Sipunculans are non-segmented marine worms with an anterior retractable introvert, which are commonly included in Annelida based on molecular phylogenetic and phylogenomic analyses. They generally burrow in the soft sediments or live inside the crevices of hard substrata (e.g. calcareous/coralline rocks). However, members of some sipunculan genera (mainly Phascolion and Aspidosiphon) are known to have a peculiar habit of dwelling in vacant shells of gastropods or scaphopods. In this study, we investigated the shell utilization and preference pattern of the species of Phascolion and Aspidosiphon in Japan. We collected 302 sipunculans, comprising 273 and 29 individuals in Phascolion and Aspidosiphon, respectively, from 57–800 m depth of three study sites in the Pacific coast of Honshu Island, Japan. The species of Phascolion were found in vacant shells of 38 genera of 27 families of gastropods and six genera of four families of scaphopods, whereas the species of Aspidosiphon were found in 11 genera of 11 families of gastropods and one genus of scaphopod. These results suggest that members of each genus use a wide range of gastropod and scaphopod shells. The body size of the sipunculans was positively correlated with the shell size, suggesting that they change the shells as they grow. Furthermore, we investigated the shell preference of Phascolion species by comparing morphological characteristics of shells occupied and unoccupied by sipunculans. Generalized linear mixed model (GLMM) analyses suggest that the species of Phascolion tend to use long and narrow shells. Such shells likely fit well the elongated trunk of sipunculans.

The aim of this study is to determine the phytoplankton species found on the shores of Burgaz Island and the basic ecological variables that affect their seasonal distribution between May 2013 and February 2014. Water samples were collected from four stations at six different depths and plankton samples were gathered horizontally and vertically. The analysis of phytoplankton community composition revealed 101 phytoplankton taxa belonging to five classes. As a result of the study, two taxa belonging to the Dinophyceae (Corythodinium frenguellii and Gonyaulax scrippsae) were new records for Turkish coastal waters. Additionally, a taxon belonging to Dinophyceae (Pronoctiluca pelagica) and a taxon belonging to Bacillariophyceae (Nitzschia reversa) were new records for the Sea of Marmara. Prorocentrum micans, was the dominant species throughout all sampling periods. During the study, the highest phytoplankton abundance was observed at 0.5 m depth in May 2013 (138,500 cells l−1) and February 2014 (52,620 cells l−1). Primary ecological variables, such as temperature (9.0–21.5 °C), salinity (15.23–37.22‰) and dissolved oxygen (4.89–15.84 mg l−1), were recorded on each sampling occasion. In addition, nitrite + nitrate–N (NO2 + NO3–N) (0.01–7.37 μg-at N l−1), phosphate (PO4–P) (0.05–51.95 μg-at P l−1) and silicate (SiO4–Si) (0.01–0.20 μg-at Si l−1) concentrations were measured. Chlorophyll a and suspended material values ranged between 0.01–3.17 μg l−1 and 10.0–61.5 mg l−1, respectively. Spearman's rank correlation was used to determine the relationship between phytoplankton species and ecological variables, and Bray–Curtis analysis and Euclidean distance were applied to bring out the similarity between stations.

We present the first record of the very large deep-sea mytilid Gigantidas horikoshii Hashimoto & Yamane, 2005 outside of its type distribution, based on a juvenile specimen collected by fishermen at the Tashi fishing grounds east of Taiwan. This record extends the distribution of the species in about 20 latitudinal degrees to the west (about 1916 km), and it is the first record for this species outside the Izu-Ogasawara Island-arc, southern Japan, representing the westernmost geographic limit of this species and the first record in Taiwanese waters.

