Torres et al.: Energetics of larval Saaenops ocellatus 



757 



is being combusted. Values higher than 20 suggest 

 that lipids are the primary source of energy (Bayne, 

 1973). Larvae depleting their yolk-sac lipid reserves 

 should thus exhibit high 0:N values, whereas fast- 

 growing larvae actively synthesizing protein with 

 little or no lipid deposition or combustion should have 

 low 0:N values. 



The aims of this study were to determine the res- 

 piratory costs of red drum larvae in their first 2 weeks 

 of life, to determine the amount of energy lost as ni- 

 trogenous waste in the form of ammonia and urea, 

 to determine the effects of starvation on both nitro- 

 gen excretion and respiration, and to determine, by 

 using 0:N ratios, the main biological fuel being com- 

 busted for energy. 



Methods and materials 



Maintenance of specimens 



Fertilized eggs were obtained from the Florida De- 

 partment of Environmental Protection (FDEP) hatch- 

 ery in Port Manatee, Florida. Broodstock was main- 

 tained at a temperature of 25°C and a salirity of 30%o. 

 Eggs were obtained from six spawnings over the 

 course of six months to complete the respiration and 

 excretion experiments described below. 



Eggs were transported to the University of South 

 Florida Marine Science Laboratory in St. Petersburg, 

 and 2,500—3,000 individuals were placed into each 

 26-L experimental aquarium. The high mortality 

 associated with first-feeding (cf. Roberts et al., 1978) 

 resulted in a concentration of 250-300 individuals 

 per aquarium after the first 3 days of life. Aquaria 

 were kept in a photoperiod- and temperature-con- 

 trolled incubator; water was maintained at 25°C and 

 30%p. A 13-hour light and 11-hour dark photoperiod 

 was used throughout all experiments. Larvae were 

 fed rotifers (Brochwnus plicotilis) starting at day 3 after 

 hatching until transformation (approximately day 14), 

 when experiments were terminated. Aquaria were aer- 

 ated and 10% of the water in each was changed daily 



Rotifers were cultured by using the procedure of 

 Hoff and Snell (1987). Seawater for culturing was 

 obtained from offshore in the Gulf of Mexico. The 

 seawater was coarse-filtered, treated with bleach 

 (sodium hypochlorite, 5.25%) to remove any additional 

 plankton, and neutralized with sodium thiosulfate. 

 Salinity was adjusted with distilled water and Tropic 

 Marine Seasalt to achieve a final salinity of 30%c . 



Rotifers were provided at 5.0 per mL from first 

 feeding (day 3) through day 14. Prey concentrations 

 were monitored twice daily by removing a 25-mL 

 sample from each aquarium, counting the number 



of prey in 0.5-mL aliquots, and taking the average. 

 Concentrations were adjusted to maintain prey con- 

 centrations as necessary. 



Oxygen consumption rate 



Oxygen consumption was measured in red drum lar- 

 vae ranging in age from 3 to 18 d. Individuals used 

 in respiratory determinations were of two types: 

 those fed ad libitum and those that had been starved 

 for 24 h. Oxygen partial pressure was monitored in 

 respiratory chambers with both "micro" and "needle" 

 polarographic oxygen electrodes (Mickel et al., 1983; 

 Revsbech and Ward, 1983) as individuals or groups 

 of individuals reduced the oxygen levels to low (0-40 

 mm Hg) partial pressures. Cathode diameters on 

 both the micro- and needle electrodes were suffi- 

 ciently small to preclude the need for stirring; both 

 types were manufactured in our laboratory. Starved 

 individuals were monitored with the needle elec- 

 trodes and fed larvae were monitored by both micro 

 and needle electrodes. Electrodes were calibrated 

 before and after each experimental run with air- and 

 nitrogen-saturated seawater. 



Respiratory chambers were manufactured from 

 plastic 1-mL and 10-mL syringes. Each chamber was 

 filled with seawater filtered through a 0.45 u.m 

 millipore filter and capped with an electrode fitted 

 with an O-ring. The respiratory chamber within the 

 syringe barrel was thus defined at one end by the 

 syringe plunger and at the other end by the oxygen 

 electrode. Single individuals were run by using a 

 1-mL syringe and the needle electrode set-up. Groups 

 of 2 to 4 individuals were run in 10-mL syringes with 

 a micro-electrode. Chambers were kept at 24 (± 1.0)°C. 



Data were continuously recorded throughout each 

 run with either a computer-controlled data logging 

 system (micro-electrodes) or a chart recorder (needle 

 electrodes) (Donnelly and Torres, 1988). For data 

 acquired with the chart recorder, the oxygen con- 

 sumption rates were computed directly from the slope 

 of the recorded data. Rates were calculated from the 

 data-logger by noting the oxygen depletion over 10- 

 min intervals. Typically, the first 30 min of a run 

 were characterized by a high rate because of excite- 

 ment associated with introduction into the respirom- 

 eter. Thus, in all cases, data acquired during the first 

 30 min of the run were discarded. A mean respira- 

 tory rate for each run was determined by taking a 

 mean of all the 10-minute rates comprising the run 

 after the first 30 min. The lowest and highest 10- 

 min rates for each run were designated the minimum 

 and maximum rates. 



After each run, individuals were rinsed with dis- 

 tilled water and dried at 60°C for 24 h, then weighed 



