Biogeography of the Tropical Eastern Pacific: distribution and phylogeny of chaenopsid fishes

Abstract

The Tropical Eastern Pacific Biogeographic Region (TEP) is delimited by steep thermal gradients to the north and south, by a wide expanse of open ocean (the East Pacific Barrier) to the west, and by the Central American land mass to the east. Four provinces within the TEP have been recognized based on the distribution of rocky shore fishes and marine invertebrates: the Cortez, Mexican, Panamic, and Galapagos Provinces. For rocky shore fishes, hypothesized barriers between these provinces are areas lacking rocky outcroppings, specifically the Central American Gap between the Panamic and Mexican Provinces, the Sinaloan Gap between the Mexican and Cortez Provinces, and the Pelagic Gap between the mainland and the Islas Galapagos. The occurrence of 33 chaenopsid fish species within these provinces, as well as other oceanic islands or archipelagos in the TEP (Isla de Malpelo, Isla del Coco, and Islas Revillagigedo) were tallied based on literature records and observations of museum specimens. Chaenopsid distributions within the TEP support these hypothesized provinces and their intervening gaps. Twenty-one species (64% of the TEP chaenopsid fauna) are restricted to a single mainland province or one of the oceanic islands or archipelagos. Of the mainland provinces, the Cortez and Panamic exhibit similar levels of endemism (50%), but the Mexican Province has only one endemic (10%). Of the remaining chaenopsids in the Mexican Province, three are widespread, occurring in all three mainland provinces, four are shared only with the Cortez Province, and two are shared only with the Panamic Province. Within the TEP, the Pelagic Gap is the most effective (crossed by only 3 of 33 species adjacent to it), followed by the Central American Gap (crossed by 5 of 21 species), and the Sinaloan Gap (crossed by 7 of 17 species). Only one species, Chaenopsis alepidota, which is found off southern California and in the Cortez Province, crosses a barrier delimiting the TEP. Species-level phylogenetic hypotheses for the Chaenopsidae imply exclusively allopatric speciation for these fishes in the TEP. Of the barriers delimiting the TEP, the most important in the recent evolution of chaenopsids is the Isthmian Barrier which is implicated in six speciation events. Within the TEP, the Central American Gap and Sinaloan Gap are each implicated in three speciation events, while the Pelagic Gap is implicated in three speciation events of island endemics from mainland populations and one inter-island speciation event.