Gross biochemical and isotopic analyses of nutrition-allocation strategies for somatic growth and reproduction in the bay scallop Argopecten irradians newly introduced into Korean waters

The bay scallop Argopecten irradians, a warm-water and hermaphroditic bivalve species, is native of the Atlantic coast of North America and was introduced for aquaculture to the Chinese coast in 1982 and, in turn, to the Korean coast in 1996. The present study analyzed the gross biochemical composition and stable isotope ratios of separated organs (i.e., the adductor muscle, digestive gland, gonad, mantle, and gill tissues) of the scallop over a culturing cycle (September 2015 to May 2016), to understand its energy-storage allocation strategy in association with reproductive success for survival in the new habitat. The condition index (CI) of the bay scallop increased slowly but steadily from the beginning of deployment in the grow-out area in September and peaked in the following month of March. The gonadosomatic index (GSI) increased rapidly in April and peaked in May, when the CI decreased drastically. The seasonal somatic growth, as defined by dry tissue weight, of whole tissues and individual organ tissues differed from each other. Most of the energy reserves were stored in the adductor muscle in the summer–fall period. The subsequent sudden decreases in proteins and carbohydrates in the adductor muscle suggested that these reserves were used as catabolic substrates during winter maintenance in the December–January period, and during spring spawning in the April–May period. The δ13C values of all the lipid-free organ tissues displayed seasonal fluctuations in parallel with those of phytoplankton. Seasonal fluctuations in gross weights of biochemical constituents as well as δ13C and δ15N values of individual organ tissues confirmed a mobilization of proteins from the digestive gland to the gonad, suggesting that gonadal growth is catabolized by recently assimilated food (i.e., phytoplankton). Overall, our results clearly indicate that, although the bay scallop in a newly introduced coast of Korea possessed slightly different energy-gain-allocation processes vs the native US populations, they might adapt to new environments and keep an intrinsic trait of fast growth and reproductive success through a slight modification of adaptive strategy for survival.