HOW DO REPRODUCTIVE OUTPUT, LARVAL BEHAVIOR, AND RECRUITMENT CONTRIBUTE TO ADULT SPATIAL PATTERNS IN MEDITERRANEAN ENCRUSTING SPONGES

We studied the abundance and spatial pattern of 2 Mediterranean encrus ting sponges, Crambe crambe (highly toxic) and Scopalina lophyropoda ( non-toxic), at 4 spatial scales (0.5, 1, 2 and 4 m(2)). We also examin ed the reproductive output, larval behaviour and recruitment in these species, and assessed the relative importance of these parameters in e xplaining the abundance and spatial patterns of adults. We also determ ined, in field experiments, whether the presence of adults induces or inhibits recruitment in these 2 species. We found that C. crambe was m uch more abundant than S. lophyropoda at the site studied in both numb er of individuals per m(2) (67 +/- 2.7 vs 10.2 +/- 2.1, mean +/- SE) a nd coverage (47 +/-. 1.9 vs 11.1 +/- 1.4 %). At the smallest scale sam pled (0.5 m(2)), both species showed an aggregated pattern. Aggregatio n was also detected for S. lophyropoda, but not for C, crambe, at the scales of 1 and 2 m(2). The number of embryos incubated per cm(2) 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 o f C, crambe released into the water column was about 20 times higher t han that of S. lophyropoda. Larval behaviour was monitored in the labo ratory and in the field. Larvae of S. lophyropoda did not swim away fr om the release point. They maintained a vertical posture that minimise d horizontal dispersal, and soon began crawling. In contrast, the larv ae 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 a reas with no sponges), was monitored weekly for a month. Recruitment o f both species was higher in scraped quadrats surrounded by conspecifi cs. 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 recrui ts per unit surface area after 1 mo (all substrates pooled) was ca 3.5 limes higher for C. crambe than for S, lophyropoda. This difference w as 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 loca l population. The aggregated pattern of S0 lophyropoda at scales rangi ng from 0.5 to 2 m(2) and its discontinuous geographic distribution ma y be partially explained by strong phylopatry of its larvae due to the ir poor swimming ability and limited dispersal. The dominance of C, cr ambe in littoral assemblages, its random distribution at scales larger than 0.5 m(2), and its ubiquity along the Littoral are traits that ar e consistent with high reproductive output, the swimming behaviour of larvae which facilitates wide dispersal, and patterns of recruitment f ound in this study. Therefore, S. lophyropoda populations appear to be maintained by offspring supplied by autochthonous individuals while p opulations of C. crambe appear to be open, with a potentially signific ant flow of larvae between them.