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The larvae of many sessile marine invertebrates are able to locate and settle selectively in
physical microrefuges where the survival of the post-larval stage may be enhanced. However, it has
been suggested that clonal encrusting organisms with unlimited attachment to the substratum do not
rely heavily on microrefuges for post-larval survival, since they quickly outgrow refuges and then suffer
partial rather than total mortality. Consequently, the larva of such organisms is predicted to show little
selectivity at the time of settlement. This prediction, verified for some bryozoans and ascidians, has
also been extrapolated to sponges despite the fact that there IS virtually no field information on patterns
of settlement and post-settlement mortality in this group. In the present study, we investigated patterns
of settlement and juvenile mortality in 2 common Mediterranean encrusting sponges, Crambe crambe
and Scopalina lophyropoda. We placed a set of grooved plates (providing microrefuges for larval settlement)
and a set of smooth plates (without refuges) in the natural community, mapped larval settlement
on plates at 3 d intervals, and recorded the fate of each settled larva for 10 wk. The total number of larvae
of C. crambe that settled on the plates was significantly higher than that of S lophyropoda, and
both species settled preferentially in the microrefuges of the grooved plates. On each of the grooved
plates, larvae settled preferentially on the shaded sides of outer ridges, avoiding equivalent ridge sides
that were highly exposed to light. This pattern suggests that microrefuges are located by larvae on the
basis of photic cues. The analysis of survival time revealed that survival of post-larval stages protected
in grooves was higher in both species compared to unprotected post-larvae. The survival curve of the
juveniles did not clearly follow an exponential model. Survival decreased more or less rapidly for the
first 2 mo, but the chance of mortality was not markedly higher 1 wk after settlement than during subsequent
weeks. Because we could not exert control over the various coincidental mortality factors that
affected the sponges settled on the experimental plates, we tested the following hypotheses in the laboratory:
(1) that the abundant echinoderms inhabiting these assemblages cause significant mortality by
both unselective ingestion and abrasion, and (2) that microrefuges substantially reduce sponge vulnerability
to this mortality factor. In this experiment, we investigated differences in mortality among small
sponges as a function of microhabitat exploitation (sponges protected in grooves versus unprotected
sponges on smooth surfaces) and the presence/absence of potential predators (the sea urchin Paracentrotus
lividus, the starfish Echinaster sepositus). It was found that E. sepositus caused negligible mortality,
irrespective of sponge microhabitat. In contrast, P. lividus caused significant mortality, indicating
that bulldozing by urchins may have substantial negative effects on sponge recruitment in temperate
hard-bottom communities where urchins are ubiquitous. Our results also indicate that exploitation of
physical microrefuges, such as grooves and crevices, reduces, but does not prevent, sponge mortality
due to bulldozing by urchins. The enhancement of juvenile survival in microrefuges has probably operated
as a selective force, favoring larval mechanisms that facilitate selective settlement in refuges. |
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