AGE AND GROWTH-RATE OF THE CLAM, SPISULA-SOLIDA L, FROM A SITE OFF VILAMOURA, SOUTH PORTUGAL, DETERMINED FROM ACETATE REPLICAS OF SHELL SECTIONS

The age and growth rate of a population of Spisula solida from Vilamou ra, southern Portugal, were studied from acetate peel replicas of poli shed and etched shell sections. The growth pattern consists of alterna ting dark and light zones. During the winter, wide growth increments ( light zones) are deposited, characteristic of rapid shell growth, whil st narrowly spaced growth bands with a small depression or cleft assoc iated (dark zones) are formed in the summer between June and August. T he annual shell increments were used to determine age and growth rate. Von Bertalanffy growth curves estimated from surface growth rings dif fered from the growth curve estimated using internal lines. When surfa ce rings were used to estimate age of this species an overestimation o f age and an underestimation of growth rate occurred. Field experiment s showed that the periodicity of the microgrowth pattern is related to an endogenous rhythm of shell formation and not to exogenous factors. The growth curves obtained for both internal growth pattern (A) and s urface rings (B) are: (A) L(t) = 45.08 [1-e(-0.43(t+0.33))] (B) L(t) = 50.22 [1-e(-0.25(t+0.33))]