Relationships between reproductive cycle and energy storage in the clam Chamelea gallina from the northern Adriatic Sea.

Chamelea gallina is an infaunal bivalve of considerable commercial importance along the western coast of the Adriatic Sea, where it is subject to intense fishing. In recent years over-fishing, recruitment failure, mortality outbreaks and other concurrent factors have severely depleted clams stocks. In this context, a thorough knowledge of clam biological cycle is a fundamental requirement for a successful fishery management. During a two-year long survey, time-course of gonad maturation, gross biochemical composition, energy content and condition index were monthly assessed in commercial size clams (> 25 mm in length) from two sites offshore the Veneto coast. The gametogenic cycle and the pattern of variation in seawater temperature were strictly related: in summer, spawning always overlapped with the maximum temperature values (June-August), whereas temperature decrease at the end of summer appeared to act as a signal to restart the gametogenetic activity. Biochemical components varied scantily and irregularly, except for lipids that remarkably increased during the ripeness phase (April-May) and fell down with the spawning. The clear relationship between gonadal index and energetic content was highlighted, they showing concurrent peak values in April-May. The highest values of condition index were also found in May. Unlike other bivalve species, C. gallina is not able to recover and accumulate energy reserves before the beginning of gametogenesis. Probably, this is due to the very short sexual resting phase (no longer than one month). As a consequence, clams may be particularly susceptible to further stress conditions, when highest temperature or hypoxia can occur at the end of summer or when water temperature and food availability decrease and gametogenesis begins in autumn-winter. The features of gametogenic cycle and energy storage associated with increasing temperatures, as predicted in global change scenarios, could display detrimental effects on survival and reproductive success of clams, promoting further depletion of natural stocks.