EFFECT OF SECONDARY SUBSTRATE ON ASSOCIATED SMALL CRUSTACEANS IN A BRACKISH LAGOON

The influence of substrate provided by sessile organisms on epifaunal small crustaceans was investigated in a brackish lagoon in Sendai, Jap an. The relative abundances of small crustaceans were compared between two sites at the same tidal height on concrete structures where the s pecies composition of sessile organisms was different. In addition, th e species composition of small crustaceans was studied as a function o f three experimental substrates. Finally, by manipulating settlement p lates, the influences of some environmental factors on the sessile org anisms as well as on species interactions were studied. The field inve stigation and experiment showed that the secondary substrate created b y sessile organisms caused the spatial and temporal changes in the mos t abundant species of small crustaceans. Moreover, some environmental factors and earlier colonists have inhibitory and/or facilitative effe cts on the colonizations of barnacles and oysters which produced site differences of secondary substrate. In addition, oyster growth over ba rnacles indicated the competitive superiority of the oyster over barna cles after their colonization. These results indicate that the seconda ry substrate, especially its physical structure, is an important facto r which directly determines the species composition of associated smal l crustaceans. Environmental factors and interactions among sessile sp ecies have an indirect effect on species composition of small crustace ans, because they produce changes in the secondary substrate.