Potamid crabs are primary freshwater crabs that complete their life cycle independently from the marine environment. Geothelphusa dehaani, the only primary freshwater crab found in most temperate areas of Japan, generally inhabits the upper region of rivers or mountain streams. However, in the present survey some G. dehaani were found in the floodplains of upper tidal rivers even with some crabs inhabiting the brackish area in four rivers flowing into the Ariake Sea. This suggests that G. dehaani occasionally occurs in various environments and is tolerant to the saline environment to some degree so that their distribution is not simply limited to the narrow mountain stream area.

Adults of the varunid crab Gaetice depressus are known to engage in suspension feeding with the third maxillipeds, as well as feeding with the chelipeds. However, at what stage in life G. depressus becomes a suspension feeder has yet to be discovered. The present study aimed to elucidate the feeding behaviour of G. depressus juveniles using laboratory-reared first, third and fifth instars fed on non-suspended prey (i.e., krill and seaweed tissues) and suspended prey (i.e., diatoms and different-sized Artemia). This species fed on both non-suspended and suspended prey using their chelipeds and third maxillipeds from the onset of the juvenile stage. Experiments with Artemia of different sizes showed that the size of prey consumed by suspension feeding increased with instar, probably because of the growth or increased size of the maxillipeds. Similarly, cheliped manipulation for feeding on krill tissue improved with juvenile growth. Gaetice depressus juveniles switched between maxilliped and cheliped feeding or combined the two depending on prey size. This feeding behaviour may allow the species to consume diverse food sources.

Both ovigerous and non-ovigerous females of the cymothoid isopod Anilocra clupei Williams & Bunkley-Williams, 1986 were collected from the body surface of Japanese sardine Sardinops melanostictus (Temminck & Schlegel, 1846) in Aso-kai Lagoon, Kyoto Prefecture, central Japan. The isopod is redescribed herein based on the collected females. Japanese sardine represents a new host record for A. clupei. Based on the previous and present studies, adults of A. clupei have been reported from three species of clupeiform fishes [Japanese sardinella Sardinella zunasi (Bleeker, 1854), round herring Etrumeus micropus (Temminck & Schlegel, 1846), and Japanese sardine], which indicates that these clupeiform fishes are preferred hosts of this parasite and, because of its frequent occurrence records from Japanese sardinella, this fish species is considered to serve as the most important host for the parasite. The isopod occurs in temperate and subtropical waters of Japan, which are possibly affected by two warm currents, the Kuroshio and its branch, the Tsushima Current. An identification key to the three nominal species of the genus Anilocra reported from Japan (A. clupei, A. prionuri Williams & Bunkley-Williams, 1986, and A. harazakii Uyeno & Tosuji, 2023) is also provided.

Lernaeenicus ramosus Kirtisinghe, 1956 is a parasite of marine fishes in the Indo-West Pacific region. In Japan, this species has been reported from fishes in three families (Epinephelidae, Serranidae, and Pinguiperidae). In this study, we collected three metamorphosed females of L. ramosus from a grub fish Parapercis sexfasciata (Temminck & Schlegel, 1843) (Pinguiperidae) in Osaka Bay, part of the eastern Seto Inland Sea, off Awaji Island, Hyogo Prefecture, central Japan. This represents the fourth record for L. ramosus from grub fish in Japanese waters. Two of the three metamorphosed females of L. ramosus possessed paired egg sacs. From the present and other current records of L. ramosus, we infer that the parasite newly acquired grub fish as its host in the early 2000’s and has since utilized this fish species as a suitable host in coastal waters of Japan.

The mating behavior of the varunid crab Helicana japonica (Cyclograpsinae) was evaluated in three cases in the field. The male visited a female burrow and invaded the burrow or dug to widen its opening. The female then exited the burrow, and copulation was initiated around the burrow entrance. The duration of copulation was approximately 1–3 minutes. After the pair separated, the male left, while the female remained in the burrow. In all cases, neither courtship display nor mate-guarding was exhibited through mating behaviors. These behaviors were compared with those reported for several cyclograpsine species.

