Abstract
The reproductive characteristics of two estuarine crabs, Chasmagnathus convexus and Helice tridens, which coexist in the Shigenobu River Estuary, Shikoku Island, Japan, were examined on the basis of data collected between April 2001 and March 2002. Seasonal changes in the female gonadosomatic index and occurrence of ovigerous females showed that females of C. convexus spawned mainly from December to March and those of H. tridens spawned from April to July. The mean egg clutch size of C. convexus and H. tridens was 41,600 and 42,500 eggs, respectively (not significantly different) but the mean egg diameter of C. convexus (0.46 mm) was significantly larger than that of H. tridens (0.36 mm). The mean annual fecundity is higher in H. tridens because it produces more egg clutches per year.
