The main recruitment of Ophiothrixfragilis 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, Scop a h a 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.

We studied the defence patterns of 6 species of colonial ascidians from the Mediterranean Sea. We tested their palatability to common sympatric predators (fishes and crustaceans). The tests were performed separately for the main compartments (zooids and tunic) of the adults and for larvae. For compartments that proved unpalatable, tests were repeated with chemical extracts. We also tested toxicity with the Microtox method. Total energy content, amount of inorganic material and pH were analysed for all compartments. We looked for patterns of defence among species and compartments, and for relationships of adult/larval palatability, of toxicity/palatability, pH/palatability and food quality/palatability. Overall, we found a high variability among species, compartments and predators, but unpalatability was found in all species in at least 1 test. In general, tunic material was the least and zooid material the most palatable. Only 1 of the species studied had an acidic tunic. Toxicity was in general low and not related to palatability, while energy content was positively related to the latter. The larvae of species with low per zooid fecundity and large larvae (Cystodytes dellechiajei, Polysysncraton lacazei, Diplosoma spongiforme and Pseudodistoma crucigaster) were unpalatable to at least 2 of the predators, while larvae of the 2 species with higher fecundity and smaller larvae (Clavelina lepadiformis and Ecteinascidia herdmanni) were the most palatable. There was no relationship between adult and larval palatability. Tests with extracts substantiated a chemical basis for unpalatability in only a few cases, and there was a pattern of increased palatability of tissues with higher energy content and lower amount of structural material. We conclude that the defence strategies of colonial ascidians are highly variable among species (even of the same family), and that unpalatability may be relatively common and provided by either physical or chemical mechanisms, but that allocation to defence varies between compartments and between ontogenetic states (larvae or adult).KEY WORDS: Chemical defence · Toxicity · Palatability · pH · Caloric content · Larval defence · AscidiansResale or republication not permitted without written consent of the publisher

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