Monthly quantitative sampling of caridean shrimps and prawns, and logging of river water temperatures were undertaken over 13 months in the Kawazu-yatsu and Ogamo Rivers, Izu Peninsula, and the Morito and Shimoyama Rivers, Miura Peninsula, Japan in order to clarify the overwintering status. Low water temperature tolerances of 9 major species were also examined using culture experiments. Water temperatures in the Kawazu-yatsu River were highest throughout the year due to the inflow of hot spring water. Fifteen shrimp and prawn species were collected, the number of individuals tending to decrease in winter in most. In particular, two species, Macrobrachium australe and M.lar, which have low tolerance to low water temperature were found only in autumn in three rivers except Kawazu-Yatsu River, and it was confirmed that they died due to low water temperature in winter. The rate of decline from autumn to winter for the species established in the study area was the lowest in Paratya compressa, which has the highest tolerance to low water temperatures, and in the Kawazu-Yatsu River, which has the highest water temperatures. The decrease in the populations of M. formosense, M japonicum, Caridina leucosticta and C. typus occurred earlier than the decline in river water temperatures, suggesting that they moved to places where it is difficult to collect them, such as deep pools.

Four decapod crustaceans, Heterocarpus fascirostratus Yang, Chan & Kumar, 2018 (Caridea: Pandalidae), Uroptychus imparilis Baba, 2018 (Anomura: Chirostylidae), Munidopsis similior Baba, 1988 (Anomura: Munidopsidae) and Euryxanthops flexidentatus Garth & Kim, 1983 (Brachyura: Xanthidae) are reported on the basis of specimens collected from the Sea of Hyuga, Kyushu, western Japan, by local off shore commercial trawlers. Munidopsis similior and E. flexidentatus are recorded for the first time from Japan. Images showing living coloration are given for each species. Detailed descriptions and supplemental illustrations are provided for M. similior and E. flexidentatus to show evidence of identification.

Species in Cymothoidae (Crustacea: Isopoda) are cosmopolitan parasite infesting marine, freshwater and brackish water fishes. In Cymothoidae, Anilocra and Nerocila species have an elongated juvenile stage called the aegathoid stage. A total of 831 Japanese halfbeaks Hyporhamphus sajori (Beloniformes: Hemiramphidae) were collected from 11 sites in Japan. The aegathoid stage was seen infesting a Japanese halfbeak collected from Osaki-Shimojima Island, Kure City, Hiroshima Prefecture, and identified as Anilocra clupei by DNA barcoding using the COI and 16S rRNA sequences. The morphology of this individual was also almost identical to the aegathoid stage of A. clupei in the previous study. The prevalence of the aegathoid stage of A. clupei on Japanese halfbeaks was very low at 0.12% in all of Japan and 0.93% in the coast of Osaki-Shimojima Island. Aegathoid stages of A. clupei may have lower host-specificity than adult females, and be able to infest more comprehensive species of fish, which may increase the survivability of the aegathoid stages.

A large megalopa with an attached barnacle was collected from Shirahama, Wakayama Prefecture, Japan. The megalopa and barnacle were examined for species identification by DNA barcoding analysis using COI genes. It revealed that the specimens were Ocypode sinensis Dai, Song & Yang, 1985 and Conchoderma hunteri (Owen, 1830). This is the first report of megalopa stage on the life history of O. sinensis. Morphology of the megalopa specimen is preliminarily compared with those of previously described Ocypode species in Japan.

Alpheus dolichodactylus Ortmann, 1890 burrows muddy tidal flats, recorded from inshore coastal areas in central to southwestern Japan. Because of abrupt loss of habitats caused by human activities, the conservation status of the species is ranked as NT in the Japanese Red List. For example, the syntype of the species consists of 15 specimens from the Tokyo Bay, but there have been no records of the species since the original description from the Tokyo Bay and adjacent areas. In this study, we report on the occurrence of three specimens of A. dolichodactylus from small muddy tidal flat at the river mouth of the Heguri River in the Tatetama Bay, southern part of the Boso Peninsula, located at the outer part of the Tokyo Bay, represent the rediscovery of the species from the Tokyo Bay and adjacent areas by 129 years since the collection of the syntype. We also provide a description of the habitats where our specimens were collected. Maintenance of muddy substrate at estuarine areas will contribute to recovery of local population of the species.

The kelp crab Pugettia quadridens was recorded from Hakodate Bay, Hokkaido, Japan, representing the first record of Hokkaido and the northern extension of its range after the redescription of P. quadridens by Ohtsuchi & Kawamura (2019). Considering the collected specimen was a mature male and P. ferox, a closely related species to P. quadridens, is the common species in the sampling site, occurrence of reproductive behavior between the two species was examined by observing the mating behavior in an aquarium. The male of P. quadridens did not attempt to mate with either of three females of P. ferox, implying a behavioral reproductive isolation between the two species.

Two species of the xanthid crabs, Tweedieia odhneri and T. laysani, are reported based on specimens collected from Okinawa Island, Ryukyu Islands, southwestern Japan. Recent study suggested that T. laysani is endemic to Hawaiian Islands, but the present study confirmed its distribution in Okinawa Island. Tweedieia odhneri is newly added to the Japanese fauna. The additional distinguishing characters of these two species not mentioned in previous studies, are also provided.

Calcinus isabellae Poupin, 1997 was collected from the Chichi-jima Island, Ogasawara Islands, south Japan. This report represents the first record of this species from Japanese waters and expands the northern and western limits of the species.