HOW FOOD CONSUMPTION AND DIET QUALITY CAN INFLUNECE THE GROWTH OF TROPICAL SHRIMPS AND LOBSTERS
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, ks) and out of the protein pool
via catabolic processes (i.e. protein degradation, kd). 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.
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