MEPS 200:201-212 (2000)  -  doi:10.3354/meps200201

ABSTRACT: The main recruitment of Ophiothrix fragilis occurs in the northwestern Mediterranean in late spring-early summer. Recruits are whitish and aggregate visibly on the surfaces of some sponge species, where they can reach densities above 50 ind. cm^-2. In order to investigate this association, we monitored larval abundances in the plankton, and recruit distribution and dynamics on the benthos. Settlement took place in several batches, with peak abundance of recruits on sponges in June. The mean diameter of juveniles was 0.2 mm in June and grew to 0.4 mm in September. In July, after the settlement ceased, juveniles were more abundant on the surfaces of the sponges Crambe crambe, Scopalina lophyropoda and Dysidea avara (14 to 20 ind. cm^-2) than on the adjacent algal turf (4 ind. cm^-2). No preference could be substantiated for 6 other encrusting sponge species. In contrast, juveniles were not significantly more abundant on these 3 sponges in June while settlement was under way. Recruitment on experimental sponges covered with plastic lids was reduced by two-thirds, indicating an initial colonization directly from the water column. However, clearing experiments demonstrated the ability of the young brittle stars to recolonise sponges, and brittle stars were more abundant at the periphery than at the centre of the sponges. These results indicate that brittle star recruits can select their substrata, possibly as a result of post-settlement lateral migration, resulting in an uneven distribution among sponge species. We set forth the hypothesis that this association between brittle stars and sponges is based on a trophic relationship, small brittle stars taking advantage of the inhalant currents of the sponge. In sponges with well-defined inhalant fields, almost all recruits concentrate on them, with arms flat against the sponge. When the brittle stars grow to a size above 1 mm (disk diameter) they abandon sponge surfaces and hide in small crevices between algae and invertebrates.

KEY WORDS: Ophiothrix · Ophiuroidea · Recruitment · Settlement · Association · Sponges