Respiratory metabolism of juvenile spiny lobster (Sagmariasus verreauxi) under different feeding conditions

Energetic costs of protein synthesis account for a significant proportion of feeding respiratory metabolism, named specific dynamic action (SDA). SDA in aquatic animals can be evaluated by accumulated post-feeding oxygen consumption (ṀO₂) multiplied by an oxycalorific coefficient, or based on stoichiometric relationships amongst ṀO₂, dissolved inorganic carbon production (ṀO₂) and nitrogenous waste (ammonia-N plus urea-N) excretion (ṀN). Instantaneous ṀO₂, ṀCO₂ and ṀN measurements provide a stoichiometric approach to determine energy substrates oxidized over that time. Cycloheximide (CHX) as a protein synthesis inhibitor has been applied in SDA research in teleosts. Relatively little is known about SDA in crustaceans, we therefore aimed to investigate respiratory metabolism in Sagmariasus verreauxi juveniles under different feeding conditions using an intermittent-flow respirometer system. ṀO₂ and ṀN were measured in five treatments: i) fasted lobsters sham injected with saline; ii) fasted and injected with CHX; iii) starved and injected with CHX; iv) fed; v) fed and injected with CHX. A comparatively reduced routine metabolic rate (RMR) in starved lobsters indicated decreased protein synthesis. Considerably higher SDA duration, magnitude and factorial aerobic scope in Fed Group compared with Fed and CHX Group suggested SDA was inhibited by CHX. Energetic costs of protein synthesis accounted for 15% of RMR in fasted lobsters and 66% of SDA in fed lobsters. Ammonia-N compromised 59-92% of nitrogen output in fed lobsters and confirmed ammonia is the major metabolic N waste product. From the ammonia quotient, 16, 26, 14, 67 and 58% of ingested protein were oxidized respectively in Fasted and Saline, Fasted and CHX, Starved and CHX, Fed, and Fed and CHX groups, suggesting a secondary role of protein catabolism during starvation. This work demonstrated an increase in oxygen consumption after feeding, confirmed a causative link between protein synthesis and SDA, and indicated a secondary role of protein catabolism during starvation in Sagmariasus verreauxi. Further studies on ṀO₂ will be included to assess respiratory quotients to develop a more thorough understanding of changes in energy substrate use by crustaceans. This will provide a more accurate approach to evaluate crustacean nutritional factors such as dietary protein quality in relation to SDA.