Biotic Interactions Among Estuarine Infaunal Opportunistic Species

Biotic interactions among soft-sediment infauna were investigated in a small New England estuary in order to determine what effect(s) established opportunistic species had on subsequent recolonization. Interactions were defined according to successional models developed by Connell and Slatyer (1977), e.g. facilitation, tolerance and inhibition. Adults of the opportunistic polychaetes Streblospio benedicti, Polydora ligni and Hobsonia florida were added at 2 densities to separate cores containing defaunated sediment. These cores and control cores containing no worms were sampled at 10 d intervals for 40 d. Cores containing capillary tubes to simulate polychaete tubes were also deployed and sampled at 10 d intervals. Subsequent infaunal colonization densities of the polychaetes seeded to the cores - and also Capitella capitata, the amphipods Corophium insidiosum and Microdeutopus gryllotalpa and the anthozoan Nematostella vectensis - were analyzed for differences in recolonization with respect to the initial density of each of the established species. While more than 1 particular type of interspecific interaction operated during the study, the results indicate that the species could be divided into 2 groups, the polychaete and non-polychaete fauna. A predominance of inhibitory interactions (recolonization densities were significantly lower in cores with established species than in control cores) occurred among the polychaete fauna of the estuary. Some evidence of interspecific facilitation was found during initial sampling periods when overall densities of organisms were low. The effect of initial worm density on settlement inhibition was variable. The non-polychaete fauna appeared not to have been either positively or negatively affected by established species, thus suggesting some form of tolerance interaction or the lack of interaction. Cores containing simulated polychaete tubes generally had no effect on recolonization. Inhibitory interactions among opportunistic polychaetes may be due to intraspecific gregarious settlement and subsequent preemption of food and space resources. While biotic interactions among opportunistic species may play an important role in controlling successional dynamics, the specific type of interaction that occurs most likely depends on the species present, their density and habitat conditions. There appears to be no “characteristic” type of biotic interaction which influences soft-bottom successional dynamics