Juvenile-adult associations in sea urchins Strongylocentrotus franciscanus and S. droebachiensis: Is nutrition involved?

Abstract

Juvenile-adult associations (i.e. juvenile sheltering) in sea urchins of the genus Strongylocentrotus provide an example of an important post-settlement behaviour in benthic invertebrates. It has been suggested that these associations provide a nutritional advantage to juveniles by facilitating access to kelp. We examined this hypothesis in a series of growth experiments involving 2 sea urchin species, Strongylocentrotus franciscanus and S. droebachiensis, which show high and low levels of juvenile sheltering, respectively. Juvenile sea urchins (7.73 ± 0.09 mm [mean ± SE] test diameter) of both species had lower growth rates in the presence versus absence of adults, regardless of food type, though the effect was more pronounced in S. franciscanus (0.2 ± 0.1 vs 1.2 ± 0.3 mm mo -1) than in S. droebachiensis (0.8 ± 0.5 vs 1.2 ± 0.7 mm mo-1). This relationship was not evident in the absence of food, which leads us to conclude that the reduction in growth is due to intercohort competition. Hence, other post-settlement factors (e.g. predation and hydrodynamic disturbance) are likely responsible for maintaining the juvenile-adult associations observed in S. franciscanus. Species-specific strategies in juvenile sheltering indicate that behavioural processes are important for the recruitment of benthic invertebrates with mobile juvenile stages.