Around 400 million years ago, fish evolved into two very different ‘classes’, or groups. One of them is ‘Class Chondrichthyes’, which includes sharks, rays and chimeras. This class of fish is also known as ‘cartilaginous’ fish because they have skeletons made of cartilage.
The other class is ‘Class Osteichthyes’, which gave rise to the modern day bony fish, also called ‘teleost’, and their primitive relatives such as the sturgeon. Although the two groups once shared a common ancestor, there are several major biological differences between them.
Teleosts, the bony fish, are some of the most abundant vertebrates on the planet with nearly 25,000 living species. In contrast, the chondrichthians represent only about 1,100 species of extant species of. In the past, however, the chondrichthians were much more abundant, according to the fossil record.
There are a few differences between cartilaginous fish and bony fish that are pretty apparent.
The main difference between the two groups is what defines them, the composition of their skeleton.
Shark species have eyelids and some can even protect their eye with a tough third eyelid called a nictating membrane. Other species, like the great white shark, have muscles that can roll the eye back into the socket for protection. Bony fish lack eyelids or the ability to protect their eyes.
The circulatory system of the two groups is also different.
The outer design of sharks and bony fish is also contrasting.
Because all fish live in a three dimensional environment, they have to control their position to keep from floating up to the surface or sinking to the bottom to the sea floor. This is known as neutral buoyancy, a state that SCUBA divers know well.
Bony fish have an internal organ known as the swim bladder that keeps them achieve neutral buoyancy by exchanging gases with the blood vessels. Sharks lack this structure but rely on a lighter cartilaginous skeleton, hydrodynamic planing, the low density oils in their relatively large livers, and even some gulp air from the surface to keep neutral.
This is the process that all organisms use to control the levels of water and mineral salts in the blood. The water in the marine environment is saltier than the water inside a fish, both bony and cartilaginous and each group has developed distinct strategies to cope with this. The less salty water inside a fish is tends to rush out of the body to reach an equilibrium with the saltier seawater, which can leave a fish quite dehydrated. Bony fish deal with this uneven concentration by drinking a lot of seawater to stay hydrated and excreting the concentrated salts through the gills and gut. Sharks have evolved the ability to reabsorb the urea created by the breakdown of protein back into their tissue which helps level out the concentration differences so they do not lose as much water. Salts are still diffused into their body but are excreted over the gills, within the urine, and by the rectal gland that extracts salt from the blood. Read more...
Sharks have developed a life history strategy that requires slow growth to a relatively large size. It takes a long time for most sharks to reach maturity. When they reproduce, they have low numbers of advanced offspring (shark pups are miniature versions of adult sharks and are on their own from birth) and repeat these reproductive events over a long life.
Most bony fish demonstrate rapid growth, reach maturity younger, an increased ability to reproduce resulting in the creation of thousands of minuscule offspring that is scantily developed. Some bony fish have a single breeding event and then they die. The difference in these two strategies magnify why sharks and bony fish need to be managed in different ways. Bony fish on the fast track have the ability to replace itself in a population quickly but all of the characteristics listed above mean sharks take a lot longer.
All shark reproduction is internal. Male sharks have mixoptyerigia, or pelvic claspers, an extension of the cartilaginous skeleton that serve as the sperm conducting structure for internal fertilization. No such structure is found on bony fish. Read more...
As a group, the diversity of shark species is lowest in the Pacific Ocean region but bony fish have a pattern of lowest species diversity in the eastern Atlantic Ocean.