The dark, cold, crushing depths of one of the world’s largest ecosystems is home to about half of all known shark species. However, living at depths between 200-3000m, these deep-sea species aren’t your typical sharks. Armed with sharp defensive fin spines, large reflective eyes, and, in some cases, an ability to glow in the dark, these alien sharks could be the stars of a science fiction horror movie. But with a biology specifically adapted to deep-sea living, the sharks themselves may be the ones who experience the horror, as fisheries move ever deeper into their deep-sea world.
 |
| Portuguese dogfish (Centroscymnus coelolepis) Pic: Alan Jamieson |
If you watched the latest BBC “SHARK” trilogy, you will have seen cameos from the Greenland shark, Frilled shark and Goblin shark. Using new technologies the BBC was able to capture some truly revolutionary footage of these enigmatic creatures. But just as they have begun to whet our interest, they are becoming increasingly threatened by an expanding fishing industry. The very features that make these sharks successful deep-sea predators have ironically made them vulnerable to man and his ships.
 |
| Longnose velvet dogfish (Centroselachus crepidater) |
Life in the deep-sea is challenging due to the lack of baseline nutrient production. Instead, the deep-sea must rely on the transport of nutrients from surface waters to fuel their food webs. Put simply, food down here is quite scarce. To survive down here, sharks have had to adapt.
Like most other deep-sea fish, the deep-sea sharks have a very low metabolism. One theory behind this is that due to the lack of light, there is no need to be a fast active predator [1]. Chasing down prey is too risky and energy demanding. When feeding events are rare and often opportunistic, it makes sense to have a biology that inherently conserves energy. Furthermore, these sharks are slow-growing and long-lived. Some scientists estimate that the Greenland shark can live in excess of 100 years [2].
 |
| Velvet belly lanternshark (Etmopterus spinax) |
The way these sharks reproduce also has a substantial effect on their vulnerability. Deep-sea sharks tend to reach maturity at a much later age than shallower water sharks, have fewer offspring and have long reproductive cycles. The Leafscale gulper shark could almost star alongside Steve Carrell in the 40-year old virgin movie, as it is believed it doesn’t reach sexual maturity until 35 [1]! Investing more energy into producing fewer, large, developed offspring, females maximize the chances of their pups surviving in this challenging deep-sea world.
To handle the crushing pressures of the deep, these sharks have very large oily livers, which they use for buoyancy. In order to maintain such oily livers, it is important to feed regularly [3]. Deep-sea sharks literally face the risk of starving or sinking to their death if they don’t feed often enough. Therefore, deep-sea sharks don’t stray too far into the deep, where food is even scarcer and subsequently are rarely found below 3000m.
 |
| Leafscale gulper shark (Centrophorus squamosus) Pic: Joao Correia |
Unlike pelagic and coastal sharks that are targeted mainly for their fins, deep-sea sharks are targeted for their large livers. Squalene, a main component of shark liver oil, is used in a wide range of items including face creams, dietary supplements, vaccines and a variety of medical products, and it can fetch a high price in many markets.
It’s almost the perfect storm in terms of an animal’s vulnerability to fisheries. Having large livers, slow metabolism and specific behavioural traits, these sharks have become top deep-sea predators. But with fisheries targeting them for these livers, and a biology that means population recovery is slow, these deep-sea sharks are clearly under serious threat. Whilst Europe has recognized this and prohibited the landing of any deep-sea shark, the rest of the world has yet to follow suit.
We are still trying to unravel the mysteries of many of these alien ecosystems and assess the impact that continued exploitation will have. We are in grave danger of not only never seeing these fascinating sharks in future wildlife documentaries again, but also losing them from our oceans completely before we have fully understood their importance to the planets health.
1: Rigby, C. & Simpfendorfer, C. Deep-Sea Res II, 115, 30-40 (2015) http://dx.doi.org/10.1016/j.dsr2.2013.09.004
2. MacNeil et al. J. Fish. Biol. 80, 991-1018 (2012) http://dx.doi.org/10.1111/j.1095-8649.2012.03257.x
3 Musick, J. A. & Cotton, C. F. Deep-Sea Res. II. 115, 73-80 (2015) http://dx.doi.org/10.1016/j.dsr2.2014.10.010
Christopher Bird is a Ph.D student at the University of Southampton and The National Oceanography Centre. Having worked with many shallow water sharks during his early education, he is now dedicating his research to understanding the trophic and spatial ecology of deep-water chondrichthyans in the Northeast Atlantic. Follow Chris on Twitter @SharkDevocean & blog www.sharkdevocean.wordpress.com
