From 2008 to 2014, we monitored the abundance and daytime habitat use of the freshwater shrimp Macrobrachium yui at a fixed site on a tributary of the Xuang River in northern Laos. Throughout the monitoring period, the shrimp M. yui showed strong preference for cobble and small boulder substrates, as well as moderate preferences for 21–30 cm depth and the midstream (251–350 cm distance from the bank) as daytime habitat factors. The shrimp M. yui, on the other hand, exhibited intense avoidance for shallower water depths (less than 10 cm), stagnant (0 cm s⁻¹) and faster water velocity (greater than 60 cm s⁻¹), finer substrates (from silt to gravel), and stream margin (less than 50 cm from the bank). During the day, the habitat with the cobble and small boulder in the midstream is probably the best place for the shrimp M. yui to hide from predators. The abundance of the shrimp M. yui had positive and negative correlations with water depth and sand percentage in the bottom sediment, respectively. It suggests that a decrease in water depth caused by increased deposition of fine sediment from the catchment has a negative impact on the shrimp abundance.

Growth analysis and population size estimation of the northernmost population of coconut crabs inhabiting Ocean Expo Park, Okinawa, southern Japan, was performed using the fishmethods package on R software. In this study, we used more than 500 datasets obtained from the mark-recapture surveys conducted during 2006–2019. We also obtained 126 datasets from our previous study, which was performed using the same field protocol and manual data calculation. The generated data of growth parameters in the GROTAG model were similar to those of a previous study that calculated parameters manually using smaller datasets, and the results obtained were more accurate. To our knowledge, this is the first time that mark-recapture data were used for estimating the population size of coconut crab. The results showed that 1045 individuals (93.3/ha) inhabited the park. This indicates that despite certain environmental disadvantages in the northernmost area, the population density of coconut crabs in this region is not low compare to that at other locations.

Salinity is an important abiotic factor affecting the survival and development of decapod crustacean larvae and juveniles. We investigated the effect of salinity on survival and emigration behaviour during the sea-to-land transition of three terrestrial hermit crab species in the family Coenobitidae (genera Birgus and Coenobita): B. latro, C. brevimanus and C. purpureus. We cultured laboratory-born individuals from megalopae for 63 days in vessels containing both a hard substrate and seawater controlled at four different salinity levels (25, 30, 35 and 40 ppt). The overall, survival, shell-carrying and landing activities of the animals were enhanced at lower salinity conditions. Interspecific variability in survival and emigration behaviour was evident: C. brevimanus and C. purpureus showed relatively high performance even at higher salinity levels when compared with B. latro, indicating the differences in hypo-osmoregulatory ability among coenobitids. The microhabitats occupied by megalopae and early juveniles of coenobitids could be attributed to the interspecific variability in the osmoregulatory capacity of these animals.

Largemouth bass Micropterus salmoides (Lacepède, 1802) was introduced from North America into Japan in 1925 and 1972. The fish has spread extensively since the late 1960s and is now found in all prefectures of Japan. An adult female of the argulid branchiuran Argulus japonicus Thiele, 1900 was collected from the body surface of a largemouth bass in the Kako River, Hyogo Prefecture, Japan. This represents a new host record for A. japonicus. The morphology of the adult female collected is reported in detail. Although A. japonicus is native to Japan, the species is not strictly host-specific and can utilize largemouth bass of the North American origin as its host in Japan.

A new species of swimming crab of the genus Catoptrus A. Milne-Edwards, 1870 (Portunidae) is described from Kagoshima, southern Japan. Catoptrus lavicolus n. sp. was collected from deep inside large lava rocks at shallow subtidal bed of Sakurajima, an active volcano. The new species differs from all seven congeners by a combination of the characters of the carapace, chelipeds and ambulatory legs. Intersex specimens of C. lavicolus n. sp. were observed in this study, while no male specimen has been collected.

Three species of the sponge-dwelling lobster Eiconaxius are known from the eastern Pacific. Previous reports and descriptions were based on dead specimens. During a study by remotely operated vehicle, Eiconaxius baja was photographed in situ at the San Juan Seamount within a sponge of the family Phariidae. It tried to escape by swimming backward. Illuminated by the lights of the vehicle, the lobster had an orange color. The depth at which the lobster lives has no ambient light, so the function of the color remains uncertain.

A new species of majid crab, Sakaija amicitiae n. sp., is described from New Caledonia Exclusive Economic Zone, Vauban seamount and the Chesterfield Atolls. The species is superficially similar to S. japonica (Rathbun, 1932) and S. santo Ng & Richer de Forges, 2015, from Vanuatu. Specimens from New Caladonia previously referred to S. santo are now referred to the new species. Sakaija amicitiae n. sp. can be distinguished from congeners by features of the carapace spines, proportions of the ambulatory legs and structure of the male first gonopod.

Cannibalism is considered a major cause of mortality among juveniles in nursery cultures of the mud crab Scylla serrata. Size grading is recommended as an effective measure to mitigate cannibalistic behavior; however, little is known about how body size differences influence cannibalistic episodes between juvenile individuals. We conducted one-to-one match experiments using hatchery-raised juveniles with carapace width between 12.5 and 33.3 mm. Two crabs were placed in an experimental arena for 24 h, and survival and pereiopod loss were subsequently observed. Large crabs always preyed on small crabs, and the probability of cannibalistic behavior significantly increased with the increasing relative size difference [RSD=1−(size of small crab)/(size of large crab)]. The loss of chelipeds and walking legs in the surviving small crabs amounted to 25% and 38%, respectively, and the proportion of lost walking legs increased with the increasing RSD, indicating the stronger aggressive behavior of larger crabs. This study suggests that maintaining size differences below the RSD threshold (0.34) at which 50% of the small crabs are cannibalized by large crabs would be an effective strategy to contain the occurrence of cannibalism in mud crab nursery cultures.

