Despite a plethora of studies examining AVT in male fishes, relatively little is known about how AVT expression varies with female reproductive state or its role in female social behaviors. We used multiple methods for examining the AVT system in female African cichlid fish Astatotilapia burtoni, including immunohistochemistry for AVT, in situ hybridization for avt‐mRNA, and quantitative PCR. Ovulated and mouthbrooding females had similar numbers of parvocellular, magnocellular, and gigantocellular AVT cells in the preoptic area. However, ovulated females had larger magnocellular and gigantocellular cells compared to mouthbrooding females, and gigantocellular AVT cell size correlated with the number of days brooding, such that late‐stage brooding females had larger AVT cells than mid‐stage brooding females. In addition, we found that ventral hypothalamic cells were more prominent in females compared to males, and were larger in mouthbrooding compared to ovulated females, suggesting a role in maternal care. Together, these data indicate that AVT neurons change across the reproductive cycle in female fishes, similar to that seen in males. These data on females complement studies in male A. burtoni, providing a comprehensive picture of the regulation and potential function of different AVT cell types in reproduction and social behaviors in both sexes. [ABSTRACT FROM AUTHOR]

Anthropogenic noise has increased underwater ambient sound levels in the range in which most fishes detect and produce acoustic signals. Although the impacts of increased background noise on fish development have been studied in a variety of species, there is a paucity of information on how noise affects parental care. Mouthbrooding is an energetically costly form of parental care in which the brooding fish carries developing larvae in the buccal cavity for the duration of development. In the African cichlid Astatotilapia burtoni, females carry their brood for ~2 weeks during which time they do not eat. To test the hypothesis that increased background noise impacts maternal care behaviors and brood development, we exposed brooding females to a 3‐h period of excess noise (~140 dB) played through an underwater speaker. Over half of noise‐exposed brooding females cannibalized or pre‐maturely released their brood, but 90% of control females exhibited normal brooding behaviors. RNA‐seq analysis revealed that transcripts related to feeding and parental care were differentially expressed in the brains of noise‐exposed females. Juveniles that were exposed to noise during their brood period within the mother's mouth had lower body condition factors, higher mortality and altered head transcriptomes compared with control broods. Furthermore, onset of adult‐typical coloration and behaviors was delayed compared with control fish. Together, these data indicate that noise has severe impacts on reproductive fitness in mouthbrooding females. Our results, combined with past studies, indicate that parental care stages are extremely susceptible to noise‐induced perturbations with detrimental effects on species persistence. [ABSTRACT FROM AUTHOR]

The most commonly used technique to study the barostatic regulation of blood pressure in ectothermic vertebrates consists of determining the heart rate response to pharmacological manipulations of blood pressure, the so‐called "Oxford method." Although well established, the Oxford method has some important limitations, such as induction of hypervolemia in small animals and undesired effects of vasoactive drugs on central and peripheral baroreflex components. As an alternative, the sequence method, which consists in the computerized evaluation of naturally‐occurring baroreflex adjustments of heart rate without the need for pharmacological administrations, was developed to study baroreflexes. In the present study, we compare this sequence method with the Oxford technique in two teleost species with different life styles, and we assess the optimal software configuration for the employment of the sequence method in fish. Calculation of baroreflex gain through the sequence method was adequate and reliable when the software was configured to search for baroreflex sequences with a minimum length of three cardiac cycles with a delay of one cardiac cycle between fluctuations in mean ventral aortic blood pressure and reflex changes in pulse interval. When properly configured, the sequence and the Oxford methods yielded similar determinations of the baroreflex gain in fish. Research Highlights: The nonpharmacological sequence method was validated to study the baroreflex function of fish.When properly configured, the sequence method yielded determinations of baroreflex gain similar to that of the Oxford method. [ABSTRACT FROM AUTHOR]

With remarkably few exceptions, aquatic vertebrates maintain internal Cl− homeostasis despite strong and sometimes fluctuating Cl− concentration gradients between extracellular fluids and external environments. In this "Perspective," we discuss recent advances in the understanding of epithelial Cl− transport at the molecular level within key osmoregulatory organs in fishes. New insights into mechanisms for epithelial Cl− transport in basal lineages are highlighted to provide an evolutionary context. We describe Cl− transport processes that employ: cystic fibrosis transmembrane conductance regulator, cation‐chloride cotransporters, voltage‐gated chloride channels, and chloride‐anion exchangers. As the collective understanding of Cl− transport processes continues to expand, investigators are equipped to more precisely characterize how endocrine factors promote hydromineral balance. We, therefore, conclude our discussion by paying special attention to recently defined roles for prolactin and corticosteroids in the regulation of Cl− transport in basal and derived clades. Research highlights: This "Perspective" describes new findings regarding the cellular and molecular mechanisms that support Cl− homeostasis and aspects of their functional regulation in fishes. [ABSTRACT FROM AUTHOR]

