LIFE HISTORIES OF SYMPATRIC MUD-FLAT CRABS, HELICE JAPONICA AND H. TRIDENS (DECAPODA: GRAPSIDAE), IN A JAPANESE ESTUARY

Abstract

ABSTRACT Life-history traits of two mud-flat crabs, Helice japonica and H. tridens, were examined on the basis of monthly sampling at 13 stations from April 1986 to March 1987 in the Shigenobu River estuary, Shikoku Island, southwest Japan. Helice japonica and H. tridens co-occur at high densities in estuaries and salt marshes throughout western Japan, but their main habitats do not overlap in the Shigenobu River estuary. Helice japonica was found in the midintertidal, while H. tridens occurred from the midintertidal up to the high intertidal. The density of H. japonica was higher than H. tridens, though the habitat of H. japonica was limited to the lower part of the estuary. The size-frequency distributions of both species were complicated and the generation structures of them could not be elucidated, with the result that growth curves and survivorship curves of both species could not be established. Helice japonica matures more quickly, is smaller at first reproduction, has a shorter reproductive period, smaller number of broods, slightly smaller eggs, and a larger brood size. Moreover, only H. tridens shows a semilunar or lunar reproductive cycle, which may increase the reproductive rate at low population density. The two closely related species inhabited an estuary and had different life histories. In the present study, an example of the possibility of the diverse patterns of life-history traits under the constraint of a single set of environmental factors is provided.