Ditty and Shaw Early life stages of Rachycentron canadum in nortfiern Gulf of Mexico 



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RRCHYCENTRON CflNnOUM 



Figure 1 



Distribution of larval cobia Rachycentron canadum in the nortliern Gulf of Mexico. Plus signs are positive catch stations; diamond 

 is location of Crystal River estuary where cobia eggs and yolksac larvae were collected. 



Methods 



Eggs and larvae were obtained from museum collec- 

 tions along both the Gulf and Atlantic coasts. We ex- 

 amined 70 cobia eggs (all late-stage embryos) and 76 

 larvae 2.6-25 mm SL and determined their seasonal 

 occurrence, distribution, and ecology. Hydrographic 

 parameters were weighted by the total number of lar- 

 vae caught at each station to derive mean and median 

 values. All specimens were formalin-preserved except 

 those from Southeast Area Monitoring and Assessment 

 Program (SEAMAP) ichthyoplankton surveys of the 

 Gulf which were in ethyl alcohol. We considered lat. 

 26 ° 00 'N as the southern boundary of our survey area 

 (Fig. 1). Temperature and salinity data were from the 

 surface only. 



Body measurements were made to the nearest 0.1 

 mm with an ocular micrometer in a dissecting scope 

 and follow Hubbs and Lagler (1958) and Richardson 

 and Laroche (1979). All lengths refer to standard 

 length (SL) unless otherwise noted. A compound scope 

 was used to examine the origin and location of epithelial 

 spicules. Representative specimens were illustrated 

 with the aid of a camera lucida. Specimens were not 

 cleared and stained because of the paucity of material. 

 Fin rays were counted when their pterygiophores were 

 visible; spines when they resembled formed structures 

 (Richardson and Laroche 1979). Myomeres were diffi- 

 cult to count in fish >6mm due to heavy larval pig- 

 mentation and opacity of the musculature, even under 

 polarized light, but all specimens <6mm had 24 myo- 



meres. Cobia undergoing transition to the juvenile 

 stage were those with a full complement of formed rays 

 in all fins. Egg staging followed Moser and Ahlstrom 

 (1985). 



Egg and larval development 



Cobia eggs were spherical and measured 1.15-1. 3mm 

 (x 1.24, N 31), with a single oil globule 0.4-0.65 (i 

 0.45, A^ 13) in diameter. The oil globule was pigmented 

 and lay near the vegetal pole opposite the developing 

 embryo. The perivitelline space was narrow with the 

 embryo occupying about 85% of egg volume (range 

 70-92%, A'^ 13). The chorion was smooth and unorna- 

 mented. Cobia eggs hatch in about 24h at 29°C based 

 on Pauly and Pullin's (1988) relationship between egg 

 diameter and water temperature to predict develop- 

 ment time in marine fishes. 



The embryo of late-stage Gulf cobia eggs was heav- 

 ily pigmented except for the caudal peduncle which was 

 unpigmented. Late-stage embryos from north of Cape 

 Hatteras, NC, had pigment lightly scattered over the 

 peduncle. Early yolksac larvae (2.6-3.2 mm) were 

 heavily pigmented externally and lacked a functional 

 mouth, eye pigment, and all fins. A single oil globule 

 with pigment was also present in the middle of the 

 yolksac. External pigment occurred over the snout and 

 in a band immediately behind the primordial eye. The 

 eye remained unpigmented until larvae were 3.5-4.0 

 mm (Fig. 2). At higher magnification, tiny epithelial 



