Detection of Interspecies Hybridisation Between Australian (Carcharhinus tilstoni) and Common Blacktip Shark (C. limbatus): Implications for the Australian Fishery

Interspecies hybridisation in nature is a well-studied phenomenon, but has not been detected in the class Chondrichthyes (sharks, rays and chimeras). Two black-tip whaler shark species (Australian, Carcharhinus tilstoni; Common, C. limbatus) have overlapping distributions in Australia, distinct mitochondrial DNA sequence (ND4, COI, control region) and distinct morphological features such as length at sexual maturity, length at birth and number of vertebrae. A mismatch was observed between species identification using mtDNA sequence and species identification using morphological characters. To test whether hybridisation between the two species was responsible, a nuclear gene with species-specific SNP was sequenced. Extensive interspecies hybridisation was found to be occurring. Hybrids were found from five locations on the eastern Australian coastline, spanning two thousand kilometres. If hybrid fitness is low and hybrids are common, then fisheries recruitment may be overestimated and the productivity of the black-tip shark fishery may be well below that required to support commercial exploitation. Shark species identification using mtDNA may be inaccurate due to hybridisation, particularly for closely-related, sympatric species. This extraordinary species complex provides a unique opportunity to investigate the ability of sharks to adapt to environmental change.