404 
Fishery Bulletin 1 10(4) 
Table 2 
State of ovarian development of female cobia ( Rachycentron canadum) collected during the period from April to June in both 2007 
and 2008 for this study of inshore spawning of cobia in South Carolina. No.=number of fish; PC=percent composition. 
Inshore 
Offshore 
Unknown 
Phase 
Subphase 
No. 
PC 
No. 
PC 
No. 
PC 
Immature 
0 
0 
0 
0 
0 
0 
Developing 
1 
2 
1 
3 
1 
1 
Spawning capable 
Late developing 
51 
80 
20 
59 
97 
84 
Actively spawning 
2 
3 
0 
0 
3 
3 
Past spawner 1 — recent spawning 
3 
5 
1 
3 
4 
4 
Past spawner 2 — previous spawning 
7 
11 
11 
32 
9 
8 
Regressing 
0 
0 
1 
3 
1 
1 
Regenerating 
0 
0 
0 
0 
0 
0 
kg FW) was caught by SCDNR employees in SHS on 
8 June at 1030 hr. Both had ovaries in the late stage 
of FOM, during which yolk coalescence and hydration 
occurs immediately before ovulation. The time of collec- 
tion, state of oocyte hydration, and rapid nature of FOM 
found in other multiple-spawning species with similar 
geographic distributions (Brown-Peterson et ah, 1988; 
Fitzhugh et ah, 1993; Roumillat and Brouwer, 2004) 
indicate that these 2 specimens would have spawned 
the afternoon of capture. 
Egg-development study 
Micrographs of cobia eggs from the 2007 and 2008 devel- 
opment studies of hatchery-reared eggs were taken at 
13 and 19 sampling times, respectively. In 2007, eggs 
hatched between 39 and 43 hr postspawning at 22.5°C; 
between 29 and 33 hr at 25.0°C; and between 25 and 
29 hr at 26.5°C. Eggs incubated at 29°C were observed 
hatching 26 hr after having been spawned as micro- 
graphs were being recorded. At the end of the study in 
2007, all larvae still had yolk sacs and were 3.8 to 4.6 
mm TL. The 2008 development study ended before any 
eggs hatched. Mean egg diameters of live cobia eggs 
were 1241 and 1337 pm in 2007 and 2008, respectively. 
Mean oil-droplet diameters were 359 pm in 2007 and 
403 pm in 2008. 
Micrographs of preserved eggs from each year re- 
vealed damage to the oil droplet in many specimens 
because of preservation or handling (Fig. 3). Irregu- 
larly shaped oil droplets occurred most often in early- 
stage eggs. Damage in late-stage eggs included split 
oil droplets or droplets in which the pigmented por- 
tion had detached from the lipid (Fig. 3B). Measure- 
ments of preserved eggs were limited to specimens 
that had a single, intact oil droplet. The mean egg 
diameter of preserved eggs from both years was 1280 
pm, and the mean diameter of oil droplets was 380 
pm. The average diameters of eggs and oil droplets 
for both preserved and live cobia eggs fell within 
the published range (Table 3 [Ditty and Shaw, 1992; 
Ditty, 2006]). Upon preservation, cobia eggs shrank 
significantly; 1.7% in 2007 (P<0.05) and 1.0% in 2008 
(P<0.05). No shrinkage occurred in the diameters of 
oil droplets during either year (2007, P=0.13; 2008, 
P= 0.92). 
Five stages of embryological development were de- 
scribed for cobia (Fig. 4) and are detailed below. Dura- 
tion of each stage was approximated in hours. Overlap 
was observed in the durations of stages III and IV 
because of minor differences between eggs in the 2007 
and 2008 development studies. 
Stage I (0-7 hr; Fig. 4, A and B): newly fertilized eggs 
had a distinct, translucent oil droplet. Early cell divi- 
sions were evident at the animal pole (opposite of the 
oil droplet) and eventually formed the blastodisc. Divi- 
sions progressed until individual blastomeres were no 
longer distinguishable. Stage I ended when the germ 
ring was visible and enclosed approximately one-third 
of the yolk mass. 
Stage II (7-13 hr; Fig. 4C): the germ ring became dis- 
tinct and epiboly proceeded until the germ ring was in 
center of the egg. Stage II ended when the blastopore 
was closed and the optic cups distinguished head and 
tail regions of the embryo. 
Stage ill (13-19 hr; Fig. 4D): the embryo was greater 
than one-quarter of the internal circumference of the 
egg. Somites were distinct. Stellate melanophores were 
scattered around the outside of the yolk. Pigmentation 
on the embryo increased (from head towards anterior of 
caudal region). Stage III ended with first appearance of 
melanophores on the oil globule and when the embryo 
was approximately one-half the internal circumference 
of the egg. 
Stage IV (14-21 hr; Fig. 4E): the embryo continued to 
become more heavily pigmented. “Free” melanophores 
continued to congregate on the oil globule until there 
were few to no free melanophores around the outside 
