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Sponges (phylum Porifera) are metazoans which lack muscles and nerve cells, yet perform coordinated behaviours such as whole-body contractions. Previous studies indicate diurnal variability in both the number of contractions and the expression of circadian clock genes. Here, we show that diurnal patterns are present in the contraction–expansion behaviour of the demosponge Tethya wilhelma, by using infrared videography and a simulated night/day cycle including sunrise and sunset mimics. In addition, we show that this behaviour is at least strongly influenced by ambient light intensity and therefore indicates light-sensing capabilities in this sponge species. This is supported by our finding that T. wilhelma consistently contracts at sunrise, and that this pattern disappears both when the sponge is kept in constant darkness and when it is in constant light.

Sponges (phylum Porifera) are metazoans without muscles and nervous system. Still, they perform coordinated behaviours, such as whole body contrations. Previous studies have indicated diurnal variability in number of contractions, and in expression of circadian clock genes. Here we show that diurnal patterns are present in the contraction-expansion behaviour of the demosponge Tethya wilhelma using infrared videography and a simulated night/day-cycle including sunset and sunrise mimic. In addition, we show that this behaviour is at least strongly influenced by the ambient light intensity and therefore implicates light-sensing capabilities in this sponge species. This is backed by our finding that T. wilhelma consistently contracts at sunrise, and that this pattern disappears both when the sponge is kept in constant darkness and when in constant light.

All known cubozoans, box jellyfish, have a similar visual system. They possess four sensory structures called rhopalia, which carry-six eyes each. Two of these six eyes are true image-forming camera type eyes in several ways similar to vertebrate eyes. The rhopalia hang by a thin flexible stalk and in the distal end, there is a high-density crystal. In an earlier study of the Caribbean species Tripedalia cystophora, we showed that the crystals act as weights ensuring that the rhopalia are always upright no matter the orientation of the medusa and the vertical part of the visual field of the eyes thus kept relatively constant. Here we have examined the horizontal part of the visual field under different experimental conditions including different visual environments. We find that thehorizontal gaze direction is largely controlled by the anatomy of the rhopalium and rhopalial stalk, similar to what has previously been shown for the vertical gaze direction. In a vertically oriented medusa, the rhopalia are kept with a 90◦ angle between them with the lower lens eyes (LLE) pointing inwards. This 90◦ shift is kept in horizontally swimming medusa, resulting in the left LLE gazing right, the right gazing left, the bottom gazing orally (backwards compared to swimming direction), and the top LLE gazing aborally (forwards compared toswimming direction). The light environment was manipulated to test if the visual input influences this seemingly strict horizontal gaze direction but even in complete darkness there is tight mechanistic control All known cubozoans, box jellyfish, have a similar visual system. They possess four sensory structures called rhopalia, which carry-six eyes each. Two of these six eyes are true image-forming camera type eyes in several ways similar to vertebrate eyes. The rhopalia hang by a thin flexible stalk and in the distal end, there is a high -density crystal. In an earlier study of the Caribbean species Tripedalia cystophora, we showed that the crystals act as weights ensuring that the rhopalia are always upright no matter the orientation of the medusa and the vertical part of the visual field of the eyes thus kept relatively constant. Here we have examined the horizontal part of the visual field under different experimental conditions including different visual environments. We find that the horizontal gaze direction is largely controlled by the anatomy of the rhopalium and rhopalial stalk, similar to what has previously been shown for the vertical gaze direction. In a vertically oriented medusa, the rhopalia are kept with a 90 degrees angle between them with the lower lens eyes (LLE) pointing inwards. This 90 degrees shift is kept in horizontally swimming medusa, resulting in the left LLE gazing right, the right gazing left, the bottom gazing orally (backwards compared to swimming direction), and the top LLE gazing aborally (forwards compared to swimming direction). The light environment was manipulated to test if the visual input influences this seemingly strict horizontal gaze direction but even in complete darkness there is tight mechanistic control.

Annelids constitute a diverse phylum with more than 19,000 species, which exhibit greatly varying morphologies and lifestyles ranging from sessile detritivores to fast swimming active predators. The lifestyle of an animal is closely linked to its sensory systems, not least the visual equipment. Interestingly, many errantian annelid species from different families, such as the scale worms (Polynoidae), have two pairs of eyes on their prostomium. These eyes are typically 100–200 µm in diameter and structurally similar judged from their gross morphology. The polynoids Harmothoe imbricata and Lepidonotus squamatus from the North Atlantic are both benthic predators preying on small invertebrates but only H. imbricata can produce bioluminescence in its scales. Here, we examined the eye morphology, photoreceptor physiology and light-guided behaviour in these two scale worms to assess their visual capacity and visual ecology. The structure and physiology of the two pairs of eyes are remarkably similar within each species, with the only difference being the gaze direction. The photoreceptor physiology, however, differs between species. Both species express a single opsin in their eyes, but in H. imbricata the peak sensitivity is green shifted and the temporal resolution is lower, suggesting that the eyes of H. imbricata are adapted to detect their own bioluminescence. The behavioural experiments showed that both species are strictly night active but yielded no support for the hypothesis that H. imbricata is repelled by its own bioluminescence.

