Alex Burnett 6E
May 16, 2011

Sharks Versus Bony Fish

Sharks and bony fish have been around since the time of the dinosaurs, more than four hundred million years. Sharks have changed very little over the years, although some species have gone extinct. Bony fish, on the other hand, have changed a lot. At first fish were jawless, but slowly they evolved over time to the modern bony fish. Although sharks are a type of fish, they are very different from bony fish. Shark are mostly found in salt water oceans and seas, although some species, like the Bull shark, are known to be able to swim up rivers and survive in both salt and fresh water. One species, the Lake Nicaragua Shark, lives entirely in fresh water. In comparison, bony fish live in both salt and fresh water. Although most species stick to one type of water, some, like the Salmon, live in both. The main difference between bony fish and sharks is what their skeletons are made of. Bony fish skeletons are made of bone, while shark skeletons are made of cartilage. Bones are a rigid connective tissue containing calcium phosphate that make up the skeletons of most vertebrae. Cartilage is a firm but flexible connective tissue. Another difference in the skeleton is that a shark jaw is not connected to its skull while a bony fish jaw is. The external features of bony fish and sharks are generally the same. They both have the same type of fins, pectoral fins, pelvic fins, dorsal fins, anal fins and caudal fins, although their placement on the body might be different. An example of this is that the pectoral fins of a shark are angled like the wings of an airplane, but the pectoral fins of a bony fish are more flexible and can be folded against the body of a bony fish. Unlike bony fish, sharks do not have scales. They have a tooth-like structure called dermal denticles. The purpose of scales and dermal denticles is for external protection of the bony fish and shark. Both scales and dermal denticles overlap in the same way to prevent drag when a shark or bony fish is swimming. However, unlike scales which grow with the bony fish, dermal denticles are shed when the shark grows bigger and new ones grow in their place. The eyelids of sharks and bony fish differ, because bony fish don’t have any eyelids, but sharks have two types of eyelids; non-nictitating and nictitating. Sharks with non-nictitating eyelids have upper and lower eyelids that do not move. Sharks with non-nictitating eyelids roll their eyes back when they attack prey, exposing the less vital whites of the eye. Sharks with nictitating eyelids have upper and lower eyelids that don’t move, but they also have a third, inner eyelid, called the nictitating eyelid or membrane, that closes over the eye when a shark attacks its prey.

The internal features of a shark differ from that of a bony fish. For one, bony fish have swim bladder and sharks do not. The swim bladder lets the fish adjust its depth according to how much air it holds there. When bony fish wants to rise, they take more air into their swim bladder, and when they dive, they release some of this air. Sharks rely on their liver to keep them afloat. Their liver is very large and filled with oil, because oil is lighter than water. This allows the shark’s heavy body to float. The lighter cartilage of the shark also aids in buoyancy for a shark.

Bony fish and sharks both breathe through the use of gills, but they each use gills differently. Bony fish breathe by taking in water through the mouth, while the gill cover, or operculum, is closed. The oxygen rich water then passes through the gills which filter out the dissolved oxygen and passes it into the bloodstream. The mouth closes and the operculum opens releasing the de-oxygenated water out of the body. Most sharks have to keep moving in order to breathe. Sharks breathe by continually taking water in through the mouth while the gill slits are closed. The