Characteristics

The bony fishes are characterized by three key features:

• Bone—This material is typically harder and heavier than cartilage. The skeletons of most bony fishes contain bone.

• Lungs or swim bladder—Early bony fishes had lungs, internal respiratory organs in which gas is exchanged between the air and blood. Only a few species of bony fishes have lungs today. Most bony fishes have a swim bladder, a gas-filled sac that is used to control buoyancy. The swim bladder is thought to have evolved from the lungs of the early bony fishes.

• Scales—The body of a bony fish is usually covered with scales. Scales protect the fish and reduce friction when swimming.

There are two main groups of bony fishes. These are the lobe-finned fishes and the ray-finned fishes.

Characteristics of Lobe-Finned Fishes

The lobe-finned fishes have fleshy fins that are supported by a series of bones. Two groups of lobe-finned fishes exist today, six species of lungfishes and one species of coelacanth. Lungfishes, shown in Figure 39-9a, exchange gases through both lungs and gills. They live in shallow tropical ponds that periodically dry up. The coelacanth, shown in Figure 39-9b, lives deep in the ocean and was thought to be extinct until 1938. Extinct lobe-finned fishes are ancestors of amphibians.

objectives

• List three characteristics of bony fishes.

• Distinguish between lobe-finned fishes and ray-finned fishes.

• Describe three key features of bony fishes' external anatomy.

• Summarize the major body systems in bony fishes.

• Describe the function of the swim bladder.

• Discuss reproduction in bony fishes.

vocabulary swim bladder lobe-finned fish ray-finned fish operculum countercurrent flow optic tectum spawning figure 39-9

Lobe-finned fishes have fleshy fins that are supported by a series of bones. Lungfishes (a) have the ability to gulp air into lungs as an oxygen source. Coelacanths (b) have muscular fins with stout bones.

Characteristics of Ray-Finned Fishes

Ray-finned fishes do not have fins with a central bony axis—they have fins that are supported by long, segmented, flexible bony elements called rays. Rays probably evolved from scales. Ray-finned fishes are diverse in appearance, behavior, and habitat. Ray-finned fishes include most familiar fishes, such as, yellow perch, trout, salmon, guppies, bass, herring, goldfish, and eels.

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