We studied the abundance and spatial pattern of 2 Mediterranean enci-ust~ng 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 behaviour 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 recruitmenl in these 2 species. We found that C. crambe was much more abundant than S. lophyropoda at the site stu.died in both number of individuals per m? (67 * 2.7 vs 10.2 + 2.1, mean i SE) and coverage (47 + 1 9 vs l l . l * 1.4 Y o ) At the smallest scale sampled (0.5 m'), both species showed an aggregated pattern Aggregation \vas 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 1 4 * 1.7 (mean i 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. Larva! beha\.',ol?r was meni!ored in the !aSeratory and in !he fie!-'. La:...ae of S. !ophyropoda did not swim away from the release point. They maintained a vertical posture that minimised horizontal dispersal, and soon began crawling. In contrast, the larvae of C. crambe swam actively and had a comparatively delayed crawling phase. Recmitment of the 2 speries in scraped quadrats surrounded by individuals of C, cran~be and S lophyropoda, and in controls (rocky areas with no sponges), was monitored weekly for a month. Recruitment of both specles 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 m', and its ublquity along the littoral are traits that are consistent with high reproductive output, the swimming behaviour 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...

Benthic assemblages of two Mediterranean submarine caves were compared. Species coverage and number of species were lower in internal (dark) communities than external. This feature was specially marked in the less illuminated cave. Ordination analyses performed on species coverage per community for each cave separately, distinguished several benthic communities from the outermost to the innermost zone of each cave. Cluster analyses on species coverage, taking into account all communities in both the caves, established similarities among communities: algal-dominated communities clustered according to the level of light received independently of the cave they inhabited, while animal-dominated communities were more similar within each cave than between the caves. Moreover, among the abiotic parameters measured irradiance was the only factor that clearly diminished from the entrance to the innermost zone in both the caves. In contrast, water movement and particulate organic matter varied differently in each cave. Results indicate that the different topography, depth and geographic location of the two caves determine water movement, light penetration and nutrient availability along the caves. These factors are responsible for determining species abundance and diversity, as well as species growth habit in each community.

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The toxicity of crude extracts of 32 seaweed species from the western Mediterranean was analysed by Microtox ® assay in spring and autumn of 1996 and 1997. The species analysed represented more than 76% of seaweed coverage in the 3 algal communities studied: photophilic and sciaphilic communities from the Cabrera Archipelago (Balearic Islands), and a hemisciaphilic community from the Medes Archipelago (northeastern Iberian Peninsula). Most species showed seasonal variation of toxicity, which was greater in species from Cabrera than in those from Medes. Both, intra and interspecies variation of toxicity were found. Moreover, comparison of mean toxicity of these communities showed that toxicity was higher in November than in June in all cases, and that the photophilic community had both the highest number and the most toxic species. To make an ecologically relevant interpretation of the toxicity detected by Microtox ® , we compared the toxicity of extracts analysed by the Microtox ® test and those analysed by the commonly used sea urchin embryo assay. In addition to seaweeds, some species of invertebrates (sponges and ascidians) were compared to ascertain whether the relationship between the 2 tests was applicable to species belonging to different phyla. These comparisons allowed us to establish that 0.5 gamma units in Microtox ® assay is the threshold value between toxic and non-toxic species. Following a light gradient from the photophilic to the sciaphilic communities, the seaweed species that were occasionally toxic increased while the always-toxic seaweeds decreased. Rhodophyta and Phaeophyta had a higher percentage of toxic species than Chlorophyta. Non-encrusting seaweed forms were more toxic than the encrusting ones, and in contrast to most other seaweeds, the non-encrusting calcareous species that increased coverage from June to November simultaneously displayed a marked decrease in toxicity. We conclude that the temporal variation of toxicity observed in the seaweeds studied may be partially explained by intrinsic factors of the species (growth rates and growth shapes).