Torres et al .: Energetics of larval Sciaenops ocellatus 



763 



processes with starvation. As was demonstrated by 

 the 0:N ratios (see below), the normally fast-grow- 

 ing young larvae apparently combust their own tis- 

 sues to fuel their metabolic needs. 



Values obtained in the present study for the per- 

 centage of nitrogenous waste in the form of urea agree 

 well with previously reported values. Jobling ( 1981b) 

 found that 15% to 25% of nitrogenous waste in pla- 

 ice larvae occurred in the form of urea: his values 

 agree well with our values of 18% to 33%. Ammonia 

 as a percent of the total nitrogen excretion increased 

 with age in red drum larvae fed to satiation. 



Production of urea may play a role in survival of 

 young larvae. Griffin (1991) proposed that urea syn- 

 thesis was a mechanism for detoxifying ammonia 

 during early embryogenesis, particularly before 

 hatching, and several investigators have reported the 

 presence of urea-cycle enzymes in larval fish (Rice 

 and Stokes, 1974; Depeche et al., 1979; Anderson, 

 1995). Urea costs more energy to produce than am- 

 monia, but it is less toxic. It is used extensively in 

 osmoregulation by elasmobranchs (Anderson, 1995), 



as an excretory product to prevent poisoning during 

 estivation by African lungfish (Janssens, 1964), and 

 in response to elevated external ammonia levels or 

 to stress in toadfish (Walsh et al., 1990). It is pos- 

 sible that production and retention of urea in young 

 red drum larvae could be used to increase blood os- 

 molarity, in a manner similar to that of elasmobrachs, 

 to aid in osmoregulation. Alternatively, it may be a 

 response to the stress of starvation. 



The percent increase in urea production with star- 

 vation has been observed in other fishes under hatch- 

 ery conditions (Brett and Zala, 1975). The constant 

 mass-specific nitrogen excretion in starved fish, de- 

 spite the changes in relative contributions of ammo- 

 nia and urea, is indicative of continued protein ca- 

 tabolism. However, the protein catabolized in starved 

 individuals was the protein contained within the 

 larva's own tissues. 



Urea normally contributes less than 50% of the 

 total nitrogen excreted and is often overlooked, but 

 the results here suggest urea is an excellent indica- 

 tor of starvation in larval fishes. The decrease in 



