Reproductive cycle and gross biochemical composition of the ark shell Scapharca subcrenata (Lischke, 1869) reared on subtidal mudflats in a temperate bay of Korea

Abstract

A local shortage of wild seed has led to a recent reduction in the annual production of the ark shell, Scapharca subcrenata, cultured in natural subtidal habitats on the Korean coast. To determine the best time for seed collection from natural habitats, the annual cycles in the condition, gametogenic activity, and gross biochemical composition of the bottom-cultured ark shell were investigated in a southern coastal bay of Korea over a 16 month period. Samples were collected at regular intervals between August 2009 and November 2010. The condition and dry tissue weight of a standard animal (shell length = 31.4 mm) peaked in April−May and reached minima during late summer−fall. Histological analysis of the gonadal tissues showed a unimodal gametogenesis cycle with one spawning peak (June–August) during the year. The gametogenesis of the ark shell was initiated simultaneously with the accumulation of reserve materials (i.e., glycogen, proteins, and lipids) from winter to late spring, indicating that recently ingested food energy is used to fuel gamete production. The stored energy reserves were then rapidly exhausted during and after spawning. The high chlorophyll a concentrations in the water column and surface sediments throughout the year suggest that high local phytoplankton production and the presence of resuspended microphytobenthos may produce high food availability at the study site. The results obtained during the study period show that the gametogenic development of the ark shell has accelerated and spawning has advanced by at least a month in the past decade. These changes probably reflect the elevated temperatures in winter−early spring. Finally, our results suggest that it is necessary to advance the time of seeding from late July−August to spring to ensure the successful seeding of ark shell cultures in natural habitats.