Experimental analysis of factors influencing the aggregating behaviour of the green sea urchin Strongylocentrotus droebachiensis (Müller)

Abstract

The aggregating behaviour of the green sea urchin Strongylocentrotus droebachiensis was investigated in a multifactorial laboratory experiment. The null hypothesis of a random pattern of aggregation was rejected prior to analyses of treatment effects and patterns of aggregation were quantified using three indices of aggregation: the mean aggregation size, the index of mean crowding and the index of patchiness. Sea urchin density and body size were the two most important determinants of spatial pattern. Sea urchins, irrespective of size, had a regular pattern of aggregation at low density, a nearly random pattern at medium density and a clumped pattern at high density. Large urchins were consistently more aggregated than small urchins, even though urchin size was involved in several interactions with other experimental factors. Averaged over all densities, large urchins had a clumped, while small urchins had a regular pattern of aggregation. The presence of decapod predators had no detectable effect on the aggregation behaviour of S. droebachiensis. Significant predator effects were entirely due to the presence of wolffish, which reduced the size of sea urchin aggregations both by eliciting behavioural responses, and by decreasing the population density of the urchins. Additional spatial heterogeneity, in the form of claypipe refuges, also made the urchins less aggregated. Prefeeding, food and season had minor effects on the aggregating behaviour of S. droebachiensis.