Drivers and functional consequences of covering in a pervasive marine grazer (green sea urchin, Strongylocentrotus droebachiensis)

BACKGROUND: The circumpolar green sea urchin, Strongylocentrotus droebachiensis, exhibits a fascinating behaviour, termed "covering", which consists of coating the body with materials collected in the environment. The lack of a consensus about the drivers of covering in green sea urchin and their relative importance prevents accurate predictions about the frequency and intensity of the behaviour and its functional consequences in a globally changing ocean climate. We paired an experiment in an oscillatory wave tank with green sea urchins collected from an extensive sea urchin barren in southeastern Newfoundland (Canada), and a 3-mo survey of this barrens, to examine individual and interactive effects of wave action, light, sea urchin body size, and types of covering materials on sea urchin's displacement and covering. RESULTS: Our findings establish that covering in S. droebachiensis is: (a) predominantly controlled by hydrodynamic forces, with the existence of water current-induced covering tipping points; (b) ontogenetically determined, with a continuous inclination to cover in small individuals and a seasonal component to covering in larger individuals; (c) opportunistic, with multiple types of covering materials employed based on availability; and (d) functionally costly, as it significantly reduces mobility. We largely rule out the paradigm that light induces covering or at least clearly marginalize it as a trigger or effector. Our results and those of other studies of the behavioural repertoire of S. droebachiensis, are consistent with the notion that covering serves a complementary function to mitigation of hydrodynamic forces. CONCLUSIONS: We propose that covering in S. droebachiensis primarily serves a mechanical protection function, whereby the species shields its body surface to protect its external sensory, defensive, and locomotory organs against physical contact with moving debris.