We report the first occurrence record of the freshwater ectoparasitic platyhelminth Scutariella japonica (Matjašič, 1990) from Hokkaido, Japan. The flatworms were collected from the surface of exoskeleton and the inside of branchial chamber of the atyid shrimp Neocaridina sp. aff. davidi. Phylogenetic analysis based on the 18S + 28S concatenated dataset of rhabdocoel flatworms retrieved the Scutariella clade, and our specimens identified with Scutariella japonica were genetically distinct from those referred to S. sinensis Chen, Feng, Lin, Lu & Wang, 2018. Phylogenetic analyses using COI showed that the host shrimp specimens from Sapporo were clustered with N. koreana identified by Shih et al. (2017) and placed close to N. palmata (Shen, 1948) and N. davidi (Bouvier, 1904). Our specimens did not agree with the original description of N. koreana Kubo, 1938, particularly in the shape of the endopod of the male pleopod 1. The identity of the present host shrimp specimens could not be established satisfactorily, and thus they were referred to N. sp. aff. davidi for the time being.

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We report microhabitats used by the tanaidid tanaidacean Zeuxo ezoensis Okamoto et al., 2020 on the marine alga Sargassum confusum C. Agardh, as well as its molting behavior, based on observation of captive individuals. Zeuxo ezoensis constructed its tube-shaped nests on S. confusum (1) at the bases of lateral branches, (2) on main branches in the apical region, (3) in cracked air bladders, and (4) in clumps of a filamentous alga (Chordariaceae sp.) epiphytic on S. confusum. The behavioral sequence of a single molt observed for a male Z. ezoensis was as follows. (1) A break occurred along the ventrolateral and posterodorsal edges of the carapace, creating a dorsal opening in the old exoskeleton; (2) the animal began to extricate itself from the old exoskeleton through this opening; (3) the cephalothorax and its appendages shed the old exoskeleton; and (4) the animal completed extrication from the old exoskeleton. The entire molt took about five minutes.

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The life history characteristics of the mole crab (Hippa truncatifrons) were studied using monthly or bimonthly sampling at a cobble shore on the Miho Peninsula, located in the inner part of Suruga Bay, central Japan, between April 2013 and March 2014. A total of 523 individuals was collected, comprising 276 males (carapace length, CL, 3.4–9.7 mm) and 247 females (CL 4.9–15.0 mm). The number of individuals collected showed two peaks (>10 individuals per station), in early summer and autumn, and decreased to only a few per station in winter. The population exhibited a unimodal CL distribution, with the modes of CL gradually increasing between April and July. Smaller individuals (<4.0 and <7.0 mm CL for males and females, respectively) appeared in August. After August, bimodal distributions of CL were recorded, with fewer individuals in the larger size group. This indicates the mean lifespan was estimated to be almost one year, with some individuals surviving for up to two years after hatching. Ovigerous females were observed between May to September, but were most abundant during June and July. Ovarian development corresponded with the occurrence of ovigerous females. The size at sexual maturity for females was estimated to be 10.2 mm CL. Batch fecundity ranged from seven to 1801 and was correlated with CL for some embryonic stages in all breeding season and early embryonic stages in the early breeding season. Some ovigerous females with late embryonic developmental stages had mature ovaries, indicating multiple spawning by some females during one reproductive season. The Suruga Bay population showed a typical life history similar to that of the Sagami Bay and Hokuriku populations, but different from that of extreme areas, such as Aomori (the northern limit of their range) and an island in Mikawa Bay with large-sized individuals.

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The purpose of this study is to obtain ecological information on the freshwater shrimp Caridina typus in Honshu, which is the northern limit of the species distribution. Monthly sampling was performed from March 2010 to March 2012 at three stations set in the Heda-ohkawa River, Izu Peninsula, Shizuoka Prefecture, central Japan. A total 870 individuals were selected from the collected samples. Orbital body length (OBL) of shrimps was measured and categorized into juveniles (< 10 mm OBL), males with appendix masculina and females without appendix masculina. Few specimens of this species were collected from November to May when the water temperature recorded was below 14 ℃, in which no individuals was collected from December to February. Ovigerous females were observed from July to October, and small juveniles (<5 mm OBL) occurred from July to September 2010 and August to October 2011. The OBL frequency distributions indicated that the individuals of this species grew from June to October when water temperature was over 16 ℃. It was strongly suggested that the adult individuals were successfully overwintering in the river, because of the similar mean OBL between the autumn and the following spring.

