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The m e r a l o g i c a l features of the substrate were generally cons~dered a rmnor factor m structunng manne benthic communities The alm of t h~s work 1s to venfy whether the presence of quartz nunerals in rock may exphcate dlfferences, usually explamed m terms of substrate roughness or other factors In epibenthic cornrnun~hes Laboratory tests on the hydroid Eudendnum glomeratum showed that ~t s planulae settle preferentially on carbonat~c, rather than quartzltic substrates To test the Influence of quartz on established communities, we analysed the specles composition and quantitat~ve structure of subhttoral sesslle assemblages on Wferent rocks In several locahhes of the L~gurian and Tyrrhenian seas The observed dlfferences appeared to be related to the presence of quartz in the substrate rock The ~nteractions between organisms and minerals (bio-mmeralogy) mlght play a slgnlficant role on benthic communities affechng not only the inihal colonlsahon but also later assemblages This potential role has been largely neglected to date and further studies are needed to prove ~t s Impor tance KEY WORDS: Substrate colonisation . Mineral composition . Marine benthos distnbution . Hard substrates . Bio-mineralogy INTRODUCTIONThe spatial distribution and structure of marine benthic communities are due to numerous abiotic and biotic factors which, in turn, are influenced by the presence of the organisms, in a mutual exchange of inputs. Among the abiotic factors, the mineralogical features of the substrate were generally considered of scarce importance, but recent studies by Cerrano et al. (1998) have shown that the presence of quartz in the sand may affect the initial steps of infauna colonisation. Cerrano et al. (1998) introduced the term bio-mineralogy to explicate the interrelationships between biological systems at different hierarchies (cell, organism, species, community) and minerals.Bio-mineralogy could influence hard-bottom assemblages and explain some 'anomalies' in the structure of communities growing on rocks of different nature. A species assemblage, which may be slightly more attracted to a particular substrate, could affect succession by its subsequent interaction with later assemblages. A similar effect was evidenced in the colonisation of artificial substrates, with respect to both species composition and abundance (Anderson & Underwood 1994, Holm et al. 1997). Less information is available for natural substrates (McGuiness 1989), but it is common knowledge that the softness and asperity of a rock can favour or hamper biotic colonisation through selective larval settling, retention of water (in the littoral) and organic matter, and provision of refuges from predation or grazing (Den Hartog 1972, Levinton 1982, Walters & Wethey 1996.More is known about the influence of substrate mineralogy on bioboring, which is prevented by high percentages of quartzitic or pelitic components in the rock. Sublittoral endolitic communities are charac-

The sexual and asexual phases of reproductive cycles of two sponges, Tethya citrina and T. auranrium, living sympatrically in a Mediterranean coastal lagoon (Stagnone di Marsala, NW Sicily), were studied from samples collected over an 18-mo period. Both species are oviparous and gonochoric. They have a summer, partially overlapping, period of oocyte production, although T. citrina appear to mature earlier.No males were found, possibly due to the very short period of spermatogenesis. Both species produce asexual buds during the autumn/winter months. However, they seem to follow different reproductive strategies, with T. citrina showing a significantly lower production of buds than T. aurantium; by contrast, egg production is significantly lower in the latter species. The difference in the reproductive resource allocation is consistent with data reported in the literature on the anatomy features, genetic population structure and ecological distribution.

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