By Blaise Jones
When you look at a fish, a shark, and a whale, you might think that they are so similar that they all must be closely related, but that’s far from true.
While sharks and fishes may be related, whales are mammals. This means they are warm-blooded, give birth to live offspring, and breathe air.
Whale-specific Body Parts
Whales have special adaptations that let them live in the ocean and set them apart from sharks and fish. These include blow holes, which let them breathe air, and large reserves of fat which keep them warm in cold water. Blubber is so important to whales that in some species as much as 50 percent of their body mass is blubber.
Sharks are Fish, but Fish are not Sharks
Sharks and fishes, however, are very closely related. In fact, sharks are a type of fish. Fishes are defined as any group of cold-blooded animals that live their whole lives in the ocean and that have a backbone, fins, and gills.
Fish are divided into two groups based on their skeleton, which is why sharks are technically fish, though not what we traditionally think of when we image a fish. Sharks have skeletons made out of cartilage, the same material your nose and ears are made out of. In contrast, fishes have skeletons made out of bones. Bony fish are called Osteichthyes; and cartilaginous fish are Chondrichthyes.
Bony Fish Body Parts
Jaw is attached to the skull and some Osteichthyes have a second set of teeth located in their throat called pharyngeal jaws that help further breakdown their food. The notorious moray eel is not intimidating to look at, but when it’s method of feeding is reminiscent of the monster star of the movie Alien. When feeding, the moray attacks and holds the prey with it’s jaws while it’s pharyngeal jaws, located in the eel’s throat, darts forward and feeds on the prey. Shark jaws are unattached from their skull and can move independently to feed on larger prey. Both bony fish and sharks replace their teeth for their entire life, unlike humans have a permanent set of teeth by after childhood.
Bony fish have a specialized body organ called a swim bladder that provides buoyancy and allows them to regulate their depth in the water. Instead of a swim bladder, sharks have very large livers. In some species the liver can stretch across their entire chest cavity. These specialized livers secrete an oil that is lighter than the surrounding water, which helps keep the shark buoyant and makes moving much easier.
Bony fish often possess a protective bony plate to protect their gills, while the gills of sharks are exposed and more vulnerable. Sharks also have multiple rows of teeth, while most fish only have one. Additionally, while fish only replace teeth that have been lost, sharks teeth are constantly dropping out and being replaced by new ones, like a conveyer belt. This leads to some sharks losing tens of thousands of teeth in their lifetime, as opposed to the few dozen humans lose.
Shark-specific Body Parts
Sharks are known for their rough skin, while most fish are smooth and slick. The difference between the two is that most bony fish are covered in scales (either cycloid, ctenoid or ganoid) giving them their slick feel. Similar to determining the age of trees, a fish’s age can be determined by counting the number of rings on its scales. By contrast, sharks are covered in denticles, which are enamel-covered, toothlike body parts that cover their entire body to help increase their swimming efficiency by reducing drag.
Sharks have eyelids to protect their eyes and some species have a third eyelid, called a nictating membrane, while bony fish do not have eyelids, leaving their eyes unprotected. Some species of sharks, such as the great white shark, not only have a nictating membrane but can further protect their eyes by rolling them back into their head.
“The Encyclopedia of Sharks” by Steve Parker