Background The phyla Cnidaria, Placozoa, Ctenophora, and Porifera emerged before the split of proto- and deuterostome animals, about 600 million years ago. These early metazoans are interesting, because they can give us important information on the evolution of various tissues and organs, such as eyes and the nervous system. Generally, cnidarians have simple nervous systems, which use neuropeptides for their neurotransmission, but some cnidarian medusae belonging to the class Cubozoa (box jellyfishes) have advanced image-forming eyes, probably associated with a complex innervation. Here, we describe a new transcriptome database from the cubomedusa Tripedalia cystophora. Results Based on the combined use of the Illumina and PacBio sequencing technologies, we produced a highly contiguous transcriptome database from T. cystophora. We then developed a software program to discover neuropeptide preprohormones in this database. This script enabled us to annotate seven novel T. cystophora neuropeptide preprohormone cDNAs: One coding for 19 copies of a peptide with the structure pQWLRGRFamide; one coding for six copies of a different RFamide peptide; one coding for six copies of pQPPGVWamide; one coding for eight different neuropeptide copies with the C-terminal LWamide sequence; one coding for thirteen copies of a peptide with the RPRAamide C-terminus; one coding for four copies of a peptide with the C-terminal GRYamide sequence; and one coding for seven copies of a cyclic peptide, of which the most frequent one has the sequence CTGQMCWFRamide. We could also identify orthologs of these seven preprohormones in the cubozoans Alatina alata, Carybdea xaymacana, Chironex fleckeri, and Chiropsalmus quadrumanus. Furthermore, using TBLASTN screening, we could annotate four bursicon-like glycoprotein hormone subunits, five opsins, and 52 other family-A G protein-coupled receptors (GPCRs), which also included two leucine-rich repeats containing G protein-coupled receptors (LGRs) in T. cystophora. The two LGRs are potential receptors for the glycoprotein hormones, while the other GPCRs are candidate receptors for the above-mentioned neuropeptides. Conclusions By combining Illumina and PacBio sequencing technologies, we have produced a new high-quality de novo transcriptome assembly from T. cystophora that should be a valuable resource for identifying the neuronal components that are involved in vision and other behaviors in cubomedusae. Electronic supplementary material The online version of this article (10.1186/s12864-019-5514-7) contains supplementary material, which is available to authorized users.

Box jellyfish have an elaborate visual system and perform advanced visually guided behaviors. However, the rhopalial nervous system (RNS), believed to be the main visual processing center, only has 1000 neurons in each of the four eye carrying rhopalia. We have examined the detailed structure of the RNS of the box jellyfish Tripedalia cystophora, using immunolabeling with antibodies raised against four putative neuropeptides (T. cystophora RFamide, VWamide, RAamide, and FRamide). In the RNS, T. cystophora RF-, VW-, and RAamide antibodies stain sensory neurons, the pit eyes, the neuropil, and peptide-specific subpopulations of stalk-associated neurons and giant neurons. Furthermore, RFamide ir+ neurites are seen in the epidermal stalk nerve, whereas VWamide antibodies stain the gastrodermal stalk nerve. RFamide has the most widespread expression including in the ring and radial nerves, the pedalium nerve plexus, and the tentacular nerve net. RAamide is the putative neurotransmitter in the motor neurons of the subumbrellar nerve net, and VWamide is a potential marker for neuronal differentiation as it is found in subpopulations of undifferentiated cells both in the rhopalia and in the bell. The results from the FRamide antibodies were not included as only few cells were stained, and in an unreproducible way. Our studies show hitherto-unseen details of the nervous system of T. cystophora and allowed us to identify specific functional groups of neurons. This identification is important for understanding visual processing in the RNS and enables experimental work, directly addressing the role of the different neuropeptides in vision.

Asteroidea, starfish, constitutes a major part of the macrobenthos in most marine environments. Being members of the echinoderms, they have a nervous system with no well-defined central nervous system. Accordingly, starfish are assumed to pick up rather limited information from the surroundings, and it is also often assumed that most of their behaviors are guided by olfaction. Here, the sensory biology of starfish is reviewed in order to evaluate these assumptions. There is a vast amount of behavioral data dealing with mechanoreception, chemoreception, and combinations of the two (chemosensory-mediated rheotaxis), but the receptors have not yet been identified and almost nothing is known about the physiology behind these senses. What can be concluded from the available data is that starfish possess a sense of touch, some are able to sense gravity and many display positive rheotaxis, moving up currents. A number of starfish species use olfaction during foraging and prey localization. Interestingly, eyes are also present in most starfish, and recent studies have documented that in Linckia laevigata and Acanthaster planci vision plays a major role in seeking out their feeding grounds. The physiology and structure of the eyes filter out small moving objects while optimizing the contrast between the large stationary objects (e.g., coral boulders in the habitat) and the surrounding water. These new results demonstrate the importance of controlling the visual environment when conducting experiments on starfish behavior.

Fan worms (Annelida: Sabellidae) possess compound eyes and other photoreceptors on their radiolar feeding tentacles. These eyes putatively serve as an alarm system that alerts the worm to encroaching threats, eliciting a rapid defensive retraction into their protective tube. The structure and independent evolutionary derivation of these radiolar eyes make them a fascinating target for exploring the emergence of new sensory systems and visually guided behaviours. However, little is known about their physiology and how this impacts their function. Here, we present electroretinogram recordings from the radiolar eyes of the fan worm

Bat flies (Diptera: Nycteribiinae) are highly specialized bloodsucking bat ectoparasites. Some of the ectoparasitic bat flies are themselves parasitized with an ectoparasitic fungus of the genus Arthrorhynchus (Laboulbeniales). Ascospores of the fungus attach to the cuticle of a bat fly and develop a haustorium that penetrates the host cuticle. This interaction defines the fungus as a hyperparasite. Both the fly and the fungus are obligate parasites and this peculiar case of hyperparasitism has remained largely unstudied. We studied the prevalence of Laboulbeniales, genus Arthrorhynchus, in natural populations of bat flies infesting the bat species Miniopterus schreibersii, Myotis bechsteinii, My. blythii, My. daubentonii, My. escalerai and My. myotis in Portuguese caves. Laboulbeniales were found infecting 10 of the 428 screened bat flies (2.3%) in natural populations, with fewer infections in winter. Images obtained with transmission electron microscopy show the fungal haustorium within the bat fly host tissue, from where it extracts nutrition.

