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This paper reports on six species of the genus Copidognathus Trouessart 1888, studied from sandy habitats along the Turkish Aegean coasts (Balikesir and Izmir) and the Mediterranean coast (Antalya). Two species, Copidognathus boyaci sp. nov. and C. ozkani sp. nov. are described as species new to science. Additionally, two species, C. brevipes Viets 1940, and C. latisetus Viets 1940, are recorded for the first time from the eastern Mediterranean Sea, Türkiye. Furthermore, revised descriptions and new illustrations of C. dentatus Viets 1940, and C. longirostris are provided for both sexes.http://www.zoobank.org/urn:lsid:zoobank.org:pub:4539526C-F5C0-44ED-B727-DA2B921FDBFF [ABSTRACT FROM AUTHOR]

It remains uncertain whether diatom and nematode assemblages can reflect the same ecological status in eutrophic reservoirs. To address this uncertainty, we collected diatom and nematode assemblages and measured environmental variables in the tropical eutrophic Tri An Reservoir (TAR) in Vietnam. We assessed the ecological status of each community using various biological metrics, including the Shannon-Weiner diversity index (H′), the Trophic Diatom Index (TDI), and the index of trophic diversity (ITD) of nematodes. Multivariate direct gradient analysis was conducted to determine the environmental variables influencing the dominant diatom and nematode species. We then conducted a nonparametric permutational analysis of variance with two fixed factors to assess differences in all biological variables across communities. The analysis had a cross-sectional design that incorporated "station" and "season" as factors. Our findings indicated that the surface water in the TAR ranges from eutrophic to hypertrophic. Based on these TDI values, we classified the ecological and trophic status of the TAR into two groups: meso-eutrophic and eutrophic. However, the ITD of the nematodes suggested that the ecological status varied from poor to moderate or high. Our findings suggest inconsistency between two bioindicator groups in their ability to reflect surface water quality in the TAR. This study highlights the potential of using the TDI of diatoms and emphasizes the need to revise the H′ and ITD indices of nematodes if they are to be incorporated into lake monitoring programs for assessing ecological status. [ABSTRACT FROM AUTHOR]

An illustrated description of Microstomum rogozini sp. n. is given based on original material collected over the period of 1994–2008; it can be concluded that this new species is the most common and most numerous representative of microstomids that inhabit Lake Baikal. Individuals of this species are bright orange and transparent and consist of one–two zooids; in the mature state, they have a solid spirally twisted stylet with 3.5 whorls. Its length ranges from 120 to 170 µm at an average value of about 145 µm. The distal opening is slit-shaped and is located on the distal half-whorl. Comparisons, as well as the distribution, variability, and ecology of this species, are also given. [ABSTRACT FROM AUTHOR]

Meiofauna can act as remediation organisms by stimulating microphytobenthos, sequestering carbon dioxide, and degrading organic debris. Sediments from two basins in Lake Mariut, Egypt, which had undergone multiple rounds of restoration, were used in microcosm experiments to assess the role of meiofauna in organic matter degradation. Treatments included sediments with and without fauna, and four chlorophyll-a additions (0.002, 0.035, and 0.005 mg/ml, with 0.000 mg/ml as the reference). Meiofauna, chlorophyll-a, and organic matter were measured over two 8-month periods in 2014. Most treatments exhibited a rapid loss of organic matter, reducing organic content by two to eight times by study end. By the end, meiofaunal populations increased one- to 13-fold in microcosms with algae additions of 0.035 and 0.005 mg/ml chlorophyll-a in the Main and Northwest basins but had no change in those with 0.002 and zero mg/ml. Meiofauna abundance rose with rising temperature and oxygen levels, while organic matter declined. There was no correlation between chlorophyll-a levels and meiofauna abundance indicating that meiofauna likely play a role in the aerobic decomposition of organic matter at high temperatures. The meiofauna contribute to the diversity of ecosystems and have a potential role in ecosystem processes; therefore, conservation efforts should also include meiofauna. [ABSTRACT FROM AUTHOR]

Phylosymbiosis, the association between the phylogenetic relatedness of hosts and the composition of their microbial communities, is a widespread phenomenon in diverse animal taxa. However, the generality of the existence of such a pattern has been questioned in many animals across the tree of life, including small‐sized aquatic invertebrates. This study aims to investigate the microbial communities associated with poorly known marine interstitial nemerteans to uncover their microbiota diversity and assess the occurrence of phylosymbiosis. Specimens from various Central American sites were analyzed using morphology‐based taxonomy and molecular techniques targeting the host 18S rRNA gene whereas their microbial association was analyzed by targeting the prokaryotic 16S rRNA gene. Phylogenetic and statistical analyses were conducted to examine the potential effects of host nemertean taxa and sampling locations on the host‐associated microbial communities. The results provide compelling evidence of phylosymbiosis in meiofaunal nemertean species, highlighting the significant impact of host genetic relatedness on microbiome diversity in small‐sized animals. This finding supports previous studies that demonstrate how certain nemertean species harbor distinct microbial communities with functional and ecological implications. Given the remarkable diversity of meiofaunal animals—spanning numerous phyla with varying lifestyles and co‐existing in the same habitat—combined with advancements in multi‐omics approaches, there is a promising opportunity to deepen our understanding of the evolutionary and ecological interactions between hosts and their microbiota throughout the animal tree of life. [ABSTRACT FROM AUTHOR]

Hurricanes are natural periodic events that disrupt marine ecosystems along their path, altering the distribution and abundance of organisms. In October 2016, Hurricane Matthew struck the Santa Marta region (Colombia) and impacted its coastal zone, yet there have been insufficient studies to measure its effects. The aim of this study was to assess the impact of Hurricane Matthew on the meiofaunal community in the region. To achieve this, sediment samples were collected from the intertidal zone of three beaches 5 days after the hurricane had passed. The results were compared with previous data collected in October 2014, on the abundance and diversity of taxonomic groups of intertidal meiofauna, as well as physicochemical variables such as granulometry, organic matter in sediments, salinity, and water temperature. Following Hurricane Matthew, the physicochemical variables showed changes due to the effect of the cyclonic surge in the coastal zone. The average organism abundance not only experienced a decline in both abundance and richness but also underwent changes in composition. Nematoda, originally the fourth most abundant group in 2014, became the most dominant taxon post‐hurricane. Conversely, the relative abundance of Copepoda decreased. The total number of recorded taxa was 13, which is close to the 15 previously reported. The ANOSIM and PERMANOVA tests confirmed differences in the meiofaunal community before and after Hurricane Matthew, with Tardigrada, Cnidaria, Ostracoda and Mollusca, being the main groups that contributed the most to the dissimilarity between periods. The environmental variables of organic matter and temperature best explained the structure of the meiofaunal community after the natural phenomenon. This study highlights the importance of meiofauna as a monitoring tool for beaches and the effects of environmental disturbances such as hurricanes, which are expected to become more common in coastal areas due to climate change. [ABSTRACT FROM AUTHOR]

