Settlement patterns of two sesarmid megalopae in the Sai River Estuary, Ishikawa Prefecture, Japan

Two semi-terrestrial crab species, Chiromantes haematocheir and Chiromantes dehaani, are closely related and have a sympatric distribution. The ecological characteristics of these species, including habitat and behavior during the adult stage, have been investigated. However, the larval stage remains poorly understood because it is difficult to identify larvae morphologically. Our study objectives are to clarify the settlement patterns involved in recruitment of the two species of megalopae using genetic analysis and the effects of ecological factors on settlement, including 1) seasonal changes, 2) spatial variations (dominated by conspecific or heterospecific adults), and 3) tidal effects. The Chiromantes megalopae were collected in the lower reach of the Sai River during spring and neap tides from August to November 2017. Megalopa larvae were collected from three areas: one dominated by C. haematocheir adults, one dominated by C. dehaani adults, and a revetment area. Both species of megalopae showed seasonal and spatial variations in settlement, which also differed with tidal fluctuations. Chiromantes dehaani megalopae settled from mid-August to early October and C. haematocheir megalopae settled from early September to early November. Chiromantes haematocheir megalopae preferentially settled in the area dominated by conspecific adults over the area dominated by C. dehaani and the revetment area, while no difference in the settlement of C. dehaani megalopae was observed among areas. Chiromantes haematocheir megalopae returned on the spring tide; however, C. dehaani megalopae showed no tidal pattern.