The Vertebrates: Fish (Rev, 1985)

[Music] Nearly 34s of our planet is covered by water. Water teameming with living creatures of a great variety of different species. A starfish making its way along the bottom. A jellyfish weaving through the water. In spite of their names, these animals are not fish at all. They belong to a major division of animal life called invertebrates. Animals without backbones. True fish are part of another major division. Fish along with amphibians, reptiles, birds, and mammals are vertebrates, animals with backbones. Most fish have skeletons made of bone. But there are two small groups of fish with skeletons made of cartilage. One group includes hagfish and lampres, jawless fish with long snake- like bodies. The second group of fish with cartilage skeletons includes sharks along with rays and skates. These fish have jaws and most of them live in the ocean. The vast majority of fish belong to a third group, fish with skeletons of bone. We know of more than 20,000 different species, and there are probably many more that we haven't yet discovered. There are more species of fish than of all the other vertebrates combined. And there is enormous diversity in size and shape and other adaptations that help each species to survive. [Music] But survival also depends on several characteristics that are shared by all fish. All of them have gills that are used to breathe oxygen dissolved in water. On each side of the head, under a gill cover, a fish has four gills supported by an arch of bone. The thin skin of the gills is folded into gill filaments. Water enters through the fish's mouth and flows over the gills which are well supplied with blood vessels. Oxygen dissolved in the water is absorbed by the blood moving through the gills and from the blood carbon dioxide is released into the water. Gills are part of the body structure of all fish. Another typical characteristic of body structure is a skin covering of scales. There are a few species, including the catfish, that have a smooth skin with no scales. [Music] But most fish do have them. Scales are plates of bone embedded in the skin. The scales overlap one another to form a tough, flexible covering. There are other features of body structure that adapt a fish for smooth movement through the water. Even though there is a wide variety in body shape, the body of a fish is always more or less streamlined and the skin is covered with a slippery mucus secretion that reduces friction as the fish pushes its way through the water. Fins are another adaptation for movement. Most fish have a pair of fins, the pectorals, one on each side of the body, located just behind the gills. On the underside of the body are a pair of pelvic fins and an anal fin. On top is a dorsal fin which may be divided into two parts. And there is also a tail fin. Each type of fin has its own function to help maintain balance or to steer or to push the fish through the water. [Music] And many fish have another adaptation for movement. [Music] In the body cavity under the spine is an airtight sack called the swim bladder. By increasing the amount of air in the bladder, a fish can rise in the water and a decrease in the amount of air makes the fish sink deeper. Along with these adaptations for movement, most fish share some other characteristics. Usually the eyes of a fish are located on opposite sides of its head, giving it two nearly separate fields of vision. A fish has no eyelids. Its eyes are continually being cleaned by the surrounding water. Fish have the same five senses as most other vertebrae animals, but they also have another type of sense organ located in a row of scales called the lateral line. These organs are sensitive to the vibrations and pressure waves sent out by objects in the water and by the movement of the water itself. And that makes it possible for a fish to avoid obstacles or to find a hiding place that it cannot see. So there are several characteristics of body structure that are shared by all fish. Some of these characteristics are also found in a few other animals. A porpus has a body shape very much like that of a fish. But porpuses along with dolphins and whales are mammals. They have lungs and they breathe oxygen from the air rather than from the water. And like all mammals, they are warm-blooded animals. They maintain a constant high body temperature using energy from the food they eat. But fish along with amphibians and reptiles are called coldblooded which means that their body temperature changes with the temperature of their surroundings. And water is a favorable environment for a cold-blooded animal. From summer to winter, the changes in water temperature are not as great as the changes in air temperature, and they usually take place more slowly. As winter approaches, most fish can move to a place where the water is warm enough for them to remain active. But even though all fish are alike in some ways, there are also many differences in body structure and behavior. Like all animals, each type of fish must be adapted in certain ways to survive in its environment. And the waters of the earth contain many different environments. Some fish live in the salt water of the ocean. and others live in the fresh water of inland lakes and streams. This difference in environment has life and death importance for a fish. That's because body fluids contain a certain amount of salt and water is drawn to areas of greater salt concentration. And so freshwater fish must avoid taking in too much water. They don't drink at all. and the water absorbed through the skin is released with their waste products. Ocean fish have the opposite problem. To avoid losing body fluid to the saltier water around them, they must drink a great deal of water and release the salt. There are other variations that also help to create different environments. The water may be in motion as in a river or an ocean current or it may be relatively still as in a lake or pond. And the environment in shallow water or near the surface is very different from the conditions in deeper water where there is greater pressure and less light and where the water is much colder. The temperature of the water plays an important role in the lives of all fish in every environment. For each species, the mating season begins when the water temperature is right for it. For some, including the trout, mating season comes in springtime. Each female lays several thousand eggs. That's necessary for the survival of the fish species. Most of the eggs and most of the young fish will be eaten by predators. The eggs contain a yolk sack that provides food for the developing embryo. When the eggs hatch, the yolk sack is still attached to the young fish. The yolk is used as food until the mouth of the fish develops. Although many fish leave their eggs unprotected, others, like the bluegill, dig out a shallow nest for the eggs and guard them until they hatch. Some tropical fish, including the Siamese fighting fish, build a nest of bubbles to protect their eggs. A few kinds of fish reproduce without laying eggs. These young guppies were born directly from their mother's body. For some fish, mating involves a difficult and hazardous journey. Salmon leave the ocean where they live as adults and swim into a river, fighting their way upstream, even leaping small waterfalls to reach the place where they were hatched and where they will mate. How they guide themselves on this dramatic journey is still largely a mystery to us. So among the various types of fish, there are several different patterns of reproduction. And there are many other variations that adapt each species for survival. Color and markings may serve as camouflage, making it possible to hide from a predator. The spotted fish at the left near the rock is difficult to see unless you look very closely. When a flounder settles down in the sand, it almost seems to disappear except for one eye. A fish may also be adapted for eating certain types of food. Some fish are planteaters and many others feed on plankton seen here under a microscope. Plankton consists of several types of very small plants and animals as well as single-sellled organisms. Some fish are predators. Their diet may include invertebrate animals like this crab. And they may also eat other fish. The food chain that begins here also extends beyond the water. [Music] Commercial fishing is a major industry around the world. Most of the large fishing fleets operate in the coastal waters of the ocean. The yield of fish is greater here than on the high seas because these waters are rich in plankton and have large populations of the plants and animals that fish use for food. [Music] In many parts of the world, the supply of fish is increased by means of aquaculture or fish farming. Ponds and streams can be stocked with small fish, and fish can also be raised in specially constructed pens in a hatchery. [Music] Fish farmers have increased their yield dramatically by dissolving additional oxygen in the water so it can support a larger population of fish. [Music] Whether raised in a fish hatchery or living in the open water, fish are a major source of food for the people of many countries. As world population continues to increase, they will become even more important. Like other animals, fish are part of an intricate web that links all of the many forms of life on the earth. [Music] [Applause] [Music]