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This section introduces two major groups on the vertebrate tree: the rayfin fishes, whose fins are supported solely by bony rays, and the lobefin fishes and tetrapods, whose defining feature is some kind of limb.
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Spines with many uses
Rigid, spiny rays in their flexible fins enable the spinyrayed fishes to swim with finely controlled movements. Their spines can also be used for defense. The spines of this red lionfish (Pterois volitans), for example, are like hypodermic needles and can deliver deadly poison.
Norbert Wu / www.norbertwu.com
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Rayfin fishes have evolved to fill nearly every aquatic habitat on Earth. Their flexible fins have become adapted to swim with beautiful efficiency, walk along the ocean bottom, climb vertical walls, glide along the water's surface and even fly through the air.
The number of species in the very successful rayfin group exceeds the total number of all other vertebrates combined. Over 25,000 species of rayfin fishes have been identified, and about 200 more are discovered each year.
This is a group of tremendous diversity. Some species live in mountain springs 5,000 meters (16,000 feet) above sea level, while others live in the deepest ocean 11,000 meters (35,000 feet) below the surface. Some live only in tropical water, while others live in ice-cold polar seas. Various rayfin species produce light, venom, sound or electricity. Some of these fishes live just a year, others 150 years. Some never stray more than a few meters from the site of their birth; others migrate thousands of kilometers. Most lay eggs, but a few give birth to live young, and some are devoted parents.
Though the rayfins include some relatively primitive fishes such as bichirs and sturgeons, over 95 percent belong to the teleost group, which means "perfect boned." Their skeletons provide a powerful yet versatile foundation for many ways of life in water.
There are roughly 24,000 species of four-legged vertebrates alive today. All frogs, lizards, turtles, birds, mammals—including humans—and many ancient species such as dinosaurs share similar bone structures because they are all related. Each inherited its four-limbed skeleton from a common ancestor: the first lobefin fish.
Unlike those of rayfin fishes, the paired fins of the lobefin fishes have a single bone connected to the shoulder and pelvis. These lobed fins evolved into the limbs of four-legged tetrapods. Tetrapod limbs were used first for swimming and only later became adapted for use on land. Lungs, which enabled tetrapods to survive when they left the water, were also first used in the ocean by lobefin fishes.
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A Fish With Lungs
This African lungfish (Protopterus annectens) is more closely related to land tetrapods than to other fishes. But instead of developing into limbs like those of its tetrapod relatives, its lobed fins degenerated into long, skinny tendrils. For years, the lungfishes were known only from the fossil record. They were believed to have gone extinct until the first living lungfish was discovered in the 1830s.
Norbert Wu / www.norbertwu.com
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Although just a few species of lobefin fishes are still living today, their lineage is of tremendous importance. All four-limbed vertebrates, or tetrapods—including humans—evolved from early lobefin fishes.
Life first arose in the ocean, and only a few evolutionary branches adapted to live on dry land. Many animals completed a long evolutionary detour by moving back into the sea. Whales and manatees, for example, descended from land-dwelling mammals but now live exclusively in the water. And many birds, turtles, frogs and snakes also moved from land back to aquatic environments.
Most fishes have a gas bladder for buoyancy, but lungfishes and many primitive rayfins have lungs that can extract oxygen from air. For years it was assumed that gas bladders evolved first for buoyancy and later evolved into lungs. But scientists now know that gas bladders arose first for breathing and only later became adapted for controlling buoyancy.
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