GROWTH AND POPULATION-STRUCTURE IN THE INFAUNAL BIVALVE YOLDIA-EIGHTSI IN RELATION TO ICEBERG ACTIVITY AT SIGNY ISLAND, ANTARCTICA

Growth rates in the infaunal nuculanid bivalve mollusc Yoldia eightsi (Couthouy) were assessed for field populations in Factory Cove, Signy Island, Antarctica, between February and April 1992. Daily increments in length (maximum shell dimension) ranged from 2.3 mum d-1 for a 30 mm individual to 5.1 mum d-1 for a 10 mm specimen. These growth rates were converted to annual increments, based on a growing season for the Signy population of around 5 mo, and ages for the largest individuals in the population (35 mm in length), were calculated to be congruent-to 65 yr. Specimens of 43 mm in length have been found near this site and, if their growth rates are similar to this population, their ages would be in the order of 120 yr. Size distributions from two sites in the same Y. eightsi bed 300 m apart showed significant differences. At the more exposed site the distribution was dominated by small juveniles, with 86% of the population < 10 mm in length, while 13% were > 20 mm in length. At the less exposed site the values were 27% < 10 mm and 56% > 20 mm. Icebergs have often been seen grounded on the Y. eightsi bed in this study, especially on the exposed outer portion. This factor, in association with inhibition of larval settlement by high densities of large individuals, is proposed as an explanation of the observed population distributions and the absence of very large specimens in the Factory Cove population.