Modelling retinoid content in live prey: A tool for evaluating the nutritional requirements and development studies in fish larvae

This study was conducted to evaluate the accumulation of different levels of total vitamin A in live prey (Brachionus plicatilis and Artemia nauplii) enriched with a commercial emulsion (0.15 and 0.6 g l- 1 for rotifers and Artemia nauplii, respectively) , which contained increasing levels of all-trans retinyl acetate. Emulsions used for rotifer enrichment contained 124, 138, 151, 165, 178, 192, 226, 259 and 327 μg total vitamin A l- 1, whereas those used for Artemia nauplii contained 494, 548, 602, 629, 710, 764, 899, 1034 and 1334 μg total vitamin A l- 1. Total vitamin A incorporation in rotifers was constant until a threshold comprised between 226 and 327 μg total vitamin A l- 1, above which the incorporation of total vitamin A from the emulsion was maximum (806 ng total vitamin A mg DW- 1 in rotifers enriched with 327 μg total vitamin A l- 1). In Artemia nauplii, total vitamin A increased from 4.0 ng mg DW- 1 up to 52 ng mg DW- 1 in nauplii enriched with an emulsion containing 1334 μg total vitamin A l- 1. Retinoid levels in live prey increased as the content of all-trans retinyl acetate augmented in the emulsion, although they did not accumulate in a dose-dependent manner because retinoid incorporation in live prey was found to be not proportional to the content in the emulsion. Rotifers exhibited a higher retinoid incorporation pattern than Artemia nauplii, which seemed to be related to species-specific differences between both live prey. Both live prey were able to absorb and metabolize the vitamin A compounds administered through the emulsion, according to the results regarding retinol and retinoic acid content although the levels were higher in the rotifers than in the nauplii. The differential pattern of total vitamin A accumulation between rotifers and Artemia nauplii should be considered when designing nutritional studies dealing with this vitamin and first feeding marine larvae reared on live prey due to the difficulty in maintaining constant levels of total vitamin A especially during the transition feeding phase from rotifers to Artemia nauplii.