Defence mechanisms of adults and larvae of colonial ascidians: patterns of palatability and toxicity

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 unpalatdbility 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 lejoadiformis 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).