Vulnerable species: Disappearance of this apex predator would destabilize local ecosystems, affecting species diversity and productivity. Photo: Henley Spiers
It is one of the planet's most iconic top predators, but like many other shark species, the great white is classified as vulnerable. In the last fifty years, the number of white sharks in the world has nearly halved. In Europe, the species is considered critically endangered.
Collaborating with a world-wide network and using new approaches, researchers at Nord University have now mined the genes of the great white shark in unprecedented detail.
Using genetic markers, known as SNPs (single nucleotide polymorphisms), they have found that the world's white sharks are distributed as three different populations or lineages which rarely interbreed: a North Atlantic/Mediterranean population, a population in the southern Pacific Ocean (Indo-Pacific), and a population in the northern Pacific Ocean.
The findings were published recently in the prestigious journal Current Biology.
“Knowledge of genetic diversity is important when it comes to protecting vulnerable and endangered species”, says Professor Galice Hoarau at Nord University.
International collaboration
Together with Professor Leslie Noble, he leads the research group that has headed up an international collaboration to develop the new genetic markers and apply them to white sharks. These markers can be used to study genetic diversity and population history in a variety of endangered shark species.
“As top predators, sharks play an extremely important role in marine ecosystems. They are something that occupies a large space in our imagination, but when it comes to research, there is still much lacking. Especially when it comes to genetics”, says Hoarau.
“Using our new tools, and collaborating with shark researchers around the world, we have produced the first truly global view of connectivity in white shark populations” adds Noble.
The researchers estimate that the white shark went from being one single population to splitting into three geographically separated populations between 100,000 and 200,000 years ago.
“This coincides with the Penultimate Ice Age, when low sea levels, lower temperatures, and changed ocean currents created new barriers for migrating animal species” says Dr. Catherine S. Jones, lead collaborator from the University of Aberdeen.
“When sea levels and temperatures rose, white sharks moved into new habitats further north but became isolated, perhaps when currents changed”.
“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”, says Hoarau.
The Penultimate Ice Age, also called the Saale Glacial, lasted from 240,000 to 130,000 years ago.
“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”.
New genetic tools
To examine the great white shark's genome, the researchers used a technique called Genome-wide Target Capture.
“What we do is select a set of genes across the entire genome. In total, we are talking about 5-6000 genes. To fish out these genes, we use a bait, which is then attached to small magnetic beads”, explains Hoarau.
The article continues below the fact box.
Genome-wide target capture
• This is a technique where specific regions or DNA sequences from a genome are isolated and sequenced using a molecular bait.
• The bait consists of biotin probes, which are molecules designed to bind to specific (pre-selected) DNA sequences.
• When the bait binds to the selected DNA sequences, it is said to hybridise.
• After hybridisation, all DNA that did not bind to the bait is removed.
• The bait with the selected DNA sequences is retrieved by binding it to small magnetic beads.
• The sequences are now ready to be analysed. The genetic code can be read, and deviations such as mutations or other genetic variations can be identified.
Inside these gene sequences, one can then find the small variations - the SNPs - where a single base pair in the DNA strand is different from the other individuals' DNA.
“We have analysed tens of thousands of SNPs, which have shown us that there are three completely isolated lineages in the white shark. To be sure those results are correct, we have also sequenced the entire genome of some individuals” says Noble.
“The result is that we now have a tool that can be used to study a whole range of other shark species”, affirms Hoarau.
The different populations of white sharks are exposed to various dangers, both natural and due to human activity.
“In the Mediterranean, where the population is declining sharply, white sharks are caught as bycatch in fisheries. Also, their natural food, tuna and marine mammals, have declined sharply in the last 50 years, while there is increasing pollution in the coastal areas, where sharks mate and breed”, asserts Jones.
Around Cape Town, South Africa, and off California, local population collapses have been experienced, attributed to attacks from killer whales, illegal shark fin trade, and climate change. Around Australia and the Eastern Cape of South Africa, attempts to keep them away from beaches have led to increased mortality among the white sharks.
A shift in conservation perception
Professor Hoarau emphasizes that genetic knowledge must be used when implementing measures to protect white shark populations.
“We need to know which population we are dealing with, as these are the management units for conservation. 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”, says Hoarau.
Noble states that the output of this global collaboration shifts conservation perception of white sharks. During migrations individuals of these genetically isolated lineages may 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”, he says.
“Should these barriers change lineages may interbreed more frequently, but our findings suggest the offspring may not be viable. Add that to other challenges, and without refocused conservation management the outlook for this apex predator and their ecosystems looks less than promising.”
