Overgrowth competition in an assemblage of encrusting bryozoans settled on artificial substrata

Abstract

Patterns of intra-and interspecific competition on artificial substrata were studled in Patagonia, in an epifaunal community composed of 19 species of bryozoans, 4 spirorbid polychaetes and 1 colonial tunicate. The latter was found to be dominant over bryozoans and both groups were able to overgrow spirorbids. Apparent cessation of growth occurred in intraspecific encounters among bryozoan colonies. Several cases of homosyndrome were recorded in the dominant species Umbonula alvareziana. Competitive ability was correlated with zoarial height in cheilostome but not in cyclostome bryozoans. Lack of correlation between abundance and competitive ability could be related to the relatively early successional stage of this assemblage, where a high proportion of the surface is bare. Analysis of the interaction matrix indicates a clearly hierarchical con~petitive organization of the community. Colony size was found to be an important factor influencing the outcome only in encounters between species with similar competitive abilities. Relationships among number of ovicells, colony shape and overgrowth ability were analyzed in 2 closely related species of Celleporella: the runner-like C. patagonica compensates its poor conlpetitive performance by reaching sexual maturity earlier and having a significantly higher rate of ovicell production than the sheet-like C. yagana.