Weirdest shark

The cookiecutter shark is easily the weirdest shark around: it uses bioluminescence to lure large predators, feeds by suction, sheds whole rows of teeth at once and swallows them, and by weight can be more than one third liver.

Cookiecutter shark
Jennifer Strotman, Public domain, via Wikimedia Commons

There’s a lot of competition for weirdest shark – have you ever seen a picture of a goblin shark? – but the cookiecutter shark is my personal favourite. It is to sharks what the platypus is to mammals, a strange outlier with many unusual adaptations.

It’s a tiny little shark, rarely getting above 60cm long, but it feeds on some of the largest and most dangerous animals of the sea: blue whales, leopard seals, great white sharks, dolphins, tuna, and stingrays. It’s not a super-predator as such, because it doesn’t kill these ocean giants; instead, it latches onto their sides and takes out circular bite-sized chunks. This is where it gets its name, because the bite mark looks like a cookie-cutter has cut into the side of the victim.

How do you lure large predators? Well, the cookiecutter shark is bioluminescent: it glows green. It glows green everywhere except a small collar around its neck. Why does it have a dark collar? The current theory is that the green glow serves as camouflage if the shark is seen from below, but the non-camouflaged collar looks like a small wriggling fish. Predators swim up for a quick bite, and instead get a surprise bite of their own.

The actual mechanism of the bite is also pretty amazing: the cookiecutter shark’s top teeth are thin like needles whereas its bottom teeth are essentially one long saw. The needles jab in, the target’s flesh is pulled in by powerful suction, the teeth saw around, and then the shark detaches and swims away. Because the bottom teeth are all connected the shark cannot shed individual teeth like most sharks; instead, it sheds them a whole row at a time. Because it takes a lot of effort (and calcium) to grow a row of teeth the cookiecutter shark swallows that discarded row so it can recycle the ingredients for more.

Finally, and this is one that really surprised me, cookiecutter sharks have enormous livers. Like, in some cases more than a third of their entire body weight is the liver. Now, all sharks have large livers. Their livers are full of a substance called squalene – it’s lighter than water, so it helps to keep the shark afloat. For its size the cookiecutter shark has a very heavy skull and jaw. I imagine that this is because of how it eats. Because it weighs so much, it needs a large liver to keep it buoyant.

So there you go, a glow-in-the-dark vacuum-cleaner shark that lures and hunts great whites, eats its own teeth, and is effectively a floating liver. Weird.

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