Abstract
Protein turnover (the continuous synthesis and degradation of the proteome) is energetically expensive and strongly correlates with growth rate in ectotherms. However, in crustaceans our understanding of whole-body protein turnover is limited. This restricts our ability to accurately determine dietary protein and energy utilisation efficiency, which are critical for optimising aquaculture nutrition. This study investigated how two isoenergetic feeds containing 42% (CP42) or 64% (CP64) crude protein impact post-prandial metabolism, nitrogen flux and protein turnover in tropical rock lobster (Panulirus ornatus). We found that dietary protein had no effect on rates of respiratory gas exchange (O2 and CO2) or total excretory nitrogen (NH3-N+Urea-N) over the 48-hour post-prandial period. However, lobsters retained >2-fold more dietary protein in CP64 (50 % vs 19% in CP42). Using dietary 15N metabolic tracers, we observed 1/3rd higher whole-body protein synthesis rates in CP64 (10.2±0.9 % of body protein day−1) but no differences in protein degradation. Protein turnover (the proportional degradation of daily synthesised protein) was higher in CP42 at 77±3% (vs 67±1% in CP64) which was associated with ∼2-fold higher protein growth costs (21.5±1.5 kJ g−1 vs 10.2±0.4 kJ g−1 in CP64). Our results suggest that high protein intake in P. ornatus does not increase catabolism of dietary protein but stimulates protein synthesis leading to reduced protein turnover and more energy efficient growth. These findings clearly demonstrate the importance of whole-body protein turnover to nutritional physiology and bioenergetics for optimising aquaculture productivity.