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Chapter 2 - Evolution and Classification of Fish

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Chapter 2 - Evolution and Classification of Fish Chapter 2 - Evolution and Classification of Fish The term fish encompasses three distinct classes: 1. Agnatha or jawless fish 2. Chondrichthyes or sharks, skates and rays 3. Osteichthyes or bony fish Fish make up the largest group of vertebrates and are the most widespread of the vertebrates. Fish are very diverse however they all possess certain adaptations that allow them to live in water. Adaptations The diversity of water, which is 800 times the density of air, effects how fish live in water. Water is more resistant to movement then air. To live in water most fish have adaptations for buoyancy. This is called a swim bladder. The fish is able to regulate the amount of gas in this bladder to allow them to swim deeper or shallower depending on where it needs to go. Fish also have a slick streamlined body and a muscular tail to steer then through the water. The paired fins allow then to maneuver easily in all directions. Protective scales on fish limit chemical exchange through the skin, so most occurs across the gills. Gills are the external respiratory organs used to pull oxygen out of the water as the water passes over the gills. Fish have either gill slits or they have an outer covering over them to protect them. One prominent adaptation is the lateral line system. All fish have this except agnatha. The lateral line system runs the length of a fish and is its sensory organ for detecting the movement of the water caused by other fish. Evolution The main evolutionary trend in fish is from fish with no jaws and a skeleton of cartilage, to fish with jaws and a skeleton of cartilage, to fish with jaws and a skeleton of bone. Agnatha The only existing jawless fish are the lampreys and hagfish. Agnatha means “jawless”. Fish in this class are parasitic and attach themselves to other fish and suck out their blood. Because of their round mouths these animals are sometimes called cyclostomes meaning “round mouths”. Agnatha have no scales and no paired fins. Lampreys have a highly developed sense of smell which helps locate their prey. This organ is located on top of their head and leads directly to the olfactory nerve responsible for smell. Chondrichthyes Sharks, skates, and rays belong to this class. Like agnatha, chondrichthyes have skeletons made of cartilage. Cartilage is a tissue made of cells surrounded by tough, flexible protein structures. Chondrichthyes have jaws and almost all of them live in salt water. Most are carnivores and use their large olfactory organs (smell) and lateral line system for tracking prey. The skin of sharks are covered with placoid scales. Which are small, tooth-like spines that feel like sandpaper. Sharks swim from side to side with use of their tails. They also have a paired pectoral fins just behind the gill region. Sharks also have 6 to 20 rows of backwards pointing teeth to help hold the food in the mouth. The acute smell of sharks can detect blood up to 500 meters away. Gas exchanges through the gills requires a continuous flow of water through the gills therefore sharks must swim continuously. Sharks eggs are fertilized internally and have live born pups, they do not follow the normal pattern. Osteichthyes Of the 25,000 known species of fish, 95% are bony fish. Osteichthyes or bony fish that have a bone skeleton, scaly skin, sense organs, paired fins and reproductive patterns (spawning). Osteichthyes may be divided into 4 orders: 1. Coelacanths - or lobe-finned fish which are fish that have paddle like fins with a fleshy base. 2. Lungfish - which have lungs (internal respiratory organ). Lungfish also have gills where gas exchange occurs. However lungfish live in tropical ponds that dry up during the hot summers. Lungfish dig holes into the mud and hibernate until it rains again. During this time of hibernation they use their lungs to breath. 3. Ray-finned fish - which are fish that have fins supported by long bones called rays. These include perch, herring and lantern fish. .
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