Palatability of ascidians: a meta-analysis of the predation effect on ascidians

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Ascidians are marine sessile animals that have evolved many strategies to reduce predation. Previous manipulative experiments with ascidian tissues or pellets have shown that they have chemical defenses that render them unpalatable. However, predation-exclusion experiments on a community scale have shown that ascidians are almost entirely eaten when exposed to predators. Based on these contrasting results, we performed a meta-analysis to assess the importance of study site, experiment design, ascidian sociability, and predator identity to the efficacy of the ascidian defense. Our study is the first quantitative review of predation on ascidians, and it emphasizes the importance of ecological interactions beyond the specific defense tactics of the organisms. We found that multiple factors can interfere with the effectiveness of the ascidian defense. Palatability studies have shown evidence for ascidian defense mechanisms; however, they depend on the identity of the predators (e.g. fish, crab, amphipod). We did not find evidence of ascidian defense in community studies. There is a lack of field experiments, mainly on solitary ascidians, that evaluate their predation risk in communities. Research on ascidian defense mechanisms is also geographically biased toward the temperate region in the Northern Hemisphere. The commonly held belief that ascidians possess active defenses may be overestimated, and the defenses that do exist are probably restricted to only a few species. This misconception has been caused mainly by methodological and geographical bias that has resulted in tests being performed only on species with previous evidence of defenses. Therefore, we need more worldwide studies focusing on the ecological relationships between ascidians and predators, specifically in natural communities under field conditions.