In Japan, the invasive entocytherid ostracod Ankylocythere sinuosa was first recorded in 2017, as a symbiont on the invasive crayfish Procambarus clarkii collected in an urban stream in Tokyo, central Japan. In 2022, A. sinuosa was also recorded on P. clarkii and on the Japanese mitten crab Eriocheir japonica, both collected in Shizuoka City, central Japan, but there have been no other Japanese records of A. sinuosa except for these two localities. In this study, we found many individuals of A. sinuosa on P. clarkii collected in an agricultural channel in Okayama City, western Japan. This new record is significant because it is the first from western Japan, approximately 370 km west of the nearest previously known locality, suggesting that this invasive species could be widespread in Japan.

Ceratothoa oxyrrhynchaena Koelbel, 1879 is a parasitic isopod of the family Cymothoidae, previously recorded from more than 20 fish species in 12 families. In this study, a male cymothoid isopod was found in the buccal cavity of a sandperch, Parapercis aurantica (Döderlein, 1884), collected from Suruga Bay, Shizuoka Prefecture, Japan. Based on morphological characteristics and DNA barcoding (mitochondrial COI partial sequence), the specimen was identified as C. oxyrrhynchaena. Parapercis aurantica (Pinguipedidae) represents a new host for C. oxyrrhynchaena, providing additional insight into the host utilization of this species.

An ovigerous female of a rare axiid shrimp, Litoraxius boshu Komai & Tachikawa, 2007, was collected from the intertidal zone of Katsuyama, southern Boso Peninsula, Japan. This study described the first zoeal stage of laboratory-hatched L. boshu in detail. Larval morphological characteristics were compared with other axiid shrimps, larvae of which have been studied. The first zoea of L. boshu is distinguished from those of other axiid shrimps by a combination of the characters of posterodorsal spines on the pleonites, the antennal scaphocerite, the endopod of the maxillule, the endopod of the third maxilliped, and ornamentation of the posterior margin of the telson.

Coral-dwelling crabs of the family Cryptochiridae have been recorded across the tropical western Atlantic but are rarely reported from the eastern Atlantic. Here, we document the presence of cryptochirid crabs in the coral Siderastrea radians (Pallas, 1766) from São Tomé and Príncipe, Gulf of Guinea. This coral is likely inhabited by Kroppcarcinus siderastreicola Badaró, Neves, Castro & Johnsson, 2012, the only known gall crab to inhabit this coral host with a recorded distribution from Florida to Brazil. Our new record highlights that this crab potentially has an amphi-Atlantic distribution, similar to its confamilial Troglocarcinus corallicola, and in line with the distribution of its coral host genus.

Two arcturid isopod species, respectively belonging to the genera Idarcturus and Neastacilla, were collected by SCUBA diving from Izu Oshima, the Pacific coast of central Japan. Based on adult female specimens, these species were identified as I. trispinosus and N. paralongipectus, both of which were originally described from South Korea and represent the first records from Japan. Idarcturus trispinosus, in particular, was found in large numbers (N=25) on detrital red algae that accumulated on the seafloor. In this report, we also describe the pigmentation of these two species for the first time and provide partial sequences of cytochrome c oxidase subunit I (COI) and 16S rRNA (16S) from I. trispinosus.

Porcellanids avoid predation by autotomizing a cheliped, a strategy that enhances immediate survival but incurs costs. Previous research that did not consider sex differences has suggested that cheliped loss accelerates molting and reduces growth at molt. As female decapods typically allocate more energy to reproduction than males do, molting and growth patterns may vary between sexes. This study investigated the effects of cheliped autotomy on molting and growth in the porcellanid crab Petrolisthes japonicus. Male and female crabs were cultured under three autotomy conditions: both chelipeds intact, the larger cheliped autotomized, and both chelipeds autotomized. Loss of both chelipeds reduced growth at the first molt in both sexes, but this effect disappeared in subsequent molts for females, likely due to energy allocation to reproduction in all groups. Sex-specific differences emerged in crabs with intact or single-cheliped loss, with males exhibiting longer molt cycles and fewer molts than females, potentially enhancing male mating opportunities and female reproductive output. However, autotomizing both chelipeds increased the molting frequency in males, counteracting these patterns. These findings highlight the interplay between autotomy, molting, and sex-specific energy allocation, suggesting that cheliped loss affects growth and reproduction differently in males and females, with potential fitness trade-offs.

A new species of raninid frog crab, Umalia modesta n. sp., is described from the northern part of the South China Sea, near Tungsha Islands. The species is morphologically most similar to U. chinensis (Chen & Sun, 2002) (from southern China) and U. tenuiocellus (Davie & Short, 1989) (from Australia) but can easily be distinguished from them by its carapace form, well developed cornea, and structures of the rostrum, thoracic sternum, third maxilliped and ambulatory legs.

