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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. |
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