Brightman et al.: Energetics of larval Sciaenops ocellatus 
433 
of five individual larvae that had been anesthetized 
with MS-222 was measured with the aid of a dis- 
secting microscope. Standard length was considered 
to be the distance from the snout to the tip of the tail 
in preflexion larvae and from the snout to the tip of 
the notochord in post-flexion larvae. 
Mass measurements Growth in mass was moni- 
tored at the same intervals as those used for stan- 
dard length. At each monitoring interval, 30 indi- 
viduals were removed for wet, dry, and ash-free dry 
mass analysis. To determine mass, larvae were first 
separated into three groups of ten. Each group of 
larvae was filtered onto a preweighed 0.5-cm 
Whatman glass fiber filter (made with an office hole- 
punch) that was placed in a custom-made miniatur- 
ized vacuum funnel. Larvae were then rinsed very 
briefly by introducing distilled water into the funnel 
with a pasteur pipette and by removing the water 
immediately with the vacuum filter. To minimize 
evaporation, samples were immediately placed in 
preweighed microcentrifuge tubes which were then 
weighed to the nearest pg on a Mettler electrobalance 
to determine wet mass. Specimens were dried at 60°C 
to a constant mass (about 24 h) to determine dry 
mass. 
Average proximate and elemental 
composition of prey items 
Rotifers were collected in bulk from two 28-L cul- 
ture bags (approximately 50 mg dry mass/bag) for 
determination of proximate and elemental composi- 
tion. Proximate composition (water, ash, protein, and 
lipid content) was determined by using the methods 
of Stickney and Torres (1989) and Donnelly et al. 
(1990). Elemental composition was determined by 
using a C:H:N analyzer. 
Average proximate and elemental 
composition of fish larvae 
Methods used to estimate the proximate and elemen- 
tal composition of fish larvae were the same as those 
used for prey. Larvae were obtained in bulk (50 mg 
dry mass) for each day sampled. Each pond was 
sampled from the hatchery at 0, 2, 6, 10, and 14 days. 
Laboratory-raised larvae were sampled at prey con- 
centrations of 0, 0.1, 1.0, and 5.0 prey/mL at 0, 2, 6, 
10, and 14 days for each of four spawns. Protein and 
lipid values as percent ash-free dry mass (%AFDM) 
were multiplied by individual ash-free dry mass to 
obtain concentrations as mg/individual. The instan- 
taneous protein growth rate (Gpi) was calculated by 
using the formula from Buckley (1982): 
Gpi = 
lnM„-lnM„ xl()0 
t 2 ~ t x 
where M - mass in mg; and 
t = age in d. 
Caloric content of prey and larvae 
Caloric content was calculated from proximate com- 
positional data of the rotifers and larvae by using a 
value of 0.0048 cal/pg for protein and 0.0095 cal/pg 
for lipid (Brett and Groves, 1979). 
RNA-DNA ratio 
Ten to 20 individuals were removed for analysis of 
RNA:BNA content each time sampling occurred for 
measurements of mass. Larvae were filtered onto 
preweighed Whatman glass-fiber filters, rinsed with 
distilled water, weighed, placed in microcentrifuge 
tubes, and frozen at -80°C until analysis. RNA:DNA 
was analyzed by first homogenizing the freshly 
thawed groups of larvae in 1.2 M NaCl, then by us- 
ing the sequential enzymatic method of Bentle et al. 
(1981) to determine RNA:BNA. 
Activity of lactate dehydrogenase 
Larvae were sampled in bulk (minimum 10-20 mg 
wet tissue mass) every day at a prey concentration 
of 0 prey/mL. Samples were taken at 0, 2, 6, 10, and 
14 days for larvae fed 5 prey/mL and for those col- 
lected in the growout ponds. Tissue was introduced 
frozen into the homogenizing medium, ice-cold Tris/ 
HCL buffer (10 mM, pH 7.5 at 10°C), and homog- 
enized by hand at 0 to 4°C with conical glass homog- 
enizers having ground-glass contact surfaces ( Kontes 
Glass Co., “Duall” models). Homogenates were cen- 
trifuged at 4,500 xg for 10 minutes and the superna- 
tants saved for enzyme analysis. 
L-Lactate dehydrogenase (LDH, EC 1.1.1.27; Lac- 
tate: NAB+ Oxidoreductase) activity was assayed in 
the pyruvate reductase direction by using methods 
described in Torres and Somero ( 1988) at a tempera- 
ture of 25°C. Enzyme activity was expressed as units/ 
gWM (wet mass), where a unit was 1 pmole of sub- 
strate converted to product per minute. 
Statistical analyses 
Simple regressions for each relationship were fitted 
by using the least-squares method (Statgraphics 
Plus, Manugistics Corporation). Data from treatment 
groups were compared by using one-way analysis of 
variance (ANOVA). Differences between the means 
