HOW FOOD CONSUMPTION AND DIET QUALITY CAN INFLUNECE THE GROWTH OF TROPICAL SHRIMPS AND LOBSTERS

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Both studies confirm that the methodology, previously used to measure protein synthesis in a wide variety of mammals, fish (reviewed by Houlihan et al., [9] and amphibians [10] can be successfully applied to measure protein synthesis in crustaceans. Protein synthesis rates have been measured using the flooding method in the same species as in this study (crabs: [3] and in lobsters [5]) although the size of the experimental animals were not similar.

Protein metabolism is fundamental to the growth of all animals and therefore an understanding of such process is essential. Protein synthesis is essential to provide a supply of structural proteins to promote growth of the animal. Knowledge of the dynamics of muscle protein deposition and the relationship between diet quality, protein synthesis and muscle protein deposition is required. Protein turnover is considered to be a highly dynamic process which results in a continuous flux of amino acids into the protein pool via anabolic processes (i.e. protein synthesis, k_s) and out of the protein pool via catabolic processes (i.e. protein degradation, k_d). The efficiency of retention of

synthesised protein (protein growth rate x 100/ rate of protein synthesis) is used as a key indicator of strategies of protein metabolism (Table 1). Dietary amino acids imbalance lead to a reduction in growth rate and a decrease in the efficiency of retention of the synthesised protein (Table 1, Diet 3). High efficiencies of retention of synthesised proteins indicate reduced protein degradation rates and hence low turnover rates in growing animals. Indeed, this study, shows that in fast growing shrimps the efficiency of whole-body protein retention was 94%. In tropical shrimps L. vannamei there is an increase in protein synthesis retention efficiency with increasing growth rates. It seems that tropical shrimps sacrifice protein turnover in order to maximise retention efficiencies of synthesised protein. The present results confirm the results reported by [4] who examined the shrimp Penaeus esculentus and suggested low protein turnover rates (i.e., degradation) in invertebrates. This study offers further insight by modelling protein metabolism and by presenting amino acids flux diagrams.

The amino acid flux model for the shrimps (fig 2) showed that the animals regulate free amino acid concentrations and it appears that protein synthesis acts as a mechanism for regulation of free amino acid concentrations. Free amino acid pools in white muscle are relatively unaffected by feeding [11]. In this study the total free amino acid pool concentrations showed stability with time after feeding (fig.1). The concentration of free glycine makes up about one-third of the total free amino acids on crustaceans [12] and it is the major constituent of the FAA pool together with proline, arginine alanine and serine [13]. The lack of any correlation between the concentrations of free amino acid in whole-body and dietary amino acid composition does not preclude the possibility that amino acid requirements of crustaceans could be estimated by analysis of levels in the hemolymph [14]. The amino acid flux of the lobsters also suggests low protein conversion efficiency [15] compared to 55% found in shrimps and 63% found in larval herring. The results suggests that lobsters are slow, periodic feeders and that growth can be readily increased by manipulation of particular environmental factors such as feeding frequency.

This study showed that an increased rate of growth at temperature 27°C is a result of increased rate of protein synthesis and reduced protein turnover in shrimps. In contrast, lobsters (H. gammarus) showed lower growth rates at 19°C temperature and an increase in protein degradation. Thus, the results may be interpreting as indicating that there are species differences in protein synthesis rates. However it has been demonstrated that the efficiencies of retention of synthesised proteins increase with temperature (reduced protein degradation with increased temperature in growing animals is contrasted with increased degradation in starved animals, reviewed by Houlihan et al., [9]). The common octopus, Octopus vulgaris grows extremely rapidly and as Table 1 shows has similar protein retention efficiencies with the values obtained for shrimps in this study at nearly the same temperature (22°C). Thus, although the results of this study showed that there are specific differences in protein turnover between different species the interpretation remains ambiguous due to the effect of environmental factors such as temperature, salinity, body size and age.