Sharks of the twilight zone
What is a ‘typical’ shark? A large, toothy, familiar predator or a small, dark creature of the deep? Dean Grubbs and Charles Cotton reveal some surprises about sharks that are little known, poorly researched and increasingly overfished.
Photo by Kelvin Aitken | Marine Themes
Ask a child to describe a shark and he or she will often respond with something like ‘a big fish with big sharp teeth’. Perhaps an adjective like ‘scary’ or ‘man-eating’ will be added. Asked to name a shark, the child will typically reply ‘great white shark’, ‘bull shark’, ‘hammerhead’ or ‘tiger shark’. These responses reflect the species made most familiar to the public by books, television documentaries and films.
Few people realise how diverse sharks are or that there are close to 500 living shark species – or, indeed, that there are 1,200 living chondrichthyan fishes, which include batoids (rays, skates and sawfishes) and chimaeras, as well as sharks. Why should they? Small, dark-hued sharks with tiny teeth that live their entire lives in the darkness of the deep sea are seldom encountered and, being also not particularly scary, they don’t garner the media attention of their larger coastal and pelagic counterparts. But in reality, these small, deep-sea sharks are much more representative of a ‘typical’ shark than the familiar species featured in documentaries. In fact, two-thirds of all living shark species reach a maximum length of less than one metre, and fewer than 20% of living shark species reach lengths of more than 1.5 metres. In addition, 53% of all living shark species, and 48% of all chondrichthyans, live their entire lives more than 200 metres below the surface of the sea.
The bias favouring large, charismatic species of sharks and rays isn’t particular to the public; it pervades the scientific domain too. A survey of nearly 50,000 peer-reviewed research papers about sharks, rays and chimaeras published since the year 1820 revealed that although deep-water taxa make up more than half of the living species diversity, they represent only 7% of the scientific literature. In fact, over the past 15 years there have been almost as many published studies on white sharks and tiger sharks alone as there have been on the nearly 600 species of deep-water chondrichthyans combined.
Sharks, rays and chimaeras occupy varied habitats in the deep ocean. As is the case for most predators, the biomass and diversity of deep demersal chondrichthyans is highest in variable or edge habitats – places like submarine canyons, sea mounts and the edge between the continental shelf and slope. Most deep-sea chondrichthyans are bottom-dwelling species that probably rarely venture more than a few metres from the sea floor. This diverse assemblage is dominated by skates, catsharks, lanternsharks, gulper sharks and dogfishes. However, certain species, such as cookiecutter sharks (Isistius spp.), some lanternsharks (Etmopteridae) and the pelagic stingray Pteroplatytrygon violacea, migrate vertically each day between deep and shallow water. During the day they are found in the mesopelagic zones, between 200 and 1,000 metres down, while at night they may penetrate the epipelagic zone, which is less than 200 metres deep.
Many deep-water sharks and skates previously thought to be ‘rare’ are actually quite common in the deep ocean. The abundance and diversity of deep-sea sharks and rays often peak at intermediate depths (400–800 metres), and recent research has shown that there appears to be a limit to the depth at which deep-sea sharks and their relatives can live. More than three kilometres below the ocean surface – or in the abyssal depths, as they are known – the ocean is almost entirely devoid of chondrichthyans, and there are three prevailing hypotheses to explain why this may be so. One posits that the limited food available below 3,000 metres makes it impossible for sharks, rays and chimaeras to generate enough liver oil to provide buoyancy. Thus, below 3,000 metres these animals would have to swim continuously to avoid sinking. Another hypothesis suggests that since food resources are so limited in the deep ocean, predators at the top of the food chain are unable to find enough food in this habitat and would essentially starve. Lastly, all chondrichthyans need a vital compound in their bodies – trimethylamine N-oxide (TMAO) – which helps to maintain osmotic balance and protects proteins from the destructive effects of extreme pressure; it’s the molecular constraints of that compound that could be imposing this depth limit.
The relatively similar pressure, temperature, salinity and light levels across and between deep ocean basins mean that many deep-water shark species have very broad, often global distributions. Recent technological advances in telemetry have facilitated greater understanding of the distribution and movement patterns of some deep-water shark species. The limited studies that have been conducted reveal that many species make daily vertical migrations from depths of 600–1,000 metres to the edge of the thermocline (200–300 metres) at night and back to the depths during the day, following a massive migration of smaller fish and other marine organisms. As technology advances and logistical constraints are overcome, we predict telemetry studies will become more common for deep-water sharks in the near future, helping scientists to unravel some of the mysteries surrounding the ecology and behaviour of these little-known species of the deep.
