New gene-sequencing tools have just unlocked a well-kept secret – that there are three different kinds of great white shark and, unfortunately for them, they tend not to mix.
The white sharks that populate the north Pacific are distinct from those of the south Pacific and Indian Ocean – and both groups differ from those of the north Atlantic and Mediterranean.
Though still all belonging to the species Carcharadon carcharias the three groups were physically separated during the “Penultimate Glacial Period” between 240,000 to 130,000 years ago. This ice age saw sea levels and water temperatures fall then rise, and ocean currents alter, creating invisible barriers to shark migration.
Each group developed its own distinct genetic profile and remains isolated from the others when it comes to breeding. As this process continues, and if each group survives, it will eventually develop into a distinct species.
Researchers from Nord University in Norway, led by professors Galice Hoarau and Leslie Noble, collaborated with an international scientific team to study great white sharks in what they say is unprecedented detail.
They examined 106 specimens and used genetic markers called SNPs (single nucleotide polymorphisms) to sequence the entire genomes of 17 of them, along with selected parts of the genomes of the others.
Each shark could be readily identified as belonging to one of the three populations – with one exception, a single hybrid shark that displayed a combination of Indo-Pacific and North Pacific genes.
Threats to sharks
Genetic divergence dating from the Penultimate Glacial Period has been noted before in a wide range of species. “For example, the same type of divergence has been found in manta rays around South Africa at the same time,” says Prof Hoarau.
But the finding is worrying for white sharks in that each group depends on a smaller group to pass on its genes than previously realised, and the loss of any one group would remove from the pool genes unlikely to have survived elsewhere.
The global white shark population is thought to have almost halved during the past half-century. The Mediterranean population continues to decline sharply as sharks become bycatch in fisheries, their natural prey of tuna and seals declines and pollution increases.
Around Cape Town in South Africa and off California local populations have collapsed through orca predation, illegal shark-finning and climate change, while around Australia and South Africa’s Eastern Cape anti-shark measures designed to keep them away from beaches kill many.
Current signposts
During migrations it is thought that individual sharks use major ocean currents as signposts, keeping to specific areas and potentially maintaining their separation.
“Recent work points to climate change modifying several ocean currents, with a tipping point forecast to change their strength and position in less than 50 years,” says Prof Noble.
“Should these barriers change, lineages may interbreed more frequently, but our findings suggest that the offspring may not be viable. Add that to other challenges, and without refocused conservation management the outlook for this apex predator and its ecosystems looks less than promising.”
Prof Noble says that by using the new tools “and collaborating with shark researchers around the world we have produced the first truly global view of connectivity in white shark populations”. Now the team hopes that the tools can be applied to study a range of other shark species. The study is published in Current Biology.
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