The mitochondrial genome provides important information for phylogenetic analysis and an understanding of evolutionary origin. In this study, the mitochondrial genomes of Ilisha elongata and Setipinna tenuifilis were sequenced, which are typical circular vertebrate mitochondrial genomes composed of 16,770 and 16,805 bp, respectively. The mitogenomes of I. elongata and S. tenuifilis include 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA), two ribosomal RNA (rRNA) genes and one control region (CR). Both two species' genome compositions were highly A + T biased and exhibited positive AT-skews and negative GC-skews. The genetic distance and Ka/Ks ratio analyses indicated that 13 PCGs were affected by purifying selection and the selection pressures were different from certain deep-sea fishes, which were most likely due to the difference in their living environment. Results of phylogenetic analysis support close relationships among Chirocentridae, Denticipitidae, Clupeidae, Engraulidae and Pristigasteridae based on the nucleotide and amino acid sequences of 13 PCGs. Within Clupeoidei, I. elongata and S. tenuifilis were most closely related to the family Pristigasteridae and Engraulidae, respectively. These results will help to better understand the evolutionary position of Clupeiformes and provide a reference for further phylogenetic research on Clupeiformes species.

Hermit crabs have soft abdomens and therefore must use empty gastropod shells as protection. Shell choices do not occur at random, and the animals often occupy shells with the best fit among the available ones. To live in intertidal and sublittoral zones, regions subjected to daily variations in abiotic factors, hermit crabs present a locomotory circadian rhythm. Clibanarius antillensis has been the object of several studies in Brazil's south-east, so this study was aimed at describing its population in the south of Brazil and its movement activity under laboratory conditions during night-time and daytime, while using two different shells. The animals were sampled from the Sepultura and Lagoinha beaches, in the state of Santa Catarina through free diving. Sex ratio was skewed for males, with ovigerous females present during the whole year but with a spring peak. Males were more abundant than females, ovigerous females and intersex individuals, being predominant in all size-classes analysed. The hermit crabs occupied mainly two gastropod shells: Cerithium atratum and Agathistoma viridulum, and ovigerous females used only the first. Clibanarius antillensis showed nocturnal activity and the animals using C. atratum shells were more active than those using A. viridulum. Cerithium atratum shells are lighter and less bulky than A. viridulum, implying that there might be a tradeoff in which the hermit crabs use lighter shells, that provide less protection in order to expend less energy and be more active.

An organism's maximum gonad investment (MGI) typically indicates its reproductive season and is often measured by the peak of the gonadosomatic index. Since external sexual dimorphism is often not evident, intrinsic sex differences remain unstudied. We analysed the reproductive seasonality of each sex of the broadcast-spawning sea cucumber Holothuria (Halodeima) inornata in two populations (Caleta de Campos, Michoacán ‘CC’ and Puerto Madero, Chiapas ‘PM’) in the southern coast of the Mexican Pacific by examining: intensity and duration of MGI, frequency of each gonadal developmental stage (GDS) through time, sexual asymmetry in GSI, sexual asymmetry in GDS, and adult sex ratio. We observed a trade-off between the intensity (%) and duration (months) of each sex's MGI: as intensity decreases, duration increases and conversely. The frequency of ripe and spawning stages was consistently higher in females than males. Sexual asymmetry in GSI was slightly female-biased in the PM population, but male-biased in the CC population. Sexual asymmetry in GDS showed a more recurrent sexual equality at PM than at CC. The adult sex ratio of each population did not differ significantly from unity, but showed a near-significant trend for male bias in the CC. Although H. inornata exhibited different MGI responses between males and females and more markedly in CC than in PM, it also showed a synchronized relation between its mass investment (GSI) and physiological investment (GDS), possibly reflecting an optimum reproductive strategy. Lastly, changes in GSI were not underpinned by changes in local temperature, but rather by regional temperature.

Deployment of artificial wreck reefs (AWRs) near natural reefs can influence fish assemblages on both reefs. Here, we assessed fish attitudes (abundances, biomass and species richness) according to different level complexity areas of wrecks, feeding guilds of fish and body size of fish of two identical AWRs deployed at the same depth (36.6 m) and different distances from natural reefs. Scuba divers used visual census method to sample fish communities at wrecks for two years. An average of 2346 individuals per dive belonging to 52 species were observed on wrecks combined. Both mean fish abundance and mean biomass did not differ significantly between wrecks. Mean species richness was significantly greater at 9 Eylül than Alaybey. Planktivore was the most abundant feeding guild on wrecks, and its abundance statistically differed between AWRs. The comparisons of fish assemblages at the different level complexity areas of the wrecks show differences, the highest mean abundances were recorded at the areas of low complexity of the wrecks. The results of this study show that although deploying artificial wreck reefs near to natural rocky reefs catalyses fish colonization, it does not present an exact influence on the fish communities of AWRs. We have not known yet how creating an artificial wreck reef affects fish community of natural reefs. With giving extra importance to the choice of deployment area, AWR applications may transform to win-win projects for both the marine environment and recreational users, such as divers and anglers.

