Researchers from Norway’s Nord University have revealed for the first time that there are three distinct types of great white shark, following a split in the global population some 100,000 to 200,000 years ago.
Although the differences between the three populations are not great enough to mark them as different species, the scientists say that they are at least sufficient to prevent the three groups – with rare exceptions – from mingling.
Great whites (Carcharodon carcharias) – often referred to nowadays as simply ‘white sharks’ – are one of the most iconic (and, perhaps, misunderstood) ocean predators. They are a very cosmopolitan species, found in temperate to cool waters across the globe, but their numbers have been greatly reduced over the last 50 years.
Although there has recently been a resurgence of the northwestern Atlantic population, great whites are globally listed as Vulnerable to extinction by the IUCN Red List of Threatened Species, with the European/Mediterranean populations listed as Critically Endangered, and the overall population trend given as Decreasing.
In order to differentiate between the populations of great whites, the scientists examined the DNA of the sharks to look for single nucleotide polymorphisms (SNPs, pronounced ‘snips’), genetic markers which can help scientists identify different traits within a species. They are often used in humans to determine which groups are genetically susceptible to a particular disease.
Identifying the SNPs is carried out through a technique known as ‘genome-wide target capture’, in which specific parts of a DNA sequence are isolated for analysis, making it easier to identify the tiny variations between DNA taken from different individuals.
‘We have analysed tens of thousands of SNPs, which have shown us that there are three completely isolated lineages in the white shark,’ said Professor Leslie Noble, one of the new study’s lead authors. ‘To be sure those results are correct, we have also sequenced the entire genome of some individuals.’
The researchers estimate that the genetic split began 100,000 and 200,000 years ago, during the Penultimate Glacial Period, when sea levels were much lower and water temperatures and currents very different from today.
The physical and temperature-related barriers present during this period would have prevented animals from migrating, effectively trapping the three populations of great whites until sea levels began to rise again
‘When sea levels and temperatures rose, white sharks moved into new habitats further north but became isolated, perhaps when currents changed,’ said Professor Galice Guillaume Hoarau, also one of the study’s lead authors. ‘Placing a timestamp on such population split is always difficult, and this is a hypothesis where we calibrate divergence – the genetic separation – with the molecular clock.
‘What strengthens our hypothesis is that we have looked at the entire genome,’ said Hoarau. ‘Genetic divergence during this period is a pattern we find in a wide range of species, both algae and animals. For example, the same type of divergence has been found in manta rays around South Africa at the same time.’
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Understanding that populations of great whites are genetically distinct has several implications for the conservation of the species. Firstly, it appears that the three populations are not – or, at least, very rarely – interbreeding.
Although the study acknowledges that the data collected does not conclusively prove that the populations are genetically unable to reproduce, the presence of only a single hybrid shark identified during the study does seem to suggest that the populations are ‘reproductively isolated’.
Secondly, if the different types of great white really are not capable of interbreeding, it makes the conservation of the Critically Endangered Mediterranean population even more important.
The different populations are subject to a number of different threats, most of them man-made. In the Mediterranean, this includes accidental bycatch, the depletion of food resources such as tuna, and pollution of the coastal areas the sharks frequent.
In South Africa, predation by orcas (killer whales) has been implicated in local population collapse; attempts to protect bathers from sharks has led to similar declines in Australian waters, and the illegal trade in shark fins is an ever-present threat.
‘We need to know which population we are dealing with, as these are the management units for conservation,’ said Hoaru. ‘If all the white sharks belonged to one large, integrated global population, it would not matter so much if they disappeared from one area.
‘But if white sharks in, for example, the Mediterranean, constitute a separate genetically distinct population, and this were to be wiped out, we would lose unique genetic diversity. Therefore, we must know the distribution of the different management units and the health of each.’
The complete paper, ‘Genome analysis reveals three distinct lineages of the cosmopolitan white shark’, by Leslie R Noble, Catherine S Jones, Galice Hoarau et al is published under an open access licence in the online journal Current Biology.
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