Protein turnover in juvenile tropical rock lobster (Panulirus ornatus) and its implications for growth bioenergetics

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 affect 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-h post-prandial period. However, lobsters retained >2-fold more dietary protein in CP64 (50% versus 19% in CP42). Using dietary 15N metabolic tracers, we observed one-third higher whole-body protein synthesis rates in CP64 (10.2±0.9% of body protein per day) but no differences in protein degradation. Protein turnover (the proportional degradation of daily synthesised protein) was higher in CP42 at 77±3% (versus 67±1% in CP64), which was associated with ∼2-fold higher protein growth costs (21.5±1.5 versus 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.