This study designates a lectotype for and redescribes Thelphusa dehaani White, 1847, a Japanese primary freshwater crab species belonging to the potamid genus Geothelphusa Stimpson, 1858. Geothelphusa dehaani was described based on material exported to the Netherlands by von Siebold. Although the provenance of all his specimens is uncertain (they may be from several locations), our morphological comparison of the lectotype with fresh specimens led us to ascertain that it was almost certainly collected from Nagasaki, where von Siebold stayed. Two new Geothelphusa species are also described from Amakusa Islands and Aomori Prefecture.

The pedunculate barnacle Octolasmis unguisiformis is a rare epibiotic barnacle attached to a macrophthalmid crab, Macrophthalmus (Macrophthalmus) milloti Crosnier, 1965, on tidal flats in the Ryukyu Archipelago. It has a rare sexual system called androdioecy, a coexistence of hermaphrodites and males in the population. In June 2020, we found two hermaphrodites and one dwarf male of O. unguisiformis attached to a female M. (M.) convexus at Kise Bay, Amami Oshima Island. Here we describe the symbiotic conditions between the pedunculate barnacle and the new host crab species.

A male gnathiid specimen was captured at a depth of 374 m off Cape Muroto, southern Japan. The specimen exhibited small mandibles, extremely large eyes covering most of the lateral cephalon, a slender body, and J- or L-shaped narrow pylopods. These characteristics correspond to those of the monotypic genus Tenerognathia. This specimen, distinct from the type and sole species of Tenerognathia visus, is identified as a new species based on differences in pylopod articulation and shapes of cephalon and maxilliped basis.

The Japanese spiny lobster Panulirus japonicus has been reported to harbor a numbers of nuclear mitochondrial pseudogenes (NUMTs) and heteroplasmy. However, distinguishing phylogenetically young NUMTs, heteroplasmy, and PCR-cloning artefacts may be challenging. In addition, greater degradation for mtDNA than nuclear DNA in elderly tissue specimens may promote amplification of NUMTs. In this study, we performed clone library-based nucleotide sequence analysis of the partial mtDNA COI gene using genomic DNA and cDNA obtained from fresh tissues of the Japanese spiny lobster and genomic DNA obtained from three crustacean and three fish species. Minor nucleotide substitutions between clones in an individual were ubiquitously observed in all species examined including the lobster cDNA, suggesting that most of these were artefacts. Rarely, a few clones were most likely to have originated from heteroplasmic copies, as they had skewed nucleotide substitutions at the third codon. The Japanese spiny lobster is more likely than others to detect NUMTs, while the detection of NUMTs may be somewhat suppressed using genomic DNA obtained from fresh tissue.

Sand shrimp Crangon uritai is distributed in shallow waters including the intertidal zone in eastern Asia and is known to be a prey item and a predator of juveniles of commercially important fishes. Specimens were collected on the Kawaradzu tidal flat, using a rake sledge net (mesh size 2.0 mm) from June to December 2016 and 2017, and in the adjacent shallow bottom (3–4 m depth at high tide) using a sledge net (mesh size 0.76 mm) from January to August 2018 and 2019. Seasonal changes in the density index and carapace length distribution were studied in both sites. The frequency of ovigerous females were indicated for the tidal flat. Settlement mainly occurred from May to July although low intensity settlement seemed to occur throughout the year except for September and October. Ovigerous season in the tidal flat finished in August or September by disappearance of the cohort including ovigerous females and resumed in October with the oviposition of 0-year individuals. Ovigerous females might occur throughout the year except for August or September, although their occurrence was not checked in the shallow bottom. Research on the offshore area is needed to detail the whole life history.

One and two specimens of Argulus japonicus Thiele, 1900 were collected from the body surface of big-scaled redfin Pseudaspius hakonensis (Günther, 1877) (Cypriniformes: Leuciscidae) in the Babame River, Akita Prefecture, and Lake Jusan, Aomori Prefecture, northern Honshu, Japan, respectively. This is two new prefecture records in Japan and a new host record for A. japonicus. Lake Jusan is the northernmost collection locality of this parasite in Japan recorded from wild fishes. As Lake Jusan is a brackish water lake and big-scaled redfin is known to migrate from fresh to salt waters, and since A. japonicus completes its life cycle in fresh waters, we infer that the two individuals of A. japonicus parasitized the big-scaled redfin in an inflowing river, then this fish was collected along with the surviving argulid individuals after migrating to the lake. The collection of A. japonicus in Lake Jusan represents the second record for the species from brackish water lakes in Japan.