Hybridisation among coral reef fishes at Christmas Island and the Cocos (Keeling) Islands

Abstract

Hybridisation is common among terrestrial and freshwater species, but is considered rare and insignificant in marine systems. Christmas and Cocos (Keeling) Islands represent a recently discovered marine suture zone that contains a high number of hybridising coral reef fishes. In this study we document a further eight species that are hybridising, bringing the total number of reported hybrid crosses to 15, involving 27 species across eight families. So far, eight of the 15 hybrid crosses have been genetically confirmed. There is a taxonomic bias to this hybridisation,with chaetodontids and acanthurids containing the greatest number of hybridising species. Hybridisation commonly involves Indian and Pacific Ocean sister species that are coming into secondary contact. In most cases, at least one parent species is rare and the lack of conspecific partners is leading to the formation of heterospecific social groups and the breakdown of assortative mating. The discovery of this hybrid hotspot provides a unique and importantopportunity to examine speciation in the marine environment.