Microrefuge exploitation by subtidal encrusting sponges: patterns of settlement and post-settlement survival

The larvae of many sessile marine invertebrates are able to locate and sett le selectively in physical microrefuges where the survival of the post-larv al stage may be enhanced. However, it has been suggested that clonal encrus ting organisms with unlimited attachment to the substratum do not rely heav ily on microrefuges for post-larval survival, since they quickly outgrow re fuges and then suffer partial rather than total mortality. Consequently, th e larva of such organisms is predicted to show little selectivity at the ti me of settlement. This prediction, verified for some bryozoans and ascidian s, has also been extrapolated to sponges despite the fact that there is vir tually no field information on patterns of settlement and post-settlement m ortality in this group. In the present study, we investigated patterns of s ettlement and juvenile mortality in 2 common Mediterranean encrusting spong es, Crambe crambe and Scopalina lophyropoda. We placed a set of grooved pla tes (providing microrefuges for larval settle ment) and a set of smooth pla tes (without refuges) in the natural community, mapped larval settlement on plates at 3 d intervals, and recorded the fate of each settled larva for 1 0 wk. The total number of larvae of C. crambe that settled on the plates wa s significantly higher than that of S. lophyropoda, and both species settle d 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 pos t-larval stages protected in grooves was higher in both species compared to unprotected post-larvae. The survival curve of the juveniles did not clear ly 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 a fter settlement than during subsequent weeks. Because we could not exert co ntrol over the various coincidental mortality factors that affected the spo nges settled on the experimental plates, we tested the following hypotheses in the laboratory: (1) that the abundant echinoderms inhabiting these asse mblages cause significant mortality by both unselective ingestion and abras ion, 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 (s ponges protected in grooves versus unprotected sponges on smooth surfaces) and the presence/absence of potential predators (the sea urchin Paracentrot us lividus, the starfish Echinaster sepositus). It was found that E. seposi tus caused negligible mortality, irrespective of sponge microhabitat. In co ntrast, P. licidus 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 bulldo zing by urchins. The enhancement of juvenile survival in microrefuges has p robably operated as a selective force, favoring larval mechanisms that faci litate selective settlement in refuges.