Greenland sharks: the old man of the sea

One species can really push the boat out when it comes to longevity. Greenland sharks rank as one of the longest living species on the planet, surviving for hundreds of years. Jena Edwards told Chris Smith about this elusive ocean dweller.

Jena - Well, Greenland sharks are very large, sluggish animals that have sort of a zombie-like appearance. They have very dark grey skin, sometimes with a light coating of algae, because of their very slow swim speeds. And they have these very pale milky eyes that give them a death-like appearance.

Chris - So if they've got pale milky eyes, what would they make of me?

Jena - So they have a great sense of smell. So they might know that you were there. However, they probably wouldn't be able to see you because they have a lot of corneal damage from parasitism by copepods that attach to their eyeballs. So these parasites anchor into the eyeball and they actually eat the tissue on the surface of the eye, which causes damage and thickening of the lens making it so that they can't see very well.

Chris - And what would these sharks eat then?

Jena - So they eat a wide range of things. They're very astute scavengers, but they also feed on things such as squid and fish and even Marine mammals. They also eat flat fish that typically hang out on the bottom. And also that's a great place to find dead animals that they can scavenge such as whale falls.

Chris - And this question about how long they live for, how did that surface in the first place, and why do they live so long?

Jena - So, one thing that makes Greenland sharks interesting is the fact that they're the only shark species that lives as far north as they do. So they're inhabiting very, very deep, very cold waters. And this gives them traits such as slow swim speeds and very low metabolism. So this led to the question of age, given that many shark species that are able to live quite a long time, this cold water just enhances that ability.

Chris - Terms of actually how we know that they live as long as that, how did that come to light?

Jena - Yeah. So typically sharks are aged by looking at rings on the vertebrae. However, Greenland sharks don't have any calcified structures such as these. So researchers had to actually look at the centre of their eye lens in their cornea. And the tissue that's laid down in the core of their eye lens contains carbon atoms that were incorporated during their prenatal development, so when that shark was still in the womb. And this carbon signature reflects that of the environment at the time of the shark's formation, which allows us to determine the date of its development and therefore giving us an estimate of its age.

Chris - So that's what researchers have done. They've happened to have gotten hold of some eyeballs from the sharks and then done carbon dating on the lens because it doesn't change over their lifetime. And that gives you the sort of timestamp of when this shark was born.

Jena - Exactly.

Chris - And how long does the average Greenland shark live?

Jena - So what researchers found looking at female Greenland sharks of average size, so ranging from 81 to about 500 centimetres, they found that most of these sharks were centuries old, with the oldest being 272 years, but possibly over 500 years old,

Chris - Goodness. And when they live that long, do they, like a human, kind of get to adolescence, become sexually mature, and then they're just adults for a really long time, or is the whole of their life course spread out in the sense that they take a really long time before they can even reach reproductive age and then they start reproducing?

Jena - Yeah. So that's a really interesting point. The same paper, these Danish researchers estimated that the age of sexual maturity is around 150 years. So the adolescent and juvenile stage of these sharks is very, very long and it takes quite a while before they reach adulthood and are able to reproduce.

Chris - And there must therefore be implications if a species takes 150 years before an animal is of reproductive age, that has got to have consequences for its sustainability. If any kind of threat comes along, if you see something that causes your numbers to drop, or if there's some kind of pressure on your population, it's got to have an impact.

Jena - Yeah, absolutely. And this is something we see in long-lived fish species as well. Having this really late age at maturity means that the population can't bounce back as quickly from drops in numbers. And it means that we really have to take preemptive steps to prevent population declines that exceed the point at which these animals can recover. So for something that matures at 150 years, we have to be very cautious.

Chris - Does that mean then, that actually these sharks have survived in order to have evolved to a point where they have to be 150 before they can reproduce, that actually, they've had a pretty stable and a good time over evolutionary time because they can afford to have arrived at that situation. And it's only now we've come along or other pressures in the environment have come along, that actually they could be in jeopardy?

Jena - Yeah. That would be a good thing to assume. We don't know exactly the mechanism of this aging, how they've managed to survive so long. That's still something we're looking into, but it does suggest that they have lived a very stable, um, evolutionary history.