Variable effects of ascidian competitors on oysters in a Florida epifaunal community

It is often assumed that sessile animals die when they are overgrown by others. Three settling plate experiments were run to determine whether ascidian overgrowth reduces the survivorship of the oyster, Ostrea equestris Say, in a Florida epifaunal community. Each experiment was run under a different set of conditions. Ascidians reduced the number of living oysters in two experiments but enhanced the number in a third. Ascidians had no effect on oyster size in one experiment, but enhanced size in the other two. Thus, there was no consistent effect of ascidians on oysters. At the end of all experiments, many living oysters were found under ascidians. Gaps beneath ascidians permitting water flow to underlying oysters may explain the ability of oysters to survive.