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

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