Atyopsis spinipes is a freshwater shrimp species found in streams and rivers on tropical and subtropical Pacific islands. Juveniles of A. spinipes are also found in temperate regions of Japan, but adults are not, suggesting that juveniles do not successfully overwinter there. The present study investigated the low temperature tolerance of A. spinipes juveniles as a basis for predicting the northern limits of the species’ geographical range under global warming. Artificially raised juveniles were cultured in containers in which the temperature was adjusted to decrease from 20°C by 1°C every 48 h until all test juveniles had died. The survival rates of the juveniles largely declined below 16–17°C, and all died at around 12°C. The mean lethal temperature for juveniles was 14–15°C, which appears to be higher than winter temperatures in temperate regions and equivalent to those in the subtropical islands of Japan. A winter temperature of around 15°C may be the limit for successful overwintering by A. spinipes juveniles.

The mysids Siriella sp., Anchialina lobata, Heteromysis domusmaris and H. kaufersteinae sp. nov. were extracted from sponges sampled in shallow water at the coast of Lizard Island. The range of A. lobata is extended ≥5400 km along waterways from the coast of SW-Australia to the Coral Sea off NE-Australia. Heteromysis domusmaris is first recorded from nature; this species was previously known only from coral reef aquaria. Heteromysis kaufersteinae sp. nov. differs from closely related species of the subgenus Olivemysis by the basal segment of the antennular trunk being furnished with a mid-dorsal lobe bearing a blade-like spine (modified seta), in combination with an unsegmented antennal scale, the disto-mesial edge of eyestalks bearing a tooth-like projection, by the numbers of segments of the carpopropodus of thoracic endopods 5–8, and by the numbers of spines on modified male pleopods 3–4.

Temperature and salinity are important abiotic factors that affect the survival and development of decapod crustacean larvae. Atyoida pilipes is an amphidromous freshwater shrimp species with a wide distribution in tropical and subtropical Pacific islands. Therefore, it is hypothesised that A. pilipes larvae have the potential to survive and develop in an open sea environment with high temperature and salinity conditions. We cultured A. pilipes larvae under 16 combinations of temperature (20, 24, 28 and 32°C) and salinity (8.5, 17, 25.5 and 34 ppt) treatments to infer suitable environments for larval survival and development in the sea. Early moulting events up to stage 4 zoeae were determined. Larvae did not survive to the juvenile stage. However, temperature and salinity significantly affected larval survival and duration to reach stages 2, 3 and 4 zoeae. Larval survival was high at 25.5–34 ppt and 28–32°C, especially at 34 ppt and 32°C. Thus, our results support the initial hypothesis of larval adaptation to temperature and salinity in this species. Larval adaptation to high temperature and salinity conditions may lead to a wider eastward distribution and a restriction of the northern range limit of this species in the Pacific.

The commercial value of snow crabs and red snow crabs is low immediately after molting due to low meat quantities and soft shells. As time passes, the meat content and shell hardness increase, making them more valuable. In Japanese market, male snow crabs are arbitrary divided into soft and hard shells. Previous studies also categorized crabs soft-shell and hard-shell arbitrarily and compared the quality between the two groups. A durometer and a penetrometer have been used to determine the shell hardness in the stock management of the snow crab and the red snow crab, but they destroy the crab shells. In the present study, we developed a new non-destructive technique using a portable near-infrared spectrometer to evaluate the dry matter content of pereiopods, which were correlated with the shell hardness. The cheliped was found to be sufficient for measurement in both species, providing a wider range of values and higher precision than the first walking leg. This non-destructive technique would be valuable for assessing both commercial value and the approximate time since the terminal molt of snow crabs and red snow crabs.

This study designates a lectotype for and redescribes Thelphusa dehaani White, 1847, a Japanese primary freshwater crab species belonging to the potamid genus Geothelphusa Stimpson, 1858. Geothelphusa dehaani was described based on material exported to the Netherlands by von Siebold. Although the provenance of all his specimens is uncertain (they may be from several locations), our morphological comparison of the lectotype with fresh specimens led us to ascertain that it was almost certainly collected from Nagasaki, where von Siebold stayed. Two new Geothelphusa species are also described from Amakusa Islands and Aomori Prefecture.

The pedunculate barnacle Octolasmis unguisiformis is a rare epibiotic barnacle attached to a macrophthalmid crab, Macrophthalmus (Macrophthalmus) milloti Crosnier, 1965, on tidal flats in the Ryukyu Archipelago. It has a rare sexual system called androdioecy, a coexistence of hermaphrodites and males in the population. In June 2020, we found two hermaphrodites and one dwarf male of O. unguisiformis attached to a female M. (M.) convexus at Kise Bay, Amami Oshima Island. Here we describe the symbiotic conditions between the pedunculate barnacle and the new host crab species.

