Dispersion and Food Differences Between Two Populations of the Sea Urchin Strongylocentrotus franciscanus

The spatial pattern differs between two allopatric populations of the adult sea urchin Strongylocentrotus franciscanus. The amount of macroscopic algal food is correlated with this difference. In the population with abundant food, the urchins are highly aggregated; in the other food is limited and the spatial pattern of the urchins is random. Urchins aggregate in laboratory aquaria when well supplied with food, while those without food tend to be positioned more at random. This tendency toward random dispersal in food-limited areas is due to the increased and apparently random movement of the urchins in their search for food.