A submersible descends into a volcanic vent off Las Gemelas seamount.
Figure 1. A deep sea submersible exploring the ocean floor in darkness (source)

The deep ocean is one of the least researched places on earth, with less than 5% having actually been explored, it places extreme limitations on our knowledge of deep sea biota. Originally it was thought to be a desolate abyss due to Edward Forbes’ Azoic theory. Whilst on an ocean expedition in the mid 19th century, Forbes observed that marine life became decreasingly abundant with depth and proposed that below 550m the conditions would be uninhabitable for life completely. This theory was disproved 25 years later with the discovery that the deep ocean was in fact teeming with life, supported by the work of Michael Sars and Charles Wyville Thompson. Since then many discoveries have been made (17,000 new species) and we now know that the deep ocean is home to many weird and wonderful creatures despite the extreme environment, however, much is left undiscovered (up to two thirds of all marine species in fact).

How deep is deep?

Over 90% of the ocean reaches depths of over 1000m, this is classified as the beginning of the aphotic zone where sunlight can no longer penetrate and is generally considered as the ‘deep ocean’. Conditions at this depth are so extreme it is surprising to think that organisms could thrive there. The main reason for their success is the adaptations they have evolved to cope under such severe stresses.

  • Light – Low to non-existent light levels limit photosynthesis in the deep so photosynthetic bacteria, phytoplankton, corals and algae cannot grow. This means ecosystems have had to depend on alternate energy sources.
  • Pressure – Below 1000m the pressure is over 100atm – That’s equivalent to 1470lbs of force per square inch, or the weight of a grizzly bear!
  • Temperature – At 1000m deep, water temperature is 5° on average and slowly decreases to 0° with depth.

 

Sixgill sharks

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Figure 2. A bluntnose sixgill shark in the deep ocean showing its physical characteristics and size (source)

The deep ocean is our planet’s final frontier and its ecosystems are dominated by sharks. One of which is the bluntnose sixgill shark (Hexanchus griseus). This is one of the last remaining species of the hexanchidae family, or ‘cow sharks’. They are an extremely primitive species, with fossils of close relatives dating back to the Triassic period 200 million years ago. Their morphology has not significantly changed in that time as they are so well adapted to the deep environment, however, little is still known about their biology as they only venture to shallower waters at night. The sixgill was discovered in 1788 and originally named Squalus griseus by Bonnaterre before being renamed. Sixgill sharks (shockingly) have 6 gill slits unlike the usual 5 you would find in the majority of other shark species. This is common in deep water and primitive sharks due to the low oxygen levels, and like other primitive elasmobranchs, reproduction is ovoviviparous. They have only one dorsal fin located towards the posterior end of its long body which is larger than its anal fin and an elongated caudal fin. They are one of the largest species of sharks, reaching up to 5.5m in length and possibly more. This along with their saw-like and serrated teeth and fluorescent green-blue coloured eyes makes for an eerie sight in pitch black waters.

 

 

Habitat and range

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Figure 3. A world map showing the distribution and range of the bluntnose sixgill shark (source)

Sixgill sharks are a highly migratory species with one of the widest ranges and can be found almost anywhere in the world, from boreal to temperate and tropical seas. They prefer to coast along the ocean floor at depths of anywhere from 100 – 2,500m during the day and display diel vertical migration behaviour by moving towards the shallower surface waters of the continental shelf at night to feed. They have even been seen in waters as shallow as 12m! The bluntnose sixgill is one of the top predators on most of the worlds continental shelves, preying on other elasmobranchs like rays and sharks, large crustaceans, squid and even some seals. However, they do have their own predators such as the orca, great white shark and humans that fish them.

 

Adaptations to the deep

Having evolved in the deep ocean, Sixgill sharks have evolved very specific adaptations in order to inhabit such a hostile environment. A 6th gill slit developed out of the low levels of oxygen at depth to aid with efficient gas exchange. Their unusually large size and long lifespan of up to 80 years is thought to be due to the low temperatures and scarcity of food in its habitat. Low surrounding temperatures mean a slower metabolic rate and these sharks in particular were found to have one of the lowest enzyme activity levels ever observed, explaining their slow growth rate. Adopting a live slow, die old outlook on life, these sharks are able to conserve heat and energy, resulting in a concept called deep-sea gigantism. In order to maintain neutral buoyancy under pressure sharks have a large lipid-filled liver which allows them to vertically migrate thousands of meters without a swim bladder. Their cartilaginous skeleton also benefits this as it is light, neutrally buoyant and flexible to cope with high pressures. When mating, the male is thought to nip at the female’s fins to entice her as a visual display of courtship would go unnoticed in the darkness.

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Figure 4. A section of upper and lower jawbone of the bluntnose sixgill shark showing serrated and cusped teeth. Used for feeding and courtship (source)
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Figure 5. A close up open-mouthed shot of the sixgill shark showing its internal gill structure, tooth placement and one green-blue fluorescent eye visible (source)

Threats and conservation

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Dr. Demian Chapman taking a DNA sample from a large six gill shark before it was released (Photograph by Lance Jordan) (source)

Although this deep-sea species is rarely seen in surface waters it is often caught as bycatch in industry, is a valuable species in trophy/ sport fishing and sometimes eaten as food although its slow growth rate renders it unsustainable.  The IUCN red list assessed it as Near Threatened, although it appears to be very vulnerable to overfishing. We still know so little about this species’ ecology and behaviour it is difficult to know how to protect them from anthropogenic threats which is why research into the deep oceans is so vital as it is an important step to protect our oceans and its inhabitants.

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