A female specimen of Alitropus typus H. Milne Edwards, 1840 (Isopoda: Aegidae), a micropredator of fresh- and brackishwater fishes, was found from nurseryfish, Kurtus gulliveri Castelnau, 1878 (Kurtiformes: Kurtidae) at the Tokyo Sea Life Park, Japan. The isopod crawled out from the fish, representing the first time that the micropredator, A. typus, has been discovered in Japan. The present paper provides a description of the morphological characters of the isopod, and molecular barcod for identification. The morphological characters of the specimen are in well agreement with the original and the previous descriptions of A. typus. The molecular analysis, A. typus formed an independent clade in the maximum likelihood trees of Aegidae based on cytochrome c oxidase subunit I (COI) gene. However, K2P distance of the partial COI gene sequences determined from the present specimen is 17.85% from that of A. typus present in the GenBank. This isopod is an introduced species imported from north Australia that attaches to the nurseryfish. However, great care is taken to ensure that these animals imported from overseas do not come into contact with Japan’s natural environment.

The morphological characters of the freshwater shrimp called “Ryugu-hime-ebi” in Japan are not always consistent with those of Caridina laoagensis, which has been given its Japanese name. Recently, a cryptic species of C. weberi species group that includes C. laoagensis was newly described as Caridina tupaia. We aimed to resolve the historical confusion between C. laoagensis and C. tupaia in Japan. For DNA barcoding and morphological examination, we recognized the two taxa in “Ryugu-hime-ebi,” i.e. C. laoagensis and C. tupaia. Both species were clearly identifiable by the morphology of the distal setae on the posterior margin of the telson. Furthermore, we presented two new diagnostic characters: the presence (C. tupaia) or absence (C. laoagensis) of black spots on the lateral side under fresh conditions and the number of postorbital dorsal rostral teeth (0–2 vs. 0): these characters do not completely segregate the two species and are therefore recommended to be combined. Based on these characters, we reviewed previous records and found that the taxon that was given the Japanese name “Ryugu-hime-ebi” was most likely C. tupaia. Finally, we proposed to give the standard Japanese name “Ryugu-hime-ebi” to C. tupaia and the new Japanese name “Un-mon-hime-ebi” to C. laoagensis.

Seven specimens of the pilumnid crab Pilumnopeus marginatus were collected from the Shioiri and Heguri Rivers in Chiba Prefecture and the Tagoe River in Kanagawa Prefecture, eastern Japan. Based on these specimens, this study is the first record of this species in these prefectures. Moreover, two specimens collected from the Tagoe River represent the northernmost record of the species. From the Shioiri River, several individuals of different sizes were collected. Therefore, the species was possibly overwintering with established populations in the river. In the Tagoe River, one female specimen was carrying eggs, suggesting that the species is likely reproducing.

An invasive alien signal crayfish Pacifastacus leniusculus (Dana, 1852) is a native to a region spanning southern West Canada to the northwestern United States of America. In July 2021, September and October 2023, we captured four signal crayfishes in the Ogouchi reservoir (Lake Okutama), Tokyo, Japan, representing the first record of this species from this prefecture. To effectively manage this invasion, further research is crucial to clarify the signal crayfish’s habitat range and reproduction state in Tokyo. Additionally, educating the public about the potential risks of these invasive species is essential.

A mangrove sesarmid crab, Neosarmatium daviei Schubart & Ng, 2002, is reported from Japan for the first time based on a female specimen collected from Ishigaki Island, Ryukyu Archipelago. This study demonstrates that although the morphological features of the examined specimen and the holotype of N. daviei differ from those of the type series of N. punctatum (A. Milne-Edwards, 1873), they conform to the specimens collected from Taiwan identified as N. punctatum. Consequently, a reevaluation of the specimens from Taiwan is recommended.

Macrobrachium ustulatum (Nobili, 1899) is the sister species of Macrobrachium australe (Guérin-Méneville, 1838), and they have been historically confused because of their morphological similarity. The northern edge of the distribution range of both species is Japan: M. australe has been recorded from the Ryukyu Islands to a wide area of Honshu, while M. ustulatum has only been recorded once each from Okinawa Island and the Izu Peninsula. Therefore, the distributional records and habitat information of M. ustulatum in Japan are lacking. Here we report a total of 10 specimens of M. ustulatum collected from three localities in the Ryukyu Islands, including new localities. The environments in which these specimens were collected suggest that the habitat of this species is a weakly flowing environment in the middle reach of a river. This differs slightly from M. australe, which prefers stagnant environments. Furthermore, our morphological examination of the two species suggested that the width/length ratio of the palm and carpus of the major second pereiopod is useful in distinguishing this species from M. australe without correlating with body size: both characters are thick in M. ustulatum (0.13–0.19, 0.14–0.18) and thinner (0.10, 0.08–0.10) in M. australe.

A freshwater shrimp, Neocaridina denticulata (De Haan, 1844), distributed over a wide area of western Japan, has a land-locked life history and lives its entire life in freshwater. This species also occurs on adjacent islands, and some populations are genetically distinct from mainland populations. Since this species cannot disperse beyond the sea, the island populations are believed to be remnants of past connections with the mainland. Here we report N. denticulata from Uku Island in the Goto Islands. Phylogenetic analysis based on the mitochondrial DNA COI region showed that the Uku population was included in the lineage of N. denticulata, which is mainly found in mainland Kyushu, and had a unique haplotype, suggesting that it is native to the island. Compared with the N. denticulata population from Fukue Island in the Goto Islands, southwest of Uku Island, the Uku population was more closely related to the mainland Kyushu population. This suggests that the geographic isolation of Uku Island from mainland Kyushu is weaker than that of Fukue Island. We also report N. ikiensis Shih, Cai, Niwa & Nakahara, 2017, endemic to Iki Island, from mainland Kyushu and an invasive species, N. davidi (Bouvier, 1904), in a wide area of Kyushu, including Fukue Island.

Japanese records of the deep-sea calappid species identified as Mursia danigoi Galil, 1993 are reexamined by literatures and museum specimens, and collected new materials from Suruga Bay and Sea of Hyuga. As the results, M. danigoi and M. mameleu Galil & Takeda, 2004 are recognized. The reliable Japanese M. danigoi record is recognized by only one specimen from Suruga Bay, central Japan. Although Japanese specimens of M. danigoi have not been identified carefully since Takeda (1997), most records are reidentified as M. mameleu by the shape of carapace, cheliped and male gonopod 2 herein. The certain distribution records of this species in Japan are from Sagami Bay to Sea of Hyuga on the Pacific coast and from Tottori Prefecture to Tsushima Island on the Sea of Japan coast. Based on the present identification, “Kostuno-kinsen-modoki”, the well-known Japanese name, is adopted for M. mameleu, and the Japanese name “Suruga-kinsen-modoki” is adopted for M. danigoi.