Seasonal variation in weight and biochemical composition of the tissues of Ruditapes decussatus in relation to the gametogenic cycle

Abstract

Seasonal variations in the body weight and biochemical composition of the bivalve Ruditapes decussatus were studied over a period of 15 months. Separate analyses were made of foot, mantle edge, siphons, gills, adductor muscle and gonad-visceral mass. Variations in weight, body growth, gonad growth and spawning depended on environmental conditions, especially food availability. The gametogenic cycle comprised two phases: a resting phase (November–December) and gametogenesis, including ripeness and spawning, during the rest of the year. Gametogenesis usually took place during the spring, and spawning in summer (June–August). The highest variation in biochemical composition was largely attributable to a change in the glycogen content (average 14.7%±5.1 S.D. of dry weight). Protein (48.1%±1.8) and lipid contents (5.6%±0.6) remained relatively constant throughout the year. Adductor muscle, foot and siphons contained mainly proteins (67.8%, 51.2% and 65.8% of the dry tissue weight, respectively) and gonad-visceral mass and gills contained the highest amount of lipids (≈7%). The gonad-visceral mass showed the largest variations in all components during the gametogenic cycle. Lipid and glycogen concentrations in the gonad-visceral mass were inversely related: maximum concentrations of glycogen occurred during the resting phase or initial gametogenesis and corresponded with minimum concentrations of lipids, and minimum concentrations of glycogen occurred at maturity when lipids reached maximum concentrations. The gonad-visceral mass contained lower amounts of proteins (<40%) than the other body parts. Variations in total fatty acid content in neutral and polar lipids of the gonad-visceral mass followed a seasonal cycle related to the gametogenic cycle, with increases during the period of maximum ripeness and decreases during spawnings. Maximal contents of polyunsaturated fatty acids, especially in the polar lipids corresponded with oocyte maturation and minimal contents with the post-spawning period. However, variations in the neutral lipids appeared to be related to the available food, in association with an increase in the chlorophyll a concentration.