Factors affecting spat ial patterns in encrusting sponges

We studied the abundance and spatial pattern of 2 Mediterranean encrusting sponges, Crambe crambe (highly toxic) and Scopalina lophyropoda (non-toxic), at 4 spatial scales (0.5, 1, 2 and 4 m2). We also examined the reproductive output, larval behavior and recruitment in these species, and assessed the relative importance of these parameters in explaining the abundance and spatial patterns of adults. We also determined, in field experiments, whether the presence of adults induces or inhibits recruitment in these 2 species. We found that C. crambe was much more abundant than S. lophyropoda at the site studied in both number of individuals per m2 (67+- 2.7 vs 10.2 +- 2.1, mean +- SE) and coverage (47 +- 1 9 vs l l .l +-1.4 %). At the smallest scale sampled (0.5 m'), both species showed an aggregated pattern Aggregation was also detected for S. lophyropoda, but not for C crambe, at the scales of 1 and 2 m2 The number of embryos incubated per cm2 by C. crambe and S. lophyropoda was 76.2 +- 12.5 and 14 +- 1.7 (mean +- SE), respectively. We estimated that the potential number of larvae of C. crambe released into the water column was about 20 times higher than that of S. lophyropoda. Larval behavior was monitored in the laboratory and in the field. Larvae of S. lophyropoda did not swim away from the release point. They maintained a vertical posture that minimized horizontal dispersal, and soon began crawling. In contrast, the larvae of C. crambe swam actively and had a comparatively delayed crawling phase. Recruitment of the 2 species in scraped quadrats surrounded by individuals of C crambe and S lophyropoda, and in controls (rocky areas with no sponges), was monitored weekly for a month. Recruitment of both species was higher in scraped quadrats surrounded by conspecifics. This effect was notably more marked for S. lophyropoda than for C. crambe recruits. The toxicity of C. crambe did not inhibit settlement of S. lophyropoda with respect to controls. The mean number of recruits per unit surface area after 1 mo (all substrates pooled) was ca 3.5 times higher for C. crambe than for S. lophyropoda. This difference was smaller than expected given that larval production of C. crambe was ca 20 times higher. This indicates that a significant proportion of C. crambe's offspring did not contribute to the maintenance of the local population. The aggregated pattern of S. lophyropoda at scales ranging from 0.5 to 2 m' and its discontinuous geographic distribution may be partially explained by strong phylopatry of its larvae due to their poor swimming ability and limited dispersal. The dominance of C crambe in littoral assemblages, its random distribution at scales larger than 0.5 m2, and its ubiquity along the littoral are traits that are consistent with high reproductive output, the swimming behavior of larvae which facilitates wide dispersal, and patterns of recruitment found in this study. Therefore. S. lophyropoda populations appear to be maintained by offspring supplied by autochthonous individuals while populations of C. crambe appear to be open, with a potentially significant flow of larvae between them.This research was funded by projects DGCYT PB94-0015 and CICYT MAR95-1764 of the Spanish Government and 1997SGR 00084 of the "Generalitat de Catalunya"Peer reviewe