Abstract. - CoMa is a highly 

 prized recreational species of world- 

 wide distribution in tropical and sub- 

 tropical seas, but the development, 

 distribution, and ecology of its early 

 life stages are poorly known. Eggs 

 are spherical, average 1.24 mm in 

 diameter, and have a single oil glob- 

 ule (mean diameter 0.45 mm). The 

 perivitelline space is narrow and the 

 embryo heavily pigmented. Eggs 

 hatch in about 24h at 29°C based on 

 the relationship between egg diam- 

 eter and water temperature to pre- 

 dict development time in other ma- 

 rine fishes. Larvae hatch at about 

 2.5mmSL. Cobia spawn in both es- 

 tuarine and shelf waters during the 

 day, and eggs and larvae are usual- 

 ly collected in the upper meter of the 

 water column. Larvae are recog- 

 nized by the large supraorbital ridge 

 with a single spine, laterally swollen 

 pterotics, heavy body pigmentation, 

 minute epithelial spicules covering 

 the body integument, and a pair of 

 moderate-to-large, simple spines on 

 either side of the angle of the pos- 

 terior preoperculum. Only 70 larvae 

 <20mmSL were collected and iden- 

 tified from the Gulf of Mexico be- 

 tween 1967 and 1988; most occurred 

 between June and September at sur- 

 face temperatures >25°C, salinities 

 >2T7™,, and within the 100 m depth 

 contour. Similar patterns of head 

 spination provide evidence of a 

 sister-group relationship between 

 cobia and dolphinfish rather than 

 that previously hypothesized be- 

 tween cobia and remoras. 



Larval development, 

 distribution, and ecology of 

 cobia Rachycentron canadum 

 (Family: Rachycentridae) in the 

 northern Gulf of Mexico* 



James G. Ditty 

 Richard F. Shaw 



Coastal Fisheries Institute, Wetland Resources Building 

 Louisiana State University, Baton Rouge, Louisiana 70803-7503 



Cobia, in the monotypic family Ra- 

 chycentridae, is distributed world- 

 wide in tropical and subtropical seas 

 (Briggs 1960), except the eastern 

 Pacific, and is found seasonally in 

 temperate waters (Hassler and Rain- 

 ville 1975). A highly prized recrea- 

 tional species, most of the U.S. land- 

 ings are from Gulf of Mexico (Gulf) 

 waters; they are also caught inciden- 

 tally in commercial fisheries (Shaffer 

 and Nakamura 1989). Recreational 

 landings are not well documented, 

 but cobia are reportedly not abun- 

 dant and recruitment is considered 

 low (Gulf of Mexico & S. Atl. Fish. 

 Manage. Counc. 1985). Cobia are 

 migratory and usually absent from 

 commercial and recreational catches 

 of the northern Gulf during late fall 

 and winter at which time they are 

 caught off the Florida Keys. They 

 migrate north and west along the 

 Gulf coast during the spring (Shaffer 

 and Nakamura 1989) and reappear in 

 northern Gulf waters during March 

 and April (Springer and Pirson 1958). 

 Cobia are usually caught in shallow 

 coastal waters (Shaffer and Naka- 

 mura 1989), although they are often 

 taken offshore along the Louisiana 

 and Texas coasts in association with 

 oil and gas platforms and rafts of 

 Sargassum (RFS, pers. observ.). 



Manuscript accepted 16 July 1992. 

 Fishery Bulletin, U.S. 90:668-677 (1992). 



'Contribution LSU-CFI-91-6 of the Coastal 

 Fisheries Institute, Louisiana State Univer- 

 sity. 



Despite the recreational value of 

 cobia, its ecology, distribution, and 

 morphological development during 

 early life stages are poorly known. 

 Only 23 specimens <20mmSL are 

 reported in the historical literature 

 for the Gulf (Dawson 1971, Finucane 

 et al. 1978ab, Houde et al. 1979). 

 Richards (1967) reviewed cobia gen- 

 eral life history, Shaffer and Naka- 

 mura (1989) compiled biological data, 

 and Hassler and Rainville (1975) de- 

 veloped techniques for hatching and 

 rearing cobia. In addition, Johnson 

 (1984) discussed the utility of cobia 

 early life stages for examining pre\i- 

 ous phylogenetic hypotheses and the 

 evolutionary interrelationships of ech- 

 eneoids (Rachycentridae-Corj'phae- 

 nidae-Echeneididae). Aspects of early 

 egg development have been described 

 (Ryder 1887, Joseph et al. 1964, Has- 

 sler and Rainville 1975) but not devel- 

 opment of early larvae <12.6mmSL. 

 Most larval illustrations and photo- 

 graphs are of poor quality (Ryder 

 1887, Dawson 1971, Hassler and 

 Rainville 1975, Finucane et al. 1978a, 

 Johnson 1984). The objectives of this 

 study are to describe cobia egg and 

 larval development, to provide data 

 on the seasonal occurrence, distribu- 

 tion, and ecology (i.e., relationship of 

 lai-vae to water temperature, salinity, 

 and station depth at time of capture) 

 of early life stages of cobia in the 

 Gulf, and to further examine the in- 

 terrelationships of the echeneoids. 



668 