There is an apparent contradiction between the sedentary lifestyle and direct development of meiofaunal organisms and their widely observed distribution. This study is situated within the theoretical framework of ecological connectivity, metacommunities, and the impact of substrate type, particularly in the unique context of oceanic islands, offering a crucial perspective to unravel the complexities of meiofaunal dispersal and connectivity. We here studied the spatial distribution of meiofauna on natural and artificial beaches of an oceanic island, that is, Gran Canaria (Canary Is., NE Atlantic Ocean). The results revealed higher abundance and richness of meiofauna on artificial substrates compared to natural ones, with nematodes, copepods, and annelids being the dominant groups. The meiofaunal community composition differed significantly between natural and artificial beaches, suggesting colonisation of artificial beaches by natural beach communities. Notably, certain species were exclusively found on artificial beaches. The study highlighted the importance of substrate type in influencing meiofaunal composition, with artificial beaches providing new ecological niches and resources for meiofauna. The dissimilarity between beaches was attributed to morphospecies exclusive to either artificial or natural substrates, emphasising the role of dispersal mechanisms. These findings contribute to understanding the metacommunity dynamics of meiofauna in oceanic islands and call for further research on dispersal potential and biogeography. [ABSTRACT FROM AUTHOR]

Free‐living marine nematodes play crucial roles in the benthic ecosystem and serve as valuable bioindicators for monitoring environmental stress. We explore the community structure of free‐living marine nematodes in the beach ecosystem near Bandar Abbas port city of Iran. The research focused on assessing the impact of domestic and industrial influences during winter and summer seasons in 2018 and 2019. Three coastal locations, namely KhurGursuzan (KG), Power Plant (PP), and Resalat (RE), were sampled, with KG under domestic wastewater influence, PP characterized by industrial activities, and RE serving as the control site. A total of 41 nematode genera were identified during the study, and dominant genera varied across locations, namely, Daptonema, Sabatieria, Promonhystera, Ptycholaimellus, and Eleutherolaimus at KG; Spirinia, Chromadorina, Terschellingia, Eleutherolaimus, and Promonhystera at PP; and Oncholaimus, Pomponema, Viscosia, Ptycholaimellus, Daptonema, and Eleutherolaimus at RE. These dominant genera accounted for 80% of the total nematode community during the sampling periods. The study revealed significant differences in abundance and genus diversity among the sites, with pollution‐affected sites exhibiting the lowest nematode diversity. Sediment granulometry showed a positive correlation with genus diversity across the sites, and the structure of the nematode assemblage was negatively influenced by polychlorinated biphenyls and total organic matter. Surprisingly, the spatial patterns in nematode assemblage structure and diversity remained consistent across both studied years. These findings contribute to our understanding of marine ecosystem health and the role of free‐living nematodes as bioindicators in the Persian Gulf. [ABSTRACT FROM AUTHOR]

Schizorhynchia is a group of free-living interstitial flatworms within Kalyptorhynchia, characterized by possessing a split proboscis. To date, only seven species of Schizorhynchia have been recorded from Japan. Here, we describe two new species of schizorhynchs, Proschizorhynchella quadricaudata sp. nov. and Cheliplana izuensis sp. nov., based on specimens collected from the intertidal zone of Japanese waters. We also explored the phylogenetic position of the new taxa based on partial sequences of the nuclear 18S rRNA and 28S rRNA genes. Proschizorhynchella quadricaudata sp. nov. is characterized by (i) proboscis lips with a row of glands; (ii) a simple cone-shaped stylet; and (iii) a caudal end with four finger-shaped projections. Cheliplana izuensis sp. nov. can be distinguished from its congeners by (i) two strong hooks with bifurcate tips, encircling the distal part of the spiny cirrus; (ii) the fact that the proximal 20% of the cirrus length is unarmed; (iii) proboscis' sidepieces lacking needles; (iv) paired seminal vesicles; and (v) the presence of a vagina. The phylogenetic analyses support the position of P. quadricaudata sp. nov. within Schizorhynchidae and the position of C. izuensis sp. nov. in a clade containing all species of Cheliplana within Cheliplanidae. [ABSTRACT FROM AUTHOR]

Gastrotrichs of the genus Chaetonotus Ehrenberg, 1830 (Chaetonotida, Chaetonotidae) are known to be a speciose and pervasive group in both the marine and freshwater ecosystems, and recent investigations indicate that there is a significant amount of species diversity yet to be discovered. In the present study, a new species of this genus is described from two quarry lakes in northern Italy and characterized using an integrative morphological and molecular approach. Chaetonotus polites n. sp. is recognized by a rather stocky body, a five-lobed head, and the cuticular covering resolved in 13 alternating columns of partially imbricated scales, trilobed on the head and pentagonal on the trunk; each scale bears a peculiar simple, very thick, curved spine with a truncated apex. The new species is also distinguished by four putative molecular apomorphies at the 18S rRNA gene and two at the 28S rRNA gene. The location and type of these unique traits in the predicted secondary structure of the ribosomal genes is provided. The phylogenetic analysis based on concatenated sequences of three genes (the nuclear 18S and 28S rRNA genes and the mitochondrial COI gene) derived from 123 selected chaetonotidans, including the novel species, confirms that the genus Chaetonotus is a polyphyletic group, with several of its members resolved together with species of other genera. The new species forms a cluster with species of the subgenus Hystricochaetonotus Schwank 1990, suggestive of its potential stem lineage. [ABSTRACT FROM AUTHOR]