The present study aims to investigate the reproductive biology of the small pelagic Sardinella aurita Valenciennes, 1847 in the Canary Islands, to enable its reliable assessment and advise on appropriate management measures for a fishing resource showing a declining trend in landings. Reproductive biology and sexual patterns of round sardinella were examined from monthly random samples of commercial catches landed by the artisanal purse-seine fleet. The landings' length frequencies, ranging between 9–32 cm (based on the total lengths, TL), were recorded from 2013–2019. The overall mean value of TL was 20.9 cm, with annual mean values between 20–22 cm, except in 2016 (TL = 19 cm). The overall sex ratio M:F was 1:0.92, with males significantly predominant. Sex ratios fluctuated as a function of size and month: females were more abundant in the larger length classes, as well as before and after spawning, whereas males were more abundant in the smaller length classes and during spawning. Based on gonad maturity stages and gonadosomatic index, round sardinella spawns during almost all the year, with a peak in January–February and a resting period during October–November. The length at first maturity was estimated at TL of 18.2 cm, notably smaller than the value obtained for the NW African coastal waters where the demographic structure in round sardinellas' landings is totally different.

A recent survey of The Canyons Marine Conservation Zone provided noteworthy records of several crustacean species, which are presented here. The geographic or bathymetric ranges of several species are considerably extended, while the genus Rhachotropis, which has not previously been reported from British waters, is represented by two species. Other species that were previously only known in British waters from very few, often historical, records are also reported.

Kelp forests are regarded as important nursery and foraging habitats for commercially important species of finfish and shellfish despite an absence of fishery-independent data in many regions. Here, we conducted targeted surveys at 12 subtidal reefs, distributed across 9° of latitude in the UK, using three complementary techniques (Underwater Visual Census (UVC), Baited Remote Underwater Video (BRUV) and deployment of prawn pots) to quantify the abundance of crustaceans within kelp forests. Commercially important species were recorded at all sites; Cancer pagurus (brown/edible crab) and Necora puber (velvet swimming crab) were the most abundant and commonly observed, although Maja brachydactyla (spider crab), Homarus gammarus (European lobster) and Palaemon serratus (common prawn) were also recorded. The abundance of some species exhibited pronounced regional variability, with higher abundances of C. pagurus within northern regions and, conversely, higher abundances of M. brachydactyla and P. serratus within southern regions. Each sampling technique yielded similar spatial patterns for the most abundant species but had varying sensitivity to some species. Most individuals observed were juvenile or sub-adults, suggesting kelp forests serve as important nursery grounds for commercially and ecologically important crustaceans. Further monitoring efforts, conducted across greater spatiotemporal scales and in different habitat types, are needed to provide a robust baseline against which to detect changes and to inform management and conservation actions.

Recent losses of seagrass beds have resulted in a decrease in seagrass-associated fishes, highlighting the need for the improved management of such habitats and resources. Analyses of feeding patterns of fish associated with seagrass habitats are useful for understanding their habitat dependence, which contributes to achieving their effective conservation. The gut contents of 61 fish species were examined to clarify the food habits of subtropical seagrass fishes in Nagura Bay, Ishigaki Island, southern Japan. A cluster analysis based on dietary overlaps showed that the seagrass fish assemblage comprised six trophic groups (small crustacean, large crustacean, detritus, plant, fish and zooplankton feeders). Small and large crustacean feeders were the most abundant, followed by detritus, plant and fish feeders. Zooplankton feeders were represented by only two species. Ontogenetic changes in food preference were recognized in four species, including Lutjanus fulviflamma, Ostorhinchus ishigakiensis, Gerres oyena and Siganus fuscescens. Small individuals of the four species generally fed on small crustaceans or detritus, subsequently switching to other larger prey items with growth (e.g. crabs, shrimps and seagrass fronds). The most important food items for the assemblage comprised harpacticoid copepods, shrimps, crabs and detritus, representing high cumulative gut volumes for all fishes. Some species showed diets remarkably different from those determined for fishes in other seagrass beds off a nearby neighbouring island, evidence of the food-use patterns of seagrass fishes varying on a small spatial scale. Such dietary flexibility may increase the adaptability of fishes to seagrass beds characterized by significant variations in available food resources.

