Factors influencing the distribution and coexistence of Pachygrapsus crassipes and Hemigrapsus oregonensis (Decapoda: Grapsidae) in a California salt marsh

Abstract

Pachygrapsus crassipes Randall and Hemigrapsus oregonensis (Dana), coexisting grapsid crabs, were studied in a tidal marsh near Santa Barbara, California, USA. Both species are found in intertidal mud burrows along banks of the slough channels. H. oregonensis occupies the entire intertidal vertical range on the mud banks, whereas P. crassipes inhabits primarily the uppermost zone (above +1.4 m). In areas where both species occur in large numbers, the density of H. oregonensis in the uppermost zone is less than in areas with only a few P. crassipes. P. crassipes does not excavate burrows, but moves into and enlarges the existing burrows of H. oregonensis. P. crassipes also preys upon H. oregonensis, but burrowing provides the latter with partial refuge from predation. Population structures of the two species differ in the number of small crabs that occur in different areas of the slough. Few small specimens of P. crassipes are found along the upstream muddy banks of the slough, whereas young specimens of H. oregonensis are abundant in these areas. Physical factors such as salinity and siltation may restrict settlement or reduce survivorship of small P. crassipes. A balance between the predatory/competitive abilities of P. crassipes and the greater tolerance of H. oregonensis to physical stresses is proposed as a mechanism of coexistence.