20,000 Leagues Under the Sea, but Where are the Sharks?
Sharks are often seen as “voracious killing machines”, perfectly adapted and evolved to sit at the pinnacle of the food chain. So well adapted that they have changed little in their 400 million years on this planet. And yet even with these myriad of adaptations sharks are uncommon below 2000 meters and have yet to be discovered below 4000 meters depth.
The deepest living species of shark is Centroscymnus coelolpis (the Portuguese Dogfish) which has been found at a maximum depth of 3675 meters, while teleost species ate routinely found to a depth of 8400 meters. This means that nearly 70 % of the entire ocean is free of sharks, which are confined to the
surface layer. The number of individuals and species diversity of both elasmobranch and teleost’s decreases with depth but at a much greater rate with Elasmobranchs. If sharks had the same species extinction rate with depth as teleost’s you would expect to find them down to depths of 7500 meters; far greater than the depths at which they are actually found. This presents us with the compelling biogeographical problem: what is excluding sharks and their relatives from the deep ocean?
Challenges of the Deep
The deep sea is an extreme marine habitat: experiencing cold temperatures of between 1-2oC (in the abyssopelagic zone); incredibly high pressures of 300 atmospheres and severe food limitations. Light penetration and hence photosynthesis are limited to the first few hundred meters of the ocean so that, with the exception of hydrothermal vents, deep sea ecosystems are entirely dependent on dead organic matter sinking from the epipelagic zone as a source of energy. These conditions are more suited to the physiology of teleosts rather than elasmobranchs which may explain their exclusion from this region.
Buoyancy and Energy Demand
The way in which elasmobranch and teleosts propel themselves through the water column and maintain buoyancy are fundamentally different. Sharks as predators tend to be faster moving and so maintain buoyancy by a combination of hydrodynamic lift provided by the pectoral fins and hydrostatic lift created by a lipid rich liver which is less dense than the surrounding water. This creates a fundamental flaw for deep sea elasmobranchs. The abyssopelagic zone is food limited, prey are much scarcer than at the surface. The cost to reward ratio at depth is low whilst hunting, sharks expend too much energy maintaining hydrodynamic lift to warrant going deeper in search of prey. Whilst some deep sea species have adapted by evolving enlarged livers to generate a greater degree of hydrostatic lift this creates its own problems. To even approach neutral buoyancy the ratio of liver to body mass must be between 15-20% while the lipid content has to be between 50-80%, producing squalene or other lipids is very energetically demanding (costs 100-1000 times more energy than inflating a swim bladder) and to do so to this volume is simply unfeasible in a food limited environment.
Deep sea teleosts in contrast rely solely on hydrostatic lift, generated by watery tissues, low-density fluid-filled spaces, reduced skeletal systems, air filled swim bladder and secretion of low density lipids (but in much smaller quantities). All of this enables them to save energy by moving very slowly without wasting energy maintaing hydrodynamic lift.
Pressure Effects on Osmoregulation
Sharks are ion-regulating osmoconformers and use urea and TMAO (trimethylamine N-oxide) to regulate osmolarity. In shallow waters TAMO occurs in a 1:2 ratio with urea, optimal for osmoregulation. However, urea decreases and TMAO increases with increasing depth. Muscle urea content intercepts zero urea at approximately 4700 m, almost the depth at which elasmobranchs have been caught. There may be a maximum depth for elasmobranchs beyond which they will be unable to osmoregulate and hence die.
The very adaptations which make sharks such good predators in the upper layers of the water column work to exclude them from depths of greater than 3700 m. Whereas teleosts are adapted to live down to much greater depth (a maximum of 8400m), other prey can also survive at greater depths. The main reason sharks and other elasmobranchs cannot survive at great depths is the scarcity of food and which would not satisfy the high energy demands placed on sharks by their methods of locomotion and buoyancy control.