“To find a wild hybrid animal is unusual,” the scientists wrote in the journal Conservation Genetics. “To find 57 hybrids along 2,000 km [1,240 miles] of coastline is unprecedented.”
James Cook University professor Colin Simpfendorfer, one of the paper’s co-authors, emphasized in an e-mail that he and his colleagues “don’t know what is causing these species to be mating together.” They are investigating factors including the two species’ close relationship, fishing pressure and climate change.
Australian blacktips confine themselves to tropical waters, which end around Brisbane, while the hybrid sharks swam more than 1,000 miles south to cooler areas around Sydney. Simpfendorfer, who directs the university’s Centre of Sustainable Tropical Fisheries and Aquaculture, said this may suggest the hybrid species has an evolutionary advantage as the climate changes.
As a result, he wrote, “We are now seeing individuals carrying the more tropical species genes in more southerly areas. In a changing climate, this hybridization may therefore allow these species to better adapt to different conditions.”
The researchers — who had been working on a government-funded study of the structure of shark populations along Australia’s northeast coast — first realized something unusual was going on when they found fish whose genetic analysis showed they were one kind of blacktip but their physical characteristics, particularly the number of vertebrae they had, were those of another. Shark scientists often use vertebrae counts to distinguish among species.
The team also found that several sharks that genetically identified as Australian blacktips were longer than the maximum length typically found for the species. Australian blacktips reach 5.2 feet; common blacktips in that part of the world reach 6.6 feet.
Demian Chapman, assistant director of science of Stony Brook University’s Institute for Ocean Conservation Science, said the idea that sharks can interbreed is “something a lot of shark biologists thought could happen but now we have evidence, and it’s fantastic evidence.”
He added, however, that the fact that these two species were so closely related made it easier for them to mate than wildly-divergent ones.
“It doesn’t mean we’re going to see great-white-tiger sharks anytime soon, or bull-Greenland sharks,” he said. “If any species was going to hybridize, it was going to be this pair.”
Chapman, who first documented in 2008 that some female sharks can reproduce without having intercourse, said this latest discovery suggests “there’s yet another path to reproduction that these species can do. It just reinforces that sharks can do it all when it comes to reproduction.”