Cubozoans have a complex lifecycle in many ways similar to the scyphozoan lifecycle. The sexual reproduction within cubozoans varies between species with one clade having copulation and internal fertilization and the release of planula larvae. This cubozoan clade, the family Tripedaliidae, includes three species, Copula sivickisi, Tripedalia cystophora, and Tripedalia binata. In a recent study, it was suggested that in C. sivickisi cnidocytes play a new and important role during the sexual reproduction. Male derived cnidocytes anchor sperm packages to the female gonads and female derived cnidocytes protect the externalized embryo strand. Here, we have examined the gonads and gametes of T. cystophora and our results reveal that the male produced spermatozeugmata have a high number of isorhiza type cnidocytes, which are transferred along with the sperm during copulation. This adds further support to our hypothesis that they are important for sperm anchorage. The female gonads are lacking cnidocytes all together showing that cnidocyte production is not just a default state of the epithelium in these animals.

Fan worms (Annelida: Sabellidae) possess compound eyes and other photoreceptors on their radiolar feeding tentacles. These eyes putatively serve as an alarm system that alerts the worm to encroaching threats, eliciting a rapid defensive retraction into their protective tube. The structure and independent evolutionary derivation of these radiolar eyes make them a fascinating target for exploring the emergence of new sensory systems and visually guided behaviours. However, little is known about their physiology and how this impacts their function. Here we present electroretinogram recordings from the radiolar eyes of the fan worm Acromegalomma vesiculosum (Montagu, 1813). We examine their spectral sensitivity along with their dynamic range and temporal resolution. Our results show that they possess one class of photoreceptors with a single visual pigment peaking in the blue-green part of the spectrum around 510 nm, which matches the dominant wavelengths in their shallow coastal habitats. We found the eyes to have a rather high temporal resolution with a critical flicker fusion frequency around 35 Hz. The high temporal resolution of this response is ideally suited for detecting rapidly moving predators but also necessitates downstream signal processing to filter out caustic wave flicker. This study provides a fundamental understanding of how these eyes function. Furthermore, these findings emphasise a set of dynamic physiological principles that are well-suited for governing a multi-eyed startle response in coastal aquatic habitats.

Photoreception in echinoderms has been studied for several years with a focus on the dermal photoreceptors of echinoids. Even though spatial vision has been proposed for this dermal photosystem, by far the most advanced system is found in a number of asteroids where an unpaired tube foot at the tip of each arm carries a proper eye, also known as the optical cushion. The eyes resemble compound eyes, except for the lack of true optics, and they typically have between 50 and 250 ommatidia each. These eyes have been known for two centuries but no visually guided behaviors were known in starfish until recently when it was shown that both Linckia laevigata and Acanthaster planci navigate their coral reef habitat using vision. Here we investigate the visual system of A. planci and find that they have active control of their visual field. The distalmost tube foot holding the eye is situated on a movable knob, which bends to adjust the vertical angle of the visual field. On the leading arms the visual field is directed 33° above the horizon, whereas the eyes on the trailing arms are directed 44° above horizontal on average. When the animal traverses an obstacle the knob bends and counteracts most of the arm bending. Further, we examined a previously described behavior, rhythmic arm elevation, and suggest that it allows the animal to scan the surroundings while preventing photoreceptor adaptation and optimizing image contrast.

Animals relying on vision as their main sensory modality reserve a large part of their central nervous system to appropriately navigate their environment. In general, neural involvement correlates to the complexity of the visual system and behavioural repertoire. In humans, one third of the available neural capacity supports our single-chambered general-purpose eyes, whereas animals with less elaborate visual systems need less computational power, and generally have smaller brains, and thereby lack in visual behaviour. As a consequence, both traditional model animals (mice, zebrafish, and flies) and more experimentally tractable animals (Hydra, Planaria, and C. elegans) cannot contribute to our understanding of systems-level visual information processing-a Goldilocks case of too big and too small.However, one animal, the box jellyfish Tripedalia cystophora, possesses a rather complex visual system, displays multiple visual behaviours, yet processes visual information by means of a relatively simple central nervous system. This-just right-model system could not only provide information on how visual stimuli are processed through distinct combinations of neural circuitry but also provide a processing algorithm for extracting specific information from a complex visual scene.

Cubozoans have the most intricate visual apparatus within Cnidaria. It comprises four identical sensory structures, the rhopalia, each of which holds six eyes of four morphological types. Two of these eyes are camera-type eyes that are, in many ways, similar to the vertebrate eye. The visual input is used to control complex behaviors, such as navigation and obstacle avoidance, and is processed by an elaborate rhopalial nervous system. Several studies have examined the rhopalial nervous system, which, despite a radial symmetric body plan, is bilaterally symmetrical, connecting the two sides of the rhopalium through commissures in an extensive neuropil. The four rhopalia are interconnected by a nerve ring situated in the oral margin of the bell, and together these structures constitute the cubozoan central nervous system. Cnidarians have excellent regenerative capabilities, enabling most species to regenerate large body areas or body parts, and some species can regenerate completely from just a few hundred cells. Here we test whether cubozoans are capable of regenerating the rhopalia, despite the complexity of the visual system and the rhopalial nervous system. The results show that the rhopalia are readily regrown after amputation and have developed most, if not all, neural elements within two weeks. Using electrophysiology, we investigated the functionality of the regrown rhopalia and found that they generated pacemaker signals and that the lens eyes showed a normal response to light. Our findings substantiate the amazing regenerative ability in Cnidaria by showing here the complex sensory system of Cubozoa, a model system proving to be highly applicable in studies of neurogenesis.

