A Bite-Sized Shark With a King-Sized Appetite

By Carolyn Sandoval

A shark lurks in the ocean’s wide depths, vigilant green eyes searching for the next meal to fit in its toothy grin. A crafty hunter, both master of stealth and disguise, it feeds on everything from the smallest fish to the largest whale – this shark’s menu has everything. Even submarines are not spared, forcing the most sophisticated machines humanity has to offer back to the scrapyard. This description brings to mind some kind of gigantic monster, no doubt – a demon from the deep, a modern Megalodon, a hideous horror that consumes anything that makes the mistake of crossing it. This shark has a name – and a rather unassuming one at that – the Cookiecutter Shark.

The Cookiecutter Shark (Isistius brasiliensis), despite being among the smallest of sharks, feeds on the largest animals in the sea

For the standards the word ‘shark’ sets up, this creature seems to not quite hit the mark at first glance. A plump, cigar-shaped little fella that’d be lucky to grow larger than fifteen inches looks tame compared to its cousins who have chainsaws for noses, caudal fins that crack like whips, and lifespans that can just about touch 500 years. But don’t let that fool you – this guy’s got it covered.

The Cookiecutter makes its mark by being parasitic. Like a suction cup, its big lips can create a pressure difference, attaching and forming a tight seal between parasite and prey. It then digs in with hacksaw-like teeth, twisting until it steals a 5-cm deep cookie-shaped bite from the host. It’d be difficult to find a more upsetting use of the word ‘Cookie-cutter’. (Content warning if you wish to research what the resulting ‘crater wound’ looks like.)

Cookiecutter attacks leave a distinct circular scar on its victims. While not lethal on its own, the animal would need to allocate resources to heal. The effect this has on its growth and development is still unknown.” (Source: https://www.flickr.com/photos/jkirkhart35/5380517372)

This feeding style allows this tiny shark to sample the largest animals in the sea – this hunter is more than happy to latch on to anything that exceeds its own size. This includes everything from rays, fish, squid, other sharks (including the Great White), seals, dolphins, and even the gargantuan blue whale, whose length exceeds that of two buses. Anything too small the shark can hork down the old-fashioned way leading to an unbelievably diverse carnivorous diet.

Fortunately for you and me, attacks on people are rare and nonlethal. The cookiecutter prefers temperate waters far from shore – no place you’d be on your average beach excursion. Most attacks that do occur are under the cover of night on long-distance swimmers who are surrounded by bright artificial lights that unintentionally attract them.

The menu may be long, but the sea operates through self-service, and a tiny shark like this could just as easily become lunch or starve if it doesn’t be both careful and clever. Small size does come with the benefit of stealth, and exactly that is another one of the Cookiecutter’s specialties.

Camouflage works differently for pelagic – or open ocean – species; there are no surfaces or plant life to disguise as or to hide in. There’s only the darkness below and the brightness above. But that’s just enough, as the Cookiecutter is bioluminescent.

In a technique known as counter-illumination, the underside of the creature is lined with light-producing organs called photophores that mask the shark’s silhouette against the sunlight above. Great for staying hidden, but for this parasite, it’s not just a cloak. It’s a masquerade. A dark-pigmented ‘dog collar’ wraps around the shark’s throat, interrupting the bed of photophores on its underside. The effect of this causes the camouflaged Cookiecutter to appear invisible save for a small dark band. This is believed to act as a lure, mimicking the appearance of a small fish. This sneaky trick dupes predators into approaching and getting a nasty bite, so combine this with the Cookiecutter’s schooling behavior, and you’ve got a rude awakening in store for any predator seeking a nice helping of forage fish.

Like many other pelagic species, the Cookiecutter engages in ‘diel vertical migrations,’ a phenomenon where an organism migrates up the water column at night to feed, before diving back to the depths during the day to avoid predators. The shark migrates around three kilometers each way, and the migration also serves to preserve its disguise, moving up and down the water column to preserve the amount of background light.

The shark inevitably meets countless species in its migrations to taste-test, and some are far stranger than others.

Imagine, for a moment, you were this little gourmand. Lurking in the shadows and luring in prey ten times your size with your buddies – you’d be forgiven if this life made you a little complacent. Fortune favors the bold, and you love to brave new heights. Though maybe you learn that you can swim too close to the sun when you take a big chomp out of the rubbery skin of the greatest animal you’ve ever seen – a leviathan as long as the Washington Monument is tall.

It has a strange taste, like motor oil. Disgusted, and maybe a little confused, you move on.

Back on the surface world, a mystery plagues the U.S. Navy. It’s the Cold War, and they’ve just rolled out their shiny new top-of-the-line Ohio-class nuclear submarines. There’s a problem – crews are reporting unexpected equipment damage and malfunction, some severe enough to warrant a return back to base. By 1980, 49 subs reported such damage, 19 of which rendered completely blind by broken sonar equipment.

These events caused the Navy to suspect sabotage or some kind of secret Soviet weapon – one which delivered surgical strikes to precisely the submarine’s most sensitive and crucial components. It would be a weapon that is undetectable, quiet, swift, and dangerous.

The culprit? None other than the diminutive Cookiecutter, eager to try a bite of the ocean’s newest whale. The sea told the Navy to back off, and the Navy responded by installing fiberglass over sensitive components. The shark would eventually get its revenge by trashing random pieces of oceanographic instrumentation (and sinking a catamaran). When it’s not causing mischief, it’s back to the food tour.The Cookiecutter, perhaps most of all, demonstrates the incredible diversity sharks have maintained for over the 400 million years they’ve enjoyed being the ocean’s premiere predators.