Sea Urchin (Strongylocentrotus droebachiensis) Aggregating Behavior Investigated by a Subtidal Multifactorial Experiment

We performed a multilevel factorial field experiment to identify the effects of five factors on sea urchins' (Strongylocentrotus droebachiensis) aggregating behavior. The factors were (1) source of urchins (kelpbed or barrens), (2) density of urchins (high or low), (3) location of treatment (kelpbed or barrens), (4) the presence and type of invertebrate predators (crabs or lobsters), and (5) season (summer or winter). These manipulative experiments were performed in flexible, resilient cages designed to withstand the severe wave surge in Nova Scotia's shallow subtidal environment. Interactions identified by ANOVA among the various factors showed that urchins aggregate more at high (20∙m−2) than at low (4∙m−2) density. The presence of lobsters in the kelpbed, and of crabs in the barrens, triggered the formation of even larger aggregations. These aggregations remain in the open even in the presence of predators. We argue that this behavioral mechanism (a defensive aggregation response to lobsters in the kel...