Fish can be found everywhere on planet Earth; from the tiniest streams to the mighty oceans. There are thousands of species of fish, and new species are still being discovered with the help of new technology. Fish represent a large percent of the ocean's population and their existance is very important to the global economy and to the overall health of the world's lakes and oceans.
Fish are members of the Chordata phylum. A phylum is a group of animals that have common characteristics. This means that fish and all other members of the Chordata phylum have dorsal, hollow nerve cords, a notochord (a supportive yet flexible rod between the digestive tract and the nerve cord), pharyngeal slits (slits in the area behind the mouth, sometimes called gills), and a post-anal tail.
Fish are vertebrates. They have skeletons made of bone and cartilage and have a distinct spinal column, or backbone. All fish also have hinged jaws. This trait evolved about 470 million years ago and is one of the most important classifying characteristics in fish. The jaws of fish are hinged, meaning that the upper and lower jaw bones are connected, and the jaws are supported by skeletal parts. These skeletal parts evolved from the ancient skeletal structure of fish into bones that support the jaws.
Well no, actually, there is another distinction that sets fish apart from other vertebrates. Fish have gills. Gills are slits in the pharynx, which is the area of the head behind the mouth. These slits are protected by flaps and supported by thin, boney structures. These sturctures hold the gill tissues or fillaments that are full of blood and act as a site for gas exchange. Gills act as lung derivatives for fish, since for the most part fish do not have real lungs.Fish intake water through their mouths and pass it across their gills. Dissolved oxygen from the water exchanges places with the carbon dioxcide in the fish's bloodstream. This ability to breathe underwater with the help of gills is probably the most important characteristics of fish.
Fish also have something called the lateral line system. These are rows of sensory organs along both sides of the fish. These organs register changes in pressure or temperature, and even pick up sounds and vibrations made by prey. Another fish adaptation is paired fins. The development of paired pectoral and pelvic fins provides fish with increased stability and greater movement control. Paired fins developed as a result of increasing mobility among fish millions of years ago.
Fish can be broken up into three different types. Each of these types still have the main characteristics of fish, but they are each unique in their own way. In the next sections we will further explore these groups: the chonodrichthyes, ray-finned fish, and lobe-finned fish.
The first fish group is the Chondrichthyans. This group can also be called "cartilage fish" because of their flexible, cartilage skeletons. Sharks and sting rays are two good examples of Chondrichthyans. Chondrichthyans can be either bottom feeders or suspension feeders, like rays, or predators like sharks who chase their food. Generally, though bottom dwelling rays are an exception, Chondrichthyans must constantly swim to pass water across their gills and breathe. Chondrichthyans generally have strong eyesight and a strong sense of smell. These senses make Chondrichthyans formidable predators.
Sharks come in many shapes and sizes. The biggest sharks, which are also the biggest of all fish, are generally suspension feeders. They filter krill and plankton from the water as they swim. Other sharks, like the classic Great White (photo above) swim constantly in search of prey. These predator sharks have evolved over the years to have streamline bodies, strong muscles, and powerful jaws. Sharks are an ancient species of fish, they have seen relatively few changes to their species over the years, and they are one of the oceans top predators.
Rays are fish that are flattened dorsoventrally, or along their backs. This adaptation helps rays as they feed off the bottom or lie still in wait of pray. Rays also developed eyes on the tops of their heads to better see their food. Some rays, like the giant Manta Ray (photo above) are filter feeders. They, like the large sharks, filter their microscopic food from the water around them. Rays are actually poisonous fish. Their long tails have barbs that contain venom which acts as a powerful defense mechanism.
Lobe-finned fish make up the second group of fish. Lobe finned fish are ancient fish, and few species survive today. Three present day lineages of lobe-finned fish are coelacanth (a type of deep sea fish), lungfish, and tetrapods. Throughout years of evolution, the tetrapods grew legs and slowly lost their fish characteristics and adapted to life on land. Lobe-finned fish are characterised by their lung derivatives or in some cases even lungs and their muscular fins. The fins of lobe-finned fish differ greatly from the fins of the ray-finned fish, who we will look at next, because they are so muscular. Their fins are supported by sturdy bones and muscles and are often covered by scales or skin.
The African Lungfish (photo above) is a prime example of a lobe-finned fish. It is an ancient species of fish, and you can clearly see in this picture the muscular fins that classify it as a lobe-finned fish. Lungfish are very different from other fish, even other lobe-finned fish, because they have lungs. The Lungfish lives in stagnant water that often does not have a high dissolved oxygen content. The Lungfish can survive in these conditions because it can swim to the surface and breathe air through its mouth straight into its lungs. This amazing adaptation reveals the Lungfish as an important part step in the evolution of life on Earth.
Ray-finned fish are the most common of the three fish families; there are over 27,000 recorded species! Ray-finned fish, like the lobe-finned fish, have bony skeletons and are covered by an outer layer of flattened scales. They also secrete a mucus onto their scales to reduce resistance as they swim through the water. The major defining characteristic of Ray-finned fish is their fins. The fins of Ray-finned fish are supported by thin, flexbile rays of bone or cartilage. The fins are usually made of thin membrains and can be raised or lowered easily. Ray finned-fish, and a few lobe finned fish, also have something called an operculum flap. This flap is positioned over the gills and can be moved to bring water across the gills. This allows the fish to breathe without swimming and is very helpful for fish that live on the bottom or lie in wait for prey.
The Largemouth Bass (photo above) is a classic example of a Ray-finned fish. This image clearly shows the fins supported by thin bony structures. Largemouth Bass live in fresh water across North America and are a prized fish to many sports fishermen. Many ray-finned fish are familiar fishes like this bass, tuna, trout, or goldfish. Others, like seahorses, flouders, and even pufferfish, have fascinating adaptations.
This is a Leafy Sea Dragon (photo above). It is not a real dragon, but actually a kind of seahorse. This Leafy Sea Dragon is an excellent example of a Ray-finned fish's ability to adapt to its environment. The Sea Dragon lives in large kelp forests, and thus it has developed fins that help it blend in.
This flounder (picture above) is a final example of a Ray-finned fish with specific adaptations. Its body is dorsoventrally flat, like the Manta Ray, and its eyes are on the top of its head. The flounder spends most of its time burried in the sand waiting for its food, small fish, to swim by. Then it attacks out of the sand.
I hope you have learned something new or interesting about fish and enjoyed my website in the process!
Great White Shark
http://www.hoax-slayer.com/images/original-shark-pic.jpg
Manta Ray
http://www.honusports.com/HonuImages/mantaray.jpg
Largemouth Bass
http://www.flmnh.ufl.edu/fish/gallery/Descript/LargemouthBass/lbassusda.JPG
Leafy Sea Dragon
http://www.tnaqua.org/Newsroom/HighRes/Leafy_Seadragon.jpg
Lungfish
en.wikivisual.com/images/6/61/Australian-Lungfish.jpg
Flounder
http://surfnail.com/200610-hawaii/20061017-flounder.jpg
