Publication Abstract

The increased utilization of egg lipid reserves following induction of triploidy in the Manila clam (Tapes philippinarum)

S.D. Utting and J. Doyou

Manila clam (Tapes philippinarum Adams and Reeve) broodstocks were conditioned in the hatchery in sand-filtered sea water with diet supplements of either Dunaliella tertiolecta or Skeletonema costatum. Eggs spawned by the Dunaliella broodstock had higher levels of 18:3 (n-3) and 20:4 (n6), and lower levels of 20: 5 (n-3) than eggs from the Skeletonema broodstock; this reflected the fatty acid composition of the algal diets.

Attempts to induce the clams to spawn were made after 4, 6 and 7 weeks (Experiments 1, 2 and 3, respectively). Spawning occurred after 6 and 7 weeks, equivalent to 480 and 580 "day-degrees" (D°) of broodstock conditioning. Eggs were treated with 0.5 mg cytochalasin B (CB) 1^-' to induce triploidy in Experiments 2 and 3. The percentage of triploids was 67% and 73% for embryos from broodstock fed on Dunaliella and Skeletonema, respectively. Survival of embryos from the CB treatments compared to the diploids varied between experiments but growth and survival of larvae from the CB treatments were similar to the diploids. Larvae from broodstock conditioned for 580 D° grew and survived better (P<0.01 and P<0.05, respectively) than larvae from broodstock with only 480 D° of conditioning.

Lipid ranged from 7.48 ng to 8.07 ng/egg and was not related to broodstock diet. Lipid in D-larvae ranged from 1.86 ng to 7.01 ng/larva. During development from egg to D-larva, clams from the Skeletonema-fed broodstock utilized 13% (diploid) and 22% (CB-treated) of the lipid reserves. Respective values for the Dunaliella-fed broodstock were 36% and 76%, which were significantly higher (P<0.05).

Eggs from the Dunaliella broodstock contained 18:3 (n-3) and 20:4 (n-6), (mean contents of 7.4% and 3.6%, respectively,of total fatty acids), but these fatty acids were not found in eggs from Skeletonema broodstock. Levels of 20:5 (n-3) were 3.6% and 10.3% in eggs from Dunaliella and Skeletonema respectively. During embryo development, long-chain polyenoic fatty acid content decreased, in particular (n-3) fatty acids.

Reference:

S.D. Utting and J. Doyou, 1992. The increased utilization of egg lipid reserves following induction of triploidy in the Manila clam (Tapes philippinarum). Aquaculture, 103: 17-28