Influence of salinity on the horizontal distribution of soldier crab Mictyris guinotae in a subtropical river in Japan

Abstract

ABSTRACT The horizontal distribution of Mictyris guinotae was studied in relation to gradients of salinity and sediment characteristics in sandy flats between 0 and 8.2 km from the mouth of a subtropical river. Lower salinity tolerance was also tested at seven salinity levels (0, 2, 4, 8, 12, 24 and 32) by a survival experiment for 14 days. Environmental clines were found in salinity but not in sediment characteristics. The upper limit of the occurrence of crab was 5.07 km upstream where seawater could reach during the spring flood tide. The densities of age-0 crabs tended to be higher at lower stations (0–0.65 km upstream), suggesting more recruits settle near the river mouth. Carapace length was significantly larger in the upper section (2.50–5.07 km upstream) in which salinity fluctuated or remained low (<10) for longer periods. It is unlikely that low salinity has a negative effect on post-larval growth. In contrast, body size was smaller in the lower section, possibly due to the effect of density. Some individuals died at the end of the experiment at salinity ≤12. The upper limit of appearance was explained by the salinity of riverine water and low salinity tolerance of the crab.