WHAT IS A FISH Fish is a general term used to identify more than 27,000 species of animals that inhabit fresh water and salt water Fish are diverse and they come in all shapes and sizes For instance the smallest fish are 0.3 inches 0.76 cm long while the largest fish span 39 feet 12 m CLASSIFYING FISH Classifying fish into species can be difficult because they are very diverse However most fish have some common characteristics All fish spend at least some time in water They have heads with obvious eyes sense organs and teeth Fish also have brains that are protected by braincases Fish are classified as vertebrates which means they have backbones A fish's skin often contains scales that help protect it from injuries and predators Many fish have fins that come in pairs These limbs help to propel fish through the water Most fish are cold blooded This means they cannot regulate their own body temperatures They depend on their environments and movements to generate heat Fish have gills that can be used to get oxygen from water Many fish species have a sense organ called a lateral line system This system helps fish find prey or sense predators Fish may also have swim bladders that help them remain buoyant 4
Some fish do not share all of these characteristics For example tuna are warm blooded fish Lampreys are classified as invertebrates because they lack backbones Lungfish can use their lungs to breathe CATEGORIES OF FISH Fish fall into three specific categories Agnatha fish are primitive fish which means they have existed for many millions of years In fact scientists believe these were the first type of fish to appear on Earth The first Agnatha appeared 450 million years ago Agnatha fish do not have jaws Chondrichthyes are fish that are made up of cartilage These include sharks rays and skates Osteichthyes have bones and are the largest group of fish They are divided into ray finned fish and lobe finned fish 5
HOW TO USE THIS BOOK Tab shows the fish category The fish's common name appears here The fish's scientific name appears here Images show the fish Sidebars provide additional information about the topic 54 6
This paragraph provides information about the fish FUN FACT Some scientists estimate that a female carp can lay around 300,000 eggs each breeding season How to Spot features give information about the fish's size range habitat and diet Fun Facts give interesting information related to fish 7
