MEPS 138:189-197 (1996)  -  doi:10.3354/meps138189

Many bryozoans, exemplified by species of Membranipora, are obligate epiphytes. We used laboratory and field experiments to ascertain whether colonies of Membranipora isabelleana benefit from contact with the fronds and exudates of Lessonia trabeculata, their natural substratum in central Chile. Type of substratum, presence or absence of microalgal food and of L. trabeculata fronds were the experimental factors, while colonial growth and percentage of non-degenerated zooids were the measured response variables. Colonies on L. trabeculata fronds did not grow faster than those attached to glass slides, either in the laboratory or in the field. Colonies grown for 1 mo on fronds, however, retained a significantly higher proportion of non-degenerated zooids than colonies grown on slides (88 and 73% respectively in the laboratory; 100 and 84% respectively in the field). When grown on slides, without microalgal food, colonies retained 79% of non-degenerated zooids in the presence of fronds but only 28% in their absence. These results suggest that colonies absorbed algal exudates when fronds were present, thus preventing the early degeneration of zooids. Even when fronds were separated from colonies by borosilicate membranes, allowing only the diffusion of molecules smaller than 1200 daltons, the above effect was still pronounced. In the field, colonies on fronds had a higher survival rate than those on slides. Association with an algal host therefore enhances at least 2 important life-history parameters in M. isabelleana: the percentage of non-degenerated zooids and colonial survivorship. Our results suggest that the trophic association with seaweeds is likely to be particularly important to epiphytic bryozoans under conditions of reduced particulate-food concentration.

Membranipora isabelleana . Lessonia trabeculata . Epibiosis . Seaweed exudates . Polypide degeneration . Bryozoa . Phaeophyta