The use of tetracycline marking has enabled the daily production rate of calcareous spicules in the Mediterranean calcisponge Clathrina cerebrum to be measured. Twenty-four hours after starting the experiment newly formed spicules represent about 10% of the total spicule number. No differences in this rate of spicule secretion were detected between the samples tested under light and dark conditions. The growth rate of the tetracycline-tagged spicules in Clathrina cerebrum is similar to that found for other calcispongiae using other methods.Spicules constitute the skeleton of sponges and are the most important taxonomic character in this group. In calcareous sponges many details of the process of spicule secretion are well known (see Simpson, 1984 for review), particularly their chemical composition and crystallographic structures (Jones, 1955, 1967; Jones & Jenkins, 1970; Ledger, 1976). Ultrastructural studies were carried out on the cellular processes involved in spiculogenesis (Jones, 1970; Ledger & Jones, 1977), but little is known of the rate of spicule secretion. This phenomenon has been studied here by tetracycline marking of newly calcified spicules, triactines. Tetracycline forms complexes with calcium or other metal ions which are easily incorporated into biominerals; consequently it can be used to label the growth of biocalcified structures (Ibsen & Urist, 1962). This method is commonly used in medical research (Urist & Ibsen, 1963; Sandhu & Tonna, 1986; Milch et al., 1957; Van Linthoudt et al., 1991), and has also been employed in studies on biocalcification in marine organisms, such as echinoderms (Gage, 1992) and molluscs (Nakahara, 1961).
The Chemical Composition of the Cell Wall in some Gram-positive Bacteria and its Possible Value as a Taxonomic Character