Specimens of Argulus coregoni Thorell, 1864 were collected from the body surface of rainbow trout, redspotted masu salmon, and masu salmon reared in a fish farm at Shijo in Mihama, Fukui Prefecture, central Japan. This collection represents the first record of A. coregoni from farmed fishes in Fukui Prefecture. Two (one male and one female) specimens of A. coregoni were examined in detail for their morphology and found to have four and six plumose setae on the posterior margin of each coxa of the fist legs and 58 and 65 supporting rods in the marginal membranes of the first maxillae. The second leg coxa of the male specimen has two protrusions adorned with small spines and a digitiform projection on the ventro- and dorsoposterior margins, respectively, whereas that leg coxa of the female specimen bears no such protuberances. Specimens of A. coregoni collected from rainbow trout on 26 September 2024 were comprised of large (6–11 mm long) and small (2–5 mm long) individuals, both of which are inferred to have hatched from eggs in the spring and summer of 2024, respectively, in fish raceways.

We investigated the horizontal and vertical distributions of Macrobrachium larvae within a range of 20 km offshore from the Niyodo River mouth in Kochi Prefecture during the daytime, once a month in August 2022, July and August 2023, and July–October 2024. Zoeal larvae at stages II–IX (IX being the final developmental stage) of M. nipponense and M. spp. thought to include M. formosense and M. japonicus were observed. The horizontal distributions of M. nipponense and M. spp. overlapped but differed, with their densities being high within 1.5 km and 5 km offshore, respectively. The densities of both Macrobrachium taxa were higher in the upper layer than in the lower layer; however, the frequency of occurrence in the lower layer was higher for M. spp. Considering the tidal current directions during the survey, M. nipponense was assumed to remain close to shore by being distributed in the upper layers, where coastal or onshore currents were dominant. Macrobrachium spp. were also distributed in the lower layers where rip currents were more likely to occur, thereby dispersing further offshore than M. nipponense.

The known distribution range of the coastal isopod Armadilloniscus notojimensis primarily covers western Japan, with previous records in south of Ishikawa and Kanagawa Prefectures on the Sea of Japan and Pacific side of Japan, respectively. The present study reports findings this species in Matsushima Bay, Miyagi Prefecture, northeastern Japan. This is the first from the Tohoku region and sets a new record for the northernmost sighting of this species. While the number and arrangement of projections present on the dorsal surfaces of the cephalon, one of the morphological characteristics of this species, has been previously described incompletely, we here thoroughly describe our observations of 10 projections (large or small) present on the dorsal surface of the cephalon and their arrangement. Our observations also permit distinguishment of that this species from A. hawaiianus, which has previously been indicated to be a possible senior synonym of this species, by the presence of two anterior tubercles in front of prominent bosses, a pair of inner tubercles on the dorsal surface of the pleonite 1, and a pair of outer tubercles outside the inner tubercles on the dorsal surface of pleonites 2–4.

Three mancae and nine adult specimens of the parasitic cymothoid isopod Cinusa nippon Nagasawa, 2021 were collected in Moune Bay, Kesennuma City and Samenoura Bay, Ishinomaki City, Miyagi Prefecture, northeastern Japan. The adult specimens were found in the buccal cavity of the host fish belonging to the genus Takifugu Abe, 1949. This parasitic isopod species has been known from central to southwestern Japan: off Murotsushimo, Shimonoseki City, Yamaguchi Prefecture; off the coast of Shima Peninsula, Mie Prefecture; and the south of Boso Peninsula, Chiba Prefecture. Therefore, this study updates the northernmost record of this species to Moune Bay. This is the first report of T. alboplumbeus (Richardson, 1845) and hybrid individuals between T. snyderi (Abe, 1988) and T. stictonotus (Temminck and Schlegel, 1850) as hosts of this species. We found intraspecific morphological variations that have not been reported by previous studies. The number of the antennal articles ranged from 7 to 10 and the number occasionally differed between the left and right antennae of the same specimens. The number of recurved robust setae on the mesial and lateral lobes of the maxilla ranged from 0 to 2 and 0 to 3, respectively. In addition, we also found intraspecific morphological variations in the presence/absence of aesthetascs of the antennula, shape of anterior margin of the head, and the number of setae on pereopod 6 and on pleopod 1 of manca stage.

The diogenid hermit crab, Clibanarius snelliusi Buitendijk, 1937, is reported for the first time based on a single specimen from Chiba Prefecture, Honshu mainland, central Japan. It represents the first Honshu record and the northernmost distributional record of the species.

The coral crab Trapezia rufopunctata is widely known from the Indo-West Pacific Ocean and has been recorded from Wakayama Prefecture and southward in Japan. Here, we collected one specimen of T. rufopunctata from the southwest coast of Chiba Prefecture in Japan. This is the first record in this prefecture and the northernmost record worldwide. With the advancement of global warming, overwintering and reproduction areas are expected to expand.

Abstract: This study reports the northernmost records of three species of the hermit crab genus Clibanarius (Decapoda: Diogenidae) collected from Shikine Island in the Izu Islands, eastern Japan. Specimens of the two species, C. corallinus (H. Milne Edwards, 1848) and C. eurysternus (Hilgendorf, 1879), were immature, suggesting that their present occurrence could be attributed to sink dispersal. However, the third species, C. striolatus Dana, 1852, could potentially reproduce in this area because of the collection of mature specimens. Moreover, with an advancement in ocean warming, distributional ranges are expected to expand northward.

A rafting crab, Plagusia immaculata Lamarck, 1818, is distributed widely from the Indian Ocean to the Pacific Ocean, with several records known from Japan. In October 2024, a single specimen of the species was collected on Hiraiso coast in the central part of Ibaraki Prefecture, Pacific coast of eastern Honshu Island. This represents the first record of the species in Ibaraki Prefecture and the northern range extension. It is likely that the specimen was transported by the Kuroshio Current from a more southern region, either during its larval stage or after it had grown into an adult.

Macrobrachium formosense Spence Bate, 1868 is a freshwater prawn with an amphidromous life history. Although the species is widely distributed across Japan, there is no consensus on whether its distribution range includes the Ogasawara (Bonin) Islands, as previous studies have reported conflicting information. Here, we reexamined the species’ occurrence in the Ogasawara Islands through literature and museum specimen surveys. We found no confirmed records of the species based on specimens from the region. The earliest reference was by Balss (1914) based on the collection from the Museum Tokio. However, there was no robust evidence that the museum’s collection included M. formosense specimens from the Ogasawara Islands. Given the absence of recent records and the lack of verifiable historical specimens, we conclude that it is inappropriate to include the Ogasawara Islands within the distribution range of M. formosense.

A small sponge crab, Cryptodromia laevis, was originally described from Sanguisiapo Island, Sulu Archipelago, Philippines. In this study, the species is recorded from the Okinawa Group, Ryukyu Islands, representing the second distributional record of the species as well as first record from Japan. Based on morphology, this study provides a brief discussion on the systematic relationship between C. laevis and Cryptodromiopsis dubia, belonging to other dromiid genus in the current taxonomic studies.