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We report the presence of mobile macro-epifauna on rafts of pumice generated by the Fukutoku-Oka-no-Ba volcano in 2021 in Oku Port, at the northern end of Okinawa Island. Most of the water surface in the harbor was covered by a thick pumice raft at the time of our observations. Grapsus crabs were the dominant species on the surface of the raft, and many crabs were visible near the quay. No crab was seen to burrow into the raft. These results show that mobile macro-epifauna can expand their distribution to the surface of a pumice raft. However, the presence of a raft is likely to make it difficult for crabs to move between land and water.

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The sailfin sandfish Arctoscopus japonicus is a common demersal fish in the Sea of Japan and one of the important fishery resources in Japan and Korea. Despite the need for information on genetic population structure for resource management, population genetic studies in this species have been based on only RFLP and sequence analysis of a part of the mitochondrial DNA and classical allozyme analysis. These analyses have presented the roughly group structure of the species, but have not resolved enough to mention the fine scale genetic structure within each group. Here we screened microsatellite DNA markers suitable for fine scale population structure from previously reported papers based on fit of Hardy-Weinberg equilibrium, the presence or absence of null allele, large allele dropouts, and linkage disequilibrium. Using the resulting 10 loci selected, population genetic analyses of two sample populations belonging to different genetic groups showed relatively high heterozygosity (He: 0.764 and 0.755) in both sample populations, and high degree of genetic differentiation between them (FST = 0.0709, RST = 0.1598) which was consistent with predictions from previous studies. Furthermore, Bayesian clustering revealed that group attributes can be correctly estimated at the individual level. These results suggest that the 10 loci selected have the potential to make a significant contribution in future population genetic studies of this species.

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A gilled (final) stage slipper lobster phyllosoma larva (BL = 23.8 mm) collected in the western North Pacific (27º N, 130º E) was determined to be Chelarctus aureus using mitochondrial 16S rDNA sequence analysis. Morphological characteristics of this larva agreed well with the final stage larva of this species reported previously (Ueda et al. 2021). Mid to final stage phyllosoma larvae of this species can be distinguished from those of the other congeneric species based on their large cephalic shield width and length ratio (1.404‒1.587) and the sub-rectangular shape cephalic shield.

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The fish fauna in the middle and lower reaches of Sakura River flowing into Lake Kasumigaura was investigated with casting nets, hand nets and scoop nets in May, July, and October 2020 and from April to October 2021, and compared with data from previous studies conducted in 1984–1987, 1996–2002 and 2010–2015. A total of 36 fish species in 11 families, including 20 introduced exotic and Japanese species and 16 native species, were collected in 2020–2021. The analyses of long-term changes in presence/absence of fish species from the 1980s to the 2020s demonstrated the disappearance of four threatened species (three native bitterling species and the crucian carp Carassius buergeri subsp. 2) by various artificial environmental changes and a gradual increase in introduced exotic and Japanese species spreading mainly from Lake Kasumigaura.

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The batillariid snails Batillaria multiformis and B. attramentaria are designated as a Category X (Extinct/Untraceable species) in the Red List of Chiba City. In the present study, populations of these species were confirmed at the mouth of the Oyumi River in Chiba City. In particular, the former species has been thought to be extinct in Chiba City since the 1970s. A clear habitat segregation was observed between the two species, in which B. multiformis was distributed on the surface of sandy mud in the upper intertidal zone and B. attramentaria was distributed on the sandy flats of the riverbed. The tidal flats and salt marshes at the mouth of the Oyumi River should be conserved as the only habitat for these endangered batillariid snails in Chiba City.

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Four species (Chelarctus aureus, C. crosnieri, C. cultrifer, and C. virgosus) have been described in the genus Chelarctus of the subfamily Scyllarinae, of which mid to final stage phyllosoma larvae in three species (C. aureus, C. crosnieri, and C. virgosus) have been reported. Ueda et al. (2021) found phyllosoma larvae morphologically assigned to the genus Chelarctus but having distinct DNA sequences from the above four species, designating ?Chelarctus sp-1. Additional specimens (n = 8) of mid to final stage phyllosoma larvae of this species were collected in the western North Pacific. The 16S rDNA sequence data supported ?Chelarctus sp-1 to be a member of the genus Chelarctus. Based on the shape of the cephalic shield, the ratio of the cephalic shield width and length, length of posterolateral spines on the telson, and body size of each stage, mid to final stage phyllosoma larvae of ?Chelarctus sp-1 could be discriminated from those of the other species of the genus reported to date.

