Most people probably think of a shark as a swimming animal with big fins and pointy teeth. But how exactly does it fit into the Animal Kingdom? For starters, some of you may be surprised to know that sharks are indeed fish. But they really don’t look all that similar to your average gold fish, do they?
There’s a good reason for these distinct appearances: while they are both fish, the creatures shown above actually represent two very different “Classes” of animals (see our short article on Scientific Nomenclature).
What we think of as the typical fish – goldfish, minnows, salmon – are “bony fish” belonging to Class Osteichthyes. They have a skeleton that is made mostly of bone. In addition, they are agile swimmers that can move forward and backward, and they are found in rivers, lakes and seas.
How Are Sharks Different From Other Types Of Fish?
Sharks, on the other hand, belong to an entirely different group called Class Chondrichthyes (which oddly enough are thought to have evolved from an earlier group of bony fish). Some newer classification schemes instead refer to this as Class Elasmobranchii. Mostly these are marine fish, meaning that they live in salt water. “Cartilaginous” fish or “elasmobranchs” include sharks along with rays, skates, and chimaeras (ratfish): those that lack true bone. Instead, they have a skeleton made of cartilage, reinforced with calcification (calcium builds up in the tissue, causing it to harden). Sharks can only swim forward. This is because unlike in bony fish, their pectoral fins can’t bend upwards. Many sharks have to keep swimming in order to breathe.
Among other key distinctions, cartilaginous fish have a unique type of tooth attachment and replacement. Teeth are arranged in many rows that are constantly shed and replaced. Unlike bony fish, whose males and females eject their sperm and eggs into the water to mix, these fish have internal fertilization. “Chondrichthyan” males have organs known as claspers. These are a modified part of the pelvic fins that are inserted into the female cloaca (an opening in the body), so that sperm can be placed inside the females during copulation. Compared to bony fish, Chondrichthyans mature sexually at much greater ages, produce fewer young at a time, and can have very long gestation periods.
Sharks Have Been Around For A LONG Time!
Would you believe that sharks have existed for millions of years? In fact, they evolved long before dinosaurs! The fossil record of cartilaginous fish goes back 450 million years to the Paleozoic era. Sharks themselves began to appear about 400 million years ago, during the Devonian period, which is often called “The Age of Fishes”. The prehistoric nature of sharks is just one of many things that makes them utterly fascinating creatures.
The Misunderstood Species
Unfortunately, sharks are greatly misunderstood. They have an unfair reputation for being dangerous, but for the most part are normally harmless. As the famous shark biologist Eugenie Clark once said, “It’s more dangerous to drive to the beach than it is to go into the water with sharks”.
Sharks worldwide are threatened by human activities such as overfishing, exploitation for certain body parts, habitat degradation, and sometimes, outright slaughter. A major conservation challenge is that we are missing basic information. Although many species are widespread, a lack of fisheries data makes it impossible to assess the status of most species. Sharing information about these splendid animals can only help to raise awareness of them.