Asteroids, starfish, are important members of the macro-benthos in almost all marine environments including the deep sea. Starfish are in general assumed to be largely olfactory guided, but recent studies have shown that two tropical shallow water species rely on vision alone to find their habitat at short distances. Their compound eyes are found at the tip of each arm and they vary little between examined species. Still, nothing is known about vision in the species found in the aphotic zone of the deep sea or whether they even have eyes. Here, 13 species of starfish from Greenland waters, covering a depth range from shallow waters to the deep sea below 1000 m, were examined for the presence of eyes and optical and morphological examinations were used to estimate the quality of vision. Further, species found in the aphotic zone below 320 m were checked for bioluminescence. All species, except the infaunalCtenodiscus crispatus, had eyes, and two were found to be bioluminescent. Interestingly, one of the species found in the aphotic zone,Novodinia americana, had close to the highest spatial resolution known for starfish eyes along with being bioluminescent. Accordingly, we hypothesize that this species communicates visually using bioluminescent flashes putatively for reproductive purposes. Other species have greatly enhanced sensitivity with few large ommatidia but at the sacrifice of spatial resolution. The discovery of eyes in deep-sea starfish with a huge variation in optical quality and sensitivity indicates that their visual ecology also differs greatly.

Background Opsins are G protein-coupled receptors used for both visual and non-visual photoreception, and these proteins evolutionarily date back to the base of the bilaterians. In the current sequencing age, phylogenomic analysis has proven to be a powerful tool, facilitating the increase in knowledge about diversity within the opsin subclasses and, so far, at least nine types of opsins have been identified. Within echinoderms, opsins have been studied in Echinoidea and Ophiuroidea, which do not possess proper image forming eyes, but rather widely dispersed dermal photoreceptors. However, most species of Asteroidea, the starfish, possess true eyes and studying them will shed light on the diversity of opsin usage within echinoderms and help resolve the evolutionary history of opsins. Results Using high-throughput RNA sequencing, we have sequenced and analyzed the transcriptomes of different Acanthaster planci tissue samples: eyes, radial nerve, tube feet and a mixture of tissues from other organs. At least ten opsins were identified, and eight of them were found significantly differentially expressed in both eyes and radial nerve, with R-opsin being the most highly expressed in the eye. Conclusion This study provides new important insight into the involvement of opsins in visual and nonvisual photoreception. Of relevance, we found the first indication of an r-opsin photopigment expressed in a well-developed visual eye in a deuterostome animal. Additionally, we provided tissue specific A. planci transcriptomes that will aid in future Evo Devo studies. Electronic supplementary material The online version of this article (10.1186/s12862-018-1276-0) contains supplementary material, which is available to authorized users.

Animals relying on vision as their main sensory modality reserve a large part of their central nervous system to appropriately navigate their environment. In general, neural involvement correlates to the complexity of the visual system and behavioural repertoire. In humans, one third of the available neural capacity supports our single-chambered general-purpose eyes, whereas animals with less elaborate visual systems need less computational power, and generally have smaller brains, and thereby lack in visual behaviour. As a consequence, both traditional model animals (mice, zebrafish, and flies) and more experimentally tractable animals (Hydra, Planaria, and C. elegans) cannot contribute to our understanding of systems-level visual information processing—a Goldilocks case of too big and too small.

Bat flies (Nycteribiidae) of the order Diptera are highly specialized bloodsucking ectoparasites living on bats. The life-cycle of the bat flies emphasizes their obligate relationship with their hosts as they spend almost their entire life on bats. Upon mating, the female bat fly carries the larvae internally until the 3rd-instar when it deposits the larvae on the ceiling of the roost occupied by bats. The larvae then form a puparium. After 3-4 weeks the adult bat fly emerges from the puparium and starts searching for a host bat to colonize. Some of these ectoparasitic bat flies themselves are infected with an ectoparasitic fungus of the genus Arthrorhynchus (Laboulbeniales). Ascospores of the fungi attach themselves to the cuticle of the bat fly and develop a very conspicuous haustorium that penetrates into the soft tissues from where it presumably extract nutrition from the hemolymph of the bat flies. This interaction converts the fungus into a hyperparasite. Both the parasite and hyperparasite are obligates and cannot live separate from their hosts. This peculiar case of hyperparasitism remains highly unknown. The bat flies were collected in caves of Portugal, in maternity and hibernation bat seasons, and in the autumn migration period. The most common species of cave-dwelling bat species in Portugal is Miniopterus schreibersii, frequently parasitized with Nycteribia schmidlii and Penicillidia conspicua bat flies. We have studied the prevalence of the Laboulbeniales of the genus Arthrorhynchus in natural populations of bat flies. The site and position of the fungus on male and female bat flies unveils the mechanism of fungal transmission among bat flies, indicating that it occurs during mating behavior. This study is the starting point towards the understanding of this unique case of fungus-insect-vertebrate hyperparasitism interaction. See Suppl. material 1.

