Ditty and Shaw: Early life stages of Rachycentron canadunn in northern Gulf of Mexico 



673 



fins. Dorsal- and anal-ray anlagen, however, began to 

 develop along the posterior fin base and proceeded 

 anteriorly. Development of anal rays consistently 

 preceeded that of dorsal rays. All anal fin elements 

 were countable by 17 mm. Dorsal spines were very dif- 

 ficult to count in specimens not cleared and stained 

 because spines were short and often covered by integu- 

 ment. One partially cleared 16.7 mm specimen had 7 

 poorly-formed, short dorsal spines, 11 precaudal and 

 14 caudal vertebrae (including urostyle), and 7 bran- 

 chiostegal rays. Pelvic buds were visible by 6 mm, with 

 the full complement of elements (1,5) present by 

 16.5-17mm. Pectoral rays were first visible at about 

 10 mm and the full complement (20-21 rays) was pres- 

 ent by 19-20 mm. A full complement of rays in all fins 

 (around 20 mm) marked the beginning of transition to 

 the juvenile stage (Table 2). 



phores by about 10.5mm. Fin pigmentation pro- 

 gressed anteriorly along the dorsal base and extended 

 onto posterior dorsal rays by about 18-19 mm. Pelvic 

 rays were first pigmented at about 13 mm, but pectoral 

 fins remained unpigmented at all sizes examined 

 (Fig. 2). 



A 49mm juvenile had a jet black caudal fin except 

 for the distal tips of the upper principal rays. Pelvic 

 fins were completely black, but pigment was present 

 basally on only the upper few rays of the pectoral fin. 

 Pigment also covered all of the posterior rays of both 

 the dorsal and anal fins. Fin pigment decreased ante- 

 riorly, however, such that only the basal portions of 

 the anterior dorsal and anal rays were pigmented. All 

 dorsal spines were visible and the integument was en- 

 tirely jet black. 



Fin development 



A continuous median finfold extended posteriorly along 

 the body from the nape to the cleithral symphysis of 

 early larvae. About 5 mm, a ventral thickening occurred 

 near the tip of the unflexed notochord. Anlagen began 

 to form obliquely downward in the caudal finfold 

 during flexion (~6.5-8mm). As the hypural complex 

 shifted to a terminal position, caudal ray development 

 proceeded both dorsally and ventrally until the adult 

 complement of 9-i-8 principal rays was present at 

 10.5-llmm. By 19.5mm, the caudal fin was distinctly 

 spatulate and heavily pigmented. Dorsal- and anal-fin- 

 base development coincided with notochord flexion. 

 These fin bases began to differentiate centrally, and 

 development proceeded both anteriorly and posteriorly 

 with all pteryiophores countable by 10-1 1mm in both 



Distribution and ecology 



The only confirmed collection of both cobia eggs and 

 yolksac larvae from the Gulf was from the Crystal 

 River estuary, Florida, during July 1984. These speci- 

 mens were collected from waters 28.1-29.7°C and 

 30.5-34. rVoo, except for a single 3.2 mm yolksac larva 

 from a power plant discharge canal at 36.7°C and 

 25.2"/u(.. All other eggs and early larvae were from 

 stations along the outer perimeter of the study area 

 at station depths of 3-6 m. No eggs or larvae were col- 

 lected at stations located over oyster reefs, in the salt 

 marsh, or in tidal creeks. Gulfwide, larvae were first 

 collected during late May, with most (98%) collected 

 June-September. Cobia larvae also primarily (85%) 

 occurred at 25-30°C (x 28.2°C, range 24.2-32.0°C), 



at >27'7o,i (x 30.8"/nn, range 18.9-37.7"/ ), and most 



(75%) at station depths <100m (median 50m, range 

 3.1-300m) (Fig. 3). 



Discussion 



Our data suggest that cobia eggs hatch in about 24 h 

 at 29°C. Ryder (1887) projected a 36h incubation time 

 at an unspecified temperature. Based on Pauly and 

 Pullin's (1988) predictive relationship to derive incuba- 

 tion time and a mean egg diameter of 1.24 mm from 

 this study, Ryder's cobia eggs were probably incubated 

 at about 25 °C. In cooler waters of the mid-Atlantic 

 Bight and northward during the spring/early summer 

 (i.e., ~20°C), projected incubation time increases to 

 56h. Cobia hatch at about 2.5mm based on collection 

 of vdld-caught early yolksac larvae (2.6-3.2 mm) with 

 unpigmented eyes and on the work of Hassler and 

 Rainville (1975). 