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Of phyllosoma larvae of the subfamily Scyllarinae collected in the western North Pacific, 19 individuals (BL = 10.1‒38.5 mm) having an onion-shaped cephalic shield were selected. Phylogenetic analysis using partial mitochondrial 16S rDNA and COI sequences revealed that these larvae were of Galearctus lipkei. Gilled (final) stage phyllosoma larvae (BL = 34.2‒38.5 mm) of G. lipkei were the largest in this subfamily reported to date.

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The freshwater shrimp Palaemon paucidens C type is likely to be endemic to Amami-Oshima Island. We investigated the distribution of the shrimp in 34 rivers on Amami-Oshima and Kakeroma Islands, and implemented DNA analysis and rearing experiment for the seawater requirement of the larvae. This type was found only in Katoku River in the southeastern part of Amami-Oshima Island. Mitochondrial (16S rDNA) and nuclear (18S rDNA) DNA analyses revealed substantial sequence differences between the C type and other two types (A and B) collected from the Japanese Archipelago. This result strongly indicates that the C type is a unique group that is geographically isolated and has diverged genetically from the two types in the Japanese Archipelago. The zoeal larvae of C type showed higher survival rate in 30 % and 70 % seawater than 0 % or 100 %, suggesting that this type has amphidromous life history as the B type. Considering the limited distribution and low genetic diversity of the C type, the risk of extinction is probably high. For the viability of this type, conservation of the landscape of Katoku River, including its estuary, is needed.

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Of scyllarine phyllosoma larvae collected in the western North Pacific, a larva with a large body length (28.1 mm) for a subfinal stage and having a large kidney-shaped cephalic shield was selected. Mitochondrial 16S rDNA and COI sequence analyses indicated this larva to belong to the genus Galearctus but failed to identify species. We tentatively designated the species of this larva as Galearctus sp-A.

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We have performed DNA barcoding and morphological analyses for scyllarine phyllosoma larvae. Five phyllosoma larvae having subrectangular-shaped cephalic shields with straight posterior margins were selected from plankton samples collected in the central North Pacific. Mitochondrial 16S rDNA sequence analysis indicated Crenarctus crenatus to be the closest species for these larvae. Based on the nucleotide sequence divergence (K2P: 1.66 ± 0.61 %) between these five larvae and C. crenatus and the geographic distribution of C. crenatus restricted to the South Pacific, we tentatively determined these larvae to be a subspecies of C. crenatus (designated as C. crenatus sub sp. 1).

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Nineteen phyllosoma larvae (BL = 21.2 to 48.9 mm) of slipper lobster of the genus Scyllarides were extracted from plankton samples collected in the western and central North Pacific. DNA barcoding and phylogenetic analyses using mitochondrial 16S rDNA and COI sequences clearly segregated these larvae into three clades (designated by A, B and C). Clade A phyllosoma larvae (n = 8) collected in the western North Pacific were identified to be Scyllarides squammosus. Highly homologous nucleotide sequences for eight larvae in the clade B (designated by Scyllarides sp-B) and one (designated by Scyllarides sp-C) in the clade C were not found in the database. Nucleotide sequences of Scyllarides sp-B larvae collected in the central North Pacific were similar to those of S. squammosus, but the differences were substantial (K2P distance: 4.515 ± 1.019 % SE in 16S rDNA and 6.788 ± 0.844 % in COI). Nucleotide sequence of Scyllarides sp-C larva collected in the western North Pacific was substantially different from its closest kin Scyllarides deceptor in the clade C (K2P distance: 3.232 ± 0.945 % in 16S rDNA). No distinct morphological difference was observed among these phyllosoma larvae examined, but the ratio of cephalic shield width to thorax width was significantly larger in Scyllarides sp-B larvae than those in S. squammosus and Scyllarides sp-C larvae.

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We investigated the period until diet acceptance in the wild red-spotted grouper Epinephelus akaara under captive condition. Experiments were conducted under three dietary treatments: two commercial diets, floating pellet (FP) and sinking pellet (SP), and combination of SP and sliced fish (SP+SF). We repeated the observation twice using different-sized fish with average initial body weights 390 g (n = 23) and 290 g (n = 18), respectively. The fish accepted SP and SP+SF within four days both in the two trials. In the SP + SF treatment, fish accepted SF at first and then fed on SP 12-days later in the large fish and 3-days later in the small fish, respectively. The first feeding of large fish on FP (11-days) was later than small fish (4-days). Therefore, wild red-spotted grouper is recommended to feed the SF or SP.

