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

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Hannah Sheppard-Brennand, Alistair G. B. Poore, and Symon A. Dworjanyn

Unusual defense deployed by a sea urchin

A juvenile collector sea urchin (Tripneustes gratilla) reaches out its tube feet to explore. Spines and pedicellariae are abundant on the test.
(Credit: Hannah Sheppard-Brennand)


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 water-borne 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. Read the Article