Within cubozoans, a few species have developed a sexual reproduction system including mating and internal fertilization. One species, Copula sivickisi, is found in a large area of the indo pacific. They have separate sexes and when mature males and females meet they entangle their tentacles and the males transfer a sperm package, a spermatozeugmata, which is ingested by the female fertilizing her eggs internally. After 2-3 days, the females lay an embryo strand that sticks to the substrate and after another 2-3 days, the fully developed larvae leave the strand. We have examined the ultrastructure of the gonads and spermatozeugmata to look for structural adaptations to this specialized way of reproduction and understand how the fertilization takes place. Surprisingly, we discovered that the male gonads were heavily packed with cnidocytes of the isorhiza type and that they are transferred to the spermatozeugmata. The spermatozeugmata does not dissolve in the female gastrovascular cavity but is attached to the female gonad probably using the isorhizas. Here, the sperm cells are partly digested and the nuclei are released. The actual fertilization seems to happen through phagocytosis of the released nuclei by the epithelial cells. The female gonads are likewise packed with cnidocytes but of the eurytele type. They do not mature inside the female and putatively serve to protect the developing larvae once the embryo strand is laid. This specialized way of fertilization is to our knowledge novel and so is this first account of cnidocytes being directly involved in cnidarian reproduction.

Opsins are G protein-coupled receptors used for both visual and non-visual photoreception, and these proteins evolutionarily date back to the base of the bilaterians. In the current sequencing age, phylogenomic analysis has proven to be a powerful tool, facilitating the increase in knowledge about diversity within the opsin subclasses and, so far, nine paralogs have been identified. Within echinoderms, opsins have been studied in Echinoidea and Ophiuroidea, which do not possess proper image forming eyes, but rather widely dispersed dermal photoreceptors. However, most species of Asteroidea, the starfish, possess true eyes and studying them will shed light on the diversity of opsin usage within echinoderms and help resolve the evolutionary history of opsins. Using high-throughput RNA sequencing, we have sequenced and analyzed the transcriptomes of differentAcanthaster plancitissue samples: eyes, radial nerve, tube feet and a mixture of tissues from other organs. At least ten opsins were identified, and eight of them were found significantly differentially expressed in both eyes and radial nerve, providing new important insight into the involvement of opsins in visual and nonvisual photoreception. Of relevance, we found the first evidence of an r-opsin photopigment expressed in a well developed visual eye in a deuterostome animal.

Highlights: • First long-term study on microplastic in the marine environment • Case study based on a unique sample set from the highly human impacted Baltic Sea • Water column microplastic concentration constant over past three decades • Microplastic concentration in forage fish constant over past three decades • We hypothesise that household waste is the dominant source of Baltic marine plastics. Abstract Microplastic is considered a potential threat to marine life as it is ingested by a wide variety of species. Most studies on microplastic ingestion are short-term investigations and little is currently known about how this potential threat has developed over the last decades where global plastic production has increased exponentially. Here we present the first long-term study on microplastic in the marine environment, covering three decades from 1987 to 2015, based on a unique sample set originally collected and conserved for food web studies. We investigated the microplastic concentration in plankton samples and in digestive tracts of two economically and ecologically important planktivorous forage fish species, Atlantic herring (Clupea harengus) and European sprat (Sprattus sprattus), in the Baltic Sea, an ecosystem which is under high anthropogenic pressure and has undergone considerable changes over the past decades. Surprisingly, neither the concentration of microplastic in the plankton samples nor in the digestive tracts changed significantly over the investigated time period. Average microplastic concentration in the plankton samples was 0.21±0.15particlesm-3. Of 814 fish examined, 20% contained plastic particles, of which 95% were characterized as microplastic (

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Animals adapt their behavior according to the environment and their specific needs in a given situation. In order to do so in an appropriate way, they need to detect, analyze, and code the relevant sensory cues. This task is handled by sensory systems and their associated parts in the central nervous system. With few exceptions, the amount of information present in the environment and thus in principle available to sensory systems, is close to infinite. It is impossible and not desirable to encode and process all the information. Therefore, the first and most important task of any sensory system is to filter and select only the essential information—information, which potentially will improve the fitness of the bearer. Differences in sensory information processing occur between animals of different organization levels. Sensory coding in invertebrates and vertebrates relies on multiple stages of processing to extract information relevant to the survival of the individual. The wide range of organizational complexity, in combination with their relatively simple and accessible nervous systems, makes invertebrates excellent models to study general sensory coding principles. In addition, many invertebrate species are of socio-economic importance as pollinators, crop pests, or as disease vectors or elicitors. Therefore, understanding their communication systems and sensory biology is important for the development of insect management or plant protection strategies.

Directional swimming in the box jellyfish Tripedalia cystophora (cubozoa, cnidaria) is controlled by the shape of the velarium, which is a thin muscular sheet that forms the opening of the bell. It was unclear how different patterns of visual stimulation control directional swimming and that is the focus of this study. Jellyfish were tethered inside a small experimental tank, where the four vertical walls formed light panels. All four panels were lit at the start of an experiment. The shape of the opening in the velarium was recorded in response to switching off different combinations of panels. We found that under the experimental conditions the opening in the velarium assumed three distinct shapes during a swim contraction. The opening was (1) centred or it was off-centred and pocketed out either towards (2) a rhopalium or (3) a pedalium. The shape of the opening in the velarium followed the direction of the stimulus as long as the stimulus contained directional information. When the stimulus contained no directional information, the percentage of centred pulses increased and the shape of the off-centred pulses had a random orientation. Removing one rhopalium did not change the directional response of the animals, however, the number of centred pulses increased. When three rhopalia were removed, the percentage of centred pulses increased even further and the animals lost their ability to respond to directional information.

