There are over 400 species of sharks living  around the world today. Some live in deep, cold water. Others live in warmer, shallower waters. They are all considered fish. Shark skeletons are cartilage rather than bone.  

Some other features that help identify different sharks are fins, gills, and teeth. There are sharks with fins on their backs. Others have fins on their backs and on their undersides. All sharks have gills. But the number of gill slits ranges from five to seven. Shark teeth can be flat, sharp, or thorny. Sharks are born with teeth, and they grow new ones regularly. Some sharks, like the whale shark, can have thousands of teeth! Can you imagine such a mouthful? 

One other really important difference between sharks is where they live. Most sharks live in the oceans. You’ve heard to watch for sharks when visiting the beach, right? Every year, there are stories in the news about sharks spotted close to where people are swimming.  

Wait. Most sharks live in the oceans, right? So where do the rest of them live? Would it surprise you to know that there are some species of sharks that live in rivers and lakes? It’s true!  

The salt found in ocean water is necessary to keep a lot of sharks alive. Fresh water flooding the cells of most sharks will kill them. There are a few species that can tolerate brackish, or slightly salty, water. Others have adapted to survive only in fresh water. Some can travel between salt and fresh water. Before getting concerned about sharks in your favorite local river or lake, let’s talk about where the six species of freshwater sharks live. 

Bull sharks are probably the most widespread and dangerous of the species in rivers and lakes. They are saltwater fish that can survive in fresh water for long periods. Bull sharks grow to about 11 feet long and have thick, broad bodies. We have found them many miles inland on rivers like the Mississippi, Zambezi, and Nicaragua. There are also groups of these sharks living in a few lakes. We believe they swam there during floods in nearby rivers and had to stay. 

Speartooth sharks live in both salt and fresh water. We find them around Australia and New Guinea. The rising and falling tides help these sharks travel upriver and back out to sea. Northern river sharks live in the same area as Speartooths. Mainly young northern river sharks travel up several Australian rivers. Researchers believe them to be the rarest shark species in the world. 

Ganges sharks live in rivers in India, Bangladesh, Borneo, and Myanmar. They have eyes on top of their heads, unlike other sharks whose are on the sides. We believe this feature helps Ganges sharks to swim near the river bottom while scanning for food above. These sharks stay only in fresh water. 

No one is sure how many Pondicherry sharks there are in the world. Scientists discovered the Pondicherry shark was not extinct when a fisher caught one in Sri Lanka in 2011. We know very little about these small sharks and how long they spend in fresh water as opposed to salt water. 

Last, there is the Greenland shark. These sharks grow very large and live a long time. They reach maturity by 150 years and can live up to 500 years! They can swim deep in cold ocean waters. Researchers regularly observe Greenland sharks in Canadian rivers.  

These six types of sharks show how some animals have adapted to various habitats. Can you adjust to living in different locations? What other animals can you think of that have mastered this skill?