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Animals greater than 1 mm, found among tangled tubes of Phyllochaetopterus socialis (Chaetopteridae) from Araçá Beach, São Sebastião district, Brazil, were studied for 1 year, with four samples in each of four seasons. They comprised 10 338 individuals in 1722•7 g dry weight of polychaete tubes, with Echinodermata, Polychaeta (not identified to species) and Crustacea as the dominant taxa. The Shannon-Wiener diversity index did not vary seasonally, only two species (a holothurian and a pycnogonid) showing seasonal variation. Ophiactis savignyi was the dominant species, providing 45•5% of individuals. Three other ophiuroids, the holothurian Synaptula hidriformis, the crustaceans Leptochelia savignyi, Megalobrachium soriatum and Synalpheus fritzmuelleri, the sipunculan Themiste alutacea and the bivalve Hiatella arctica were all abundant, but most of the 68 species recorded occurred sparsely. The assemblage associated with P. socialis was similar to the endofauna of the sponge Zygomycale parishii and the bryozoan Schizoporella unicornis, and to the epifauna of seaweed Sargassum cymosum, all of which occurred nearby. 1995 Academic Press Limited

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Individual fitness and the structure of marine communities are strongly affected by spatial competition. Among the most common space holders are the colonial ascidians, which have the ability to monopolize large areas of hard substrate, overgrowing most other competitors. The effects of competition on colony growth and on gonad production of the ascidian Didemnum perlucidum were studied in southeastern Brazil by experimentally removing surrounding competitors. Colonies of D. perlucidum competing for space exhibited a growth rate 9 times less than that of colonies that were competitor free. Among the colonies subject to competition, growth rates were unrelated to the percentage of colony border that was free of competitors. However, the identity of the competitor was important in the outcome of border contacts. At the beginning of the experiment, most border encounters of D. perlucidum were with solitary organisms, which in most cases were overgrown. These were progressively replaced by colonial ascidians and bryozoans, resulting mostly in stand-off interactions. Besides reducing asexual growth, spatial competition also affected female gonad production. Colonies free of competitors had a significantly higher proportion of zooids with ovaries. Thus, our findings show that spatial competition reduces both ascidian colony size and gonad production. KEY WORDS: Space competition · Tunicate · ReproductionResale or republication not permitted without written consent of the publisher

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