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At 3 sites in Florida, USA, the con~munities of marine sessile invertebrates are dominated by oysters (Crassostrea virginica) in the mid-intertidal zone, and by ascidians (several species) in the low intertidal and subtidal zones. Our experiments tested the hypotheses that the upper depth limit of ascidians is set by (1) predation by oysters on larvae, (2) larval zonation, and (3) early post-settlement mortality. Hypothesis 1 was tested by monitoring ascidian recruitment on the shells of dead and living oysters placed in both the oyster and ascidian zones. Because recruitment of all ascidian species was confined to oysters placed in the ascidian zone, w e concluded that the upper depth limit of these ascidians cannot be due solely to predation on larvae by oysters. Hypotheses 2 and 3 were tested by monitoring the recruitment of ascidians on settling plates fixed in the 2 zones, and on plates floating at 2 depths in the water column. In the 'fixed' experiments, recruitment of all ascidian specles was confined to the ascidian zone. In the 'floating' expenments, the number of recruits of Diplosoma glandulosum did not differ between shallow and deep plates, and recruitment of Styela plicata was confined to shallow plates. Thus, we concluded that early post-settlement mortality is a major factor setting the upper depth limit of these 2 species. We vertically transplanted 2 ascidians (S. plicata, Botryllus planus) and found that the survivorship of both was greater in the ascidian zone than in the oyster zone. Thus, further support was gained for the post-settlement mortality hypothesis.

Role of early post-settlement mortality in setting the upper depth limit of ascidians in Florida epifaunal communities