Sharks inhabit diverse aquatic environments globally, from vast marine expanses to certain freshwater systems. While often envisioned solely in oceans, their habitats showcase remarkable adaptations to different conditions.
Marine Environments: Oceans and Seas
Sharks primarily inhabit marine environments, including expansive oceans and smaller, often partially enclosed seas. Oceans are vast, continuous bodies of saltwater, while seas are typically smaller, sometimes enclosed by land, and often connected to an ocean. Most shark species live exclusively in these saltwater habitats, ranging from shallow coastal areas to the deepest parts.
Different shark species occupy distinct zones within these marine environments. Pelagic sharks, such as great white sharks, blue sharks, and shortfin makos, roam the open ocean, often undertaking long migrations across vast distances. These species are adapted for life in the water column, constantly moving to find prey. Conversely, benthic sharks, including nurse sharks, wobbegongs, and horn sharks, spend most of their lives near the seafloor in coastal seas and coral reefs.
Marine shark habitats also span a wide range of temperatures and depths. Tropical species like the great hammerhead and tiger shark prefer warmer waters, often found near coral reefs and in shallow coastal areas. Temperate species, such as the great white shark, inhabit regions with moderate temperatures, migrating to stay within their preferred range. Some sharks, like the Greenland shark and black dogfish shark, are adapted to frigid polar regions, enduring water temperatures near freezing. Many deep-sea shark species, including gulper sharks and lantern sharks, live in the twilight zone between 200 and 1,000 meters deep, where light is minimal and food is scarce.
Beyond Saltwater: Sharks in Freshwater
While the vast majority of sharks are marine, some species possess the unique ability to inhabit freshwater or brackish environments, which are mixtures of fresh and saltwater. The bull shark (Carcharhinus leucas) stands out as the most prominent example of a shark that can transition between saltwater and freshwater with ease.
Bull sharks achieve this remarkable feat through a physiological process called osmoregulation. Their kidneys and rectal glands adjust to varying salinity levels, maintaining salt and water balance. When in freshwater, bull sharks reduce urea concentration in their tissues and produce large amounts of dilute urine to excrete excess water, preventing cell swelling. This adaptability allows them to travel far up major river systems, with sightings hundreds to thousands of kilometers inland.
Bull sharks have been found in the Amazon River in Peru, over 3,000 kilometers up the Mississippi River in Illinois, and in the Zambezi River in Africa. They also maintain a semi-permanent population in Lake Nicaragua in Central America. While bull sharks are the most well-known, a few other species, like some river sharks (genus Glyphis), can also live in freshwater, though less commonly encountered.
Factors Influencing Shark Habitats
Several environmental factors dictate where different shark species live and move. Prey availability is a key factor, as sharks follow food sources or settle in prey-rich areas. For instance, marine mammals or schooling fish attract large pelagic sharks to specific regions.
Water temperature plays a major role in shark distribution. Most sharks are ectothermic, meaning their body temperature largely matches surroundings, leading them to seek optimal thermal ranges. This influences seasonal migrations, with many species moving to warmer waters in winter and cooler areas in summer. Deep-sea sharks, for example, are adapted to consistently cold temperatures and low oxygen levels.
Depth and light penetration also shape shark habitats. Some species prefer shallow, sunlit waters for hunting, while others are specialized for the deep, dark ocean, relying on different sensory adaptations. Many sharks also utilize specific areas as breeding and nursery grounds, often migrating to shallow, protected coastal waters to give birth and protect their young. Finally, salinity levels, as seen with the bull shark, determine whether a species can inhabit marine, brackish, or freshwater environments. These factors collectively create the diverse habitats sharks occupy globally.