Age structure and growth in an unusual population of purple clams, Amiantis purpuratus (Lamarck, 1818) (Bivalvia; Veneridae), from Argentine Patagonia

Abstract

The purple clam, Amiantis purpuratus, is distributed in the southwest Atlantic between Vitoria (Brazil, 20º SL) and San Matías Gulf (Argentina, 41º SL). We studied the age structure and growth in its southernmost population, which sustains small artisanal and sport fisheries. Examination of individuals collected in 1995 revealed well marked external growth bands on the shells, and a puzzling pattern: most had 15 or 16 bands. Were these annual, the implication would be virtually no successful recruitment during the 14-year period 1981-1994. We recovered and processed samples collected in the same area in 1980-1983 and 1987-1990, and conducted a comprehensive survey of the population in 1995. Examination of thin shell sections showed an almost exact correspondence between internal and external bands. The time series of size-at-age data indicated a bi-univocal correspondence between years lapsed and number of bands accumulated. This confirmed that external bands are annual, and that only two significant year-classes (1979 and 1980) settled in the study area over a period of at least 15 years (1979-1994). Growth was studied at one location using size frequency distributions and size-at-age data. Comparisons were made using likelihood methods to test differences among year-classes (1979, 1980, older) and between two sites. Differences between the 1979 and 1980 year-classes were insignificant at both sites. Growth rate estimated for a pool of pre-1979 year-classes was high when compared to the 1979-1980 year-classes. Density-dependence is advanced as a hypothesis to explain the comparatively slow growth of the 1979-1980 year-classes. Longevity is above 25 years. Implications of the findings for conservation and management are discussed.