This study provides a taxonomic analysis of three newly discovered species of free-living marine nematodes in the subgenus Tricoma from the East Sea, Korea. Tricoma (Tricoma) polyringulata sp. nov. is characterized by its relatively small body size, with lengths of 280–370 µm in males and 320–390 µm in females, and 75–89 main rings. Diagnostic features include an uncovered first ring, triangular head shape, amphidial fovea with slight constriction, and a unique pattern of somatic setae, comprising 7–9 subdorsal and 8–12 subventral setae. Tricoma (Tricoma) fortiseta sp. nov. is distinguished by 65–69 main rings, 10–12 subdorsal setae, and 17–20 subventral setae, along with a hexagonal head, a well-defined labial region with six lips, and bifid-tipped cephalic setae located medially on the head. Tricoma (Tricoma) uljinensis sp. nov. exhibits 67–70 main rings, with a thick, sclerotized head that is 1.5 times as wide as its length, gently triangular in shape. Its diagnostic features include a gubernaculum bent downward near its midpoint and a conical terminal ring, with 25–39% of the desmos covered. Detailed morphological descriptions of each species are provided, along with a comparative table of morphological traits for species with 61–75 main rings and an illustrated key for identification. [ABSTRACT FROM AUTHOR]

Small-sized invertebrates inhabiting hard substrates in coral reefs (a.k.a. cryptofauna) contribute substantially to reef biodiversity, but their patterns of distribution and ecological controls are poorly understood. Here, we characterized the cryptofauna community and explored "bottom-up" and "top-down" controls by benthic cover and fish abundance, respectively. We sampled the cryptofauna inhabiting the reef terrace from 13 sites along 200 km in Jardines de la Reina (Cuba), a well-preserved and protected area in the Caribbean. We counted 23,959 invertebrates of 14 higher taxa, being the most abundant Copepoda (54%), Nematoda (21%), Mollusca (7%), Ostracoda (5%), Polychaeta (5%), and Amphipoda (3%). Richness, abundance, and community structure varied across the reefs without any geographical gradient of distribution. One-third of the variance occurred at site scale (~ 10 km), and half occurred at quadrat scale (~ 1 m). Algal cover promoted cryptofauna richness and abundance likely providing substrate and food, while live coral cover negatively influenced nematode abundances, potentially due to coral defenses. Relationships between cryptofauna and reef fishes were also present, with invertivores and herbivores negatively affecting cryptofauna abundance likely due to direct or indirect predation pressures. This research highlights the important roles of bottom-up and top-down controls, by algal/coral cover and fishes, respectively, on cryptofauna and in extension to coral reef biodiversity. Current threats by climate change are expected to alter these controls on cryptofauna resulting in changes to diversity, trophodynamics and energy flows of coral reefs. [ABSTRACT FROM AUTHOR]

The clade Scalidophora is an animal taxon composed of three distinct phyla: Kinorhyncha, Priapulida and Loricifera. These animals are currently studied by a very small but highly active number of experts, and the overall knowledge about the group remains low due to challenges in handling and describing them, as well as the relatively recent discovery of the third phyla. In this contribution, we analyse the study of Scalidophora and its constituent phyla using bibliometric tools, with the aim of understanding the state of research in the group and identifying and predicting trends in its study. Our results indicate (1) a trend towards taxonomy in Scalidophora research, likely due to the generally low knowledge of the clade; (2) a significant growth in recent years in both general scientific production and the number of species of these animals; (3) high collaboration patterns, especially among European countries and countries such as Japan and the United States; (4) a clear imbalance in productivity and knowledge between regions of the Northern and Southern Hemispheres; and (5) a substantial positive impact of the Scalidophora Workshops and the associated Special Issues on scientific production and collaborations within the group. [ABSTRACT FROM AUTHOR]

The Arctic, despite being profoundly affected by global climate change, remains largely understudied. Its local ecosystems and fauna are undergoing significant changes, including marine meiobenthic invertebrates, such as kinorhynchs. However, although they are an important component of the benthic meiofauna, our understanding of their communities remains relatively limited. The present study focuses on the structure and connectivity of the populations of three Echinoderes species: Echinoderes aquilonius , Echinoderes eximus and Echinoderes remanei , inhabiting the coastal area of Disko Island (Western Greenland). A hypothesis of low population diversity was tested through analyses based on both morphology and the molecular cox1 marker. For the first time the distribution and relationships between and within species were described. Phylogenetic analysis and haplotype networks reflected the results of population statistics (nucleotide and haplotype diversity, Tajima's D and Fu's F s), revealing expected homogeneity and low diversity, without any noticeable patterns of geographical distribution. The results indicate that Arctic kinorhynchs form progressively developing and expanding communities. Additionally, the analyses revealed a taxonomically important example of intraspecific polymorphism. Presence or absence of tubes are usually considered to be species diagnostic characters, and the finding an isolated population of E. eximus -like specimens with sublateral tubes on segment 9, suggested that this could be a new, yet undescribed species. However, the genetic divergence between these specimens and other populations of E. eximus without tubes on segment 9 was so low that they all have to be considered conspecific. This is the first time molecular sequence data has provided evidence for intraspecific tube polymorphism among kinorhynchs. [ABSTRACT FROM AUTHOR]

The genus Echinoderes , currently consisting of over 150 known species, has seen a surge in new descriptions since 2015. Species identification traditionally relies on morphological traits and morphometrics, but distinguishing closely related species is more and more challenging due to overlapping features and character variability. Cryptic diversity, where species are genetically distinct but morphologically identical, is common in meiofauna and suspected in kinorhynchs, though not previously confirmed. This study examines the northeastern Pacific Echinoderes fauna, using newly collected material and historical collections of the Smithsonian National Museum of Natural History. We describe a new species, Echinoderes quasae sp. nov., using a molecular and morphological approach and provide the first evidence of cryptic diversity in kinorhynchs, revealing at least two new species within a previously unrecognized cryptic complex. [ABSTRACT FROM AUTHOR]

So far, 24 named kinorhynch species have been reported from the Southern Gulf of Mexico, and out of these only five are Echinoderes species. The present study focuses on Echinoderidae species at 29 sites from sGoM and includes new information for non-Echinoderidae species. The study revealed the presence of 28 Echinoderidae species in total, and a rich undescribed fauna, including one species of Fissuroderes and 18 Echinoderes species. The species composition through the depth gradient was significantly different. The variability of environmental conditions measured suggests a decreasing intensity of disturbance as depth increases, which influenced the diversity increasing from shallow sites (16–200 m) to medium depth sites (391–725 m) and decreasing again when reach deep sites (>953 m). The non-Echinoderidae species did not show significant trends. The total kinorhynch community was represented by 46 species and statistical analyses revealed similar trends as observed for the Echinoderidae assemblage. The richness recorded in sGoM was greater than the 32 species reported in nGoM. [ABSTRACT FROM AUTHOR]

