A COMPARATIVE-STUDY OF THE GAMETOGENIC CYCLES OF THE CLAMS TAPES-PHILIPPINARUM (ADAMS,A. AND REEVE 1850) AND TAPES-DECUSSATUS (LINNAEUS) ONTHE SOUTH COAST OF IRELAND

A comparative study was carried out on the gametogenic cycles of the i ntroduced Manila clam Tapes philippinarum and the native carpet-shell clam Tapes decussatus during the period from September 1990 to Septemb er 1991. Examination of histological preparations showed that obvious gonadal development began in March for the Manila clam, and in April f or the carpet-shell clam. Histological evidence showed that, in the Ma nila clam, ripe specimens appeared in May with major spawning starting in September. Spawning of this exotic species was not previously reco rded in Ireland. In the carpet-shell clam, first ripe specimens were o bserved in June and major spawning commenced in August. Both species h ad a unimodal gametogenic cycle in southern Irish waters. Measurement of oocyte size was facilitated by application of an image analysis sys tem, and the resultant data were used to determine the various stages of gonadal development. Mean diameter of free oocytes was 43.2 mu m fo r the Manila clam and 42.6 mu m for the carpet-shell clam.