UV radiation evokes negative phototaxis and covering behavior in the sea urchin Strongylocentrotus droebachiensis

Intertidal and subtidal Strongylocentrotus droebachiensis (Muller) often hide among rocks or cover themselves with debris, including macroalgae, mussel shells, and pebbles. Similar reactions in other species of sea urchins have been interpreted as a response to bright sunlight. This study examined the response of S. droebachiensis specifically to ultraviolet radiation (UVR). In laboratory studies using artificial irradiation, S. droebachiensis exposed to UVR (290 to 400 nm) and photosynthetically active radiation (PAR, 400 to 700 nm) sought shade and covered themselves significantly more frequently than those exposed only to PAR. In outdoor aquaria, individuals were exposed to ambient solar radiation that was filtered to create 4 treatments (dark, PAR, PAR + UVA, or PAR + WA + UVB) and observed for 6 h as total solar irradiance changed with time of day. Sea urchins covered themselves with significantly more material when exposed to PAR + UVA + UVB than in all other treatments, and in response to total irradiance. The amount of covering by sea urchins exposed to PAR + UVA (320 to 400 nm) varied over the course of the day, but were typically less than those exposed to UVB (295 to 320 nm). These sea urchins covered themselves more than those exposed to PAR alone or held in the dark. Sea urchins exposed to PAR alone did not differ in the amount of covering from those held in the dark, regardless of time of day. The amount of covering correlated significantly with WE and UVA irradiance independently, but not PAR irradiance. This study does not rule out that multiple cues may cause the covering response, but it demonstrates that S. droebachiensis seeks shelter and covers itself in response to UVR, primarily UVB wavelengths or a combination of WA and WE, presumably to avoid W-induced damage.