Johnston Atoll: Reef Fish Hybrid Zone between Hawaii and the Equatorial Pacific

Abstract

Johnston Atoll is isolated in the Central Pacific Ocean (16°45′ N 169°31′ W) about 1287 km (800 miles) southwest of Honolulu, Hawaii and 1440 km (900 miles) north of the equatorial Line Islands, Kiribati. The labrid species, Thalassoma lutescens, has a wide range of distribution in the equatorial Pacific. The related species, Thalassoma duperrey, is endemic to the Hawaiian Islands. The pelagic larvae of both species dispersed to Johnston Atoll, where we found a mix of adult phenotypes representing a range of hybridization events over generations. A hybrid acanthurid was also documented. In addition, the arrival and colonization of two pomacentrid (damselfish) species to the atoll was observed in 1999. These pomacentrid sister-species, Abudefduf abdominalis and A. vaigiensis, have become established populations with subsequent hybridization. The biogeography of the Johnston Atoll coral reef fish population shows some degree of local population retention. It is also evident that this biogeographic isolation is periodically compromised by large ocean current oscillations in the equatorial and central Pacific Ocean that bring larval fishes from either Hawaii or the Line Islands, and may distribute Johnston Atoll originating larvae elsewhere as well. The reef fauna and oceanography of this atoll provides the circumstances for improving scientific insight into marine fish speciation and island biogeography.