POPULATION-STRUCTURE AND GROWTH OF DONAX-TRUNCULUS (BIVALVIA, DONACIDAE) IN THE WESTERN MEDITERRANEAN

A comparison of shell growth in Donax trunculus (collected between 198 8 and 1990 of Cullera, Spain) has been carried out using an analysis o f cohort progression in monthly length frequency distributions, hyalin e surface shell growth rings and internal microgrowth bands. In the Me diterranean there are two periods of recruitment of D. trunculus, one in the summer (July to September) and the other in winter (December to February). Clams recruited to the population in winter display a clea r cessation in shell growth during the following summer which may poss ibly be correlated with spawning, whereas individuals of the summer re cruited cohort show no growth cessation the following summer and conti nue to deposit shell during this period. The normally opaque shell of D. trunculus reveals the presence of translucent hyaline growth rings when the shells are backlit by a strong light source, and these have b een shown to be laid down in the shell during summer months. Formation of a hyaline ring is accompanied by a narrowing of the microgrowth pa tterns present in shell sections. Both the hyaline rings and the lengt h frequency distributions have been used to determine the age and grow th rate of D. trunculus.