Gold and Richardson: Sciaenops ocellatus from Mosquito Lagoon 



63 



Table 5 



Matrix of Nei's (1978) unbiased genetic distance 

 based on allozymes (upper diagonal) and Nei and 

 Tajima's (1981) corrected interpopulational nucle- 

 otide sequence divergence based on mtDNAs 

 (lower diagonal) among red drum (Sciaenops 

 ocellatus) from the northeastern Gulf of Mexico, 

 Mosquito Lagoon, Florida, and the U.S. Carolina 

 coast. Interpopulational nucleotide sequence di- 

 vergence values are in percent. 



Northeastern Mosquito Carolina 

 Gulf Lagoon Coast 



Northeastern Gulf 0.000 0.001 



Mosquito Lagoon 0.006 0.002 



Carolina Coast 0.006 0.009 



Lagoon were not found in red drum from Mosquito 

 Lagoon; whereas one inferred allele and eleven 

 mtDNA haplotypes were unique to red drum from 

 Mosquito Lagoon. The distribution of low frequency 

 nuclear-gene alleles and mtDNA haplotypes is con- 

 sistent with reduced gene flow concomitant with 

 allele-frequency drift expected in isolated subpopu- 

 lations. Finally, both females with ovaries contain- 

 ing postovulatory follicles and spawned red drum 

 eggs have been documented in Mosquito Lagoon 

 (Murphy and Taylor, 1990; Johnson and Funicelli, 

 1991), clearly indicating that red drum spawn 

 within the system. 



Assuming red drum in Mosquito Lagoon represent 

 a partially isolated, self-contained subpopulation, 

 one question of interest is how long the subpopula- 

 tion has been semi-isolated. Geological evidence 

 (Mehta and Brooks, 1973, cited from Johnson and 

 Funicelli, 1991) indicates that several tidal inlets 

 once connected Mosquito Lagoon to the Atlantic, the 

 last of which is estimated to have closed about 1,500 

 years ago. Assuming some variation in the geologi- 

 cal estimate, this date does not differ substantially 

 from an estimate of 2,900 ± 1,550 (SD) years based 

 on 1) a corrected interpopulational nucleotide se- 

 quence divergence (between red drum in Mosquito 

 Lagoon and red drum elsewhere) of 0.0058 ± 0.0031 

 (SD) percent, and 2) an evolutionary rate for verte- 

 brate mtDNA of 0.01 substitutions/bp/lineage/Myr 

 (Brown et al., 1979; Wilson et al., 1985). Given on- 

 going debates about molecular clocks, the correspon- 

 dence between the two temporal estimates is note- 

 worthy. 



Because the genetic distinctness of Mosquito La- 

 goon red drum appears to stem largely from physi- 

 cal isolation, the biological reasons for subdivision 



between red drum in the northern Gulf and those 

 along the Carolina coast remain unknown. Possible 

 reasons for this subdivision could include 1) current 

 patterns between the Gulf and U.S. Atlantic, 2) 

 absence of suitable near-shore habitats along the 

 southeastern coast of Florida, or 3) differences in 

 biogeographic provinces (Gold et al., 1993, in press). 

 Similar genetic discontinuities between U.S. Atlan- 

 tic and Gulf coast fauna have been described by 

 Avise and co-workers (reviewed in Avise, 1992). 

 Their hypothesis is that the concordant 

 phylogeographic patterns provide evidence of simi- 

 lar vicariant histories that are tentatively related to 

 episodic changes in environmental conditions dur- 

 ing the Pleistocene (Avise, 1992). The relative inac- 

 cessibility of Mosquito Lagoon suggests that sam- 

 pling red drum from north or south of Mosquito 

 Lagoon may be more informative for testing hypoth- 

 eses regarding phylogeographic subdivision between 

 the northern Gulf and the U.S. Atlantic. 



A last point to consider is the use of Mosquito 

 Lagoon red drum as broodstock for stock enhance- 

 ment programs. It could be argued that red drum 

 from Mosquito Lagoon differ genetically from red 

 drum sampled elsewhere (e.g., the northeastern 

 Gulf) and should be used only for stock enhancement 

 at localities where no genetic differences exist. Al- 

 ternatively, it could be argued that the genetic dis- 

 tinctiveness of red drum in Mosquito Lagoon is rela- 

 tively small and possibly inconsequential. This fol- 

 lows from the observation that the documented ge- 

 netic difference between red drum in Mosquito La- 

 goon and red drum sampled elsewhere is consider- 

 ably less than that, on average, among races of man 

 (Cann et al., 1987). One other consideration might 

 be to cross red drum from Mosquito Lagoon with red 

 drum from elsewhere (e.g., the northeastern Gulf) 

 in order to increase performance from potential 

 heterotic effects. 



Acknowledgments 



Assistance in procuring red drum specimens from 

 Mosquito Lagoon was provided by J. Burch, J. 

 Camper, B. Denis, C. Furman, M. Murphy, G. 

 Ramos, and D. Roberts. Their assistance is grate- 

 fully acknowledged. Special thanks are extended to 

 C. Amemiya and D. Roberts for providing no-cost 

 lodging during field trips. We also thank B. Colura 

 and B. Bumguardner for carrying out age determi- 

 nations from otoliths, D. Bohlmeyer and C. Furman 

 for assistance in the laboratory, R. Taylor for pro- 

 viding historical information on the construction of 

 Haulover Canal, and M. Murphy for providing criti- 