A new species of semnoderid kinorhynchs, Sphenoderes japonicus sp. nov. collected from oxidized brown silt on the steep slope of the deepest depression in the Sea of Japan, Northwestern Pacific, is described using light and scanning electron microscopy. This new representative of the family Semnoderidae is characterized by the structure of the first trunk segment consisting of externally closed cuticular ring with indistinct intracuticular tergosternal junctions. Cuticular ring of the first trunk segment is with deep and narrow middorsal incision and much broader midventral incision filled by large midventral placid. Acicular lateroventral spines on trunk segments 3–9. Lateroventral accessory spines are minute, only on trunk segment 5. Trunk segment 10 with acicular spines in laterodorsal position in female and in midlateral position in male. S. japonicus sp. nov. constitutes the first bathial species of this family and the first species of Semnoderidae in the Sea of Japan. [ABSTRACT FROM AUTHOR]

A survey of the kinorhynch fauna in the Oki Islands revealed six species of kinorhynchs in three families and four genera. Among these, one species in the genus Echinoderes was described as a new species, Echinoderes okiensis sp. nov., based on the following characteristics: presence of middorsal acicular spines on segments 4–8; ventrolateral tubes on segment 2; lateroventral tubes on segment 5; lateroventral acicular spines on segments 6–9; midlateral tubes on segment 10; type-1 glandular cell outlets present middorsally on segments 1–3, 5, and 7, two middorsally on segments 10 and 11, one pair paradorsally on segments 4, 6, 8, and 9, sublaterally on segment 1, and ventromedially on segments 2–10; type-2 glandular cell outlets absent; tergal extension narrowing abruptly into long acicular tips, constituting 5–7% of trunk length; males with three pairs of penile spines, the middle pair being very short; females with papillae in ventrolateral position on segment 7 and ventromedial position on segment 8, as well as with lateral terminal accessory spines. This species was also found to inhabit off Iejima Island in Okinawa, more than 1000 km away from the Oki Islands. Phylogenetic analysis based on 18S and 28S rDNA sequences suggested that this species is closely related to Echinoderes aureus Adrianov et al., 2002 in the family Echinoderidae. These two species, along with the Echinoderes dujardinii species group, the Echinoderes spinifurca species group, Echinoderes truncates Higgins, 1983, and Cephalorhyncha , compose one of the two major clades detected in Echinoderidae. [ABSTRACT FROM AUTHOR]

Flatworms in the family Otoplanidae Hallez, 1892, are ubiquitous inhabitants of the swash zone of marine sandy beaches worldwide. We present new insights into the morphology and phylogenetic position of the otoplanid Itaspiella helgolandica (Meixner, 1938) based on specimens sampled in northern Norway. While the species was originally described from the German coast, it has since been reported in the High Arctic as well as in Greenland, Canada and Alaska, indicating that it has a circumpolar distribution. Apart from these reports, I. helgolandica has received marginal scientific attention beyond the original species description, and apart from drawings, there is not much detailed data available of its morphology. We employ antibody staining and confocal laser microscopy to provide detailed morphological information on the ciliation, myoanatomy and genital organs of specimens of I. helgolandica and compare their morphology with other accounts of the species from across the Arctic. We investigate the phylogenetic relationships of Itaspiella Ax, 1955 through a Bayesian/RAxML analysis of 28S sequences and confirm the morphologically underpinned notion that this genus sits within the subfamily Otoplaninae Hallez, 1910, as the sister taxon to Xenotoplana Ax, Weidemann & Ehlers, 1978. In light of our findings, we explore the question of the apparent circumpolar distribution of Itaspiella in the context of the larger “meiofauna paradox” and touch upon various evolutionary phenomena that may have contributed to phenotypic stability despite its limited dispersal capabilities and the vicariance imposed by the complex recent geological history of the Arctic. [ABSTRACT FROM AUTHOR]

Life is not a beach for those animals that survive in the rough ecological conditions found in marine sandy beaches – and yet, microscopic animals thrive on them. We explore the drivers for meiofaunal diversity in beaches by analysing taxonomic and functional patterns of 348 flatworm communities across 116 reflective beaches in the western Mediterranean, totalling 152 species (61.2% new to science). First, we confirm that species richness does not differ between beach hydrodynamic levels (swash, shoaling and surf) but rather depends on the characteristics of each beach. Second, we demonstrate that species composition across those levels depends on the species traits, in addition to geographical and abiotic factors. Third, we highlight that the species functional space has a lower richness than expected and a lower redundancy in the wave‐exposed swash level compared to the shoaling and subtidal levels, suggesting a trait‐based ecological filtering. Finally, we show that those differences depend on the higher frequency of hydrodynamics‐related traits in the species of the swash level. Our results suggest that the rough hydrodynamic conditions in the swash level favour a unique combination of species traits, which might be linked to ecological speciation in flatworms but also in other interstitial animals. [ABSTRACT FROM AUTHOR]

This review synthesizes recent developments in the field of tardigrade biogeography, tracing back to the origins of Baas Becking's hypothesis that 'everything is everywhere but the environment selects' to the latest empirical studies that favour biogeographic structuring of these microinvertebrates. I recapitulate evidence for diverse means of meiofauna dispersal, focusing on the two principal mechanisms suggested for tardigrades: wind (aeolian dispersal/anemochory) and animals (zoochory). I argue that tardigrade species with relatively well-documented geographic distributions display climatic regionalization. The literature treating purportedly cosmopolitan tardigrades is critically scrutinized and evidence for strict cosmopolitanism is deemed insufficient. Due to the presence of numerous endemics restricted to mountains, elevational diversity gradients can be recognized at a local scale (for example, within a single biogeographic region). Finally, the tardigrade faunas of a given region should be likely regarded as part of a regionalized macrometazoan fauna containing a small fraction of widespread, ubiquitous species (called 'biogeographic noise' herein). [ABSTRACT FROM AUTHOR]

During a survey of marine biodiversity in the deep sea off northeastern Guam, two marine desmoscolecid nematodes belonging to the subgenus Tricoma were discovered. Tricoma (Tricoma) disparseta sp. nov. was described based on specimens collected from sponge and starfish habitats on a seamount at depths ranging from 1300 to 1500 m. Tricoma (Tricoma) disparseta sp. nov. is distinguished by having 59 to 62 main rings, 9 to 10 subdorsal setae, and 14 to 18 subventral setae on each side. Notable features include the differentiation in length and insertion between the subdorsal and subventral setae, as well as the amphid extending to the second or third main ring. Additionally, the spicules have a relatively small capitulum at the proximal end, while the gubernaculum is bent into a hooked shape. The specimen of T. (T.) longirostris observed in this study closely resembles previously reported specimens, characterized by 78 main rings, a long and narrow head shape, eight to nine subdorsal setae, 14 to 15 subventral setae, and a gubernaculum with a knobbed apophysis. Two Tricoma species from the Northwest Pacific Ocean are described in detail, and pictorial keys and comparative tables for species identification are provided for groups with 50 to 64 main rings. [ABSTRACT FROM AUTHOR]

