Quantifying behavioural determinants of the coastal European sea-urchin Paracentrotus lividus

Abstract.

Field observations of covering (placement of objects on upper surface) and migration (movement between upper and lower rock surfaces) by the sea urchin, Paracentrotus lividus (Echinodermata: Echinoidea), were conducted at Lough Hyne, Ireland. Numerous hypotheses have been proposed to explain such behaviour in echinoids but have failed to consider the importance of multiple abiotic and biotic factors. Our study aimed to quantify the influence of interacting variables upon both behaviours in P. lividus using log-linear analysis. Our resultant model ranked the factors influential in covering behaviour as follows: (1) availability of covering item; (2) migratory behaviour; (3) size of P. lividus; (4) time of day; (5) time of year; and (6) predation intensity. The same variables were ranked differently in terms of their importance to the migratory behaviour of P. lividus. Moreover, the model showed, for the first time, an unequivocal relationship between covering and migration . Covering was mostly restricted to upper-rock-surface individuals, and therefore daylight periods, and was most intense in summer. This supports the theory that echinoids may cover to avoid over-exposure to light. We also conclude that rock upper surfaces are important as the principal source of food; lower surfaces may provide refuges from predators, with migration between these surfaces as a result. Certain individuals become too large to fit in boulder interstices and therefore incapable of migration to lower surfaces, whereas small individuals rarely migrate to upper surfaces. Models such as these provide a baseline for understanding the relative importance and interrelationship of factors of complex behaviour.