The probiotic yeast Debaryomyces hansenii and a characterized essential oil from Mexican oregano Lippia graveolens (OEO) were assayed, separately or in combination, as feed additives in Longfin yellowtail Seriola rivoliana. Effects on skin mucosal immunity and gene expression were studied after a 6‐week experimental test, also carrying out routine histological examination of key organs (gut, liver and head kidney). Four diets were administered to fish: control (basal diet without additives); yeast (1.1% of live D. hansenii); OEO (0.05% of oregano essential oil); and Yeast + OEO (1.1% of live D. hansenii and 0.05% of OEO). Histological examination of key organs revealed a normal pattern from a healthy situation with any of the treatments. Skin mucosal immunity (protease, antiprotease, myeloperoxidase, lysozyme, superoxide dismutase, catalase and immunoglobulin M (IgM)) were evaluated. Furthermore, gene expression of two immune‐related genes (tumour necrosis factor alpha (TNF‐α) and piscidin) was analysed at head kidney and spleen by real‐time PCR. Interestingly, the Yeast diet produced an increase in most skin mucus immune‐related parameters—except for myeloperoxidase—and also an up‐regulation of TNF‐α expression. Meanwhile, the fish fed the Yeast + OEO and OEO diets had significantly higher values of IgM (both), lysozyme (OEO diet), antiprotease, myeloperoxidase and superoxide dismutase (Yeast + OEO diet), and both down‐regulated TNF‐α expression in spleen. Based on our results, D. hansenii and Mexican oregano essential oil, to a lesser extent, can improve the humoral immunity of skin mucus of Longfin yellowtail juveniles. Nevertheless, the combination of both has no synergistic effect. [ABSTRACT FROM AUTHOR]

Xenotoca eiseni is a viviparous teleost belonging to the family Goodeidae. Here, we report histological observations of the reproductive organs in an adult male, an adult female, a pregnant female with intraovarian embryo and an extracted embryo of X. eiseni. High‐resolution images of haematoxylin–eosin‐stained sagittal sections revealed the detailed structure of gonads, gametes and reproductive components of the mother–embryo relationship. In the male, mature spermatozoa in the epididymis formed sperm packages. In the female, oogenesis proceeded asynchronously in the ovarian wall, and various stages of oocytes were observed in single ovary. In both sexes, genital openings were located between the anus and anal fin. Developing embryos were observed in an ovary of the pregnant female. Fine structures of components of the mother‐to‐embryo nutrient supply, ovarian septum and trophotaenia were observed in the pregnant ovary. An immature gonad prior to gamete formation was identified in the extracted embryo. With the aim of supporting the development and extension of studies on this viviparous teleost, we have shared our histological images as raw data in an open online archive, the 'NAGOYA repository (http://hdl.handle.net/2237/00032456)'. Our goal is a comprehensive understanding of the viviparous system in fish using both histological observation and molecular biology methods including genomics and proteomics. [ABSTRACT FROM AUTHOR]

Across the teleost skeleton, cartilages are diverse in their composition suggesting subtle differences in their developmental mechanisms. This study aims to elucidate the regulatory role of bone morphogenetic protein (BMPs) during the morphogenesis of two cartilage elements in zebrafish: the scleral cartilage in the eye and the caudal fin endoskeleton. Zebrafish larvae were exposed to a BMP inhibitor (LDN193189) at a series of timepoints preceding the initial appearance of the scleral cartilage and caudal fin endoskeleton. Morphological assessments of the cartilages in later stages, revealed that BMP‐inhibited fish harbored striking disruptions in caudal fin endoskeletal morphology, regardless of the age at which the inhibitor treatment was performed. In contrast, scleral cartilage morphology was unaffected in all age groups. Morphometric and principal component analysis, performed on the caudal fin endoskeleton, revealed differential clustering of principal components one and two in BMP‐inhibited and control fish. Additionally, the expression of sox9a and sox9b were reduced in BMP‐inhibited fish when compared to controls, indicating that LDN193189 acts via a Sox9‐dependent pathway. Further examination of notochord flexion also revealed a disruptive effect of BMP inhibition on this process. This study provides a detailed characterization of the effects of BMP inhibition via LDN193189 on zebrafish cartilage morphogenesis and development. It highlights the specific, localized role of the BMP‐signaling pathways during the development of different cartilage elements and sheds some light on the morphological characteristics of fossil teleosts that together suggest an uncoupling of the developmental processes between the upper and lower lobes of the caudal fin. Research Highlights: Treatment of zebrafish larvae with LDN193189, a bone morphogenetic protein inhibitor, caused severe morphological disruptions in caudal fin cartilage but spared the scleral cartilage.Expression of sox9a/b was reduced during the early development of caudal fin cartilage after LDN193189 treatment compared to controls.Dorsal flexion of the notochord was disrupted in zebrafish treated with LDN193189. [ABSTRACT FROM AUTHOR]