| Summary: | The cheilostome bryozoan Antarctothoa bougainvillei (d'Orbigny) is the most frequent epibiont on the ribbon-like red alga Hymenena laciniata (Hooker f. & Harvey) Kylin in San Sebastián Bay (Tierra del Fuego, Argentina). Twenty-one thalli and 1,484 colonies were examined to analyse the relationship between both species. In most cases, number and area of colonies did not differ significantly at both sides of the thallus. Ancestrulae (i.e., founder zooids originating colonies by asexual budding) were mostly oriented facing the algal growing edge. Colonies were more frequent on central than on marginal zones of the thalli. The population of A. bougainvillei was mainly composed of very small colonies (<10 mm2). Larger colonies predominated and intraspecific competition was more intense near the basal portions of the thalli. Fecundity (number of ovicells) increased at a significantly higher rate in colonies with margins obstructed by conspecific neighbours than in free-growing colonies. Colonies were significantly larger on somatic than on reproductive algal tissues. As total and reproductive surfaces covered by colonies of A. bougainvillei were on average very low (4.43% and 0.53%, respectively), this epibiont is not supposed to produce a negative effect on H. laciniata. Fil: Liuzzi, Maria Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: López Gappa, Juan José. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina
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