Tringali and Wilson mtDNA analysis of two stocks of Sardinella aurita 



369 



(1981) conducted a protein electrophoretic comparison 

 at 24 presumptive loci of Brazilian Sardinella which 

 had been first identified as either S. brasiliensis or S. 

 aurita by high or low gill-raker counts, respectively. 

 They reported no electrophoretic differences between 

 the two putative species, which is consistent with our 

 finding of significant variation in gill-raker counts be- 

 tween specimens from the Florida Panhandle and 

 Tampa Bay in the apparent absence of any genetic 

 differences (Table 1). Thus, the higher gill-raker counts 

 which supposedly distinguish S. brasiliensis from S. 

 aurita are probably due to non-genetic causes, and are 

 of no greater taxonomic importance than the intraspe- 

 cific variation in gill-raker counts observed between S. 

 aurita inhabiting the Panhandle and Tampa Bay loca- 

 tions of the eastern Gulf of Mexico (Table 1). Until the 

 present classification of Western Atlantic Sardinella is 

 supported by a thorough study of variation in the char- 

 acters on which it is based, the validity of S. brasiliensis 

 as a species is in question. 



Given the non-reticulate nature of mtDNA trans- 

 mission, it is not difficult to understand intraspecific 

 divergence when there is a mechanism between popu- 

 lations affecting the balance between gene flow and 

 genetic drift. There are a number of potential barriers 

 to gene flow within the trans-equatorial range of S. 

 aurita, including diverging oceanic currents (e.g., the 

 North Equatorial current) and the major river plume 

 of the Orinoco which can extend far out into the Carib- 

 bean. Even without discrete physical barriers, the sto- 

 chastic process of mtDNA lineage survival or extinc- 

 tion might in itself be enough to alter haplotype 

 frequencies in this continuously-distributed species if 

 the effective gene flow is low relative to the sizes of 

 the populations or their geographic ranges (Neigel & 

 Avise 1986). However, the mtDNA variability among 

 S. aurita might be a product of differential selective 

 forces because the species is distributed over so great 

 a latitudinal (environmental) gradient. For the Span- 

 ish sardine of the western Atlantic, there are not yet 

 enough data to choose between the alternative hypoth- 

 eses of drift vs. selection in accounting for the evident 

 genetic structuring. 



To our knowledge, this report represents the first 

 substantiated case of a holopelagic teleost with a sup- 

 posedly continuous distribution between sample regions 

 (Gulf of Mexico to southern Brazil) exhibiting signifi- 

 cant heterogeneity in mtDNA haplotype frequencies. 

 Whereas other studies (e.g.. Graves & Dizon 1989) have 

 compared sample populations of holopelagic fishes re- 

 moved by great longitudinal distances, or on opposite 

 sides of land masses (Ovenden et al. 1989), ours is the 

 first to compare specimens at near-opposite ends of a 

 trans-equatorial distribution spanning many degrees 

 of latitude (30°N to 26°S). Not only do these results 



further confirm the potential usefulness of mtDNA in 

 studies of hypothesized fishery stocks, but also sug- 

 gest the need for additional studies on fishes having 

 similar distributions. 



Acknowledgments 



We thank Dan Woodsen, Harry Mofield, and all those 

 who assisted in the collection of S. aurita specimens 

 from the Gulf of Mexico. We also thank Drs. Y. 

 Matsuura (Instituto Oceanographi, Sao Paulo, Brazil) 

 and Frank Miiller-Karger (University of South Florida) 

 for help in obtaining the Brazilian specimens. Joyce C. 

 Miller kindly provided the Restsite v. 1.2 computer pro- 

 gram. 



Partial support for this research came from an 

 Aylesworth Marine Scholarship (Florida Sea Grant) 

 and from grants-in-aid-of-research to MDT from Sigma- 

 Xi, Cortez Chapter of the Organized Fishermen of 

 Florida, the USF Department of Marine Science, and 

 by NOAA Grant #NA89WC-H-MF028 (MARFIN) to 

 RRW. 



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