Sixgill Sharks

Unlike its more evolved relatives, which display five gills, this shark’s set of six is a relic of the Jurassic when its ancestors first evolved. It also has only a single dorsal fin. This primitive shark is one of only few surviving members of the Hexanchidae family, related to dogfish and Greenland sharks, and it appears nearly identical to fossils that date back to around 200 million years ago.

Survival in the deep sea is a challenging task, unless a species evolves to cover the key necessities of reproductive success, feeding success, and efficiency. So why has this shark retained these ancient features and hardly changed at all in 200 million years?

It is perhaps because this shark is perfectly efficient as it is - with its unique enduring adaptations allowing sixgills to live for up to 80 years and grow to immense sizes. The six pairs of gills are thought to be an advantage for a species that lives in the oxygen-poor depths, providing a larger surface area where gas exchange can take place. Luminous eyes allow it to absorb as much light as possible in the deep-sea light conditions. Its dark coloration allows it to blend in with the dark of the midnight zone, meaning it can sneak up on prey more efficiently without using up much energy.

Furthermore, sixgill sharks are highly variable in their feeding mechanisms. It is this which could have contributed to their evolutionary success and global distribution, for in order survive in the deep where food is scarce, they have become opportunistic feeders, known to predate chimaeras and other bony fish. They can protrude their jaws to pull prey into their mouths, or tear chunks from prey that are too large to swallow whole. But they are also scavengers, forming part of the early stage of whale-fall communities when they come across sunken carrion on the sea floor. Here, they will use their serrated teeth as knives to saw and tear away at the flesh.

At 26 feet long, they demonstrate the phenomenon of deep sea gigantism, which is the tendency for deep dwelling species to grow larger than shallower water relatives. This occurs because having a larger body size decreases their surface area to volume ratio, and makes the sharks more efficient.

The fact that sixgill sharks are relatively slow moving is another reason for their success as a species - in moving slow, they use up less energy, for in the deep sea, energy is far more scarce than in the shallows, and it must be conserved.

Occasionally, sixgill sharks will rise to shallower waters under the veil of the night sky. Their reason for doing so is not well understood, but it is theorised that they do this in order to feed.

Overall, this serves as evidence that the bluntnose sixgill shark is indeed a well-adapted inhabitant of the deep sea. These ancient but effective adaptations have allowed this shark to become the most widely distributed of all sharks, rivalled only by the Great White Shark. They have also enabled it to predate a huge array of organisms, from dolphins and seals to salmon and anglerfish, meaning there simply hasn’t been a great enough selection pressure in 200 million years to justify a significant shift in the species’ appearance. It is an example of an organism near perfectly adapted to an unforgiving and extreme, but stable and unchanging environment, down in the isolated deep sea.