Warming ocean temperatures, shifting sea currents and significant changes to the way that the food pyramid is structured are all reasons that scientists will be considering when contemplating why sharks are starting to hybridise, or cross breed in Australian waters.
Scientists have identified the first-ever hybrid shark off the coast of Australia, a discovery that suggests some shark species may respond to changing ocean conditions by interbreeding with one another, the ABC has reported.
A team of 10 Australian researchers identified multiple generations of sharks that arose from mating between the common blacktip shark (Carcharhinus limbatus) and the Australian blacktip (Carcharhinus tilstoni), which is smaller and lives in warmer waters than its global counterpart.
“To find a wild hybrid animal is unusual,” the scientists wrote in the journal Conservation Genetics. “To find 57 hybrids along 2000 km of coastline is unprecedented.”
James Cook University professor Colin Simpfendorfer, one of the paper’s co-authors, emphasized in an email 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 1000 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.
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