Conditioning of Manila clam broodstock on natural and artificial diets.

Two trials in 1991 and another two in 1992 were carried out on the broodstock \nconditioning of Manila clams, Tapes philippinarum. The main objective was to manipulate \nthe lipid and polyunsaturated fatty acid content of Manila clam eggs by maintaining \nbroodstock in different dietary regimes. The second objective was to assess dried algae as \nalternative diets for conditioning broodstock. \nClams were brought into the laboratory from the natural environment early in the \nyear, before gametogenesis had started. Supplements of cultured live (Dunaliella \ntertiolecta, Skeletonema costatum, Tetraselmis suecica and Isochrysis galbana) and dried \nalgae (T. suecica) diets were fed to the broodstock, usually in a range of mixed diets, at \nrations equivalent to 3% or 6% of the initial dry meat weight of the broodstock in dry \nweight of algae per day. The microalgae differed in their long-chain polyunsaturated fatty \nacid content (PUFA). Unfed control clams received only the organic material which \nremained in the sea water after sand filtration. The nutritional value of these diets in \nrelation to gametogenesis, fecundity, quality of eggs, and viability and growth of larvae \nwere assessed. \nDry T. suecica was the same food value as live T. suecica but Manila clams produced \nmore eggs if supplements of live algae were added. The requirement for conditioning \nManila clams (32 mm shell length) to spawn with live or dry T. suecica+S. costatum was \n500 to 700 "day-degrees" (D°). With dry T. suecica on its own or mixed with I. galbana, \nS. costatum and D. tertiolecta it was 500 to 600 Do (44 mm shell length). In one trial \nclams spawned in the tanks (equivalent to 462 Do) before the first attempt to spawn them \nwas made. \nSuccessful spawning was dependent on the quantity and quality of the algal diet \nduring gametogenesis. With a 6% food ration, clams fed dry T. suecica+S. costatum or \ndry T. suecica+l. galbana produced the highest number of eggs (an average of 3.2 and \n4.5 million eggs per female, respectively). The average fecundity was 83% lower when the \ndiet was reduced to a 3% food ration. The dry meat weight, condition index and fecundity \nof fed broodstock were significantly higher than for unfed animals. \nThe quantity of lipid in the eggs, usually between 4 and 9 ng egg-1, was similar \nwhatever the broodstock diet. However, levels of the essential polyunsaturated fatty acids \n20: 5w3 and 22: 6w3 in the eggs were low if the broodstock diet was deficient in these \nPUFAs. \nEven though diet manipulation caused changes in the fatty acid composition of the \neggs, growth and survival of Manila clam larvae was not reduced in a hatchery situation.