Background: The genus Anomalohalacarus Newell, 1949, which is known to occur exclusively inhabiting sand, has not been previously reported from Korea. During a recent survey of the meiofauna from several intertidal sandy beaches in South Korea, we found both sexes of two psammobiont halacarids, Anomalohalacarus angustus sp. nov. and A. biformis Abé, 1996. New information: Both species are similar to each other in sharing an undivided posterior dorsal plate, three and two setae on the anterior and posterior epimeral plates, respectively, three bipectinate setae on tibia IV, and four ventral setae on tarsus I. However, A. angustus sp. nov. is distinguished from A. biformis by the absence of areolae posteriorly on the anterior dorsal plate, a much more slender posterior dorsal plate, a pair of subgenital setae on the genital foramen in females, and seven branched perigenital setae on the genitoanal plate in males. The feature of branched perigenital setae in males is reported for the first time in the genus Anomalohalacarus. The Korean specimens of A. biformis agree well with the original description from Japan, except for the number of ventral bipectinate setae on tibia II. This study presents (re)descriptions of both species with detailed illustrations and provides a dichotomous key to Anomalohalacarus species, including the new species, based on morphological characters. The genus Anomalohalacarus is reported for the first time in South Korea. [ABSTRACT FROM AUTHOR]

A new species of meiobenthic sipunculan, Phascolion kamchatikus sp. nov. collected from coarse sand and shell hash in shallow waters in the northwestern Pacific, is described and illustrated using light and electron microscopy. This new species is well distinguished from all other congeners by a unique combination of main taxonomic characters. This is the only sipunculan having only two primary tentacles in both juvenile and sexually mature worms. There are no hooks and holdfast papillae in the adult stages. Because of the presence of three nearly equal in size retractor muscles forming a retractor column divided for most of its length, this species is assigned to the subgenus Isomya of the genus Phascolion. Unlike most other species of the genus Phascolion , P. kamchatikus sp. nov. has an interstitial mode of life moving among the coarse sand grains and shell hash accumulated between rocks and stones in shallow water areas alongside the Pacific coast of Kamchatka, Aleutian Islands and Alaska Peninsula. Phascolion kamchatikus sp. nov. constitutes the only third interstitial species of the phylum Sipuncula. This is also the ninth species of Phascolion in the northwestern Pacific and the first interstitial representative of this genus in the Pacific Ocean. [ABSTRACT FROM AUTHOR]

Assessments of biodiversity and ecosystem status can benefit from DNA metabarcoding as a means to streamline sample processing and specimen identification. Moreover, processing the fixation medium instead of the precious material introduces straightforward protocols that allow subsequent focus on certain organisms detected among the preserved specimens. In this study, we present a proof of concept via the analysis of freshwater invertebrate samples from the Tatra Mountain lakes (Slovakia). Besides highlighting a match between the lake-specific environmental conditions and the results of our fixative DNA metabarcoding, we observed an option to fine-tune the fixation time: to prefer two weeks over a day or a month. This effect emerged from the presence/absence of individual taxa rather than from coarse per-sample records of taxonomic richness, demonstrating that metabarcoding studies—and efforts to optimize their protocols—can use the robust metrics to explore even subtle trends. We also provide evidence that fixative DNA might better capture large freshwater species than terrestrial or meiofauna. [ABSTRACT FROM AUTHOR]

Meiobenthic samples collected on the Côn Đảo archipelago (Vietnam) from two biotopes, in mangroves and on the adjacent reef flat areas, are analyzed. Quantitative characteristics of the dominant groups of meiobenthos are presented, and taxonomic composition of dominant taxa, nematodes, and harpacticoid copepods, are analyzed. It is shown that analysis of species composition is uninformative for a high species richness and a large number of unique species in each sample. It is more informative to use the generalized characteristics of communities, such as spectra of families and ecological and morphological groups of species. At the level of families and subfamilies, there are well-defined differences in the population of the sandy bottom and mangroves. The proportion of life forms in the different biotypes is informative for harpacticoid copepods. Big differences in the taxonomic composition of the studied bays are shown, interbiotopic differences make a smaller contribution to the overall diversity; i.e., each bay has its own set of species, which forms the population of each biotope. Nematodes and harpacticoids are distributed in space independently of each other. [ABSTRACT FROM AUTHOR]

The article describes the results of studies of two main components of meiobenthos—nematodes and harpacticoid copepods—conducted in Blagopoluchiya Bay (Novaya Zemlya, Kara Sea) in 2013 and 2020. The mosaic distribution of these two taxonomic groups was due to high species diversity and low occurrence of most species. Nevertheless, the authors identified patterns in the changes in the species structure of both groups related to the effect of two main environmental gradients (a spatial gradient related to the distance from the bay mouth to the sea and a depth-related gradient), as well as to the patchiness of sediment distribution. In 7 years between the two surveys in Blagopoluchiya Bay the abundance of nematodes increased tenfold, the copepod abundance also increased, but to a much lesser extent. Taxonomic changes in these two groups were observed at both species and family levels, with no differences in the taxa functional structure. The observed changes can be related to changes in abiotic factors and to general changes in the trophic system of the bay due to the establishment of the snow crab Chionoecetes opilio. [ABSTRACT FROM AUTHOR]

Human influence on Antarctic marine ecosystems is a growing concern, despite limited information being available. This study investigated the coastal meiofauna and environmental parameters of 10 locations, 4 of which served as reference points (OB1, OB2, OB3, and McGrady) and 6 which were impacted by different levels of human activity in the past and present (Wharf, Wilkes, BBIN, BBMID, BBOUT, and Shannon) in East Antarctica. Environmental variables such as metals, total petroleum hydrocarbons (TPHs), polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), grain size, organic matter content, and nutrients were measured for analysis. Locations close to human activities showed higher concentrations of pollutants (metals, TPHs, PBDEs, PCBs) and greater variations in meiofauna diversity compared to the reference sites (OB1, OB2, OB3, and McGrady). In the area impacted by the Casey Station waste dump (Brown Bay), the meiofauna community at the location furthest from the pollution (BBOUT) source exhibited greater diversity compared to the closest location (BBIN). In addition to metals and TPHs, particle size was also correlated with community patterns, with finer sediments associated with more impacted sites, facilitating the accumulation of toxic compounds. These results contribute to the understanding of the role and impact of pollutants on meiofauna biodiversity in Antarctic coastal ecosystems. [ABSTRACT FROM AUTHOR]

