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10.6: Bony Fish

Difficulty Level: At Grade Created by: CK-12
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Can fish have bones?

Of course. Many fish have bones. They serve the same function as our bones: protection and support. Notice how the skeleton protects the fish's brain. Also, notice the bones along the body of the fish would allow muscles to attach to aid in movement.

Bony Fish

There are about 27,000 species of bony fish (Figure below), which are divided into two classes: ray-finned fish and lobe-finned fish. Most bony fish are ray-finned. These thin fins consist of webs of skin over flexible spines. Lobe-finned fish, on the other hand, have fins that resemble stump-like appendages.

The fins of a ray fin and a lobe fin fish

Fins of bony fish: ray fin (left) and lobe fin (right).

Characteristics of Bony Fish

Most fish are bony fish, making them the largest group of vertebrates in existence today. They are characterized by:

  1. A head and pectoral girdles (arches supporting the forelimbs) that are covered with bones derived from the skin.
  2. A lung or swim bladder, which helps the body create a balance between sinking and floating by either filling up with or emitting gases such as oxygen. Controlling the volume of this organ helps fish control their depth.
  3. Jointed, segmented rods supporting the fins.
  4. A cover over the gill called the operculum, which helps them breathe without having to swim.
  5. The ability to see in color, unlike most other fish.

Ray-finned Fish

Most vertebrates are ray-finned fish, with close to 27,000 known species. By comparison, there are "only" about 10,000 species of birds. The ray-finned fish have fin rays, with fins supported by bony spines known as rays. The ray-finned fish are the dominant class of vertebrates, with nearly 99% of fish falling into this category. They live in all aquatic environments, from freshwater and marine environments from the deep sea to the highest mountain streams.

Lobe-finned fish

The lobe-finned fish are characterized by fleshy lobed fins, as opposed to the bony fins of the ray-finned fish. There are two types of living lobe-finned fish: the coelacanths and the lungfish. The pectoral and pelvic fins have joints resembling those of tetrapod (four-limbed land vertebrates) limbs. These fins evolved into legs of amphibians, the first tetrapod land vertebrates. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of ray-finned fish. All lobe-finned fishes possess teeth covered with true enamel. The lungfish also possess both gills and lungs, solidifying this class as the ancestors of amphibians.

How Big Are Bony Fish?

The ocean sunfish is the most massive bony fish in the world, up to 11 feet long and weighing up to 5,070 pounds (Figure below). Other very large bony fish include the Atlantic blue marlin, the black marlin, some sturgeon species, the giant grouper, and the goliath grouper. The long-bodied oarfish can easily be over 30 feet long, but is not nearly as massive as the ocean sunfish. In contrast, the dwarf pygmy goby measures only 0.6 inches. Fish can also be quite valuable. In January 2013, at an auction in Tokyo's Tsukiji fish market, a 222-kilogram (489-pound) tuna caught off northeastern Japan sold for 155.4 million yen, which is $1,760,000.

Picture of an ocean sunfish

An ocean sunfish, the most massive bony fish in the world, can reach up to 11 feet long and weigh up to 5,070 pounds!

Science : Oarfish: The Ultimate Fish Tale

Thought to the be inspiration of "sea serpent" stories, the monstrously-long Oarfish provokes wonder in nearly all that witness it. In this video by Science Friday, Professor Misty Paig-Tran provides us with insights into this real-life "sea monster's" unusual physiology.


  • The bony fish are divided into two classes: ray-finned fish and lobe-finned fish.
  • The bony fish are characterized by a lung or swim bladder, a cover over the gills, and bones covering the head and pectoral girdles.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

  1. What is one of the purposes of the operculum?
  2. What is the lateral line used for? Where is it located?
  3. Not all fish have swim bladders, but, for those who do, what are they used for? Why do you think some fish don't have swim bladders?

Explore More II

  1. What type of fish is a seahorse?
  2. What is unusual about seahorse reproduction?
  3. How wide is a seahorse's field of vision? How is it so wide?
  4. What do seahorses feed on?


  1. What is the largest group of vertebrates in existence today?
  2. Name three characteristics of the bony fish?
  3. What are the two classes of the bony fish?
  4. What is the swim bladder?
  5. What is the operculum?

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operculum A hard bony flap covering and protecting the gills.
pectoral girdles Bony structure supporting the fins.
swim bladder Gas-filled organ that helps a fish to control its buoyancy.

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Difficulty Level:
At Grade

Concept Nodes:

7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Dec 24, 2016
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