Cubomedusae all have a similar set of six eyes on each of their four rhopalia. Still, there is a great variation in activity patterns with some species being strictly day active while others are strictly night active. Here we have examined the visual ecology of the medusa of the night active Copula sivickisi from Okinawa using optics, morphology, electrophysiology, and behavioural experiments. We found the lenses of both the upper and the lower lens eyes to be image forming but under-focused, resulting in low spatial resolution in the order of 10 – 15 degrees. The photoreceptor physiology is similar in the two lens eyes and they have a single opsin peaking around 460 nm and low temporal resolution with a flicker fusion frequency (fff) of 2.5 Hz indicating adaptions to vision in low light intensities. Further, the outer segments have fluid filled swellings, which may concentrate the light in the photoreceptor membrane by total internal reflections, and thus enhance the signal to noise ratio in the eyes. Finally our behavioural experiments confirmed that the animals use vision when hunting. When they are active at night they seek out high prey-concentration by visual attraction to areas with abundant bioluminescent flashes triggered by their prey.

Photoreception in echinoderms has been known for over 200 years, but their visual capabilities remain poorly understood. As has been reported for some asteroids, the crown-of-thorns starfish (Acanthaster planci) possess a seemingly advanced eye at the tip of each of its 7–23 arms. With such an array of eyes, the starfish can integrate a wide field of view of its surroundings. We hypothesise that, at close range, orientation and directional movements of the crown-of-thorns starfish are visually guided. In this study, the eyes and vision of A. planci were examined by means of light microscopy, electron microscopy, underwater goniometry, electroretinograms and behavioural experiments in the animals’ natural habitat. We found that only animals with intact vision could orient to a nearby coral reef, whereas blinded animals, with olfaction intact, walked in random directions. The eye had peak sensitivity in the blue part (470 nm) of the visual spectrum and a narrow, horizontal visual field of approximately 100° wide and 30° high. With approximately 250 ommatidia in each adult compound eye and average interommatidial angles of 8°, crown-of-thorns starfish have the highest spatial resolution of any starfish studied to date. In addition, they have the slowest vision of all animals examined thus far, with a flicker fusion frequency of only 0.6–0.7 Hz. This may be adaptive as fast vision is not required for the detection of stationary objects such as reefs. In short, the eyes seem optimised for detecting large, dark, stationary objects contrasted against an ocean blue background. Our results show that the visual sense of the crown-of-thorns starfish is much more elaborate than has been thus far appreciated and is essential for orientation and localisation of suitable habitats.

Box jellyfish respond to visual stimuli by changing the dynamics and frequency of bell contractions. In this study, we determined how the contrast and the dimming time of a simple visual stimulus affected bell contraction dynamics in the box jellyfish Tripedalia cystophora. Animals were tethered in an experimental chamber where the vertical walls formed the light stimuli. Two neighbouring walls were darkened and the contraction of the bell was monitored by high-speed video. We found that (1) bell contraction frequency increased with increasing contrast and decreasing dimming time. Furthermore, (2) when increasing the contrast and decreasing the dimming time pulses with an off-centred opening had a better defined direction and (3) the number of centred pulses decreased. Only weak effects were found on the relative diameter of the contracted bell and no correlation was found for the duration of bell contraction. Our observations show that visual stimuli modulate swim speed in T. cystophora by changing the swim pulse frequency. Furthermore, the direction of swimming is better defined when the animal perceives a high-contrast, or fast dimming, stimulus.

SUMMARYThe dinoflagellate Kryptoperidinium foliaceum possesses one of the largest eyespots among the autotrophic dinoflagellates. Until now they were believed to be negatively phototactic using a non-opsin photopigment. Here we provide evidence that in newly established cultures they are positively phototactic and that the dynamic range of phototaxis is ∼2.5 log units. Additionally, we find that the spectral sensitivity of the phototaxis agrees reasonably well with the absorption curve of a theoretical opsin, with a peak sensitivity around 500 nm. The sensitivity in the short wavelength end of the tested spectrum is unexpectedly low, but this is probably due to selective filtering. Interestingly, the phototaxis could be temporarily overruled by tactile stimuli. After physical contact with the light guide, the cells escaped the area, and we suggest that this may serve as predator avoidance.

Cubozoan medusae have a stereotypic set of 24 eyes, some of which are structurally similar to vertebrate and cephalopod eyes. Across the approximately 25 described species, this set of eyes varies surprisingly little, suggesting that they are involved in an equally stereotypic set of visual tasks. During the day Tripedalia cystophora is found at the edge of mangrove lagoons where it accumulates close to the surface in sun-lit patches between the prop roots. Copula sivickisi (formerly named Carybdea sivickisi) is associated with coral reefs and has been observed to be active at night. At least superficially, the eyes of the two species are close to identical. We studied the diurnal activity pattern of these two species both in the wild and under controlled conditions in laboratory experiments. Despite the very similar visual systems, we found that they display opposite patterns of diurnal activity. T. cystophora is active exclusively during the day, whereas C. sivickisi is actively swimming at night, when it forages and mates. At night T. cystophora is found on the muddy bottom of the mangrove lagoon. C. sivickisi spends the day attached to structures such as the underside of stones and coral skeletons. This species difference seems to have evolved to optimize foraging, since the patterns of activity follow those of the available prey items in their respective habitats.

