Cymothoid isopods (Crustacea: Peracarida) are large ectoparasites of fishes. Mature females and males inhabit the buccal or branchial cavities, or burrow into the abdomen, or attach to the body surface of marine, brackish, and freshwater fishes. The mancae, which are released from the brood pouch of adult females, spend a planktonic period and develop into early juveniles infecting host fishes. Currently, 399 species in 49 genera of the family Cymothoidae have been recorded worldwide, including 46 species known from Japan and its adjacent waters. This paper provides a comprehensive review of cymothoid taxonomy, morphology, parasitology, molecular characteristics (cytochrome c oxidase subunit I gene, 16S rRNA gene, or both), life cycle, and host associations, based mainly on Japanese literature and the own observations. This review aims to make these findings more widely accessible.

Seven specimens of Echthrogaleus asiaticus Ho, Liu & Lin, 2012, were collected from a white shark, Carcharodon carcharias (Linnaeus, 1758), in the Seto Inland Sea off Suo Oshima, Yamaguchi Prefecture. This copepod parasite has been recorded previously only from Taiwan and Hokkaido, and the present collection of this copepod represents its second record in Japan. A partial COI sequence was generated as the first DNA barcode for this species. This new record reinforces the East Asian distribution of this species and, because of the migratory nature of the host and its record in Japanese waters, it is possible that E. asiaticus is distributed from Hokkaido to Taiwan. It is possible also that previous records have conflated E. asiaticus with E. denticulatus from the coast of Hokkaido, and in order to clarify its distribution it will be necessary to conduct further research on this species, both morphological and genetic. A review of published records showed that pandarid copepods dominate the parasitic fauna on the fins and other external surfaces of the white sharks worldwide. The few records of parasitic copepods of C. carcharias in Japanese coastal waters emphasizes the need for further studies.

Adult specimens of Argulus matuii Sikama, 1938 were collected from Japanese amberjack, Seriola quinqueradiata Temminck & Schlegel, 1845, and red seabream, Pagrus major (Temminck & Schlegel, 1843), reared in the coastal waters of Saiki Bay (32°56′56″N, 131°59′29″E) in Saiki, Oita Prefecture, Japan, in June 2010, July 2011, and June 2025. Frozen and thawed specimens of A. matuii exhibited six yellow streaks fringed with brown pigments on the dorsal surface of the carapace. Similar coloration was observed in specimens preserved for about seven months in either 70% or 90% ethanol. The yellow streaks slightly faded but the fringed dark brown pigments remained in specimens kept in 90% ethanol for about 16 years, but those streaks and pigments almost disappeared in specimens preserved in 70% ethanol for the same period. These results suggest that the dorsal color patterns of the carapace, particularly the fringed dark brown pigments, can be used as one of characters to identify marine argulid specimens preserved in 90% ethanol as A. matuii for at least up 16 years. All specimens were also characterized by foot-shaped natatory lobes on the fourth pair of legs.

Stomatopods are ecologically and economically important crustaceans, yet larval identification remains challenging due to limited morphological data. We examined stomatopod larvae collected from Osaka Bay, Japan, during September and October 2025, aiming to identify species using DNA barcoding and morphological observations. Larvae were measured and observed for diagnostic characters particularly of the carapace and telson. Partial sequences of the mitochondrial 16S rRNA gene were determined to compare with reference sequences from six superfamilies. The phylogenetic analysis reveals that the larval samples consist of seven species, which includes six species previously recorded as adults in Osaka Bay: Anchisquilla fasciata, Levisquilla inermis, Lophosquilla costata, Oratosquilla oratoria, Harpiosquilla harpax, Acanthosquilla multifasciata, and Acanthosquilla sp. This study provides the first larval records for six species other than O. oratoria in Osaka Bay. The simultaneous occurrence of multiple species’ late-stage larvae suggests overlapping settlement periods. Notably, L. inermis, previously considered rare, appeared as frequently as common species, indicating possible population extension by recent larval dispersal from its large population such as central Seto Inland Sea. Our findings highlight the utility of integrating molecular and morphological approaches for accurate larval identification and contribute baseline data for stomatopod diversity and resource management in Japanese waters.

A single male specimen of Parahelice pilosa (Sakai, Türkay & Yang, 2006) (Crustacea: Decapoda: Varunidae) was collected from Ishigaki Island, southern Ryukyu Islands. This is the first Japanese record of this species, and extends the known distributional range approximately 200 km northward. This species was previously regarded as having a smooth outer surface of cheliped palm, however, the present study identified scattered microscopic pits by staining with a methylene blue solution.

This study provides the first specimen-based confirmation of the invasive freshwater shrimp Neocaridina davidi in five prefectures across the Chubu and Hokuriku regions, Japan (Fukui, Toyama, Nagano, Niigata, and Shizuoka). We conducted mitochondrial DNA (COI) and morphological analyses at 10 sites. Phylogenetic analysis revealed that among the three N. davidi clades, Clade B is widely dominant in introduced populations. Morphological comparisons with the native N. denticulata showed significant overlaps in diagnostic traits, underscoring the extreme difficulty of species identification based solely on morphological traits. Furthermore, the presence of individuals with N. denticulata haplotypes displaying intermediate morphology is consistent with ongoing hybridization in the wild. To accurately monitor the invasion and assess genetic impacts on native populations, detailed investigations using nuclear genome analysis are needed to inform future conservation strategies.

For freshwater shrimp, population declines and extinctions have become significant issues due to factors such as habitat loss and degradation, as well as the introduction of invasive species. Information on their native ranges is crucial for the conservation and management of freshwater shrimp. Museum specimens provide direct evidence of a species’ past occurrence and status; however, due to shortages of personnel, funding, and storage space, specimen cataloging is often insufficient. Here, we investigated and re-identified both registered and unregistered freshwater shrimp specimens collected before the late Showa period (<1989) and housed at the Osaka Museum of Natural History. As a result, a total of 63 specimen lots (59 of which were unregistered) were identified, including six species of the family Palaemonidae and six species of the family Atyidae. These specimens are not only valuable natural history records of freshwater shrimp, for which historical distribution data are limited, but also constitute a collection of high scientific value that provides insight into the fauna before the establishment of non-native freshwater shrimps.

Three species of the xanthid genus Luniella Lasley, Klaus & Ng, 2015 [L. pubescens (Dana, 1852), L. pugil (Dana, 1852), L. scabricula (Dana, 1852)] are reported on the basis of specimens from the Okinawa Island Group, central Ryukyus, Japan. The morphological features and habitats of the present specimens and distributional records in Japan of the three species are provided.