By the Census of Marine Life

A worldwide review of depth distributions reveals a notable absence of sharks and other cartilaginous fishes from the deepest regions of the world’s oceans.

Most of the world’s oceans are very deep and the oceanic abyss is one of the largest environments on the planet. The abyss is deeper than 3 000m and is characterised by the absence of solar light, high pressures and remoteness from surface food supply. These challenging characteristics mean that any creature living in the abyss will have unique molecular, physiological, behavioural and ecological adaptations. Some of the most amazing anatomical adaptations which have been discovered include eyes which are sensitive to low light levels and the possession of light organs. Deep-sea exploration over the last two decades has revealed that Chondrichthyans (fishes with cartilaginous skeletons like sharks, rays and chimaeras) are absent from, or very rare, in the abyssal region and only exist in 30% of the world’s oceans.

Deep-sea bony fishes were first discovered in the 1860s, the deepest being the Ganostoma microdon, which was discovered in the Pacific Ocean at an incredible depth of 5 300m (six times deeper than the tallest building on Earth). Since then, the deepest ray has been reported at 1 033m and the deepest shark at 915m in depth. Chondrichtyans have successfully colonised the world’s oceans, yet records show that bony fishes have consistently been discovered at much greater depths. Scientists hoped that modern deep-sea exploration would reveal a hidden diversity of Chondrichthyans in abyssal areas, however, based on research to date, it appears that the deep sea is largely shark-free.

More than 100 scientists from over 16 countries were involved in the Mid-Atlantic Ridge Ecosystems Project (MAR-ECO), which was part of the 10-year Census of Marine Life initiative concluding in 2010. It explored the abundance, distribution and diversity of life in the world’s oceans. When the University of Aberdeen’s Oceanlab started developing “landers”, which are remotely operated vehicles (ROVs) used to explore deep waters, the team had a means to investigate findings that have built up over the past two decades. Expeditions using ROVs explored the deepest abyssal plains on the planet, namely the north of Hawaii, the south Atlantic Ocean off the Falkland Islands, the northwest African slopes off Angola, the northeast Atlantic Ocean, the west of Ireland and the northeast Atlantic Ocean. Scientists believe that it is very unlikely that major new populations will be discovered in the abyssal regions. Professor Priede from the University of Aberdeen added that as far they can see, there is no hidden reserve of sharks in the deep sea.

This revelation renders the oceans 70% free of sharks, rays and chimaeras. Their fragmented distribution amongst seamounts, ocean ridges and margins, and the fact that they are often found near the surface in open waters, confine them to a small proportion of the vast oceanic environment.

Because such a fact places all the world’s shark populations within reach of human fishing activities, the discovery has serious environmental implications. This new finding suggests that sharks may be more vulnerable to overexploitation than what was previously thought.

The deepest living Chondrichthyans are described as demersal, thus living near or on the seabed. To date, no evidence of any midwater (pelagic) Chondrichthyan has been found living at depths greater than 1 500m. Recent data from a number of studies using depth-sensing devices, trawl nets and baited cameras indicate that some demersal Chondrichthyans were found down to 3 000m in depth, with rare occurrences at just over 4 000m in depth. A possible reason as to why Chondrichthyans have failed to colonise these deep waters could be because of the lack of food resources to support their high-energy demands and oil-rich diets. Yet, like many creatures which are well-adapted to deep-sea life with low-light-sensitive eyes and/or bioluminescent properties to attract food, sharks are not completely ill-adapted to these depths. Trends in data show that deep-sea scavengers are often bigger at greater depths, suggesting a possible advantage for their survival in the abyss.

The fact of the matter still stands that more research is needed to determine why Chondrichthyans are absent from these depths, which are characterised by their other-worldly remoteness. It is not clearly understood why it is that bony fishes have colonised the abyss while sharks, which have a longer evolutionary history, have not. These questions could lead to new research and discoveries in the field of ecology and evolution.

References

Priede IG, Froese R, Bailey DM, Bergstad OA, Collins MA, Dyb JE, Henriques C, Jones EG & King N. 2006. The absence of sharks from abyssal regions of the world’s oceans. Proceedings of the Royal Society of London B. 273(1592):1435-1441.

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