Sharks are one of the most iconic marine animals, having swum in Earth’s oceans for over 400 million years. Their sleek, streamlined bodies and rows of serrated teeth conjure up images of apex predators patrolling the depths in search of prey. So it may seem obvious that sharks live in the sea, right? But while this is largely true, the full answer is more nuanced.
The Majority of Shark Species Live in Saltwater Seas and Oceans
The class Chondrichthyes contains all of the cartilaginous fishes, including sharks, rays, and chimaeras. Of the over 1,000 species in this group, around 500 are sharks. The vast majority of sharks – over 95% of species – live in saltwater seas and oceans around the world.
Sharks inhabit tropical waters, temperate seas, and even frigid polar regions. Different species frequent the sunny surface waters, twilight zones, and the dark midnight zone depths. Sharks demonstrate a wide range of lifestyles, from active apex predators to sedentary bottom dwellers.
Many sharks, especially larger species like the great white, tiger, and bull sharks, roam vast distances around ocean basins and migrate across entire seas. Other species inhabit smaller coastal regions, coral reefs, or deepwater habitats. But all these sharks share the common need for saltwater environments.
Key Reasons Most Sharks Live in Saltwater
There are several key reasons why most sharks inhabit marine waters:
- Osmoregulation – Sharks maintain internal salt and water balance using special cells in their gills and rectal gland to retain urea and other salts while excreting excess water. This internal chemistry is adapted for saltwater.
- Respiration – Sharks extract oxygen from seawater as it passes over their gills. Some species can respire in freshwater but are adapted for saltwater.
- Reproduction – Many sharks give birth to live young rather than laying eggs. The growing embryos require the buoyancy and support of seawater.
- Food sources – Sharks are carnivores that feed on other marine animals like fish, seals, and shellfish that live in the oceans.
The saltwater environment provides the right balance of salts, water pH, buoyancy, and prey sources that sharks need to survive and thrive.
A Small Number of Shark Species Have Adapted to Freshwater
While the vast majority live in oceans and seas, a small number of shark species have adapted to survive in freshwater rivers and lakes. Here are some of the most notable freshwater sharks:
- Bull shark – Also known as the Zambezi shark, bull sharks can swim between salt and fresh water using special physiological adaptations to maintain the right internal salt balance. They have been recorded 4,000 km up the Amazon River.
- River shark – River sharks are a rare species found in parts of Borneo and Australia. They live and breed in freshwater systems but may travel downstream to estuaries.
- Speartooth shark – A threatened species, speartooth sharks inhabit freshwater lakes and fast-moving rivers in New Guinea and northern Australia.
- Ganges shark – Critically endangered, these sharks patrol the shallow waters of the Ganges River and other river systems in India and Bangladesh.
- Irrawaddy river shark – Another endangered river shark species found in tropical freshwaters of South and Southeast Asia.
These rare freshwater sharks have evolved physiological and anatomical adaptations such as changes in kidney function, gill structure, and rectal gland processes to survive in low-salt environments.
A Few Shark Species Can Thrive in Both Seawater and Freshwater
Very few shark species can actually thrive in both marine and freshwater habitats. The salmon shark is one example. This close relative of the great white shark migrates between the Pacific Ocean and rivers along the West Coast of North America. Salmon sharks feed on migrating salmon runs, following the fish between the ocean and inland waters.
Perhaps the best example of a shark flexible across saltwater and freshwater is the bull shark. This adaptable predator inhabits tropical and subtropical coastal waters worldwide. But bull sharks can also swim into and survive in freshwater rivers, lakes, and estuaries for extended periods. They have special kidneys that allow them to change their internal water and salt balance.
By altering the concentrations of urea and other solutes in their blood, bull sharks can go back and forth between salt and freshwater. This gives them access to river and estuary food sources and broad habitat ranges along coasts.
Shark Habitats in Seawater vs. Freshwater
The habitats sharks occupy differ greatly between marine and freshwater environments:
| Habitat Characteristics | Seawater | Freshwater |
|---|---|---|
| Salinity | High (30-40 ppt) | Low (0-0.5 ppt) |
| Buoyancy | Saltwater provides more buoyancy to sharks | Freshwater provides less buoyancy |
| Available prey | Fish, marine mammals, invertebrates like squid and crabs | Fish, river dolphins, crabs, frogs |
| Vegetation | Sea grasses, kelp forests, corals | Aquatic plants, roots, overhanging branches |
| Other habitats | Open ocean, coral reefs, rocky coasts, sandy bottom | Rivers, streams, lakes, flooded forests |
| Oxygen levels | High oxygen in shallow water, low oxygen in deep zones | High oxygen in shallow rapids and riffles, low in still backwaters |
| Threats | Fishing, marine pollution, ocean acidification | Habitat destruction, river obstructions like dams |
As the table summarizes, while many sharks can physiologically tolerate freshwater, the ecological conditions differ greatly from their ancestral saltwater homes. Very few species are true habitat generalists.
Conclusion
In conclusion, the vast majority of shark species live in the saltwater oceans and seas that have been their domain for hundreds of millions of years. A small number of shark species have adapted to survive in freshwater rivers and lakes, with physiological adaptations that allow them to balance their internal salt levels. But only a very few species like bull sharks can truly thrive in both marine and freshwater realms. Sharks owe their evolutionary success to the riches of the seas, which offer abundant food sources and expansive habitats for these iconic ocean predators.