# Contrasting population genetic structure of three semi‐terrestrial brachyuran crabs on the coast of the Japanese archipelago

## TL;DR

This study examines the genetic diversity of three crab species along Japan's coast and finds that one species shows significant genetic differences in certain regions, highlighting conservation concerns.

## Contribution

The study provides new insights into the genetic structure of semi-terrestrial crabs and identifies vulnerable populations.

## Key findings

- Orisarma dehaani shows clear genetic differentiation between the Tohoku Pacific coast and other regions.
- Chiromantes haematocheir has a genetically isolated population at the edge of its range.
- Ocean currents likely limit gene flow in Orisarma dehaani populations.

## Abstract

Anthropogenic activities have reduced ecotones between the ocean and land, which is likely to threaten the population of brackish‐water brachyuran crabs. To assess the current status of these crabs, we examine the population genetic structures of three semi‐terrestrial brachyuran crabs widely distributed along the coast of the Japan and to clarify factors determining their genetic structures. We collected 184 Orisarma dehaani, 252 Chiromantes haematocheir, and 151 Helice tridens crabs from 36 localities of the Japanese archipelago. Genome‐wide SNP data from these crabs were analyzed using MIG‐seq. Bayesian clustering of STRUCTURE and DAPC analysis were used to identify genetically disturbed populations and to visualize genetic differentiation between local populations. Genetic population structure showed clear differentiation between populations on the Pacific coast of the Tohoku region and on other Japanese coasts in O. dehaani, but not in C. haematocheir or H. tridens. The inbreeding coefficient of O. dehaani was significantly higher on the Pacific coast of the Tohoku region compared to other Japanese coasts. C. haematocheir and H. tridens had homogeneous genetic structures along the Japanese coast, but showed genetic differentiation of a local population at their range limits. Thus, O. dehaani showed little gene flow and clear genetic differentiation between populations in the Tohoku Pacific region and those on other Japanese coasts due to ocean currents. Although such a regional differentiation was not found in C. haematocheir and H. tridens, one population of C. haematocheir was genetically isolated at the edge of its distribution range and likely vulnerable to environmental changes.

This study highlights the potential extinction threat faced by semi‐terrestrial crabs. We analyzed the population genetics of these species distributed along the coast of the Japanese archipelago using modern high‐resolution genome‐wide SNP analysis. We found that one crab species had clear spatial genetic differentiation among populations, despite its ability to disperse long distances during larval stages. In addition, a population of another species was isolated at the edge of its range. Based on these results, we emphasized the need to conserve local populations of these species, as their local populations have evolved independently.