It is widely known that seagrass plants provide shelter to marine organisms. However, their effects on meiofaunal communities in the sediment around their shoots are still unknown. We investigated spatiotemporal changes in the community composition of benthic copepods in an eelgrass (Zostera marina) bed off the beach of Kamishima Island, which belongs to the Amakusa Islands. While no significant direct effect of grass shoots on copepods in the surrounding sediment at the centimeter scale was detected, their community compositions significantly changed between the seasons. Furthermore, proportions of mud in bottom sediment particles, which were larger when the eelgrass shoots were grown, were detected as the most important environmental factor controlling the copepod composition.

Morphological features and some ecological notes of the manca of Sandythoa moritakii (Saito & Yamauchi, 2016), released from a larvigerous female infesting the deepwater bullhead sculpin, Ereunias grallator Jordan & Snyder, 1901, were provided for the first time. The manca can be distinguished morphologically from the congeners by the typical enlarged pleopodal exopod 5. A total of 222 individuals were collected and reared in the laboratory of the Toba Aquarium. The final specimen survived 59 days, but 90.1% of mancae died within seven days. This study will be an important aid in understanding the life history of cymothoid isopods.

This study examined the growth dynamics, enzyme activity, and histological development of Neocaridina denticulata juveniles in controlled aquaculture conditions. Individual body weight and length variations were attributed to microenvironmental factors and social interactions. Transparent tank systems and localized environmental niches likely influenced resource competition and growth. Despite suboptimal temperatures (16.9–19.1°C, below the ideal 22–26°C), juvenile development remained unaffected, indicating broader temperature tolerance than previously recognized. Histological analysis revealed that the digestive structures of juveniles (15–60 days after hatching, DAH) and adults (>75 DAH) were remarkably similar, supporting isometric growth and highlighting a robust digestive system across life stages. Enzyme activity analysis, conducted with minimal sample sizes (10 individuals), effectively detected growth-related trends. Amylase and lipase activities showed dynamic patterns shaped by developmental programs and environmental factors, while protease activity increased steadily, reflecting the nutritional demands of growing juveniles. Adults exhibited significantly higher lipase activity than shrimplets, likely due to reproductive and metabolic requirements. This study underscores the efficiency of direct development in N. denticulata, providing valuable insights into its physiological and behavioral adaptations. The findings offer significant implications for aquaculture practices, particularly in enhancing shrimplet growth and survival under controlled environmental conditions.

Accelerated aquaculture production has driven demand for novel ingredients in aquaculture feeds. Potential alternatives include single cell organisms, transgenic crops and terrestrial arthropods for example the black soldier fly, Hermetia illucens. Underutilised aquatic taxa, such as amphipods, possess a wide range of unsaturated and polyunsaturated fatty acids valued by the aquaculture feed industry. However, information regarding composition of semi-terrestrial talitrid amphipods remains at an exploratory stage. For instance, Talitrus saltator is a common European sandhopper currently lacking published, comprehensive body composition and lipid analysis data. In this study, monthly sampling of T. saltator (July–September) showed a distinct increase in body size, energy value, protein content and storage lipids (triacylglycerols) likely due to increased foraging potential during the warmer and more productive summer period. Toward the end of the summer, sandhopper proximate composition was comparable to other arthropod meals (Krill, Euphausia superba, amphipod Gammarus sp., and H. illucens). Whilst T. saltator fatty acid composition was biased toward monounsaturated fatty acids (ranging from 44.1–57%), samples also contained a range of polyunsaturated fatty acids, including valuable omega 3 long chain fatty acids. However, there remains scant information on T. saltator harvesting or culture potential, likely essential to produce biomass of sufficient quantity or composition for commercial use.

Salmincola californiensis, an ectoparasitic copepod, infects salmonid fishes’ branchial cavities and fins, mainly of Oncorhynchus species. While this species is widely distributed along the Northern Pacific Ocean rim, only three populations have been documented in Japan, and these records are primarily from highly fragmented local areas at high altitudes or latitudes. Here, we report S. californiensis parasitizing masu salmon Oncorhynchus masou masou captured in upstream reaches of the Naka River, Tochigi Prefecture, central Honshu. This is the fourth locality record in Japan and for the first time in 40 years. Copepods were identified as S. californiensis based on partial 28S rDNA sequences. Infection levels were relatively high (Prevalence 66.7%, mean intensity 5.0), and some copepod individuals had egg sacs, suggesting that this population is naturally sustained. Given their fragmented distributional records in high-altitude areas and considering that Japan represents the southernmost margin of their distributional range, these populations are assumed to be glacial relics.