The coral reefs of the Pitcairn Islands are in one of the most remote areas of the Pacific Ocean, and yet they are exposed to the impacts of anthropogenic climate change. The Pitcairn Islands Marine Protected Area was designated in 2016 and is one of the largest in the world, but the marine environment around these highly isolated islands remains poorly documented. Evidence collated here indicates that while the Pitcairn Islands' reefs have thus far been relatively sheltered from the effect of warming sea temperatures, there is substantial risk of future coral decalcification due to ocean acidification. The projected acceleration in the rate of sea level rise, and the reefs' exposure to risks from distant ocean swells and cold-water intrusions, add further uncertainty as to whether these islands and their reefs will continue to adapt and persist into the future. Coordinated action within the context of the Pitcairn Islands Marine Protected Area can help enhance the resilience of the reefs in the Pitcairn Islands. Options include management of other human pressures, control of invasive species and active reef interventions. More research, however, is needed in order to better assess what are the most appropriate and feasible options to protect these reefs.

This study examines global zoogeographic patterns of estuary-associated fishes using meta-assemblages compiled at the marine ecoregion scale. The classification of biogeographic regions based on estuary-associated fish species is assessed in relation to marine and freshwater zoogeographic patterns. Historical (palaeobiogeographic) and contemporary environmental factors that contribute to the establishment and maintenance of these regions are also discussed. Global patterns in estuary-associated fish species richness demonstrated that tropical ecoregions are more diverse and richer in species than temperate ecoregions; the tropical Indo-Pacific and the tropical western Atlantic were two major centres of high global species richness. Multivariate analyses identified several ecoregion groupings that broadly corresponded with both freshwater and marine biogeographic regions. These groupings could also be related to global marine palaeobiogeographic processes such as the closure of the Tethys Sea linkage during the Cenozoic as well as contemporary factors such ocean currents, upwelling areas and climatic factors, particularly rainfall and runoff.

The annelid mitochondrial genomes (mitogenomes) have been well documented, and phylogenetic analyses based on the mitogenomes provide insightful implications for annelid evolution. However, the mitogenomes of some families remain unknown. Herein, we determined the complete mitogenome of the lugworm Abarenicola claparedi oceanica (15,524 bp), representing the first mitogenome from the family Arenicolidae. The gene order of this species is the same as the various lineages in Sedentaria. The maximum likelihood phylogenetic analyses were performed based on six different datasets, including 43 ingroups (oligochaetes, hirudineans, echiurans and closely related polychaetes) and two outgroups (Siboglinidae), namely, aligned and trimmed datasets consisting of the nucleotide sequences of protein-coding genes (PCGs) and rRNAs, and amino acid sequences of PCGs. Phylogenetic analyses based on the nucleotide sequences yielded trees with better support values than those based on the amino acid sequences. Arenicolidae is clustered with Maldanidae in all analyses. Analyses based on nucleotide sequences confirm the monophyly of Terebellidae, which was paraphyletic in recent mitogenomic phylogenetic studies. We also performed the phylogenetic analysis based on the RY-coding of the nucleotide sequences of PCGs only to yield phylogeny with generally low support values. Additional mitogenome sequences of related ingroup species would be needed to comprehensively understand the phylogenetic relationship, which was not present in this study.

