Factors affecting the spatial distributions of thinly encrusting sponges from temperate waters

Interactions along borders between pairs of species of abundant thinly encrusting sponges from a hard substratum subtidal community were investigated. Over a nine month study period the majority of borders between species did not shift in position. Several different forms of overgrowth were found but none lead to the death of the underlying species or subsequent attainment by the overgrower of the space occupied by the underlying sponge. Overgrowth in these types of sponges may be a defensive strategy aimed at maintaining presently occupied space. Only one species, Aplysilla rosea, gained space along borders and this occurred without any overgrowth of the space-losing species. This Dictyoceratid sponge was found to be very toxic toward a variety of indicator organisms and it is possible that these toxins form the basis of the mechanism behind its success in maintaining and obtaining space. In this community free space (unoccupied primary space) was available over 16% of the substratum due primarily to the grazing activities of an abundant echinoid Evechinus chloroticus. The encrusting sponges were damaged experimentally to investigate the influence of their different rates of regeneration on the outcome of competitive interactions. Again, Aplysilla rosea gained the most space but some species which did not gain space along borders gained space from neighbouring sponges in this experiment. Aplysilla rosea might be expected to be the dominant species in this community but this sponge was one of the least abundant species in this community. Although competitive interactions along sponge borders and clearings are useful to predict some aspects of sponge spatial patterns, other life history processes such as recruitment may be more important.