Aspects of reproduction and biochemical metabolism in Chamelea gallina from the North Adriatic Sea

The Venus clam Chamelea gallina is an infaunal filter-feeder, widely distributed throughout the Adriatic Sea, where it has great economic importance. In the present study, some aspects of the biological cycle of C. gallina have been investigated by evaluating the time-course of gonad maturation and variation in gross biochemical composition and energy values. Specimens with homogeneous size (length range 25-35 mm) were collected monthly from January to December 2001 for biochemical measurements (protein, lipid, total carbohydrate and glycogen contents and energy values) and from November 2001 to November 2002 for the study of the gametogenetic cycle, performed by microscopic observations of histological sections of gonadic tissue. Protein content varied narrowly and irregularly, whereas lipids remarkably increased from April to June 2001. Total carbohydrates and glycogen exhibited a similar erratic trend, and various relative maxima and minima were recorded. Total energy content, evaluated by converting carbohydrate, lipid and protein values into calorie contents, was especially stable all through the year, ranging from the maximum of 13.2 kJ/g in September to the minimum of 10.7 kJ/g in December. Histological observations indicated that gametogenesis started in September and kept on throughout winter until late spring. Ripe gonads were first observed in February, and the highest percentage of ripe individuals was detected in May, when also spawned clams were found. Spawning appeared to persist during warm months up to July, and the maximum number of spawned clams was recorded in June. The gonadic index was pretty suitable as a proxy for the reproductive status of the natural population: in particular, it showed an increasing trend from January, peaked in May, decreased sharply to the minimum value of August, (when most animals resulted sexually inactive from the histological observations) and started rising from September-October, highlighting the gradual onset of gametogenesis (as observed in the histological sections). An inverse relationship was established between the gonadic index and glycogen content, particularly between the lowest levels of glycogen and the time-span of sexual maturity and gamete emission. The good correlation between gametogenesis and depletion of energetic reserves have been already documented in various bivalve species (Mytilus edulis, M. galloprovincialis, Ruditapes philippinarum, R. decussatus). However, our results did not evidence the conversion from glycogen to lipids, already observed in the clam R. philippinarum as well as in other bivalve species, as only a slight increase of lipid content was detected during the period of gamete maturation. As a consequence, glycogen is indicated as the most important component in the metabolic energy storage for C. gallina.