SOCIAL-BEHAVIOR OF JUVENILE RED-SEA URCHINS, STRONGYLOCENTROTUS-FRANCISCANUS (AGASSIZ)

This study examined social behaviour of juvenile red sea urchins, Stronglocentrotus franciscanus (Agassiz). At four coastal locations in British Columbia, Canada, distributions of juvenile sea urchins were examined in relation to adults. One-third of all juveniles were found underneath an adult "spine canopy", another third outside the spine canopy but close to an adult, and the remainder were found apart from adults. Juveniles associated with adults were contagiously distributed with each other. In the laboratory, juveniles chose adults over other target locations. They selected adults in particular locations over others, and in one case showed selection among adults; but they did not select between fed and non-fed adults, nor between adults that had been "protective" and "non-protective" of juveniles in the field. In time-lapse film studies, starved juveniles showed little change in social behaviour when food was introduced, but showed a dramatic change in behavior when predators were active. We suggest that juvenile red sea urchins are found under the spine canopies of adults as a result of juvenile behavioir; and that this interaction functions to protect juveniles from predation. The strong gregarious nature of juveniles among themselves, may have evolved in a different selective situation as a defense against predation and to ensure reproductive success.