Comparative reproductive cycles of two clam species reared in the intertidal and subtidal habitats on the southern coast of Korea: Tegillarca granosa (Linnaeus) and Scapharca subcrenata (Lischke)

Abstract

Seasonal variations in somatic growth and gametogenesis of two genetically allied-species dwelling in different environmental conditions, Scapharca subcrenata in subtidal zone and Tegillarca granosa in intertidal flat, were compared. Condition, gross biochemical composition and gonadal development of both species were monthly measured with environmental conditions during a 19-month period from January 2016 to July 2017 in Yeoja Bay of Korea. The food availability in the subtidal zone was higher than that in the intertidal zone: chlorophyll a concentration in the water column of the subtidal zone was higher than that in the intertidal zone during the critical period but food quality was vice versa. The condition and flesh tissue weight of S. subcrenata and T. granosa peaked in late winter-spring and spring-summer, respectively. Both species showed an annual reproductive cycle with unimodal peak in gametogenic index (GMI) in spring-summer and an opportunistic strategy for gametogenesis. However, these two species displayed differences in seasonal patterns in initiation and duration of gametogenesis and spawning. In addition, the distinction in somatic growth between S. subcrenata and T. granosa was also observed and the amplitude of the growth of S. subcrenata was nearly threefold higher than that of T. granosa. This result suggests that S. subcrenata was more preferential to their circumstance than T. granosa. These differences may be occurred by environmental heterogeneity. The complicated interactions among environmental conditions [i.e. temperature, quantity and quality of available food, and the gap of the feeding time induced by the tidal regime] make the genetically allied species, S. subcrenata and T. granosa, have the different phenotypic plasticity on somatic growth and reproductive activities. Therefore, these results suggest that environmental heterogeneity let not only the same species dwelling heterogeneous circumstances but also the genetically allied-species in different environments to take the different phenotypic flexibilities in growth and/or gametogenesis against their circumstances. Figuring out the associations between a condition and reproductive strategy of the cultured marine organisms and characteristics of their circumstance may deliver the fundamental information to select the cultivated farm culturing more efficiently and allow to plan the management for sustainable exploitation of the commercially important shellfish in Korea.