Interactive effects of diet and temperature on the scope for growth of thescallop Argopecten purpuratus during reproductive conditioning

Mature individuals of Argopecten purpuratus from suspended culture in Tongo y Bay, northern Chile were conditioned at two temperatures (16 and 20 degre es C) and three different diets (mixture of pure microalgae, microalgae sup plemented with lipids and microalgae supplemented with carbohydrates). The food, equivalent to 3% daily of the animal dry weight was supplied continuo usly by a peristaltic pump. The rates of different physiological processes were measured on 18 scallops (three replicates per temperature/diet combina tion) during the third week of conditioning. Nine scallops came from the gr oup conditioned at 16 degrees C and other nine from the group conditioned a t 20 degrees C (each scallop from a different tank). As in the conditioning experiment, physiological measurements were made in controlled temperature rooms using the same experimental diets. Argopecten purpuratus did not sho w significant differences in clearance rate between 16 and 20 degrees C whe n fed with pure microalgae and microalgae + lipids, showing a well known ca pability of bivalves to acclimate their physiological rates within a certai n range of temperature. On the other hand a clear effect of the composition of the diet on the clearance rate was observed. When microalgae were suppl emented with a lipid emulsion, clearance rate was highly stimulated, showin g values significantly higher in comparison with either pure microalgae, or a mixed diet of microalgae + carbohydrate. These highest feeding rates can be related with the presence of essential fatty acids in the diet, which a re considered as very important compounds during the gametogenesis of inver tebrates as well as during the development of ova into normal larvae. Thus A. purpuratus can actively regulate clearance rate and does not simply swit ch between feeding and non-feeding states. The data also suggest the presen ce of chemical receptors at the level of the gills and/or labial palps, whi ch seem able to detect specific nutritive compounds present in the diet. Ab sorption efficiency was independent of temperature and was higher with pure microalgae and with microalgae + lipids. The lowest efficiencies were reco rded with the diet supplemented with carbohydrates. The energy expended in oxygen uptake and ammonia excretion was very similar in the different exper imental diets and temperatures. Scope for growth in A. purpuratus appears m ainly affected by the diet and not by temperature. While the lower SFG seem s to be associated with diets composed of pure microalgae and microalgae carbohydrates, the highest values being found for a diet rich in lipids. Th e data were in agreement with the reproductive conditioning of A. purpuratu s, where the highest percentage of ripe scallops occurred in individuals fe d with a diet of microalgae supplemented with lipids at both temperatures. Similarly the highest larval survival rate was obtained from gametes releas ed by scallops conditioned with the diet containing lipids. (C) 2000 Elsevi er Science B.V. All rights reserved.