The Manila clam has the second-highest aquaculture production, after Pacific oysters, on the U.S. West Coast. P ost-harvest spawning by Manila clams in summer , however , results in substantial losses of product and reputation . I nducing triploidy (three sets of chromosomes) ought to solve this problem, since triploid (3n) bivalves are typically effectively sterile. We investigated the reproductive status of 3n and 2n Manila clams to confirm this assumption.
We produced a 1:1 mixture of 3n and 2n M anila clams on O ctober 9, 2020. Ploidy was assess ed by flow cytometry at 48 h, and the 1:1 ploidy ratio was confirmed in seed of 2 mm shell length . Larvae were reared in the hatchery for 23 days and then transferred to a downwelling tank and over-wintered in a temperature- controlled system with steady food. On M ay 28, 2021, we sorted clams into size cohorts and planted 400 clams, holding on an 850 μm screen, in Thorndyke Bay, WA, USA. C lams were planted at +0.3 m tidal height, in 15.2 mm diameter tubes covered with predator-exclusion netting , at 10 clams per tube . On August 31 , 2021, we harvested 40 clams ( confirmed: 18, 3n; 21, 2n) in order to analyze size (length, live weight) and reproductive status.
Although 3n were smaller than 2n, the differences were not statistically significant. All 2n appeared to have normal gonad development, with most having nearly fully or fully developed follicles or tubules (2+). All but one 3n had abnormal reproductive development. Shown below are a normal, 2n female (left) and an abnormal, 3n female (right). 3n Manila clams are sterile.