Background

The Three Major Classes of Fish

Class Agnatha
Class Agnatha (also known as jawless fish) include lampreys and hagfish. These fish have long, cylindrical bodies; smooth, scaleless skin; and unpaired fins. Agnathans have cartilage instead of a bony skeleton and do not have moveable jaws. Hagfish are bottom dwellers that burrow into the openings of decaying animals and suck the decaying internal organs into their mouth. Lampreys are parasitic animals that attach to their hosts with a round, sucking mouth. They will inject an anticoagulant into the blood of their host and suck their blood until full. Nine out of ten fish that have been attacked by a lamprey will die.

Class Chondrichthyes
Class Chondrichthyes includes sharks, rays, and skates. They are commonly known as cartilaginous fish because their skeletons are made almost exclusively of cartilage. All species are carnivores and some are scavengers. Their skin is covered with placoid scales , smooth, toothlike spines that feel like sandpaper. Sharks, rays, and skates differ from hagfish and lampreys in that they have moveable jaws, and paired fins. Rays and skates have flattened bodies with paired wing-like pectoral fins and in some species, whip-like tails. Rays have diamond or disk-shaped bodies. Skates have triangular bodies.

Sharks vary in size and shape. The typical shark will have 6 to 20 rows of teeth that point inward. If a tooth breaks or wears away, a replacement will move forward to take its place. A shark may use more than 20,000 teeth in its lifetime.

Class Osteichthyes
Class Osteichthyes is commonly referred to as bony fish. The scales on these fish are different from cartilaginous fish. These scales are thin and round and overlap like shingles. There are three major groups of bony fish: lobe-finned fish, lungfish, and ray-finned fish. Lobe-finned fish have muscular, paddle-like fins supported by bones. The coelacanth is the only known lobe-finned fish. Lungfishes have lungs as well as gills and a long, eel-like body. Ray-finned fish have a swim bladder. This is a thin-walled sac filled with gas and found in the abdominal cavity. Oxygen, carbon dioxide, and nitrogen from the fish’s bloodstream move into the swim bladder inflating it like a balloon. This enables the fish to move up in the water. When those gases diffuse back into the bloodstream, the swim bladder deflates and the fish is able to move down in the water.

Do fish breathe?

Regardless of the class of fish, all fish have gills through which they take oxygen from the water . Oxygen is not as abundant in water as it is in air. Oxygen makes up about 20% of the air we breathe, but dissolved oxygen, which fish absorb through their gills, represents only about 0.0001% of water. (In fact, the maximum amount of oxygen that water wants to hold is only about 8.3 mg/L at standard temperature and pressure. That is equivalent to 8 red Ping-Pong balls in a classroom filled to the brim with white ones!) When a fish breathes, it must work to move water or air over the surfaces it uses for gas exchange. Water is 800 times more dense than air and about 50 times more viscous. Therefore, fish must expend more energy getting oxygen from their environment than we do.

How does a fish control body temperature?

Thermoregulation is an animal’s control of its body temperature. There are two types of thermoregulation: endothermic and ectothermic. Endotherms are animals that control their body temperatures internally by way of metabolism. Mammals are endotherms. Ectotherms are unable to control their own body temperatures and must rely on the outside environment for regulation. Reptiles are an example of ectotherms. Most animals that are “water-breathers” are ectotherms – their body temperatures are closely tied to the water temperature that surrounds them. As the temperature of the water rises, so does their body temperature and metabolic rate. Therefore, rising temperatures cause water breathers to need more oxygen in their bodies.

What is respiration?

Respiration is also known as gas exchange. Animals take in oxygen and release carbon dioxide. In fish, gas exchange takes place as water enters the fish’s mouth, passes over the gills, and leaves through the operculum (gill plate). As a fish takes water into its mouth, the water passes over the gills and oxygen in the water passes or diffuses directly into thousands of fine filaments containing many blood vessels thereby putting oxygen into the fish’s bloodstream. The fish’s circulatory system then brings that oxygen to other parts of the body. Carbon dioxide diffuses out of the blood stream at the same time that oxygen diffuses into the bloodstream. Each gill has a double row of hundreds of thin projections of these gill filaments. The water then passes through the gills and out into the environment when the fish opens its operculum. The gill filaments create a large surface area for oxygen to diffuse into and thus allows for rapid gas exchange. Gills are supported by four sets of curved bones on each side of the fish’s head.