The Chordates - Fishes

Fishes are aquatic vertebrates that are characterized by paired fins, scales and gills.

• Fins are used for movement
• Scales are used for protection
• Gills are for exchanging gases

Fishes were the first vertebrates to evolve. They did not arise directly from tunicates or lancelets, but fishes and nonvertebrate chordates probably did evolve from common invertebrate ancestors. During the course of their evolution, fishes underwent several important changes. 

The evolution of jaws and the evolution of paired fins were important developments during the rise of the fishes! 

The earliest fishes to appear in the fossil record were odd-looking, jaw-less creatures whose bodies were armored with bony plates. 

The Devonian Period, which is often called the Age of fishes. Fish underwent a major evolution at this point. Jaw-less fishes emerged at this points. Unlike their ancestors, they had very little armor. 

The evolution of jaws in early fishes accompanied the evolution of paired pectoral and pelvic fins.  The fins were attached to structures of cartilage or bone that supported the fin. Paired fins gave fish greater movement and jaws gave them a wider diet. 

Although the early jawed fishes soon disappeared they left beyond two groups that continue to evolve and survive today. One group - the ancestors of modern sharks and rays - evolved a skeleton made of strong, resilient cartilage. The other group evolved skeletons made of true bone. A sub-group of bony fishes called lobe-finned fishes have fleshy fins from which the limbs of chordates would later evolve. 

Adaptions to aquatic life include various modes of feeding, specialized structures for gas exchange, and paired fins for locomotion. 

• Feeding- every type of feeding from herbivores, carnivores and omnivores 
• Respiration - Most fishes exchange gases using gills located on either side of the pharynx. The gills are made up of feathery threadlike structures called filaments. Each one contains a network of fine capillaries that provides a large surface area for the exchange of oxygen and carbon dioxide. Fishes that exchange gases using gills do so by pulling oxygen-rich water in through their mouths, pumping it over their gill filaments, and then pushing oxygen-poor water out through the openings in the sides of the pharynx. 
• Circulation - Fishes have closed circulatory systems with a heart that pumps blood around the body in a single loop from the heart to the gills from the gills to the rest of the body, from the gills to the rest of the body and back to the heart. 
• Excretion - Like many other animals, most fishes rid themselves of nitrogenous wastes in the form of ammonia. Some wastes diffuse through the gills into the surrounding water. Others are removed by kidneys, which are excretory organs that filter wastes from the blood. 
• Response - Most fishes have highly developed sense organs. Almost all fishes that are active in daylight have well-developed eyes and color vision that is as good as the average human. They have extraordinary senses of taste and smell an can detect motion although they cannot hear very well. 
• Movement - Most fishes move by alternating contracting paired sets of muscles on either side of the backbone. This creates a series of S-shaped curves that propels the fish through the water. 
• Reproduction - The eggs of fishes are fertilized either externally or internally, depending on the species. Those that hatch outside of the mother are called oviparous. Those who are "born alive" are called viviparous. 

When you consider their basic internal structure, all living fishes can be classified into three groups, jaw-less fish, cartilaginous fishes and bony fishes. 

Jaw-less fishes - They have no true teeth or jaws. Skeletons are made of fibers and cartilage and they lack vertebrae. They keep their notochord as adults. They are two types - lampreys and hagfishes. 

Cartilaginous Fish- This group contains sharks and their relatives. 

Bony Fishes- These fishes are made of hard, calcified tissue called bone. Almost all of them are ray-finned. 

Some fishes spend most of their lives in the ocean but breed in fresh water. They are called anadromous. Others live in fresh water but migrate to salt water to breed. These fishes are called catadromous.