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Seventeen phyllosoma larvae (BL = 21.6 to 74.8 mm) of slipper lobster of the genus Parribacus were extracted from plankton samples collected in the western Pacific and Atlantic Ocean. DNA barcoding and phylogenetic analyses using 16S rDNA and COI sequences successfully determined species for 10 phyllosoma larvae from western North Pacific, in which five were Parribacus antarcticus and five were P. japonicus. No notable difference was observed in the sampling locations between these two species phyllosomas. No distinct morphological difference was observed between species, while the ratio of cephalic shield width to thorax width of P. japonicus phyllosomas was slightly but significantly larger than those of P. antarcticus. This is the first report on P. japonicus phyllosomas with certain identity.

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The slipper lobster Chelarctus cultfer has been known to be a wide-spread Indo-West Pacific species. Yang and Chan (2012), however, found the species under this name in Taiwan and Japan to be different species and described as a new species C. virgosus. Yang and Chan (2012) also revealed two subspecies in C. cultrifer, which were restricted to more southern localities such as the Philippines and Indonesia. Ueda et al. (2021) reported mid to final stage phyllosoma larvae of C. virgosus collected in the western North Pacific (23ºN to 27ºN), describing the unique morphological characteristics. In the present study, nine phyllosoma larvae (BL = 12.2 to 21.7 mm) morphologically assigned to C. virgosus were extracted from plankton samples collected in the western North Pacific (14ºN to 37ºN). Mitochondrial 16S rDNA sequence analysis for six of the nine phyllosoma larvae indicated these to be C. virgosus. Distribution range of C. virgosus phyllosoma larva was confirmed to be further south than in the previous study, while C. cultrifer phyllosoma was not yet observed.

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Onchidium sp. A is a marine pulmonate gastropod mollusk that inhabits tidal flats. Although it is listed as Endangered Species in the Red List of the Ministry of the Environment, the detailed ecology that is the basic knowledge for conservation has not been clarified in Japan. The purpose of this study was to investigate and clarify the ecology such as the activity and growth of Onchidium sp. A in Misho Bay, Ehime, Japan, only one area where the inhabitation of Onchidium sp. A was observed from Shikoku Island. Onchidium sp. A was uncovered on the dried tidal flats during April 21 to November 16, 2020. Feeding behavior was observed after the tidal flats began to dry out, and the observed number of individuals tended to be the highest within 2 hours before and after the lowest tide. Similar activities were observed at low tide at night. The number of appearing individuals decreased in rainy weather.

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Crustacean body color is controlled by a red pigment concentrating hormone (RPCH), which concentrates pigment granules in the chromatophore. In this study, RPCH in the sinus gland of the Alaska pink shrimp Pandalus eous was purified by reversed-phase HPLC and structurally determined. The amino acid sequence of the characterized P. eous RPCH (Pae-RPCH) was pELNFSPGW-NH2, identical to those of decapod RPCHs known to date. In an in vivo bioassay, injection of Pae-RPCH dramatically changed the body color of shrimps from red to white. This bioassay using P. eous was easier and more sensitive than similar experiments conducted previously in other decapod crustaceans.

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The red swamp crayfish Procambarus clarkii is an invasive alien species worldwide. The sterile-male-release technique (SMRT), which uses sterile but sexually active males, may help control its population. Body sizes of males and females affect mating and spawning success through mate choice. However, mating behaviour and spawning between different-sized males and females of P. clarkii remain unclear. The present study elucidated the effects of body size on mating behaviour and spawning of P. clarkii to develop effective SMRT for this invasive species. We prepared nine test groups as a combination of males and females in three size classes, and the mating behaviour of the test pairs was videorecorded for 48 h; then, spawning and egg development rate were examined. The body size difference negatively affected the copulation frequency and duration in the pairs, and the incidence of copulation was low in the pairs between the largest males and smallest females. When mating with smaller males, females delayed spawning and the egg development rate decreased, likely because the females may be waiting an opportunity to mate with larger males with higher reproductive potential. Our results might be consequences of mutual mate choice (i.e., preference for larger mates). Our findings indicate that in SMRT, sterile medium- and large-sized males with high reproductive potential are recommended to satisfy the mutual mate choice in P. clarkii populations of various-sized females.

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