Our oceans are incredibly vast! Not only are they wide, but they are also very, very deep. In fact, so much of the ocean is inhospitable to humans, that we know very little about what life exists down there. In recent years, advancing technology has allowed us to see more of this mysterious world. One of the exiting discoveries has been the presence of sharks at greater depths than we thought possible... So how far down can we find sharks? And how do they survive down there!?
Let There Be Light
Scientists use terms to divide different depths of the ocean, based predominantly upon how much light is able to penetrate through the water. The "Epipelagic zone" receives sunlight up to a depth of 200m, but the "Mesopelagic zone", also known as the twilight zone, receives weaker light. When you dive down into depths above 1000m, we reach an area termed the "Bathypelagic zone" also known as the midnight zone. No light permeates through the water to this depth and animals live under conditions of complete darkness. The "Abyssopelagic" and "Hadalpelagic zones" are deeper still, experiencing absolute darkness and immense pressure from the weight of the water above (Ebert et al, 2021).
It might surprise you to learn that we can find sharks at depths over 2,000 metres! In areas where there is no sunlight and as much as 1,270kg of pressure per square inch! (Priede et al, 2006) Let's meet some of these amazing characters...
A Kilometre Down
The aptly named goblin shark (Mitsukurina owstoni) has been found at the dizzying depths of 2000 metres! Goblin sharks live over continental slopes and over submarine canyons, existing in the almost total darkness of the midnight zone. In this environment a pretty face is not important. That mouth is critical for ambush hunting, as the goblin shark can eject its jaw out of the front of its face in order to extend its reach. This means the shark does not have to meander in the dark trying to sense prey, but can remain in one place and wait for prey to wander within its grasp. As the goblin shark is able to reach 6 metres in length, it may comfort you to know that adult goblin sharks live in deeper regions than the juveniles (Compagno, 1984).
Quick Like A Cat
The false catshark (Pseudotriakis microdon) has been recorded in depths up to 1,400 m, over continental slopes. The large size of the false catshark's liver (up to 25% of its total weight) provides this shark almost perfect neutral buoyancy, so they can hover over the ocean floor with minimal effort. This allows them to conserve energy in order to then put on short bursts of speed for hunting their bony fish prey (Compagno, 1984).
No Frills
The frilled shark (Chlamydoselachus anguineus) has been found in depths up to 1,570 m, but it is uncommon over 1,200m. This species is thought to be the most primitive of all sharks, meaning it evolved first and has remained unchanged for around 95 - 150 million years! It is thought that the frilled shark remains at great depth during the day and moves into shallower water at night to feed (this is known as a "diel vertical migration"). A relatively small shark, only reaching a maximum of 2 metres, scientists have suggested that their daily migration might be used for predator avoidance (Compagno, 1984).
Ghost Town
At depths around 2,000 m, scientists have found several species of ghost catshark (Genus Apristurus). This group are also known as the demon catsharks because their eyes glow when the light from submersibles hits them. They are not true sharks, but are a part of a closely-related group known as chimaeras (aka the Holocephalians). These species are quite small. For instance, the smalleye catshark (Apristurus microps) only reaches up to 60 cm in total length and the Iceland catshark (Apristurus laurussonii) has a maximum size of 67 cm. These creatures can be found at maximum depths of 2,000 and 1,450 m respectively. Due to their extreme habitat, very little is known about these creatures or how they survive at such immense depths (Compagno, 1984; Ebert et al, 2021).
Deep Down
A study using fishing trawls, long-lines, submersibles and baited cameras to assess their maximum depths, found that sharks do not inhabit any waters deeper than 2,128 m. Scientists have concluded that, whilst there were rare occurrences when an individual shark has been found at 3,700 m, this is an anomaly and it is very likely that sharks are absent at depths of 3,000 metres or above (Priede et al, 2006).
This means that the vertical distribution of sharks is truncated at the boundary with the Abyss. Therefore, as far as we know, there are no sharks in either the Abyssopelagic or the Halalpelagic regions of the ocean (Priede et al, 2006). This might sound unbelievable, but based on area, this means that sharks are entirely absent from as much as 70% of the ocean!
References
Compagno LJV (1984) Sharks of the World: An annotated and illustrated catalogue of shark species known to date. FAO Species Catalogue, Rome. Access online.
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
Priede IG, Froese R, Bailey DM, Bergstad OA, Collins MA, Dyb JE, Henriques C, Jones EG and King N (2006). The absence of sharks from abyssal regions of the world’s oceans. Proceedings of the Royal Society B, 273, 1435–1441. Access online.
By Sophie A. Maycock for SharkSpeak
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