To optimize the conditioning procedure to obtain better larvae with lower production costs, Ruditapes decussatus broodstock was conditioned at different periods during the year [October (C1), February (C2), and April (C3)] under two different controlled temperature regimes: a constant temperature (CT, 20°C) and a gradual increase in temperature from ambient to 20°C (GT). Samples (30 adult clams) were collected randomly every month from a control group and from each conditioning tank (three tanks for each temperature treatment). A section of the gonad-visceral mass was sampled for gonadal histology and the remaining tissue was used for biochemical analyses. Gonadal developmental stages were determined based on the classification proposed by Wilson and Seed (1974), and fertilized eggs were reared after the method of Walne (1966). In the three conditioning experiments fewer degree-days were required to successfully spawn clams in tanks with the gradual increase of temperature. Clams conditioned in October spawned in February, four months prior to spawning under natural conditions after 1,030 degree-days (D°) in CT tanks and 937.5 D° in GT tanks. Clams conditioned in February and April spawned after 410 D° and 470 D° when held at constant temperature respectively, whereas a gradual increase of temperature shortened this to 383 D° and 337.5 D° respectively. The most important difference between the two temperature regimes was the higher condition index of clams conditioned under the GT regime. The percentage of larvae that survived metamorphosis was higher in clams conditioned at GT, maximum survival ranged from 25% in C3 at CT to 40% in C1 at GT, except for CT tanks in C2 where it was less than 10%. Results showed that Ruditapes decussatus conditioned with a gradual increase in temperature, enhanced larval production and also reduced overall hatchery production costs.
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Vol. 27 • No. 5