| Sumario: | Despite recent progress, knowledge on protein and amino acid (AA) requirements of fish larvae is limited. The major differences compared to larger fish seem to be a poorer capacity to digest and/or absorb complex proteins. In addition, the cumulative needs for protein deposition, protein turnover and AA catabolism necessary for rapid larval growth dictate a higher AA requirement during the larval stages. However, fish larvae seem to have an efficient control of AA catabolism, and use dispensable preferentially to indispensable AA as energy substrates. Still, larvae of most marine fish species hatch with a simple digestive tract and a poorly developed ability to digest proteins, and a fully mature protein digestion is only available weeks later. Therefore, fish larvae need diets rich in soluble molecular nitrogen, and avoiding complex proteins with low digestibility. The use of the indispensable AA profile of fish larvae as index of their requirements needs caution. Ontogenetic changes in the AA profile during larval stages need to be considered, and some AA are more efficiently absorbed and/or retained by fish larvae. In short, despite considerable progress in understanding protein utilisation in recent years, many questions remain open in relation to AA requirements of fish larvae.
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