Gametogenesis and energy storage in a population of the grooved carpet-shell clam, Tapes decussatus (Linné, 1787), in northwest Spain

Abstract

The reproductive cycle of a natural population of grooved carpet-shell clam, Tapes decussatus, on the coast of northwest Spain was monitored over a year-long period. Gametogenic activity was characterized on the basis of stereological analysis of the gonads. In both sexes, ripe gametes were continually present and continually released between April and August; spawning efficiency over this period was 60% in females and 83% in males. Again in both sexes, major spawning occurred in August/September (spawning efficiency 94% in females, 86% in males). In parallel with monitoring of gametogenic activity, energy storage was also monitored, by staining the visceral–gonadal mass for glycogen α (rapid-access glycogen), glycogen β (protein-bound glycogen), and acid and neutral lipids. The principal storage areas are muscle tissue, vesicular cells and the digestive gland. Muscle tissue stores and utilizes protein-bound glycogen during the resting period and at the start of gametogenesis, while rapid-access glycogen appears in this tissue only during the gamete ripening period. In vesicular cells, protein-bound glycogen bodies (diameter 1–3 μm, up to 6 per cell) accumulate throughout the resting period; as gametogenesis progresses, these are transformed to triacylglycerides (bodies of about 5–9 μm in diameter, up to 5 per cell), which are subsequently transferred to oocytes (bodies of up to 14 μm in diameter). In the digestive gland, rapid-access glycogen is present throughout the year, while triacylglyceride bodies (diameter 2–15 μm) accumulate over the period of gamete ripening. Acidic lipid inclusions (about 1 μm in diameter, probably corresponding to membrane phospholipids) are observed along the borders of muscle fibres, gonadal follicles, vesicular cells and their nuclei, haemocytes and cells of the gastric diverticles. Haemocytes transport both glycogen fractions, and are observed: (a) between muscle fibres, (b) adhering to the external wall of gonadal follicles and within follicles, (c) adhering to the external wall of gastric diverticles and within diverticles, and (d) adhering to ripening oocytes. Stereological analysis likewise allowed evaluation of seasonal variation in the volume fraction (VF) occupied by vesicular cells. The period of peak vesicular-cell VF (October/November; VF 32% in females, 36% in males, 30% in indeterminate individuals) coincided with that of peak intracellular glycogen levels. Minimum vesicular-cell VF were observed during the gamete ripening period (VF 2–8% in females, 0% in males).