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25.2 Cartilaginous Fish. Describe the unique characteristics of fishes. Identify the 2 main classes of fishes. Describe at least 3 adaptations of sharks and of rays. Diversity of Fishes.
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25.2 Cartilaginous Fish • Describe the unique characteristics of fishes. • Identify the 2 main classes of fishes. • Describe at least 3 adaptations of sharks and of rays.
Diversity of Fishes • Fishes, the 1st jawed vertebrates, make up about 30,000 of the more than 55,000 species of vertebrates on Earth today. • There are two main groups of fishes: • Cartilaginous Fishes • Bony Fishes
2 Main Groups of Fishes • The cartilaginous fishes, class Chondrichthyes (kahn DRIK theez), so named because they have a flexible skeleton made entirely of cartilage. Cartilaginous fishes include the sharks and rays. • All other fishes are called bony fishes (formerly, class Osteichthyes (ohs tee IK theez), now divided into 3 classes) because their skeletons contain bone hardened by calcium compounds.
Sharks • Nearly all sharks live in marine habitats. • Whether true freshwater sharks actually exist is still somewhat of an open question. There is no doubt that there exist sharks that live in freshwater but most of these species are really marine sharks that are able to migrate up into freshwater bodies and live out their entire lives there. • An example of such a shark is the bull shark that is notorious for wandering into freshwater and that has been found over 4200 kilometers / 2,600 miles up into rivers such as the Amazon.
Sharkshave streamlined bodies and are swift swimmers. Powerful muscles in their tail propel them forward. The dorsal (back or top) fins function mainly as stabilizers. And the lateral (side) fins provide lift in the water.
Nurse shark, resting However, these fins are not very movable, and as a result sharks do not maneuver very well. A shark must swim constantly to avoid sinking, since its body is actually denser than water. Continual swimming also ensures that water flows into the shark's mouth and through its gills, enabling the exchange of oxygen and carbon dioxide. Some sharks spend time resting on the seafloor. When not swimming, they must use muscles in their jaws and pharynx to pump water over their gills.
Most sharks are predators—fast and powerful with acute senses. • Sharks have sharp eyesight, a powerful sense of smell, and highly developed electrosensor organs on their heads. • These electrosensors can detect very small changes in electrical fields produced by muscle contractions in nearby animals.
Sharks, as well as most other aquatic vertebrates, also have a lateral line system, a row of sensory organs running along each side of the body. • The lateral line system is sensitive to very small changes in water pressure. This enables a shark to "feel" minor vibrations caused by swimming animals.
Most sharks are carnivores that swallow their prey whole or use their powerful jaws and sharp teeth to tear flesh from animals too large to swallow. • In contrast, the largest sharks (and rays) are suspension feeders that consume plankton.
Sharks have been hunting in the oceans for hundreds of millions of years. In addition to highly developed senses of touch, smell, hearing, vision, and taste, the shark has one sense you do not have. This "sixth" sense is the ability to detect small electric fields from a meter or more away. • Since living organisms produce these small electric fields, this sense helps the shark find prey . . . one reason why some people call sharks "The Perfect Predator."
Rays • Although rays are closely related to sharks, they have adapted to a very different lifestyle. • Most rays are bottom-dwellers that feed by using their jaws to crush mollusks and crustaceans.
Other rays cruise in open water, scooping food into their gaping mouths.
A ray's front fins are greatly enlarged. These fins propel the animal through water much like a bird flaps its wings to move through air. • The tail of many rays is whip-like and, in some species, has poisonous spines that function in defense.
25.2 Online Review • Phylum Chordata; Class Chondrichthyes 1, 2, 3 • PBS Website: Shark Attack • PBS Website: Island of the Sharks • Sharks 1, 2, 3, 4 • Great White Sharks 1, 2, 3, 4 • Freshwater Sharks 1, 2 • Sea Rays 1, 2, 3 • Shark Quiz 1, 2, 3, 4, 5, 6