Box jellyfish (Cubomedusae) are visually orientating animals which possess a total of 24 eyes of 4 morphological types; 2 pigment cup eyes (pit eye and slit eye) and 2 lens eyes [upper lens-eye (ule) and lower lens-eye (lle)]. In this study, we use electroretinograms (ERGs) to explore temporal properties of the two lens eyes. We find that the ERG of both lens eyes are complex and using sinusoidal flicker stimuli we find that both lens eyes have slow temporal resolution. The average flicker fusion frequency (FFF) was found to be approximately 10 Hz for the ule and 8 Hz for the lle. Differences in the FFF and response patterns between the two lens eyes suggest that the ule and lle filter information differently in the temporal domain and thus are tuned to perform different visual tasks. The data collected in this study support the idea that the visual system of box jellyfish is a collection of special purpose eyes.

SUMMARY Like all other cnidarian medusae, box jellyfish propel themselves through the water by contracting their bell-shaped body in discrete swim pulses. These pulses are controlled by a swim pacemaker system situated in their sensory structures, the rhopalia. Each medusa has four rhopalia each with a similar set of six eyes of four morphologically different types. We have examined how each of the four eye types influences the swim pacemaker. Multiple photoreceptor systems, three of the four eye types, plus the rhopalial neuropil, affect the swim pacemaker. The lower lens eye inhibits the pacemaker when stimulated and provokes a strong increase in the pacemaker frequency upon light-off. The upper lens eye, the pit eyes and the rhopalial neuropil all have close to the opposite effect. When these responses are compared with all-eye stimulations it is seen that some advanced integration must take place.

Cubomedusae have a total of 24 eyes of four morphologically different types. Two of these eye types are camera-type eyes (upper and lower lens-eye), while the other two eye types are simpler pigment pit eyes (pit and slit eye). Here, we give a description of the visual system of the box jellyfish species Chiropsella bronzie and the optics of the lens eyes in this species. One aim of this study is to distinguish between general cubozoan features and species-specific features in the layout and optics of the eyes. We find that both types of lens eyes are more severely under-focused in C. bronzie than those in the previously investigated species Tripedalia cystophora. In the lower lens-eye of C. bronzie, blur circles subtend 20 and 52 degrees for closed and open pupil, respectively, effectively removing all but the coarsest structures of the image. Histology reveals that the retina of the lower lens-eye, in addition to pigmented photoreceptors, also contains long pigment-cells, with both dark and white pigment, where the dark pigment migrates on light/dark adaptation. Unlike the upper lens-eye lens of T.cystophora, the same eye in C.bronzie did not display any significant optical power.

Thoracic cirripedes are sessile crustaceans that use six pairs of thoracic appendages (the cirri) to catch and handle food. We used scanning electron microscopy to examine the cirri, which include one to three pairs of maxillipeds in six species of thoracican barnacles, in search of correlations between cirral setation and feeding mode. The species studied comprise both pedunculate and sessile forms and represent a wide range of marine habitats as well as morphologies, viz., Ibla cumingi, Octolasmis warwickii, Capitulum mitella, Pollicipes polymerus, Tetraclita japonica japonica and Megabalanus volcano. Of the pedunculates, I. cumingi has the least complex setation pattern consisting of only serrulate types. This is consistent with its very simplified feeding mode and an apparent inability to discriminate between food items. Octolasmis warwickii is slightly more modified, while both P. polymerus and C. mitella have a more diversified setation. The balanomorphan species exhibit by far the most complex cirral setation. This is consistent with the several types of suspension feeding seen in these species, their ability to identify and sort captured food items and even to perform microfiltration in the mantle cavity using the setae on their three pairs of maxillipeds. Our results indicate that in thoracican barnacles, adaptations in feeding behaviour are associated with changes in the setation pattern of the cirri. In addition, the setal types and their distribution on the cirri are potential new characters in future morphology-based analyses of the phylogeny of the Cirripedia Thoracica.

The visual system of box jellyfish comprises a total of 24 eyes. These are of four types and each probably has a special function. To investigate this hypothesis the morphology and optics of the lesser eyes, the pit and slit eyes, were examined. The pit eyes hold one cell type only and are probably mere light meters. The slit eyes, comprising four cell types, are complex and highly asymmetric. They also hold a lens-like structure, but its optical power is minute. Optical modeling suggests spatial resolution, but only in one plane. These unique and intriguing traits support strong peripheral filtering.

Box jellyfish have the most elaborate sensory system and behavioural repertoire of all cnidarians. Sensory input largely comes from 24 eyes situated on four club-shaped sensory structures, the rhopalia, and behaviour includes obstacle avoidance, light shaft attractance and mating. To process the sensory input and convert it into the appropriate behaviour, the box jellyfish have a central nervous system (CNS) but this is still poorly understood. The CNS has two major components: the rhopalial nervous system and the ring nerve. The rhopalial nervous system is situated within the rhopalia in close connection with the eyes, whereas the ring nerve encircles the bell. We describe the morphology of the ring nerve of the box jellyfish Tripedalia cystophora as ascertained by normal histological techniques, immunohistochemistry and transmission electron microscopy. By light microscopy, we have estimated the number of cells in the ring nerve by counting their nuclei. In cross sections at the ultrastructural level, the ring nerve appears to have three types of neurites: (1) small "normal"-looking neurites, (2) medium-sized neurites almost completely filled by electron-lucent vacuoles and (3) giant neurites. In general, only one giant neurite is seen on each section; this type displays the most synapses. Epithelial cells divide the ring nerve into compartments, each having a tendency to contain neurites of similar morphology. The number and arrangement of the compartments vary along the length of the ring nerve.

