Abstract 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.
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