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

JĂșlio Henrique Garcia da Silva*, Laura Carolina Leal, Gustavo Muniz Dias

*Corresponding author:

ABSTRACT: Ascidians are marine sessile animals that evolved many strategies to reduce predation. Previous experiments have shown unpalatability through chemical defenses using manipulative experiments with pellets or tissues of ascidians. However, predation exclusion experiments on the community scale showed 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 the study site, the experiment design, ascidian sociability, and the identity of predators to the efficacy of the defenses of ascidians. Our study is the first quantitative review of the predation effect on ascidians, emphasizing the ecological interactions beyond the specific defense effects of ascidians. We found that multiple factors can interfere with the effectiveness of the defenses of ascidians. Palatability studies presented evidence for the defenses of ascidians. However, they depend on the identity of predators. We did not find evidence of the defenses of ascidians in community studies. There is a lack of field experiments, mainly on solitary ascidians, that evaluate the predation of ascidians in communities. Research on the defenses of ascidians is also biased toward the temperate region from the northern hemisphere. The common knowledge that ascidians possess active defenses may be overestimated, and defenses are probably restricted to some species. This misconception is caused mainly by methodological and geographical bias that tests only species with previous evidence of defenses. Therefore, we need more worldwide studies focusing on the ecological relationships between ascidians and predators, mainly in natural communities under field conditions.