This is the first survey of marine tardigrades from the British Virgin Islands, though two species were previously reported from St. Croix, U.S. Virgin Islands. In June 2016, we collected subtidal and intertidal sand samples from various locations throughout the British Virgin Islands. We found 602 tardigrades in 18 of 21 samples, and these included 12 taxa, one of which, Batillipes wyedeleinorum sp. nov., is new to science and described here. We compared abundance and species diversity in intertidal and subtidal samples and found significantly greater abundance in intertidal habitats but no significant difference in the number of observed species between the two habitats. We calculated Chao 1 species richness, which indicated higher estimated richness in intertidal habitats and 15 ± 3.7 species (x̅ ± SD) in both habitats combined. Non-metric Multidimensional Scaling revealed differences in species composition between the two habitats with some species showing clear preferences for one or the other. [ABSTRACT FROM AUTHOR]

There are few taxonomic studies of the harpacticoid family Idyanthidae Lang, 1944 in the Pacific Ocean. As a first report of the family in this region, one species of the monotypic genus Idyellopsis Lang, 1948 and two species of the genus Idyella Sars, 1905 are described from sublittoral habitats around the Korean Peninsula. Idyellopsis orientalis sp. nov. is distinguished from the type and only species of the genus, Idyellopsis typica Lang, 1948, in the length:width ratio of the body and genital double-somite, length of caudal seta I, and structure of the female P5 baseoendopod. Idyella dolichi sp. nov. and Idyella exochos sp. nov. share the armature of the female P5 exopod with four setae with Idyella exigua Sars, 1905, Idyella major Sars, 1920, and Idyella pallidula Sars, 1905. However, the two new species differ from the three European species in the armature of the terminal segment of the P1 endopod and female P5 baseoendopod, and in the shape of the genital double-somite. The male of Idyella dolichi sp. nov. also has two distinct features: the distal two segments of the P2 endopod are completely fused, and the third segment of the antennule has a prominent outer projection. The geographical distribution of Idyellopsis and Idyella is extended by the discovery of three new species in Korean waters. We also discuss a possible phylogenetic relationship among members of family Idyanthidae at genus level.

Kermadec Islands is a remote subtropical island arc in the Southwest Pacific Ocean located 800–1000 km northeast of New Zealand’s North Island. Until now, no data was available on the nematode fauna living in the seafloor environments surrounding these islands. A single sample of subtidal coarse sediments from the Raoul Island coast yielded four new ceramonematid species: Ceramonema taikoraha sp. nov., C. taiora sp. nov., Metadasynemoides taihua sp. nov. and Pselionema huakita sp. nov. This new discovery is the first addition to the family since 2008 and brings the total number of valid ceramonematid species globally to 67 species. Dichotomous identification keys are provided for valid species of Ceramonema, Metadasynemoides and Pselionema. The desmodorid species Acanthopharynx dormitata and Desmodora bilacinia, were also recorded from Raoul Island some 1350 km away from their type locality in Wellington Harbour (New Zealand’s North Island). These species may have a relatively widespread distribution but testing this hypothesis will require further morphological comparisons and analyses of molecular sequence data to confirm the status of the Kermadec specimens.

Abstract The organic enrichment effects on the meiofauna and nematofauna were assessed for field sediment and other experimental ones enriched with organic matters conducted in the laboratory for 4 weeks. Also, dissolved oxygen (DO) and pH were monitored for each one. The abundance and diversity of meiofaunal groups and nematofauna varied. Strong significant correlations were found between DO and the studied items. Nematoda was the most abundant group in the field sediment and other experimental ones; their counts increased with the increase in organic enrichments and were dominated by deposit feeders. Amphipoda, Ostracoda and predator/omnivore nematodes disappeared in highly organic-enriched sediments. Changes in DO and organic enrichments might be the more attributable reasons for the alteration of the meiobenthic assemblages. The generic compositions of Nematoda provide a good indicator for environmental alterations.

Most information on rotifers living in lake sands comes from the 1930s, when the first reports on this subject by Jerzy Wiszniewski appeared. After some decline in the interest in lake psammon in the following years, research on lake psammon returned in the last decade of the twentieth century. The last comprehensive review of ecology of psammon Rotifera was included in the excellent 1998 publication by Schmid-Araya on Rotifera in interstitial sediments. More than 20 papers have been published since that time. So far, research conducted almost exclusively in the northern hemisphere have dealt with issues related to distribution of rotifers, their species structure and diversity as well as to their relation to abiotic and biotic factors. The aim of the present study was to collect all available information on psammon rotifers, assess the shortcomings in our knowledge and fill these gaps with the results of some of my recent research. The results of the study show that the species richness of this group in geographical terms is still unknown. It means we also do not know the level of endemism among the psammon rotifers. The role of psammon rotifers in benthic food webs is another issue requiring research. [ABSTRACT FROM AUTHOR]

Baiyangdian Lake, the largest freshwater shallow lake on the North China Plain, plays a pivotal role in maintaining the regional ecological balance and biodiversity. Meiofauna are integral components of Baiyangdian Lake; however, their community characteristics and relationship with environmental factors have not yet been studied. The aim of the following study was to evaluate the density, spatiotemporal patterns, and habitat response dynamics of meiofauna in Baiyangdian Lake. A field investigation was conducted at 33 sites spanning various habitats, including aquatic plant-dominant, trench, and pelagic areas, across the spring, summer, and autumn seasons of 2021. The results revealed that the meiofauna in Baiyangdian Lake primarily comprise freshwater nematodes (91.78%), ostracods, and copepods, with a mean abundance of 69.40 ± 35.20 ind. 10 cm−2, peaking in the spring, followed by summer and autumn. The mean biomass was 164.95 ± 99.39 dwt. 10 cm−2, with that of ostracods being the most substantial and that of copepods being the least, with both of them exhibiting seasonal fluctuations. Notably, in the summer, the abundance of meiofauna was positively correlated with the water depth and negatively correlated with ammonia nitrogen levels (R² = 0.13 and R² = 0.24, respectively; p < 0.05 and p < 0.01; n = 33). The results of our study indicate that the distribution and abundance of meiofauna are significantly affected by environmental factors, with the water depth and ammonia nitrogen levels being potential key determinants. The results of the present study are conducive to evaluating the health status of the Baiyangdian ecosystem, protecting biodiversity, and studying the impacts of anthropogenic activities and environmental changes on the lake, and can also provide scientific support for its ecological restoration and governance as well as the assessment of ecological service functions. [ABSTRACT FROM AUTHOR]

