Growth and reproductive cycle of Polymesoda radiata (Bivalvia : Corbiculidae) in Costa Rica

A population of green clam, Polymesoda radiata was sampled from August 1992 through July 1993 in Bebedero river, Guanacaste, Costa Rica (n= 978) for d emographic data. Mean values (followed by ranges) were: shell length 32.29 +/- 7.63 mm (6.9-44.10 mm). The mean total weight was 11.13 +/- 5.71 g (0.4 -23.98 g). Data analysis showed a isometric relationship between length and total weight, represented by the equation : Pt=2.07 x 10<SUP>-4</SUP> Lt<S UP>301</SUP> (r=0.987, p < 0.01). Von Bertalanffy's equations were:for leng th Lt=45 [1- e<SUP>{-1(t+0.06</SUP> (05/2) sen 2 ((t-065}))]<SUP></SUP> and for weight Pt= 26.53(<SUP>1- e [-1 (t + 0.06</SUP>]))<SUP>3</SUP>. The gro wth rate for total length was 2.25 +/- 0.66 mm/month for the first year (0. 83 +/- 0.24 the second year and 0.26 +/- 0.1 for other months). Individuals become sexually mature throughout the year. The mean sex ratio was 1.00 ma le : 0.61 females. Spawning suggested that minimum size at sexual maturity was 7.80 mm of Lt. The largest number of ripe individuals was found in the dry season. Spawning peaks were in December and March. Histologically this is a dioecious bivalve with stable gonocorism.