A male gnathiid specimen was captured at a depth of 374 m off Cape Muroto, southern Japan. The specimen exhibited small mandibles, extremely large eyes covering most of the lateral cephalon, a slender body, and J- or L-shaped narrow pylopods. These characteristics correspond to those of the monotypic genus Tenerognathia. This specimen, distinct from the type and sole species of Tenerognathia visus, is identified as a new species based on differences in pylopod articulation and shapes of cephalon and maxilliped basis.

The Japanese spiny lobster Panulirus japonicus has been reported to harbor a numbers of nuclear mitochondrial pseudogenes (NUMTs) and heteroplasmy. However, distinguishing phylogenetically young NUMTs, heteroplasmy, and PCR-cloning artefacts may be challenging. In addition, greater degradation for mtDNA than nuclear DNA in elderly tissue specimens may promote amplification of NUMTs. In this study, we performed clone library-based nucleotide sequence analysis of the partial mtDNA COI gene using genomic DNA and cDNA obtained from fresh tissues of the Japanese spiny lobster and genomic DNA obtained from three crustacean and three fish species. Minor nucleotide substitutions between clones in an individual were ubiquitously observed in all species examined including the lobster cDNA, suggesting that most of these were artefacts. Rarely, a few clones were most likely to have originated from heteroplasmic copies, as they had skewed nucleotide substitutions at the third codon. The Japanese spiny lobster is more likely than others to detect NUMTs, while the detection of NUMTs may be somewhat suppressed using genomic DNA obtained from fresh tissue.

Sand shrimp Crangon uritai is distributed in shallow waters including the intertidal zone in eastern Asia and is known to be a prey item and a predator of juveniles of commercially important fishes. Specimens were collected on the Kawaradzu tidal flat, using a rake sledge net (mesh size 2.0 mm) from June to December 2016 and 2017, and in the adjacent shallow bottom (3–4 m depth at high tide) using a sledge net (mesh size 0.76 mm) from January to August 2018 and 2019. Seasonal changes in the density index and carapace length distribution were studied in both sites. The frequency of ovigerous females were indicated for the tidal flat. Settlement mainly occurred from May to July although low intensity settlement seemed to occur throughout the year except for September and October. Ovigerous season in the tidal flat finished in August or September by disappearance of the cohort including ovigerous females and resumed in October with the oviposition of 0-year individuals. Ovigerous females might occur throughout the year except for August or September, although their occurrence was not checked in the shallow bottom. Research on the offshore area is needed to detail the whole life history.

It has been suggested that body colour variation is shaped by both genetic and environmental mechanisms. However, it is not known whether body colour variation among the offspring of a single female occurs in brachyuran crabs. Here, we qualitatively assessed body colour variation in juveniles from a single brood of the varunid crab Gaetice depressus using material reared in the laboratory. This species exhibits body colour variation, and we reared larvae hatched from two females with brownish carapaces and two females with whitish carapaces to the juvenile stage. Brownish females produced juveniles with brownish carapaces, whereas whitish females produced juveniles with either brownish or whitish carapaces. Our experiments suggest the possibility of genotypic colour polymorphism in G. depressus.

One and two specimens of Argulus japonicus Thiele, 1900 were collected from the body surface of big-scaled redfin Pseudaspius hakonensis (Günther, 1877) (Cypriniformes: Leuciscidae) in the Babame River, Akita Prefecture, and Lake Jusan, Aomori Prefecture, northern Honshu, Japan, respectively. This is two new prefecture records in Japan and a new host record for A. japonicus. Lake Jusan is the northernmost collection locality of this parasite in Japan recorded from wild fishes. As Lake Jusan is a brackish water lake and big-scaled redfin is known to migrate from fresh to salt waters, and since A. japonicus completes its life cycle in fresh waters, we infer that the two individuals of A. japonicus parasitized the big-scaled redfin in an inflowing river, then this fish was collected along with the surviving argulid individuals after migrating to the lake. The collection of A. japonicus in Lake Jusan represents the second record for the species from brackish water lakes in Japan.