Science Worksheet #1 - Answers
Fish are found in almost all of the waters of the world. They are descendants of animals that lived 400 million years ago. Fish with jaws are divided into two major groups based on their skeletal material. Those with cartilage are called the cartilaginous fishes. Included in this group are sharks, skates and rays. These fish occur most commonly in salt water in oceans, bays and estuaries. The second major group are fish with calcified skeletons. They are called the bony fishes. In addition to saltwater habitats, they are also common in freshwater lakes, rivers and streams.
Fish do not have legs. They move by fin power. They generally have two sets of paired fins; that is, one of each pair on either side of the body. The paired fins located highest behind the operculum are the pectoral fins. The lower fins are the pelvic fins. Most fish use these fins for steering and braking. Some, like parrotfish and wrasses, use pectoral fins for propulsion. Fish have three unpaired fins. The dorsal fin runs down the middle of the back and the anal fin is found underneath near the tail. On most fish these fins just act as stabilizers; but some fish, such as the triggerfish, swim by moving the dorsal and anal fins in a sine-wave(~) pattern. Most fish, however, use their tail or caudal fin for power.
Many fish, especially the fast-swimming ones, are fusiform or streamlined. But some, such as the skates and rays, are flattened from top to bottom. These animals are adapted to live on the bottom. Many fish have a laterally compressed body that is flattened from side to side. This shape is good for maneuvering in and out of cracks in a coral reef or thick vegetation.
Most fish catch prey or graze on algae using teeth. Many fish with up-pointing mouths feed on organisms near the water's surface. Bottom dwellers that feed on animals living on the bottom often have sensitive "whiskers" or barbels located on their chin to help find food. Catfish and nurse sharks have these. Many fast-swimming fish, like the swordfish, chase and catch their food, which often includes other fish. Angelfish maneuver slowly around the reef, picking at animals or algae with their small mouths. Some large fish like the basking sharks don't use teeth for feeding. Instead, they strain small animals from the water using modified gill rakers.
The outside of most bony fish is covered with waterproof scales. Members of the shark family do not have these. Instead, their skin is covered with dermal denticles, or "skin teeth." Fish get their oxygen from water rather than from air. Water flows in the mouth and over the gills where gases are exchanged with the water. Some fish like tuna and tiger sharks must swim constantly to force water over their gills or they will suffocate. Others like the flounder and nurse shark can pump water over their gills and are able to rest on the bottom. Bony fish have a gill cover or operculum; sharks have gill slits.
Reprinted courtesy of the National Aquarium in Baltimore