Sharks are a diverse group of over 500 species of cartilaginous fish, meaning their skeletons are made of tough, flexible cartilage instead of bone. These ancient predators have adapted to nearly every marine habitat on the planet. The common assumption that one of the world’s large ocean basins is devoid of them is a misconception, as various shark species have successfully colonized every major ocean.
The Global Presence of Sharks
Shark species inhabit all five of the planet’s major oceans: the Pacific, Atlantic, Indian, Arctic, and Southern (or Antarctic) Oceans. While the greatest species diversity is concentrated in the warmer tropical and temperate zones, the adaptability of these fish ensures their presence from coastal shallows to the open ocean. Their existence across such a wide geographical range is supported by diverse diets and a capacity to handle extreme environments.
The majority of species, such as the Tiger Shark or various hammerhead species, prefer the abundant prey and warmer temperatures of tropical and temperate waters. Many of these active hunters migrate vast distances, sometimes crossing entire ocean basins in search of optimal feeding grounds. Their ability to regulate internal salts (osmoregulation) also allows certain species to tolerate variations in salinity, occasionally venturing into brackish water.
Environmental Limits on Shark Distribution
While sharks are found in all oceans, their presence is not uniform, and large regions within those oceans can be sparsely populated. Temperature is the primary limiting factor for most species, as the majority of sharks are ectotherms, meaning their body temperature is dictated by the surrounding water. If the water is too cold, their metabolism slows significantly, hindering their ability to hunt and grow.
Low prey availability also restricts distribution; the scarcity of food in the deepest ocean trenches makes sustaining large shark populations difficult. While some deep-sea species have been recorded at depths up to 3,700 meters, these occurrences are rare. Furthermore, the varying salinity of coastal areas, such as estuaries, can restrict some species, particularly juveniles, whose physiology cannot tolerate the low salt content.
Specialized Species in Extreme Waters
The presence of highly specialized species proves that no ocean is truly inhospitable. The Greenland Shark, for example, is a resident of the Arctic and North Atlantic Oceans, thriving in water temperatures that hover near freezing. This species possesses a slow metabolism and a circulatory system that employs a unique chemical adaptation, utilizing compounds like urea and trimethylamine N-oxide (TMAO) as a natural antifreeze.
In the dark abyss, deep-sea lantern sharks, like the Dwarf Lanternshark, inhabit depths of up to 3,000 meters in the Atlantic and Pacific. These small sharks are bioluminescent, using light-producing organs called photophores on their undersides. This ability helps them mask their silhouette from predators below by counter-illuminating against the faint light filtering from the surface, allowing them to occupy light-starved zones.