Genetic population structure of sheepshead, Archosargus probatocephalus (Sparidae), a coastal marine fish off the southeastern United States: multiple population clusters based on species-specific microsatellite markers

Sheepshead, Archosargus probatocephalus (Walbaum, 1792), have long been separated into three subspecies based on meristic traits, particularly counts of pigmented bars on the sides of the body. Here, we evaluate sheepshead genetic variation—both mtDNA and microsatellite—in the domain of the two morphologically defined subspecies, Archosargus probatocephalus probatocephalus and Archosargus probatocephalus oviceps. Sheepshead were collected in coastal habitats of the southeastern United States, from North Carolina (Atlantic Ocean) to Texas (Gulf of Mexico). Significant variations in the mtDNA control region data indicated that the sheepshead is divided into two populations by a genetic break originating somewhere between Apalachicola (Florida gulf coast) and Indian River (Florida Atlantic coast), leaving a wide margin of uncertainty as to the exact area of the division. However, use of species-specific microsatellite markers revealed that the sheepshead is geographically structured by two genetic breaks into three population clusters. The first genetic break occurs at Apalachee Bay, close to the boundary between the two morphologically defined subspecies (i.e., between Apalachicola and Steinhatchee, on Florida's gulf coast). The second genetic break occurs off south Florida (Atlantic Ocean), where it coincided with a discontinuity of estuarine habitat between Miami and Palm Beach that likely impedes gene flow between populations. This second genetic boundary had been unrecognized for sheepshead, although it has been widely recognized as a genetic boundary for many other nearshore fish species in the southeastern United States.