Within cubozoans, a few species have developed a sexual reproduction system including mating and internal fertilization. One species, Copula sivickisi, is found in a large area of the indo pacific. They have separate sexes and when mature males and females meet they entangle their tentacles and the males transfer a sperm package, a spermatozeugmata, which is ingested by the female fertilizing her eggs internally. After 2-3 days, the females lay an embryo strand that sticks to the substrate and after another 2-3 days, the fully developed larvae leave the strand. We have examined the ultrastructure of the gonads and spermatozeugmata to look for structural adaptations to this specialized way of reproduction and understand how the fertilization takes place. Surprisingly, we discovered that the male gonads were heavily packed with cnidocytes of the isorhiza type and that they are transferred to the spermatozeugmata. The spermatozeugmata does not dissolve in the female gastrovascular cavity but is attached to the female gonad probably using the isorhizas. Here, the sperm cells are partly digested and the nuclei are released. The actual fertilization seems to happen through phagocytosis of the released nuclei by the epithelial cells. The female gonads are likewise packed with cnidocytes but of the eurytele type. They do not mature inside the female and putatively serve to protect the developing larvae once the embryo strand is laid. This specialized way of fertilization is to our knowledge novel and so is this first account of cnidocytes being directly involved in cnidarian reproduction.

In decapod crustaceans, the largest density and diversity of sensilla, referred to as setae, is in general found on the mouthparts, but little is known about their sensory properties and thereby their functions. Here data are presented from mechanoreceptors from the two largest mouthparts, maxilliped 2 and 3, of the European shore crab Carcinus maenas. The mechanoreceptors were found to respond to either displacements of the entire seta or bending of the setal shaft. The displacement-sensitive cells encode both the amplitude and the velocity of the displacement and about half were found to be directional but most in a non-exclusive way. The amplitude of the stimulus is encoded in the number of spikes produced with a linear correlation. The velocity is encoded in the interspike intervals with shorter intervals at higher velocities. In the latter case, the correlation follows a power function. The physiological data is correlated with the morphology and usage of the maxillipeds were examined with scanning electron microscopy and macro video recordings respectively. Recordings were obtained from cells associated with four different setal types and they all showed similar mechanosensory properties supporting that the external morphology of setae is more closely connected to their non-sensory functions, e.g., mechanical manipulation of the food items. The details of the sensory properties together with the high setal density, especially on maxilliped 3, suggest that a large amount of tactile information is gathered during feeding.

Cubozoans, or box jellyfish, each have twenty-four eyes of four types, but no central brain for information processing. An investigation of these eyes reveals optics as sophisticated as in vertebrates. Despite this, the retina is out of focus and the sharp image is not used to provide acute vision, but as a way of processing visual information. ‘Blurred’ vision may be perfect for avoiding large stationary objects without focusing on small floating objects and plankton. This may also be a pointer to a missing link in the early evolution of animal visual systems, likely to have involved eyes performing a single visual task only. The cover shows the two lens eyes and two pairs of pigment pit eyes in the bizarre sensory club of Chiropsalmus sp., larger but similar to those of Tripedalia cystophora used in the optical study. Cubozoans, or box jellyfish, differ from all other cnidarians by an active fish-like behaviour and an elaborate sensory apparatus1,2. Each of the four sides of the animal carries a conspicuous sensory club (the rhopalium), which has evolved into a bizarre cluster of different eyes3. Two of the eyes on each rhopalium have long been known to resemble eyes of higher animals, but the function and performance of these eyes have remained unknown4. Here we show that box-jellyfish lenses contain a finely tuned refractive index gradient producing nearly aberration-free imaging. This demonstrates that even simple animals have been able to evolve the sophisticated visual optics previously known only from a few advanced bilaterian phyla. However, the position of the retina does not coincide with the sharp image, leading to very wide and complex receptive fields in individual photoreceptors. We argue that this may be useful in eyes serving a single visual task. The findings indicate that tailoring of complex receptive fields might have been one of the original driving forces in the evolution of animal lenses.

We studied electrophysiological properties of single chemosensory neurons in the mouthparts of the spiny lobsters Panulirus argus and Panulirus interruptus to complement our growing understanding of the behavioral roles of mouthparts of decapod crustaceans. Food mixtures and 13 single compounds were used to characterize the response specificity, sensitivity, and time course of individual neurons in the endopods of maxilliped 2 and 3. Additional chemoreceptors were found in the mandibular palp and basis of maxilliped 1 but they were not characterized. Neurons were broadly tuned, with the five most potent single compounds being ammonium, adenosine-5' -monophosphate, taurine, glutamate, and aspartate. Cluster analysis indicated that the neurons constitute a heterogeneous population that could be placed into seven groups linked according to their most excitatory compound. These neurons in the mouthparts had concentration-dependent responses, with thresholds between 10(-7) and 10(-4) M and without saturation even at 10(-3) or 10(-2) M. They also quickly adapted when exposed to their best compounds at 10(-4) and 10(-3) M. A comparison of the response properties of these neurons in the mouthparts with those of chemosensory neurons in other crustacean appendages shows that neurons in the mouthparts have relatively broad tuning biased toward detecting and resolving high concentrations. Based on these comparisons, we suggest a functional distinction among the chemosensors on the different appendages: long distance detection by the antennae, precise location and collection by the pereiopods, and detailed assessment of quality by the mouthparts. (Less)