“We Have No Idea”: Decades-Old Mystery About Great White Sharks Just Got Even Stranger

The issue relates to stark differences between the DNA in their nuclei and the DNA in their mitochondria. Although these differences have been known for some time, scientists have previously pointed to the animals’ migration patterns as a way to explain them. However, this new study has tested this assumption by assessing the genetic differences between white shark populations across the planet, which has invalidated the theory.

To understand what’s going on, we have to travel back in time to around 10,000 years ago and the end of the last ice age. Before this point, sea levels were much lower than they are now, which restricted the population distribution of these sharks. In fact, the situation was so bleak for white sharks that they dwindled to just one well-mixed population somewhere in the southern Indo-Pacific Ocean, according to the new work by Gavin Naylor, director of the Florida Program for Shark Research at the Florida Museum of Natural History, and colleagues.

This was a tough time for the population and could have pushed them dangerously close to extinction. Glaciers were around a mile high and extended from the poles, locking away so much water that, around 25,000 years ago, sea levels had dropped to merely 40 meters (131 feet), greatly reducing white sharks to what the Florida Museum of Natural History refers to as an “oceanic corral”.

But once the cold snap ended and the glaciers melted, global sea levels increased, and the sharks were able to spread out. Then, around 7,000 years ago, the researchers claim, the large predatory fish began to genetically diverge, suggesting they had become two or more isolated populations. Today there are three genetically distinct white shark populations – one in the southern hemisphere around Australia and South Africa, one in the northern Atlantic, and one in the northern Pacific. And, as the apex predator, their numbers remain low despite their widescale distribution.

“There are probably about 20,000 individuals globally,” Naylor explained in a statement. “There are more fruit flies in any given city than there are great white sharks in the entire world.”

But something weird happened during the shark’s recovery that is completely unclear. Despite their efforts, Naylor and colleagues have had to concede a begrudging point:

“The honest scientific answer is we have no idea,” the researcher said.

The problem first emerged back in 2001 when scientists compared genetic samples of sharks from Australia, New Zealand, and South Africa. The DNA produced and stored in the nuclei of these sharks’ cells was all pretty much the same, but the mitochondrial DNA – the site of the cell’s energy production and metabolic functions – of the South African sharks was distinct to the others.

This could be explained by the sharks sticking close together and rarely crossing into neighboring groups. As time goes one, this would lead to genetic mutations that accumulate in each group, which could eventually lead to a whole new species.

But if this were the case, why were the differences only present in the mitochondrial DNA and not the nuclear DNA as well?

Well, the researchers of this study initially proposed that, while male sharks went off roaming vast distances throughout the year, females pretty much stayed in one place or came back to the same spot each year to breed. This type of migration pattern is called “philopatry”. As mitochondrial DNA is most often passed onto subsequent generations from the female of a species, this could settle the problem. And in fact it is now known that female white sharks do actually travel back to the same places to mate.

And yet there is just one problem with this over 20-year-old explanation – no one has actually tested it. This was mostly because white sharks are hard to find and, even when you locate one, it’s not exactly easy to get a DNA sample from a fish that can snack on you.

Naylor has now been collecting data from white sharks for around 13 years.

“I wanted to get a white shark nuclear genome established to explore its molecular properties,” he explained. “White sharks have some very peculiar attributes, and we had about 40 or 50 samples that I thought we could use to design probes to look at their population structure.”

The samples he and his colleagues collected have come from across the world and have allowed them to sequence the DNA from around 150 sharks’ mitochondrial genomes. Their results confirmed the findings first identified in 2001 – at the population level, white sharks in the North Atlantic have rarely mixed with those in the South Atlantic, just as the sharks in the Pacific didn’t mix with those in the Indian Ocean. The nuclear DNA among all these sharks remained pretty consistent, but the mitochondrial DNA had much more variation.

So does the old philopatric theory still hold up? Well, during their research the team found no nuclear data to support it.

In order to conduct a more sophisticated test for the mitochondrial genomes, they were required to reconstruct the evolutionary history of white sharks, which is what led them to the discovery that the whole species had once been reduced to its tiny southern population during the last ice age.

“They were really few and far between when sea levels were lowest. Then the population increased and moved northward as the ice melted. We suspect they remained in those northern waters because they found a reliable food source,” Naylor said.

Specifically, the sharks probably found seal populations in these waters. Once they had snuffled a few blubbery meals, they could breed and then move on. The key to the philopatry theory, however, required the team to identify when these populations diverged from here and whether there was enough time for their mitochondrial DNA to have accumulated all the differences they exhibit today.

Unfortunately the simulation the team produced to test this came back negative – philopatry may be a behavior among great white sharks, but it is not responsible for the large mitochondrial differences. So what’s going on?

Initially, Naylor believed the sharks may engage in a reproduction behavior seen in other organisms, like meerkats and cichlid fish, whereby a few females contribute to the population from one generation to the next. But again, this did not hold up to their analysis.

There is a third explanation for what’s going on here, but the scientists think it is extremely unlikely to be the case. Small populations, such as those of the great white shark, are vulnerable to something called genetic drift. This means that when mutations occur – even harmful ones – they have a much higher likelihood of being passed on to subsequent generations. 

In larger populations, a harmful genetic mutation has a greater chance of being eliminated by natural selection. But in a small one, a single animal could generate a mutation that causes big changes to the population due to this genetic drift.

But genetic drift is a random process; it cannot selectively target one type of DNA and leave another untouched. This, Naylor argues, leaves natural selection as the only other possibility, which is itself extremely unlikely because of the shark’s small population. To be able to explain this phenomenon, the selective force would have to be, as Naylor said, “brutally lethal.”

So while this research attempted to uncover the answer to the sharks’ strange genetics, it has merely deepened the mystery, which begs for further study.

The study is published in PNAS.