When we think of sharks, many of us picture them as mindless killers. But sharks are far more remarkable than mere monsters. They are highly-specialised predators, demonstrating an array of adaptations in order to survive the various habitats of the ocean. Near the surface, pelagic sharks like blue sharks, makos, and great whites exhibit fairly similar features. They are streamlined to allow for faster movement, and blueish grey, so that they can blend in with their environment. But these sharks represent just the tip of the iceberg, for lurking in deeper waters is a world of elusive, mysterious, and often gigantic shark species. From the bioluminescent lantern shark, to the parasitic cookie-cutter shark that tears chunks from whales and dolphins. To understand why sharks of the depths are so different to those at the surface, we must find out what caused them to diversify in this way.

The evolutionary history of sharks possibly dates back to the Late Ordovician Period, 450 million years ago. In the time that followed, these primitive sharks evolved in a number of ways, with one group of cartilaginous fish having diverged 420 million years ago to become the alien-looking Chimaera. Relatives of true sharks, but belonging now to a different classification.

By the Early Jurassic Period (195 million years ago) the oldest-known order of modern sharks, the Hexanchiformes or sixgill sharks, had emerged. It was now that they evolved slender, fast moving bodies, and flexible, protruding jaws, allowing them to eat prey larger than themselves. While many sharks we find in our oceans today have changed immensely since the Jurassic, the few that remain unchanged, the ‘living fossils’, appear peculiar in comparison. Most notably, they have 6 or 7 gill slits instead of the usual 5 found on most sharks. This can be observed in the frilled shark with its slim, eel-like body and snake-like head. Another member of this order, the bluntnose sixgill shark, has a single dorsal fin placed well down on its back, as well as an anal fin. Features that are remnants from their ancestors of the ancient seas.

What’s even stranger is the fact that many of these more ancient sharks, including both the Hexanchiformes and Squaliformes now seem to inhabit the deeper regions of our oceans. A feature that is best explained when we take a look at their adaptations and the conditions of their world. In the depths, there is less dissolved oxygen present in the water. This suggests that, for sixgill, frilled and sevengill sharks, it is advantageous to have more gill slits so that they are more efficient at absorbing the oxygen they need. So, there is no cause for them to evolve to lose the extra slits. Contrastingly, sharks that moved to the shallows have no need for the extra slits, so they lost them over time.

The large eyes of sixgills is also an advantage, allowing them to take in as much light as possible. Other deep sea sharks reflect light in a process known as biofluorescence. The cat shark is one of such sharks, possessing the ability to reflect blue light and re-emit it as green light in order to attract mates.

Another feature that we observe is that the deeper-dwelling sharks tend to be far larger. The Greenland shark, for example, grows to 24 feet or 7.3 metres long, making it one of the largest sharks in the ocean, larger even than the well-known great white shark. This is due to the phenomenon of deep sea gigantism, the tendency of deep sea creatures to grow to larger sizes than their relatives from the shallows. Being larger means they lose less energy as heat. And in the nutrient-poor depths, becoming more efficient is the only way to survive this extreme environment. A full video explaining deep sea gigantism can be found on our channel.

But in addition to being larger, the Greenland shark has also evolved to live its life in slow motion. With its highly slowed metabolism, it can go for very long periods of time without eating, and is able to become the longest living vertebrate on the planet, growing up to 500 years old at depths of 7,200 feet or 2,200 metres. The feeding habits of deep sea sharks are quite different from shallower species. While makos and great whites rely on speed and agility to hunt down fast-moving prey, sharks here must be opportunistic, and depend largely on carrion. At sunken whale corpses, they use their serrated teeth as knives to saw and tear away at the flesh.

Another species, the goblin shark has a flabby body and a large liver to allow it to remain buoyant in very deep water. It is an elusive species, as are many of the creatures that inhabit the deepest parts of the ocean. In fact, one of the largest shark species on the planet, the megamouth shark, was discovered for the first time as recently as 1976. It is a filter feeder, preying on zooplankton at the surface during the night, and returning to the depths during the day. This behaviour is known as diel vertical migration. The synchronised movement of zooplankton and fish up and down the water column over a daily cycle.

Overall, deep sea sharks are understandably unique. In the lightless, high pressure, and nutrient-poor depths, adopting more ancient adaptations such as more gill slits, flabbier slow-moving bodies, and larger sizes, is advantageous. It underpins a distinct difference between these deeper species, and sharks of the shallows, which have evolved differently in order to become smaller and faster. The deep sea is a different world to the surface waters, where food is plentiful, and survival is less of a challenge for a predator.