Effects of temperature and body size on covering in green sea urchin, Strongylocentrotus droebachiensis

Green sea urchin, Strongylocentrotus droebachiensis, is common in shallow subtidal rocky reef habitats in the northwestern North Atlantic. It is an important ecosystem engineer, capable of overgrazing on kelp beds to form urchin barrens. Green sea urchin often exhibits a ‘covering’ or ‘hatting’ response, whereby it adorns its test with various materials available in the habitat. Covering is presumably a response to an environmental cue, however, definitive reasons for covering have not yet been described in the literature. We carried out a 2-week laboratory experiment to test the predictions that green sea urchin covers (1) less in cold (2°C) and warm (14°C) seawater, as it is outside of thermal optima; (2) more with live rhodolith fragments than with blue mussel shell fragments or denatured rhodolith fragments; and (3) more when small (1 to 2 cm in test diameter, t.d.) than large (4 to 5 cm t.d.) in still water conditions. Sea urchins were acclimated and exposed to one of three temperatures (2, 8, or 14°C) in containers within water baths. Each container, containing one sea urchin, was given a covering material type (live rhodoliths, denatured rhodoliths, or blue mussel shells), whereby the resultant degree of covering exhibited by sea urchins was assessed. Model outputs supported the predictions that temperature and sea urchin size affect covering in green sea urchin, while rejecting the prediction that covering material type affects the degree of covering in green sea urchin. Our results help establish a baseline for temperature-induced covering thresholds in green sea urchin under still water conditions, values which are not currently covered in the literature