How are sharks and rays different from other fishes?
Sharks and rays do not have true bones like other fishes.
They have cartilage instead which is lighter and much more elastic and allows them to bend in very tight circles.
Sharks do not have swim bladders.
A swim bladder is a gas filled sack inside the body of bony fishes that allows them to stay still without sinking. Sharks compensate by having a very big liver that is filled with oil. Even so, sharks sink unless they keep swimming forward. The exception is the Sandtiger Shark which swallows air to make itself more buoyant.
All about teeth
A shark’s upper jaw is not attached to its skull like most animals.
When a shark bites a large object, it is able to move its upper and lower jaw forward in order to take a bigger bite.
Unlike other fishes, sharks are able to replace their teeth constantly. New teeth grow from the inner surface of the jaw and rotate forward when the old teeth get worn out or lost during feeding.
Sharks and rays do not reproduce like other fishes.
Most fish release clouds of sperm and eggs into the water column where they mix together. The fertilized eggs then float around until the fish larvae hatch and form schools of tiny fish. Male sharks have two organs called claspers attached to their anal fins. They insert one of these into the female shark’s cloaca (the entrance to the uterus) to transfer sperm (just like in mammals). Some sharks and rays incubate the eggs in their uteruses until the baby sharks are ready to be born. Other sharks and rays (i.e. skates) lay eggs and attach them to the reef.
Sharks have between 5 and 7 gill slits on each side of their body in front of their pectoral fins. Bony fishes only have one pair. Having many exposed gill slits probably helps transfer more oxygen into their blood faster which allows them to swim very fast when they need to.
Most shark’s skin is covered in small denticles instead of scales. Denticles are a lot like teeth. They have dentine in the centre and enamel on the surface. This makes shark’s skin very tough and abrasive like sandpaper. The shape and position of some shark’s denticles also helps reduce friction so that they can slip through the water easier.
Sharks have an extra sense that is able to detect tiny electric fields. They can use this to find food that is buried or to search for animals to eat in the dark or in turbid water.
Sharks and rays make up the sub-class of fishes called elasmobranches. This sub-class is part of a class of cartilaginous fishes called Chondrichthyes which also includes chimaeras (ratfishes).
Quick Facts About Sharks
The largest shark is the whale shark which may grow up to 21 meters long.
The largest predatory shark is the Great White Shark which may grow up to 7 meters long.
The smallest known shark is the Cylindrical Lanternshark that is only 21cm long.
The fastest shark is probably the Shortfin Mako shark which can swim fast enough to catch tuna and swordfish.
The largest ray is the Manta Ray which can have a 7 meter wing span.
The shark with the longest pregnancy is the Spiny Dogfish which may incubate its young for more than two years (the longest gestation of any vertebrate).
How dangerous are sharks and rays?
Sharks and rays are usually very wary of people and most sharks will swim away long before you see them.
There are nearly 500 species of sharks but only a few are likely to be involved in an attack. Some of the more dangerous sharks include the Great White Shark, the Tiger Shark, and the Bull Shark.
Sharks do not normally treat humans as food. If they did, it would be very easy for them to eat all of the people who play in the water at the beach because they are much better swimmers than we are. Most scientists agree that attacks are usually the result of mistaken identity or the sharks feeling threatened because they are cornered. In places like California where there are a lot of White Shark attacks on surfers, scientists believe that the sharks think that the surfers look like seals or sea lions.
Most rays are completely harmless but stingrays have one or more barbs on their tails that they use to defend themselves. If they are caught by fishermen, cornered, or trodden on in the sand, they sometimes stab their barb into whoever is threatening them. The barb has a painful venom on it but stingray wounds are rarely fatal unless there is no medical attention available. The Amazon natives fear the freshwater stingrays that live in the river because it is very difficult for them to treat the wounds.
A good way to avoid stingrays is to shuffle your feet when you are walking in water where stingrays live.
What do sharks and rays eat?
Sharks and rays have a very varied diet. They are carnivores which means that they eat animals rather than plants and algae. Some sharks commonly eat bony fishes, crustaceans (crabs, lobsters, and other animals with an external skeleton), mollusks (snails, sea slugs, octopus and squids), and different types of worms.
A shark’s diet is often determined by its habitat. For example, sharks that live out at sea (pelagic sharks) are more likely to eat fish and squid because that is all that is available.
Sometimes sharks change their diet as they get older. The Great White Shark mainly eats fish when it is young but once it reaches maturity it consumes more marine mammals like seals and sea lions.
Most sharks prefer live food but they will also consume carrion (dead fish and other animals) that they find on the sea floor.
Just like filter feeding whales, there are a few sharks that live by filtering plankton from the water. The filter feeding sharks may consume phytoplankton (microscopic plants and algae) while hunting for more nourishing zooplankton (tiny animals and larvae that drifts around on the currents). Ironically, the Whale Shark which is the largest fish in the sea, lives on plankton which is one of the smallest animals. So does the second largest fish; the Basking Shark. Although these sharks have huge mouths, their throats are tiny and they are unable to eat anything larger than a grapefruit. Their teeth which are no longer needed for feeding, have become very small.
The largest ray (the Manta Ray) is also a plankton feeder. It has a flexible projection on each side of its mouth called cephalic lobes that it uses to funnel plankton towards its mouth.
Most rays eat small fishes and benthic invertebrates; crabs, snails, and worms etc. that live on or under the sand.
Sometimes its possible to tell what type of food a shark eats by the shape of its teeth. Sharks that catch fast swimming fishes tend to have very pointed teeth that help them grasp the fish. Sharks that eat hard shelled animals have flattened teeth that form a plate to help them crush the creature’s shell like a nutcracker.
Tiger Sharks have a reputation for eating anything. They have been found with all sorts of strange things in their stomachs from clothes to license plates. Tiger Sharks have very sharp serrated teeth that are strong enough to bite through the shells of marine turtles.
How did sharks and rays evolve?
Sharks have been evolving for about 400 million years. Some of the early sharks looked very different from the ones we see now.
Sharks have been around much longer than rays. Palaeontologists believe that rays evolved from flattened sharks about 200 million years ago. The first rays were probably guitarfish. Over time, they adapted to life on the sand by becoming more and more flattened.
Fossils are formed when minerals slowly replace buried bones and tissues. The best fossils come from bones because they last a long time before they disintegrate. Because sharks have soft cartilage instead of bones we have not found many well preserved shark fossils. Fortunately, shark teeth fossilize really well so we have an excellent collection of their teeth.
The largest predatory shark that we know about was called Otodus Megalodon, or the Megalodon Shark. Its teeth look a lot like the teeth of the Great White Shark but much bigger. We don’t know how big it was but it was probably at least 40ft (13m) long and may have exceeded 80ft (26m).
Of the shark families alive today, the cow sharks (six and seven gilled sharks) are believed to be the oldest and most primitive. The youngest family are the strange looking hammerhead sharks.