An endangered freshwater crab, Ryukyum yaeyamense (Minei, 1973) (Crustacea: Decapoda: Brachyura: Potamidae), is recorded for the first time from Kohamajima Island, Yaeyama island group, southern Ryukyu Islands, Japan. Kohamajima Island becomes the third distributional island of the species. The morphological characteristics of the species, their habitats, and ecological notes of the species are provided.

Six euxanthine crabs, Euxanthus exsculptus (Herbst, 1790), E. herdmani Laurie, 1906, Hepatoporus guinotae (Zarenkov, 1971), Hypocolpus haanii Rathbun, 1909, Hy. kurodai Takeda, 1980 and Psaumis cavipes (Dana, 1852) are reported based on specimens collected from the Ryukyu Islands, southern Japan. Euxanthus herdmani and He. guinotae are recorded for the first time from Japan and the Ryukyu Islands, respectively.

For hermit crabs, a gastropod shell is an important resource that affects individual survival and reproduction. However, since shell resources are generally in short supply in the field, coexistence mechanisms are required to enable the occurrence of multiple hermit crab species in the same area. Here, we examined the shell utilization pattern of hermit crabs and gastropod distribution on the rocky shore at Kattoshi, Hakodate Bay, Japan where six hermit species coexist. Since more than 90% of them were Pagurus filholi and Pagurus middendorffii, we focused on the factors affecting the coexistence of these two abundant species in detail. The top five common gastropod species were Reticunassa spp. (Reticunassa fratercula or Reticunassa multigranosa), Littorina brevicula, Cantharidus japonicus, Lirularia iridescens, and Homalopoma sangarense. Although both P. filholi and P. middendorffii commonly used H. sangarense and Reticunassa spp., P. filholi used Ocenebra inornatus and P. middendorffii used Ca. japonicus more often. Additionally, the two hermit species differed the hermit–shell size relationship when they used Ca. japonicus. Interspecific variations in the shell utilization pattern may contribute to the coexistence of P. filholi and P. middendorffii in our study site.

The pebble-shore crab Cyclograpsus pumilio Hangai & Fukui, 2009 (Varunidae) was recorded newly from the Noto Peninsula, western and southern Kyushu and Tanega-shima Is., extending the distribution of this species northerly on the coast of the Sea of Japan and southerly on the Pacific coast. The recorded sites were the high tide level of the pebble shore, with stranded seaweeds, which is similar to the previous report on the habitat condition.

Two individuals (one ovigerous female and one male) of the cymothoid isopod Nerocila japonica Schioedte & Meinert, 1881 were found on the left pectoral fin of a yellowfin seabream, Acanthopagrus latus (Houttuyn, 1782), in coastal waters of Hiroshima Bay, an inlet of the western Seto Inland Sea, Hiroshima Prefecture, Japan. The fin was seriously damaged, and the fin membrane was mostly lost. A wound with exposed fish muscle was also found at the base of the fin. These lesions are considered to have been caused by the isopod’s pereopod insertion to and feeding on the host’s tissues. Previous studies on the pathological impacts of cymothoid isopods on the skin of Japanese marine fishes are reviewed.

Neocaridina denticulata (Decapoda: Atyidae) is a freshwater shrimp with a land-locked life-history and is widely distributed in western Japan. The taxonomic confusion of this species, caused by the loss of type specimens, unknown type locality, and genetic and morphological geographical variation, has led to inconveniences in biodiversity conservation and the identification of exotic species that recently invaded Japan. Therefore, it is important to clarify this species’ distributional range and regional genetic and morphological characteristics to solve the problem. Here we report the first record of N. denticulata from Fukue-jima Island in the Goto Islands, Nagasaki Prefecture, Japan. Morphological analyses identified the population as N. denticulata, and mitochondrial DNA-based analyses suggested that the population was distinctly differentiated from the Kyushu populations. This population is essential for inferring the N. denticulata distribution formation and biota formation processes in the Goto Islands. On Fukue-jima Island, N. denti­culata has only been found in one pond; thus, the Fukue-jima population is likely to be in critical condition.

Five species of the xanthid genus, Neoliomera Odhner, 1925, viz. N. fragraea Ho & Ng, 2014, N. insularis (Adams & White, 1849), N. richteroides Sakai, 1969, N. striata Buitendijk, 1941 and N. variolosa (A. Milne-Edwards, 1873), are reported based on specimens collected from the Ryukyu Islands, Japan. The present study represents the first records of N. fragraea and N. variolosa to the Japanese fauna.

Freshwater sculpins (Perciformes: Cottidae) are uncommon hosts of the lernaeid copepod Lernaea cyprinacea Linnaeus, 1758 worldwide, with its only one record from those sculpins (i.e., from the bullhead, Cottus gobio Linnaeus, 1758, in Scotland, U.K.). One and two postmetamorphic adult females of L. cyprinacea were collected each from two individuals of the fourspine sculpin, Rheopresbe kazika (Jordan & Starks, 1904), in the middle reaches of the Takeno River in Hyogo Prefecture, central Japan. The infected fish were sampled in a pool immediately downstream from a weir. Although the fourspine sculpin is a typical riffle dweller, the weir prevented the sampled fish from their upstream migration, and they most probably became infected with L. cyprinacea during their stay in the pool. The fourspine sculpin is endemic to Japan, and L. cyprinacea represents the first species of parasite found from the fish species.