Studies on benthic communities in eutrophicated areas of the Seto Inland Sea. II. A population ecology of the small bivalve Theora lubrica in northern Bingo-Nada.

Abstract

The settlement and growth processes of the small semelid bivalve Theora lubrica were quantitatively investigated in the northern shallow waters of Bingo-Nada, the Seto Inland Sea, based on the seasonal fluctuations in density and size composition from April 1982 to April 1983. This bivalve, inhabiting predominantly in muddy sediments, seemed to migrate gradually to the offshore soft-muddy areas with increased age, where the largest individual with a shell-length of 15.6mm was found. Settled individuals were abundant during the spring and early summer, and thereafter decreased rapidly simultaneously with the rise of sediment temperature and the depletion of dissolved oxygen. These fluctuations in the population density were due mainly to both the settlement of planktonic larvae and the migration and/or mortality of settled benthic adults. The linear growth rate for the cohort with the highest settlement abundance was calculated at 0.12mm/day over a range of 2.2mm to 8.3mm in shell-length during the spring and summer, which would be comparable to that of the first-recovering settled cohort in fall.