Skipping across the tropics: the evolutionary history of sawtail surgeonfishes (Acanthuridae: Prionurus)

Fishes described as "anti-equatorial" have disjunct distributions, inhabiting temperate habitat patches on both sides of the tropics. Several alternative hypotheses suggest how and when species with disjunct distributions crossed uninhabitable areas, including: ancient vicariant events, competitive exclusion from the tropics, and more recent dispersal during Pliocene and Pleistocene glacial periods. Surgeonfishes in the genus Prionurus can provide novel insight into this pattern as its member species have disjunct distributions inhabiting either temperate latitudes, cold-water upwellings in the tropics, or low diversity tropical reef ecosystems. Here the evolutionary history and historical biogeography of Prionurus is examined using a dataset containing both mitochondrial and nuclear data for all seven extant species. Our results indicate that Prionurus is monophyletic and Miocene in origin. Several relationships remain problematic, including the placement of the Australian P. microlepidotus, and the relationship between P. laticlavius and P. punctatus. Equatorial divergence events between temperate western Pacific habitats occurred at least twice in Prionurus: once in the Miocene and again in the late Pliocene/early Pleistocene. Three species with tropical affinities, P. laticlavius, P. punctatus, and P. biafraensis, form a clade that originated in the Pliocene. These results suggest that a variety of mechanisms may regulate the disjunct distribution of temperate fishes, and provide support for both older and younger equatorial crossing events. They also suggest that interspecific competitive exclusion may be influential in fishes with "anti-equatorial" distributions.