A new species belonging to the genus Trophoniella Hartman, 1959 (Polychaeta: Flabelligeridae) was described from the coastal area of Civitavecchia (Northern Tyrrhenian Sea). The species was collected in gravelly coarse sands from Posidonia oceanica beds at around 7 m depth. Trophoniella cucullata sp. nov. clearly differs from all other congeners species primarily by the distribution of the sediment grains on the body and by the starting point of the anchylosed neurohooks. Diagnostic morphological traits for the identification of the species are discussed. The description of Trophoniella cucullata sp. nov. in the same P. oceanica biocenosis where other species have been recently reported in the Northern Tyrrhenian Sea, testifies to the role of this marine phanerogam as a biodiversity hotspot.

Thirty-one individuals of seven Perciform and one Tetraodontiform fish species were reported from the Davao Gulf constituting new records for the Philippines: from the Labridae, Bodianus leucosticticus (9 individuals) and Bodianus masudai (3); Nemipteridae, Parascolopsis melanophrys (2); Priacanthidae, Pristigenys meyeri (2); Serranidae, Odontanthias chrysostictus (6), Odontanthias randalli (5) and Liopropoma latifasciatum (3); Monacanthidae, Thamnaconus multilineatus (1). Large range extensions were found for P. melanophrys, O. chrysostictus, O. randalli and T. multilineatus. The individuals of P. melanophrys are the third and fourth ever-reported individuals of this extremely rare dwarf monocle bream. A new maximum SL of 126 mm was reported for O. chrysostictus. We also encountered one individual of the rare wrasse Bodianus rubrisos recently observed at a more northern location. The current observations extend our knowledge of marine fishes in the Philippines and the unusually high marine biodiversity in this region. As all recorded species are considered inhabitants of mesophotic coral ecosystems, these observations also confirm that fishing pressure has increased in these deeper habitats.

Sea urchins have important effects on marine ecosystems such as rocky shores and coral reefs across the world. However, species diversity and molecular phylogeny of most echinoid taxa are poorly known in Iran. In this study, the phylogenetic relationships of one of the most abundant species of the genus Echinometra in the Persian Gulf were examined. Echinoids were collected from the intertidal zone of Qeshm Island and Lengeh Port on March and December 2017. Morphological criteria based on valid identification keys combined with molecular analysis of a fragment of the mitochondrial cytochrome c oxidase subunit I (COI) protein-coding gene were used to delineate Echinometra species. Our analyses showed that all specimens (N = 15) belong to Echinometra sp. EZ. Tree topologies indicated that our individuals from two sampling sites formed a distinct monophyletic clade with E. sp. EZ, demonstrating high support values. This is the first phylogenetic analysis of E. sp. EZ from Iran.

Seven species of chirostyloid and galatheoid crustaceans, including three new species, are reported from the eastern Arabian Sea and Bay of Bengal, India, based on material from depths ranging from 32–1420 metres. Trapezionida samudrika sp. nov. appears close to T. munin (Komai, 2011), but is distinguished by the shape of the carapace and fourth thoracic sternite, the armature of the third maxilliped merus, the length and armature of the cheliped, and the armature of the ambulatory legs. Paramunida travancorica sp. nov. differs from its closest species P. bineeshi Macpherson, Chan, Kumar & Rodriguez-Flores, 2020, P. mozambica Cabezas, Macpherson & Machordom, 2010, and P. stichas Macpherson, 1993 in the morphology of the carapace and antennular and antennal peduncles. Munidopsis bhavasagara sp. nov. is similar to M. dentifalx Osawa, Lin & Chan, 2007, but distinguishable by the absence of a pair of elevated, short ridges on the epigastric carapace region and the proportionally shorter chelipeds and ambulatory legs. The present material of Munidopsis unguifera Alcock & Anderson, 1894 represents only the third record of the species in India and a detailed description and illustrations are provided to supplement the previous descriptions. The remaining three species, Eumunida multispina Komai, Chakraborty, Paramasivam & Gidda, 2019, Munidopsis scobina Alcock, 1894, and M. stylirostris Wood-Mason in Wood-Mason & Alcock, 1891, are recorded from additional localities within their known geographic regions.