Internal Anatomy and Biochemical Content of the Cortez Geoduck Panopea globosa during Larval Development

Histological, histochemical, and biochemical changes were assessed in the Cortez geoduck Panopea globosa, during larval development to identify the main energy storage sites and biochemical patterns. Histological sections were stained with polychromic dye, whereas histochemical sections were stained for triglycerides and phospholipids (Sudan black) and for carbohydrates (Alcian blue). Spectrophotometric quantitation of proteins, lipids, and carbohydrates was carried out in 5–35 mg dry weight (DW) samples. Larvae of Panopea globosa showed the typical linear shell growth rate (21 µm d–1) and allometric, power-type relationships between shell length and DW (exponent b = 2.73) or ash-free DW (b = 2.78). Early straight-hinged larvae exhibited a rudimentary digestive system. The crystalline style, intestine, and digestive gland were conspicuous when larvae reached the early umbonate stage, whereas the gills, foot, byssal gland, and adductor muscles were first observed in late umboned larvae. Triglycerides and carbohydrates were mainly stored as droplets in the digestive gland, and their size increased with increasing larval age. Acid mucopolysaccharides were also secreted by the byssal gland. Carbohydrate content was substantially higher than triglycerides in pediveliger larvae. These finding suggests that P. globosa larvae mainly rely on a carbohydrate-based metabolism, both as a biochemical component of the larval–postlarval byssus and as an endogenous energy source during metamorphosis.