A lernaeid copepod, Lamproglena chinensis Yü, 1937, is redescribed based on the newly collected specimens from the snakehead Channa argus (Cantor) (Actinopterygii: Perciformes: Channidae) caught in Lake Imuta, Satsumasendai, Kagoshima Prefecture, Kyushu, southern Japan and the type specimens of L. ophiocephali Yamaguti, 1939, a junior synonym of L. chinensis, from the Kizu River, Kyoto Prefecture. The adult male was found from Japan for the first time. The collection of the species in this study represents the first record from Kyushu. The previous records of L. chinensis and its related species reported from Asian countries are also reviewed.

A new genus and new species of potamid freshwater crab, Xestomon tacu, is described from Ta Kou mountain in southern Vietnam. The new genus is characterised by its transversely ovate and smooth carapace surface, the very low to undiscernible epigastric and postorbital cristae, complete absence of a flagellum on the third maxilliped, relatively short ambulatory legs, a male telson which is acutely triangular with the lateral margins distinctly concave, and a short stout male first gonopod which has the terminal segment gently curved upwards.

A single ovigerous female of cymothoid isopod was collected from the branchial cavity of the African sailfin flyingfish, Parexocoetus mento (Valenciennes, 1847) in Kadogawa Bay, an inlet of the western North Pacific Ocean, southern Japan. Although the isopod specimen differs from Norileca triangulata (Richardson, 1910) in the number of articles of the antennula and the absence of small setae on the propodus of pereopods 1 and 2, it is similar to the species. However, the species was poorly originally described, and there are several differences between the original and subsequent descriptions and the specimen. Thus, this study refrains from identifying the specimen to species and reports it as N. aff. triangulata. This represents the first record of Norileca from Japan. Five species of cymothoid isopods including N. aff. triangulata are now known from Japanese flyingfishes, and a parasite-host list is provided.

We examined the effects of temperature and salinity on larval survival, duration, and growth of three amphidromous atyid shrimps, Caridina leucosticta, C. multidentata, and C. typus to infer larval dispersal strategy in the sea. Larvae were reared under 25 combinations of five different temperature (20, 23, 26, 29, and 32°C) and salinity (4.25, 8.5, 17, 25.5, and 34 ppt) levels. Interspecific variability was detected in larval performance: C. leucosticta larvae were able to survive to moult into the juvenile stage at the lower salinity condition (8.5 ppt), C. multidentata larvae exhibited an ability to adapt to the wide range of salinity condition (17–34 ppt), and C. typus larvae adapted to the higher salinity condition (34 ppt) better than C. leucosticta larvae. Larval duration was less variable between species. Thus, salinity adaptation of larvae may play an important role in mediating the larval dispersal of the three Caridina species in the sea. Larval dispersal range may be most limited near the river mouth for C. leucosticta, and be moderate for C. typus, and C. multidentata larvae may be able to disperse broadly under the high salinity condition of the open sea.

The distribution of the invasive freshwater shrimp, Neocaridina davidi, in a spring-fed stream at Kunitachi, Tokyo, Japan, was investigated in relation to the stream’s environmental parameters. Compared to previous studies, the population density of N. davidi was found to be remarkably high (up to 315 ind./m²). The increase in population density is likely because the water temperature in the spring-fed stream remains 17–20°C in the winter, prolonging the period during which the shrimps can spawn. Generalized linear mixed models showed that adult shrimps were more abundant in areas with high emergent plant coverage. In contrast, juveniles were more abundant in areas with low current velocity. These results provide essential clues to the interspecific relationships between N. davidi and native spring organisms that inhabit the same physical environments.

We investigated the seasonal variability in fecundity and egg size in a Petrolisthes japonicus population by monthly sampling for one year on an intertidal cobble and boulder shore on the Boso Peninsula, Japan. Fecundity (number of eggs per brood) was determined for ovigerous females with non-eyed or eyed eggs in early (May and June), middle (July) and late (August and September) breeding periods. The long and short diameters of non-eyed eggs were measured, and the egg volume was calculated. Fecundity was positively correlated with female body size and found to be greater in the middle breeding period than in the early and late breeding periods. Egg loss occurred during embryogenesis, the egg loss rate being calculated as 44.6%, 14.8% and 30.2% from the non-eyed to the eyed egg stages in the early, middle and late breeding periods, respectively. Egg size decreased linearly from the early to the late breeding periods. It is inferred that the seasonal variability in reproductive output, fecundity and egg size in P. japonicus may be regulated by female nutritional conditions and seasonal fluctuations of food production for larvae, as well as a trade-off between the number and size of eggs in a brood.

The nonnative freshwater shrimp Palaemon sinensis (Sollaud, 1911) has been reported at various locations in Japan. This study compared environmental DNA (eDNA) analysis with conventional capturing and investigation as a method to detect P. sinensis in the Tama River. We developed species-specific primers to amplify a 225-bp fragment of the cytochrome c oxidase subunit I gene for eDNA analysis. The invasive shrimps were detected using eDNA analysis in all areas where they were observed using the capture and investigation method. Thus, eDNA analysis can be applied to detect sparsely distributed P. sinensis in the Tama River, making it a useful method to detect invasive shrimps and to help prevent ecological disruption.

A morphological description is given for the postlarval specimen of the polychelid lobster, Pentacheles laevis, collected from Suruga Bay, on the Pacific coast of Japan. The specimen has been previously identified using the DNA barcoding method. Diagnostic larval characters distinguishing the previously known postlarvae in three genera, Pentacheles, Stereomastis, and Polycheles, are also noted.