Glow in the Shark – The Elasmobranch with a sweet tooth
Descending over 200m into the deep sea you will come across some of the most bizarre and highly adapted organisms on the planet. With no plants or algae to form the baseline of the food chain, life here is challenging and organisms must find new ways to survive. Living at the dizzying depths of around 3200m, the Cookiecutter Shark, Isistius brasiliensis, is the only species of shark that is both parasitic and bioluminescent. Being one of the smallest sharks in the ocean, around 30-40cm long, it would be easy to underestimate the Cookiecutter’s hunting abilities. But don’t be fooled, this little guy has the biggest teeth of all sharks, relative to its size, making its scoop-like bite the stuff of nightmares.
Hit and Run
The Cookiecutter spends all day two miles below surface waters and migrates up through the water column at night to hunt. As a parasitic feeder I. brasiliensis targets much larger marine species, sometimes over 20 times its size, but the Cookiecutter does not kill its victims. Instead it uses its unique teeth to take cookie-sized bites from the flesh of its unsuspecting prey.
The Cookiecutter’s teeth are unmistakeable as you can see in the picture to the right. The upper jaw of its rounded mouth contains small, sharp, hook-like teeth used to latch onto its prey, whereas the lower jaw contains strong, thick, saw-like teeth to scoop out large chunks of flesh. But, these are not the only weapons at the Cookiecutters disposal, it also has strong sucking lips which primarily attach onto the target during attacks.
Once attached, by the suction power of its fleshy lips, the Cookiecutter digs its razor sharp teeth into the flesh of its prey. Once both sets of jaws are in position the Cookiecutter rapidly twists its whole body round carving out a cookie shaped chunk of meat. Once the Cookiecutter has taken its fill it releases its unfortunate target and retreats into the darkness to find its next victim.
The Bigger the Better
With the Cookiecutters small body size, only 30-40cm in length, you would expect it to feed on smaller species of fish and crustaceans, which it does. However, the Cookiecutter is completely unfazed when facing much larger assailants. There are many examples of the small shark targeting much larger predatory species such as marine mammals, as seen in the video below, and even other sharks, including the Great White, and leaving its trademark bite. But how does the Cookiecutter get close enough to these highly adapted, spatially aware organisms without getting eaten themselves? This is where the Cookiecutter’s ambush strategy really comes into its own.
An Indo-Pacific Bottlenose dolphin sporting the Cookie-cutter’s trademark bite. Source.
A Master of Menace
The Cookiecutter shark isn’t just armed with a fierce bite, to get close enough to its large prey it must also be a master of disguise. This highly adapted camouflage comes in the form bioluminescence. In Cookiecutter sharks, bioluminescence occurs particularly on its lower belly due to the presence of photophore glands. These glands produce light by secreting an enzyme known as luciferase, which converts chemical energy into light energy triggering the characteristic glow.
Since the shark hunts closer to the surface at night, it will only be spotted by predator fish from below. This is where its bioluminescent properties prove useful. To avoid being eaten the underbelly of the Cookiecutter glows, resembling moonlight shining through the surface of the water, eliminating its shadow and making the shark almost invisible to the predatory fish below, as seen in the picture to the right.
However, around I. brasiliensis’ neck there is one wide band which does not glow and is fully visible to the nearby predatory fish. This may seem like a pitfall in the Cookiecutter’s evolutionary prowess. But, this is the Cookiecutter Sharks’ trump card. Because the rest of its body is craftily camouflaged by the spots of bioluminescence, this one strip, to the predatory fish, looks like a much smaller prey target. An attractive opportunity for a predator looking for an easy meal. When the unsuspecting fish is successfully lured in and goes for the kill, the Cookiecutter whips around and latches onto it challengers flesh to take its gruesomely efficient, trademark bite.
Keeping the Dentist Happy
With their grasping and rotating form of attack, it makes you wonder – do the Cookiecutter’s teeth ever get damaged? The answer is yes, but like all other shark species the Cookiecutter’s teeth are readily replaced in a conveyor belt type system. Most sharks lose and grow back their teeth one at a time, but the Cookiecutter, being the unconventional shark it is, does things a little differently. Instead of losing its bottom teeth individually I. brasiliensis sheds its teeth as a complete row. This is unheard of in any other shark species and is a result of all the Cookiecutter’s teeth being connected at the base. Therefore if one tooth is lost the whole row must be replaced, ensuring that the Cookiecutter always has a sharp set of teeth available.
But just losing teeth isn’t an option for the Cookiecutter. Living in the nutrient poor deep sea means nothing must go to waste. Therefore, when the Cookiecutter’s teeth are shed the whole row is swallowed, the reason for this is debated but many believe the teeth are swallowed to recycle the valuable calcium back into the shark’s body.
Cookiecutter Shark vs. Everything Else
Though the Cookiecutter was discovered in the 1800s by French Naturalists, very little was known until the early 1970s about the Cookiecutter’s life history and adaptive traits. Up until then, no one had connected the bizarre Cookiecutter to the strange crater-like scars found on larger fish. For over 100 years these wounds remained a mystery. But, in 1971 Everet Jones found small lumps of flesh in the stomach of the tiny shark and it all became clear, the Cookiecutter was the culprit for wounding some of the oceans most fearsome residents. Being a resident of the deep sea, one of the most extreme environments on Earth, it is no wonder the Cookiecutter has adapted such a unique morphology and feeding strategy within the shark species. In the present day the Cookiecutter is at no risk of extinction as it generally resides in the deep sea safe from human influence. Therefore there is no doubt this tiny terror of the deep will continue terrorising its unsuspecting, prey indefinitely. Taking a bite out of just about anything.