Origins of isolated populations of an Indo-Pacific damselfish at opposite ends of the Greater Caribbean

The non-native Indo-West Pacific (IWP) damselfish Neopomacentrus cyanomos has two, recently discovered, isolated Northwest Atlantic (NWA) populations separated by ~ 3000 km of the Caribbean Sea. One of them spans the southern and northern Gulf of Mexico (GoMx) and the other is at Trinidad, in the southeast Caribbean. We compared DNA (CO1) sequences of 71 individuals from the southwest GoMx to those of 86 fish from Trinidad to determine similarities in their origins. In the native range, there are four distinct, largely allopatric haplogroups, and the two NWA populations are composed of individuals from the same two of those four haplogroups. Over 90% of individuals in each NWA population have six haplotypes shared between those populations, and no pairwise ΦST differences in population structure (relative abundances of different haplotypes) were found between the GoMx and Trinidad populations. Levels of haplotype diversity and nucleotide diversity are similarly high in the two NWA populations. The existence of two isolated populations in areas with major, long-standing offshore oilfields is consistent with those fish being transported to those areas of the NWA by floating offshore infrastructure moved from the IWP. The strong population similarities and genetic diversity of the two NWA populations indicate that both resulted from the introduction of a substantial number of individuals from the same part of the native range. However, existing genetic and other data do not allow determination of the sequence of events that led to the establishment of those populations, and whether one established first then effectively “seeded” the other or both were established concurrently as a result of a single introduction.