Welcome to the wonderful world of underwater creatures. From seals to sharks, and whales to whale sharks, we break down just what is swimming beneath the ocean waves and where they fit in with those animals that are much happier gallivanting about on land.
Why are sharks not mammals?
To start with, let's address a common mistake. Sharks are not mammals, they are fish.
In taxonomic terms there are two broad classes of fish. There's Osteichthyes or the bony fish, which includes fish species like goldfish, trout, and piranhas. The second big class is Chondrichthyes. These are known as the cartilaginous fish; they have jaws and skeletons made of cartilage not bone. This is the class that all sharks belong to, and Australian Museum has a helpful diagram that lays it all out.
All whales, dolphins, porpoises, and seals are marine mammals. The first three belong to the infraorder of Cetacea, meaning that they are all somewhat closely related. By contrast, seals belong to the infraorder Pinnipedia, which also includes sea lions and walruses.
Zoologists can be quite confusing when it comes to naming animals. The whale shark is a prime example of this. Whale sharks (Rhincodon typus) are not whales and therefore not mammals, but are sharks and by extension fish. They can reach lengths of around 12 meters (40 feet) and weigh as much as 20.6 tons according to National Geographic. In fact they are the world's largest fish species, beating great white sharks and basking sharks to the title.
The world’s largest animal, however, is a blue whale (Balaenoptera musculus), which can be as long as 24 meters (78 feet) and weigh 150 tons. Since we now know that whales are mammals, they're also the world’s largest mammal.
The key differences
The main differences that can help you tell if an animal is a fish or a mammal are quite obvious once you know what to look for.
The first has to do with breathing. Whales breathe oxygen through the blow holes on their heads, which means that they must break the surface of the water to breathe in the oxygen before closing the blowhole and diving beneath the surface. They have lungs that the air is passed into in the same way as a human. When whales get stranded, because they can still breathe, they can often be rescued and refloated back out to sea.
Sharks do not have blow holes and do not breathe air like a whale. Instead they have gills that filter oxygen from the seawater and take waste carbon dioxide from the blood back into the ocean. They do not have lungs. Typically, if a large shark were to be stranded out of the water it would not survive, because they cannot breathe directly from the air; however, some species can survive for longer without water, like the "walking" shark.
The other key difference is in reproduction. Whales, seals, and dolphins all reproduce by internal fertilization and give birth to live young that they then care for, feeding them milk that is produced in the mother’s body. Sharks, however, depending on species have a whole range of reproductive options, including live young and laying eggs, but never feed their young milk.
Another fun difference between sharks and mammals is that while sharks never have hair, whales occasionally do.