As glaciers recede the environmental conditions in stream ecosystems shift from variable and harsh through to more stable and benign. The process through which communities assemble and develop along this successional gradient is governed primarily by changes in the environmental conditions, but mediated by the traits of organisms. Certain traits allow for organisms to be colonisers and persist under harsh environmental conditions, whereas others allow organisms to resist ecological interactions, and extreme events in later successional stages. As such, we can use trait analyses to understand the processes and mechanisms responsible for community assembly and structure. Here, we used a unique natural laboratory, Glacier Bay National Park, to further develop our understanding of how traits vary and functional diversity assembles as glaciers recede and the riverine landscape develops. Macroinvertebrate and meiofauna were investigated across two scales: (1) landscape - 16 streams of different successional stages (years since glacial recession); and (2) long-term - one postglacial stream over 40 years. Across both datasets, clear patterns were observed in community structure, trait variation and functional diversity. Traits conveying resilience to harsh environmental conditions (e.g., size, voltinism and short generation times) were shown to decrease in prevalence over time as environmental conditions changed and new taxa colonised. Functional diversity did not unilaterally increase along the successional gradient for either macroinvertebrates or meiofauna. However, the general patterns in functional richness and divergence indicated diversification in the trait space. Through comparing differences in the structure and function of meiofauna and macroinvertebrates along successional gradients, we were able to gain an improved understanding the dominant processes influencing communities within these postglacial systems. [ABSTRACT FROM AUTHOR]

Chaetonotidae is the most diversified family of the entire phylum Gastrotricha; it comprises ~430 species distributed across 16 genera. The current classification, established mainly on morphological traits, has been challenged in recent years by phylogenetic studies, indicating that the cuticular ornamentations used to discriminate among species may be misleading when used to identify groupings, which has been the practice until now. Therefore, a consensus is developing toward implementing novel approaches to better define species identity and affiliation at a higher taxonomic ranking. Using an integrative morphological and molecular approach, including annotation of the mitogenome, we report on some freshwater gastrotrichs characterised by a mixture of two types of cuticular scales diagnostic of the genera Aspidiophorus and Heterolepidoderma. Our specimens' overall anatomical characteristics find no correspondence in the taxa of these two genera, calling for their affiliation to a new species. Phylogenetic analyses based on the sequence of the ribosomal RNA genes of 96 taxa consistently found the new species unrelated to Aspidiophorus or Heterolepidoderma but allied with Chaetonotus aff. subtilis, as a subset of a larger clade, including mostly planktonic species. Morphological uniqueness and position along the non-monophyletic Chaetonotidae branch advocate erecting a new genus to accommodate the current specimens; consequently, the name Litigonotus ghinii gen. nov., sp. nov. is proposed. The complete mitochondrial genome of the new taxon resulted in a single circular molecule 14,384 bp long, including 13 protein-coding genes, 17 tRNA genes and 2 rRNAs genes, showing a perfect synteny and collinearity with the only other gastrotrich mitogenome available, a possible hint of a high level of conservation in the mitochondria of Chaetonotidae. ZooBank: Gastrotricha (hairy-bellied worms) is a phylum of microscopic aquatic invertebrates whose traditional morphology-based classification has been challenged in recent years. We report a new genus and species found using an integrative morphological and molecular approach, which is unrelated to morphologically similar species in the genera Aspidiophorus and Heterolepidoderma. The mitochondrial genome of the new taxon has the same number of genes and gene order as the only other gastrotrich mitogenome available, suggesting the conservation of mitochondria at the family level. (Image credit: M. Antonio Todaro.) [ABSTRACT FROM AUTHOR]

Since the discovery of the first Gastrotricha at the end of the eighteenth century, the study of this taxon has proven to be notoriously difficult due to the fragility of the body and its diminutive size. Consequently, this microinvertebrates group is considered part of the less-known metazoan taxa and its biodiversity knowledge is still far from being understood. To bring light to the past, present, and future understanding of Gastrotricha biodiversity literature, we conducted the first study aimed to quantitatively and qualitatively evaluating the global research productivity on this clade by carrying out a bibliographic survey in Web of Science and Scopus databases using bibliometric tools. We found 523 studies published between 1904 and 2022 and detected i) a high prevalence of "new species" as a keyword in the literature, indicating the youth of our knowledge on biodiversity of this phylum; ii) the formation of clusters of interacting researchers around well-established veterans' researchers in Europe; but also, iii) the formation of new research centers training new researchers in recent decades around the world. However, we also found that there are gaps in the accessibility of past literature in the main search tools. We believe that it is time to explore new facets of Gastrotricha research and we hope that our study can provide a guide for future research development to shape more effective policies for biodiversity discovery and conservation initiatives. [ABSTRACT FROM AUTHOR]

Background: Sandy beaches are dynamic environments housing a large diversity of organisms and providing important environmental services. Meiofaunal metazoan are small organisms that play a key role in the sediment. Their diversity, distribution and composition are driven by sedimentary and oceanographic parameters. Understanding the diversity patterns of marine meiofauna is critical in a changing world. Methods: In this study, we investigate if there is seasonal difference in meiofaunal assemblage composition and diversity along 1 year and if the marine seascapes dynamics (water masses with particular biogeochemical features, characterized by temperature, salinity, absolute dynamic topography, chromophoric dissolved organic material, chlorophyll-a, and normalized fluorescent line height), rainfall, and sediment parameters (total organic matter, carbonate, carbohydrate, protein, lipids, protein-to-carbohydrate, carbohydrate-to-lipids, and biopolymeric carbon) affect significatively meiofaunal diversity at a tropical sandy beach. We tested two hypotheses here: (i) meiofaunal diversity is higher during warmer months and its composition changes significatively among seasons along a year at a tropical sandy beach, and (ii) meiofaunal diversity metrics are significantly explained by marine seascapes characteristics and sediment parameters. We used metabarcoding (V9 hypervariable region from 18S gene) from sediment samples to assess the meiofaunal assemblage composition and diversity (phylogenetic diversity and Shannon's diversity) over a period of 1 year. Results: Meiofauna was dominated by Crustacea (46% of sequence reads), Annelida (28% of sequence reads) and Nematoda (12% of sequence reads) in periods of the year with high temperatures (>25 °C), high salinity (>31.5 ppt), and calm waters. Our data support our initial hypotheses revealing a higher meiofaunal diversity (phylogenetic and Shannon's Diversity) and different composition during warmer periods of the year. Meiofaunal diversity was driven by a set of multiple variables, including biological variables (biopolymeric carbon) and organic matter quality (protein content, lipid content, and carbohydrate-to-lipid ratio). [ABSTRACT FROM AUTHOR]

