A deceptive little nibbler from the deep

A dolphin leaps out the water. The water on its back glistens in the sun. It’s picture perfect. Well, almost. Something isn’t quite right.

On its back a perfectly circular plug of flesh is missing.

And dolphins are not the only victims. Whales often surface with the same circular wounds. Time after time tuna are hauled up in nets looking like someone has taken a cookie cutter to their sides.

What has caused these bizarre looking wounds? The culprit is the aptly named cookie cutter shark. It doesn’t look like the most vicious of sharks. In fact, next to its larger relatives it looks rather puny and unimpressive. However, it is quite a vicious little thing with a very specialized mouth.

When it feeds the shark places its mouth on the skin of its victim, retracts it tongue to create negative pressure, and creates a tight seal to attach itself securely. It then uses its top teeth to anchor itself while the lower ones slice into the flesh. Using its body the shark then rotates. This creates the characteristic circular wound and scoops out a flesh cookie!

It’s a deceptive little shark as well. It uses bioluminescence in a clever way to get close to the substantially bigger animals it feeds on. In order to understand how it does this we need to go over the basics of visual neuroscience.

We have two sets of light receptive cells in the back of our eyes; cones for seeing colour and rods for detecting whether light is present or not (seeing black and white). There are a limited number of cones but a whole range of different wavelengths of light. In order to detect what wavelengths of light are entering the eye the brain integrates the signals, adding and subtracting the information coming from each of the cones. This requires high light levels. The rods on the other hand detect whether light is present or not, meaning they are very sensitive to low light.

As you go deeper and deeper in the oceans less and less light is present – this is because gradually all the wavelengths of light are absorbed. It therefore follows that animals which live deeper in the sea don’t see in colour; they predominantly have rods so they are sensitive to the low light levels.

But what does all of this have to do with the cookie cutter shark? And how does it allow them to get close enough to dolphins and whales so they can scoop out flesh cookies?

When the larger animals, which live deeper in the ocean, look up towards the surface they simply see light and dark, black indicating there’s an object there, white meaning that there isn’t. The bellies of the cookie cutter sharks are lined with bioluminescent cells apart from a small patch that looks like a collar. When the cookie-cutter sharks light up their stomachs they become camouflaged to any animal looking up.

It is thought that the dark collar makes the cookie cutter shark look like a small, tasty fish. When larger animals are tempted to move towards this snack, the shark gets a chance to seal its lips onto the animal’s flesh.

This one awesome little shark! Deceptive little nibbler!