Optimal dietary protein levels in penaeid shrimps, measured as growth response, vary from 50–55% in Penaeus japonicus, to 40–46% in Penaeus monodon and over 30–60% in Litopenaeus vannamei

as growth response, vary from 50–55% in Penaeus japonicus, to 40–46% in Penaeus monodon and over 30–60% in Litopenaeus vannamei (Teshima and Kanazawa, 1984; Cousin et al., 1993). Commercial shrimp feeds contain 30–50% crude protein, composed mostly of marine animal protein products such as fish, shrimp and squid meal. These feed materials have high nutritive value and palatability but are expensive and not readily available (Lim and Persyn, 1989). There is much interest in finding alternative, less expensive, protein sources to replace part of the fish meal content of diets. The importance of soybean meal as a source of protein has long been recognized (Kanazawa, 1992). Its utilisation in shrimp diets has increased due to its ‘nutritional quality, lower cost and consistent availability’ (Akiyama, 1988). Casein, however, although generally used as a standard protein source for the nutrient requirement studies of fish and other vertebrates, is poorly utilised by several shrimp species (Deshimaru, 1982). Attempts have been made to optimise the utilisation of these proteins by determining the optimal dietary amino acid profile (Cowey and Forster, 1971). This optimal dietary amino acid profile will depend on the amino acid requirement of an animal for protein synthesis and the use of individual amino acids as energy substrates or for other purposes (Ronnestad and Fyhn, 1993). Deshimaru and Shigeno (1972) suggested that the amino acid composition of the food should be very similar to that of the animal’s proteins. Ogata et al. (1985) measured the total essential free amino acid concentrations of the European eel Anguilla anguilla and noted that it was correlated with dietary protein content. To minimize the effect of different sample pretreatment, the A/E ratio (the concentration of each essential amino acid as a percentage of the concentration of total essential amino acids, including tyrosine) was calculated in some studies. Arai (1981) based diets for coho salmon fry upon A/E ratios found in the whole-animal tissue of this species. Penaflorida (1989) used the profile of essential amino acids of whole shrimp to calculate an essential amino acid index (the nth root of the product of the ratios of each essential amino acid in the feed to that of a reference protein). Changes in the levels of free amino acids in tissue after a meal have been used as a criterion for determining amino acid requirements, based on the hypothesis that the concentration of an individual free amino acid will remain low until its requirement has been met (Wilson, 1994). Protein turnover can be divided into its constituent processes, protein synthesis, protein growth and protein degradation (reviewed by Houlihan, 1991). At any particular time, protein growth (kg, protein growth as a percentage of the total protein 3107 The Journal of Experimental Biology 205, 3107–3122 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB4478