The Cookiecutter shark (Isistius brasiliensis) is a small shark with large specimens barely exceeding 50 cm from tail tip to snout. They live in warmer open waters throughout the world’s oceans and feed on marine mammals such as Sea Lions and Dolphins, fish such as Tuna and Mackerel and have even been known to nibble on the sound absorbing tiles of submarines.
From this photo you can see a number of interesting things – first of all can you see how large the eyes are in relation to the body? This indicates that the Cookiecutter shark lives in deep, dark waters and in fact the Cookiecutter shark migrates vertically in the water column up to 3km each day, arriving near the surface around sundown and returning to the depths with the dawn.
Notice the dark collar around the gills? Well, the theory is that this dark patch is used as a lure to bring prey toward the shark. Now you may be wondering how a dark patch on a fish that lives in the dark can be an effective lure. Well the secret is that the Cookiecutter shark has a neat little ability in that its belly glows in the dark. With a gently glowing belly matching what light comes from above the shark becomes invisible from below. Except for the dark collar of course, which is believed to look like a small fish that isn’t doing a good job of avoiding predators.
A hungry fish or marine mammal sees what it thinks is a small, easy to collect meal but finds itself the surprise addition to the menu. With a flick of its broad tail, which is perfectly shaped for acceleration, the Cookiecutter spins and grabs a bite out of one very astounded former predator.
By twisting and dragging its massively overdeveloped lower jaw in a circle, the Cookiecutter takes a neat, round plug of flesh to feed on. This BBC Video has a great animation showing how the shark does this. Humans are not part of the diet of the Cookiecutter shark with the first recorded attack being in 2009.
Despite their unusual feeding habits the Cookiecutter shark just isn’t big or scary enough to have an actor ever say “we are going to need a bigger boat”. Pity, because to me they are just as cool as the Great White and life is richer for having the toothsome little critters swimming around cutting neat little circles out of everything that they find.