The breeding potential and biochemical composition of triploid Manila clams, Tapes philippinarum Adams and Reeve.
In 1989 and 1990, triploid Manila clam (Tapes philippinarum Adams and Reeve) seed were reared to 15-20 mm at the Fisheries Laboratory. Conwy, and planted out in the Menai Strait, North Wales, UK. In each of the summers of 1992 and 1993, 3 of these batches, at 2, 3 or 4 years old, were returned to the laboratory to assess ploidy, size, spawning potential and biochemical composition. Percentage triploidy at this time was similar to that in the seed. After 6 and 8 weeks of warm water conditioning, only 45 out of 121 triploids (37%) were induced to spawn by thermal shock, with only 1 spawning as a male. By comparison, 75% of diploid clams spawned with a 1:1 ratio of males to females. Mean fecundity of triploids was significantly lower than that for diploids, at 0.497 compared with 1.54 million eggs per female. Compared with eggs from diploid females, eggs from triploids were larger and significantly fewer of them developed into D-larvae when fertilized by sperm from diploid males. Triploid clams were heavier and had a higher condition index and carbohydrate content than diploids of the same age, but lipid levels were similar. The potential advantages of producing and cultivating 100% triploid batches of Manila clam seed are discussed.