Background: Due to the copious disposal of plastics, marine ecosystems receive a large part of this waste. Microplastics (MPs) are solid particles smaller than 5 millimeters in size. Among the plastic polymers, polystyrene (PS) is one of the most commonly used and discarded. Due to its density being greater than that of water, it accumulates in marine sediments, potentially affecting benthic communities. This study investigated the ingestion of MP and their effect on the meiofauna community of a sandy beach. Meiofauna are an important trophic link between the basal and higher trophic levels of sedimentary food webs and may therefore be substantially involved in trophic transfer of MP and their associated compounds. Methods: We incubated microcosms without addition of MP (controls) and treatments contaminated with PS MP (1-µm) in marine sediments at three nominal concentrations (103, 105, 107particles/mL), for nine days, and sampled for meiofauna with collections every three days. At each sampling time, meiofauna were collected, quantified and identified to higher-taxon level, and ingestion of MP was quantified under an epifluorescence microscope. Results: Except for Tardigrada, all meiofauna taxa (Nematoda, turbellarians, Copepoda, Nauplii, Acari and Gastrotricha) ingested MP. Absorption was strongly dose dependent, being highest at 107 particles/mL, very low at 105 particles/mL and non-demonstrable at 103 particles/mL. Nematodes accumulated MP mainly in the intestine; MP abundance in the intestine increased with increasing incubation time. The total meiofauna density and species richness were significantly lower at the lowest MP concentration, while at the highest concentration these parameters were very similar to the control. In contrast, Shannon-Wiener diversity and evenness were greater in treatments with low MP concentration. However, these results should be interpreted with caution because of the low meiofauna abundances at the lower two MP concentrations. Conclusion: At the highest MP concentration, abundance, taxonomic diversity and community structure of a beach meiofauna community were not significantly affected, suggesting that MP effects on meiofauna are at most subtle. However, lower MP concentrations did cause substantial declines in abundance and diversity, in line with previous studies at the population and community level. While we can only speculate on the underlying mechanism(s) of this counterintuitive response, results suggest that further research is needed to better understand MP effects on marine benthic communities. [ABSTRACT FROM AUTHOR]

The family Ameiridae is the third most diverse family within Harpacticoida, with 47 genera and up to 300 species. The genus Ameira, with 21 species and four subspecies, presents taxonomic challenges due to poorly defined diagnostic features. Ameira parvula, initially was described by Claus (1866) as Canthocamptus parvulus, and has been subject to taxonomic revisions. Variations in P1 endopod-1 length, A2 exopod segments, and the number of P4 exopod-3 setae have been observed by researchers worldwide. Questions regarding A. parvula's cosmopolitan nature have led to doubts about its distribution. Given the diverse specimens examined in this study from various regions, including the Mediterranean, Aegean Sea, and Black Sea coasts of Türkiye, and material from Russia, England, and Egypt, it becomes evident that A. parvula represents a species complex. The study aims to define the boundaries of species belonging to an A. parvula's species complex based on morphological disparities. This study entails the reexamination of A. parvula specimens from various collections, followed by a comprehensive redescription. This redescription is then juxtaposed with the existing literature on A. parvula, allowing for a thorough reassessment of the previously documented records. This reassessment results in describing two new Ameira species and the reevaluation of A. parvula nana Willey, 1935 as Ameira nana. Furthermore, a comprehensive literature review on the general traits within the genus revealed that A. atlantica mediterranea Noodt, 1958 and A. atlantica atlantica Kunz, 1974 should be relegated to species rank as A. mediterranea Kunz, 1974 and A. atlantica Noodt, 1958, respectively. Additionally, A. lusitanica Galhano, 1970 should be considered as incertae sedis within Harpacticoida. [ABSTRACT FROM AUTHOR]

Introduction: The current experiment investigated the multifaceted effects induced by microplastics and the statin Lipitor on marine benthic nematodes. Methods: The nematodes were exposed to a single polystyrene and polyvinyl chlorides (both at 1 mg.kg-1 Dry Weight) and two Lipitor concentrations (0.1 and 1 mg.l-1), as well as to a mixture of both types of pollutants, for 30 days. Results: The results highlighted a significant decrease in the abundance, individual biomass, and diversity of nematodes directly with the addition of polyvinyl chlorides and/or Lipitor. These treatments induced a greater mortality rate among microvores and diatom feeders compared to other feeding types of nematodes. Discussion: The nematofauna underwent a strong restructuring phase following exposure to microplastics and Lipitor when added alone, leading to the disappearance of sensitive species and their replacement by more tolerant taxa. The toxicity of Lipitor is attenuated by the physical bonding with polystyrene when added to a mixture and has no negative effect on marine nematode species. [ABSTRACT FROM AUTHOR]

Characterizations of meiofauna, as well as of Nematoda assemblages, are rare in oceanic islands due to logistical difficulties and, until now, no study on the vertical distribution of this fauna has been conducted in these environments. Therefore, this study shows vertical distribution patterns in the meiofauna community and Nematoda assemblage on two beaches with different sediment characteristic (volcanic and carbonate) of Trindade Island, a remote oceanic island in the Atlantic Ocean. Samples were collected in December 2014 at Príncipe and Portugueses beaches in three intertidal zones (high, mid-, and low) at two sedimentary column depths (0–10 and 10–20 cm). Overall, meiofauna was comprised of seven major groups, with Oligochaeta (57%) and Nematoda (12%) as the most abundant. Higher density and richness of meiofauna and Nematoda assemblage were found at Portugueses beach with the carbonate sediment. Nematoda assemblage was mainly comprised of non-selective deposit-feeders, with a total of 27 genera and 12 families; Cyatholaimidae and Xyalidae were the most abundant and had the highest diversity. Composition and density of meiofauna and Nematoda assemblage showed similar vertical distribution where the surface layer (0–10 cm) presented lower densities. Sediment characteristics (grain size, sorting, and composition) were primarily responsible for the regulation of the structure and distribution of meiofauna and Nematoda assemblage in Trindade Island beaches. [ABSTRACT FROM AUTHOR]