High genetic diversities and complex genetic structure in an Indo-Pacific tropical reef fish (Chlorurus sordidus):: evidence of an unstable evolutionary past?

Historical sea level fluctuations have influenced the genetic structure and evolutionary history of marine species and examining widespread species across their species' ranges may elucidate some of these effects. Chlorurus sordidus is a common and widespread parrotfish found on coral reefs throughout the Indo-central Pacific. We used phylogenetic, phylogeographic, and cladistic analyses to examine the genetic composition and population structure of this species across most of its latitudinal range limits. We sequenced 354 bp of the mitochondrial control region I in 185 individuals from nine populations. Populations of C. sordidus displayed high levels of genetic diversity, similar to those recorded for widespread pelagic fish species, but much greater nucleotide diversity values than those previously recorded for other demersal reef fishes. Both phylogenetic and phylogeographic analyses detected strong genetic subdivision at the largest spatial scale (i.e. among oceans). The Pacific Ocean was characterised by weak population genetic structure. Separation of the Hawaiian location from other Pacific and West Indian Ocean sites was evident in phylogenetic analyses, but not from analysis of molecular variance. NCA and isolation-by-distance tests suggested that the genetic structure of this species was the result of multiple contemporary and historical processes, including long-distance colonisation and range expansion arising from fluctuating sea levels, limited current gene flow, and isolation by distance. This pattern is to be expected when historically fragmented populations come into secondary contact. We suggest the patterns of population genetic structure recorded in C. sordidus are caused by large local population sizes, high gene flow, and a recent history of repeated fragmentation and remixing of populations resulting from fluctuating sea levels.