Sponges have evolved a variety of chemical and structural defense mechanisms to avoid predation. While chemical defense is well established in sponges, studies on structural defense are rare and with ambiguous results. We used field and laboratory experiments to investigate predation patterns and the anti-predatory defense mechanisms of the sponge Melophlus sarasinorum, a common inhabitant of Indo-pacific coral reefs. Specifically, we aimed to investigate whether M. sarasinorum is chemically or structurally defended against predation and if the defenses are expressed differently in the ectosomal and choanosomal tissue of the sponge. Chemical defense was measured as feeding deterrence, structural defense as feeding deterrence and toughness. Our results demonstrated that chemical defense is evenly distributed throughout the sponge and works in conjunction with a structurally defended ectosome to further reduce predation levels. The choanosome of the sponge contained higher protein levels, but revealed no structural defense. We conclude that the equal distribution of chemical defenses throughout M. sarasinorum is in accordance with Optimal Defense Theory (ODT) in regards to fish predation, while structural defense supports ODT by being restricted to the surface layer which experiences the highest predation risks from mesograzers.

中文翻译：

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海绵 Melophlus sarasinorum 中化学和结构防御的分配

海绵已经进化出多种化学和结构防御机制来避免捕食。虽然海绵中的化学防御已经很好地建立，但对结构防御的研究很少，而且结果不明确。我们使用野外和实验室实验来研究海绵 Melophlus sarasinorum（印度太平洋珊瑚礁的常见居民）的捕食模式和反捕食防御机制。具体而言，我们旨在调查 M. sarasinorum 是否在化学或结构上防御捕食，以及防御在海绵的外泌体和 choanosomal 组织中的表达是否不同。化学防御被测量为摄食威慑，结构防御被测量为摄食威慑和韧性。我们的结果表明，化学防御均匀分布在整个海绵中，并与结构上防御的外泌体协同工作，以进一步降低捕食水平。海绵的 choanosome 含有更高的蛋白质水平，但没有显示出结构防御。我们得出的结论是，整个 M. sarasinorum 化学防御的平均分布符合关于鱼类捕食的最佳防御理论 (ODT)，而结构防御通过限制在经历来自中食草鱼的最高捕食风险的表层来支持 ODT。