A water-borne pursuit-deterrent signal deployed by a sea urchin

Selection by consumers has led to the evolution of a vast array of defenses in animals and plants. These defenses include physical structures, behaviors, and chemical signals that mediate interactions with predators. Some of the strangest defensive structures in nature are the globiferous pedicellariae of the echinoderms. These are small venomous appendages with jaws and teeth that cover the test of many sea urchins and sea stars. In this study, we report a unique use of these defensive structures by the collector sea urchin Tripneustes gratilla. In both the laboratory and the field, globiferous pedicellariae were unpalatable to fish consumers. When subject to simulated predator attack, sea urchins released a cloud of pedicellaria heads into the water column. Flume experiments established the presence of a waterborne cue associated with this release of pedicellariae that is deterrent to predatory fish. These novel results add to our understanding of how the ecosystem-shaping sea urchin T. gratilla is able to reach high densities in many reef habitats, with subsequent impacts on algal cover.

消费者的取食选择驱动了动植物演化出种类繁多的防御策略。此类防御机制涵盖物理结构、行为模式以及介导与捕食者互作的化学信号。自然界中最为奇特的防御结构之一，当属棘皮动物（echinoderms）的球形叉棘（globiferous pedicellariae）：这类结构是覆盖于多数海胆与海星骨壳之上的小型有毒附肢，带有颚与齿结构。本研究报道了采集海胆（collector sea urchin，Tripneustes gratilla）对这类防御结构的一种独特利用方式。无论在实验室还是野外环境中，球形叉棘对鱼类捕食者均具有拒食性。当遭遇模拟捕食者攻击时，海胆会向水层中释放一团叉棘头部结构。水槽实验证实，伴随叉棘释放过程会产生一种水源性信号，该信号可驱避掠食性鱼类。这些全新的研究结果增进了我们对这类塑造生态系统的海胆——T. gratilla为何能在诸多礁生境中达到高密度种群的理解，及其后续对藻类覆盖度造成的相应影响。