Effect of Stocking Density and Biofilm-Based Microalgae on Larvae and Post-Larvae Growth and Settlement Patterns of the Clam <i>Ruditapes decussatus</i> (Linnaeus, 1758) in Captivity.

Abstract

Larval rearing is a critical phase in shellfish-hatchery production. Thus, optimizing larval survival, growth, and settlement is essential for reliable seed output. This study investigated the effect of rearing density on larvae and settlement performances of the carpet shell clam <i>Ruditapes decussatus</i> in captivity. Furthermore, the influence of natural biofilm-based microalgae on larvae settlement and post-larvae performance was also investigated herein. Three larval rearing densities (10, 20, and 40 larvae mL^-1) were studied during larvae development, and four settlement densities (35, 70, 100, and 140 larvae cm^-2) were tested during larvae settlement and metamorphosis. The effect of biofilm-based microalgae on larvae settlement and post-larvae pattern was tested at a standard settlement density (70 larvae cm^-2). High larval density significantly reduced survival (18% at 40 larvae mL^-1) compared with lower densities (26% and 23% at 20 and 10 larvae mL^-1, respectively). Significantly higher settlement rate (36% and 33%) and spat yield were observed at 35 and 70 larvae cm^-2, respectively, compared to high densities (100 and 140 larvae cm^-2). Remarkably, post-larvae reared under the biofilm treatment showed faster growth performances compared to untreated (control) ones. These findings indicated that lower larval and settlement densities enhance survival and settlement rates. Natural biofilm promoted post-larval growth, and therefore, its use can be an effective strategy to improve <i>R. decussatus</i> hatchery outcomes.