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The shallow water shrimp Acetes sibogae sibogae is an ecological and economically important organism in the Asian region. We assessed the genetic diversity and population structure of this species based on samples obtained from eight sites in Japan, the Philippines and Indonesia. Nucleotide sequence analyses were performed based on PCR‐amplified mitochondrial DNA fragments comprising the control region and part of the 12S rRNA gene. In total, 656 bp nucleotide sequences were obtained from 377 individuals. The results show a low haplotype diversity value in shrimp from the Kokuba‐gawa River (0.5) in Japan and intermediate values (0.79–0.99) in other populations. While nucleotide diversity estimates in all sampled localities were lower (0.17%–0.86%) than those reported in other crustaceans, analysis of molecular variance revealed significant genetic differentiation (p < .001) among all sites. Furthermore, the mean fixation index estimate was very high (FST: 0.27), indicating that dispersal potential among sites was low and gene flow, therefore, was restricted. The mismatch distribution of pairwise differences between haplotypes indicated that A. s. sibogae did not fit a sudden population expansion model. Overall, the present study indicates that wild stocks of A. s. sibogae on Okinawa‐jima Island and the Philippine and Indonesian Archipelagos should be assigned high importance for conservation management of this species because adapted genotypes have significant evolutionary potential during environmental changes. [Correction added on 5th February 2022, after first online publication: Values have been corrected in the Abstract section.]

Nectonema, the only horsehair worm (Nematomorpha) genus found in marine environments, was previously known to be parasitic only in decapod crustaceans. We report Nectonema sp.as the first record of a marine nematomorph parasitic in isopod crustaceans. This is also the third record of marine nematomorphs from the North Pacific. Six infected isopods (Natatolana japonensis) collected from 1425 m depth in the Sea of Japan each contained one to seven (mean 2.33) nematomorphs in the body cavity in the pereon. There was no correlation between the host body length and number of parasites. For Nectonema sp., we describe and illustrate morphological features of the parasitic juvenile stage and present nucleotide sequences for the cytochrome c oxidase subunit I gene (COI or cox1; 451 nt), 18S rRNA gene (1777 nt), and region spanning the internal transcribed spacer 1 (ITS1) and the 28S rRNA gene including the 5.8S rRNA gene and ITS2 (1218 nt in total). In an 18S maximum-likelihood tree that included 24 nematomorph species, Nectonema sp. grouped with N. agile from the northwestern Atlantic; the 18S gene from these two taxa was divergent by 11.8% K2P distance, suggesting they are different species. Nectonema species may have a broader range of host groups than previously suspected, but may have been previously misidentified as nematode parasites.

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