The deep ocean is one of the most extreme habitats on earth, with over 1000 atmospheres worth of pressure at its deepest depths, low oxygen levels, constant cold temperatures and very little to no sunlight. If that doesn’t sound bad enough, without sunlight, photosynthesis can’t occur and so food is scarce, and when you add the fact that without light it’s pretty difficult to find a meal, this makes the deep ocean a pretty awful place to live.

In fact, the deep ocean is so inhospitable that in the 19th century, a scientist named Edward Forbes formed the “Azoic theory” stating that no life could survive below 300 fathoms (1/3 mile) after some fairly limited experiments in the Aegan sea where he found that the abundance of life declined rapidly with depth. This theory was generally accepted despite evidence proving otherwise, because it just made so much sense that no lifeform could survive these extreme conditions.

This theory was definitively disproved around two decades later and since then, some 17,000 species have been discovered below this depth, many of which are often bizarre, ugly and occasionally terrifying.

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Some of the many bizarre and terrifying creatures of the deep. Top left: Frilled Shark (Citron / CC-BY-SA-3.0). Top right: Pacific Viperfish (David Csepp / NOAA). Bottom left: Deepsea Lizardfish (NOAA OKEANOS Explorer Program). Bottom right: Blobfish (NOAA, Alaska Fisheries Science Center).

Enter: The Frilled Shark

Resembling a deep sea monster, the frilled shark (Chlamydoselachus anguineus) has a long, slender, eel-like body and a mouth filled with around 300 tricuspid (trident shaped) teeth, arranged in 25 rows for snagging slippery prey.

It can grow to about 2m long and has bright red gills that extend out of its gill slits. Almost all shark species have five pairs of gills, but the frilled shark has six pairs, and is unique in the fact that its first gill slit continues down both sides of its head and across its throat forming the “frill” it is named for.

Sightings of the frilled shark are rare as it spends most of its time at depths of 50 – 200m, but it is often found as deep as 1500m. The first records of scientists getting their hands on a frilled shark were in Japan in the 1880s, but since then they have been found all over the Atlantic and Pacific oceans in places such as Norway, the British Isles, Australia, New Zealand and the USA and there has even been a discovery of a second species – the Southern African Frilled Shark.

A Living Fossil

The frilled shark belongs to one of the oldest living shark lineages, and has remained virtually unchanged by evolution in thousands of years as it is incredibly well-adapted to the stable environment of the deep sea.

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Unsurprisingly, the frilled shark may have been the origin of old sea serpent legends. Photograph: Citron / CC BY-SA 3.0

It has a huge liver that is filled with squalene, an oil that has such a low density that it allows the shark to hover almost effortlessly in the water column and, combined with an extremely slow metabolism, this allows the shark to conserve energy and survive a long period of time without food. With so little nutrition available, it has to adapted to have a poorly calcified skeleton that requires less nutrients during growth. 

These sharks also have some unusual features rarely seen in other sharks that help adapt them to their extreme environment, such as their extruding gills that extend out of the gill slits into the seawater to increase their efficiency at capturing oxygen. Another unusual feature about these sharks is that they have an open lateral line, meaning that the sensory organs along the side of the shark that detect water movement are exposed to the seawater instead of beneath the skin. Only the most primitive of sharks have this feature, and it is believed that it makes these sharks extra-sensitive to the movements of their prey around them, which is an incredibly important hunting tool in the dark abyss of the deep sea.

Life in the Deep (Deep) Blue

Squid and fish make up the majority of the frilled shark’s diet (60% and 10% respectively), hence the jaw full of hundreds of three-pronged teeth for snaring their prey. However, it is uncertain if this shark actually has the swimming capabilities to capture such fast swimming prey, or if they merely predate on squid that are weakened or ill after spawning events.

Still, food is scarce in the deep sea and a study into the feeding habits of these sharks found that over 70% of frilled sharks caught had empty stomachs. To compensate for this, it is thought that these sharks are capable of eating live prey up to half their body size by expanding their mouths and stomachs like a snake, allowing them to make the most of any feeding opportunity they might come across.

Rare footage of the frilled shark in its natural habitat shows how it swims with slow, snake-like movements. Video: Discovery – Alien Sharks.

 

The frilled shark’s reproductive habits remain somewhat obscure, but best estimates for gestation period are thought to be at least 3.5 years. The embryos grow in eggs in the uterus and receive extra nutrition from their mother during development (matrotrophy), eventually being born as live young at around 0.5m long. Yet another strange fact about this shark is that they only develop embryos in the right uterus, which is unusual amongst sharks which usually develop embryos in both the left and right uterus and it is unclear what the reasons for this are.

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Frilled shark embryos, maintained in artificial conditions, showing the external yolk sac which provides them with nutrients as they develop. After this food source runs out, the mother continues to provide the embryos with nutrients through the uterus. From: Tanaka et al. (1990).

Despite the fact that its life history traits (slow growth rate, late maturity, small litter sizes) make it vulnerable to over-exploitation it is rarely caught as bycatch and not targeted by commercial fisheries, so it has an IUCN rating of “Least Concern”. However, it is possible that in the future there will be increased encounters with these sharks as deep water fisheries expand deep into the ocean, so it is important to continue to monitor landings of these sharks (IUCN). Much is left unknown about these rare and unique sharks, such as how they hunt, if they are capable of catching fast-swimming prey and how long their gestation period actually is, and these questions can only be answered through continued exploration of the deep ocean and observation of these sharks in their natural habitat.

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