Structural aspects of sessile invertebrates as organizing forces in an Estuarine fouling community

Abstract

The role of physical structure in the development of a fouling community in the Broadkill River Estuary, Delaware, was examined over the period March 1975 through January 1976. Asbestos cement panels and “mimic” panels were used to collect sessile fouling organisms and their motile associates. “Mimic” panels contained various objects representing the structural aspects of dead barnacles, tunicates, and hydroids. These are the dominant structural forms of early successional stages in the community. Structure provided by mimics enhanced settlement of both mussels ( Mytilus edulis L.) and tunicates ( Molgula manhattensis (DeKay)). Settlement of the hydroid Tubularia crocea (Agassiz) was unaffected by mimic structure, while colonization by barnacles ( Balanus improvisus Darwin) and serpulid polychaetes ( Hydroides dianthus Verrill) was more successful on bare surfaces than on mimic panels. These responses to structure correspond with patterns observed in the unaltered community, and which eventually determined its species composition. Balanus and Hydroides selected only bare surfaces for settlement and were earlier successional species. Molgula and Mytilus prefered previously occupied substrates, and Mytilus eventually dominated. The number of motile species on panels was positively correlated with the number of sessile species and negatively correlated with sessile species dominance. Sessile and motile diversity were causally related; this resulted from responses of motile species to structure supplied by sessile forms. More motile species were found on mimic as opposed to plain panels during early development. Physical structure supplied by sessile fouling species is an important characteristic of the system that determines habitat preferences and habitat suitability for both other sessile species and associated motile forms.