Examination of the covering reaction (masking or heaping with shells) of Lytechinus anamesus in response to ultraviolet irradiation, sunlight, and surge suggests that most previous hypotheses explaining this reaction are inadequate or incomplete. Shaded L. anamesus initially reacts to short wavelength ultraviolet light (254 nm) with a fairly strong masking response, which is followed by death after an exposure of several days. Long wave- length ultraviolet light (360 nm) elicits moderate covering, while the nonirradiated controls rarely covered. These responses to ultraviolet light are probably artifacts, as L. anamesus and other sea urchins are seldom exposed to that factor in nature. There is a strong and immediate covering response to direct sunlight. Application of surge induces a cover response nearly equal to that of sunlight, regardless of light conditions. The intensity of covering during periods of surge appears to increase during the exposure period. Cessation of surge results in a rapid reduction in covering and a marked increase in movement. We suggest that covering is important in increasing the sta- bility of this echinoid during periods of increased surge by effectively increasing its specific density, reducing its surface area, and changing the configuration it presents to surge forces. Sunlight intensity may be used as a cue that indicates directly the risk of displacement by surge action.
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