Silica content and spicule size variations in Pellina semitubulosa (Porifera : Demospongiae)

The variations in both silica content and spicular size were studied in two populations of the demosponge Pellina semitubulosa (Lieberkuhn). Samples w ere collected over a period of 1 year (June 1994 to May 1995) in two Medite rranean coastal basins: Porto Cesareo (southwestern Apulia) and Marsala (no rthwestern Sicily). The values of spicule size (length and width) and spong e silica content were significantly higher in the population of Porto Cesar eo, where the highest water silica concentration was recorded. In both Port o Cesareo and Marsala the sponge silica content showed a seasonal trend, po sitively correlated with water temperature values. In both populations, the smallest spicules were found in specimens collected from summer to late au tumn, after sexual reproduction. Secretion of new spicules may be connected with the process of remodelling occurring in sponges after gamete and larv al release.