FACTS AT YOUR FINGERTIPS ENDANGERED ANIMALS FISH BROWN BEAR BOOKS
What is a Fish everyone can recognize a fish At least everyone thinks they can until they begin to explore the term fish in a little detail Exceptions to the Rules It is of course quite ridiculous to state that an animal is a fish just because it lives in water dolphins whales leeches lobsters sea cucumbers and numerous other aquatic organisms destroy this argument right away It could be said that a fish can be identified by the fact that it has scales on its body but that again is misleading There are other organisms such as reptiles crocodiles turtles lizards and so on that have scales but are not fish Even such animals as birds have scales on their legs The other side of this coin is that not all animals that are known to be fish have scales For example catfish of the family Pimelodidae are naked There are also numerous other examples of scaleless fish All fish however have gills But so have other aquatic Salmon and their relatives exhibit the classic characteristics we associate with fish but these characteristics are not common to all fish 4 The oceanic sunfish is a giant of the open seas that does not fit the traditional image of what a fish should be like organisms Newt larvae salamander tadpoles and the incredible neotenous axolotl salamandrid have gills yet they are not fish We can make another rule Fish are not only aquatic and have gills they also have fins This still fails as a definition however because there are animals such as cuttlefish and squids that are aquatic breathe by means of gills and have fins yet these animals are mollusks One important distinguishing factor though is that in cuttlefish and squids the internal organs are found inside a mantle cavity Fish do not have a mantle cavity They do however have an internal skeleton but so have cuttlefish and squids the cuttlebone and pen respectively So do starfish and their close relatives the urchins their skeleton consists of calcareous plates overlain with thin surface tissue What cuttlefish and urchins do not have is a cranium braincase or skull Here at last is the definitive characteristic or is it Not quite If it were then lampreys and the other jawless fish would also be real fish but they are not generally regarded as being so They do have a skull of sorts even though it is largely a covered cartilage trough However they have so many other nonfish characteristics the absence of a backbone two instead of three semicircular canals in the ear gills that are arranged almost back-to-front when compared with normal
WHAT IS A FISH have a braincase and limb fin skeleton consisting of cartilage or bone have fins usually but not invariably with spines or rays breathe through outwardly directed gills covered by an operculum gill cover which results in an external slitlike aperture or a series of gill slits have bodies covered in scales there are some important exceptions here have an air swim-bladder used in buoyancy control again there are exceptions as in sharks have a sensory organ known as the lateral line organ running in a head tail direction or another series of sensory pits there are exceptions as in sharks are cold-blooded poikilothermic so their body temperature matches that of the environment there are some significant exceptions such as tuna which can raise their body temperature well above that of the surrounding water The History of Fish It is generally accepted that life began in water some 3 billion years ago For many millions of years after that however there does not appear to have been any major surge leading to the emergence of complex organisms Indeed it took about 2 4 billion years for the first animals that could be regarded as invertebrates to appear From that point on the pace accelerated to the extent that it took the relatively short time of about 120 million years for the first vertebrates to evolve These aquatic creatures known from fossils dating back about 480 million years were fish in jawless fish they are directed internally that they cannot be regarded as true fish In addition jawless fish Agnatha as their name suggests do not have jaws The almost inevitable conclusion of all this is that there really is no such thing as a fish A Common-Sense Approach When we use the term fish we should appreciate that it is a very broad-ranging word indeed as broad in fact as the term tetrapod which means four legged animal When the term fish is used it generally refers to aquatic species that can be split into two groups Cartilaginous fish class Chondrichthyes the first fish At least we tend to refer to them as sharks rays chimeras about 700 species Bony fish class Osteichthyes all other species from guppies to seahorses well over 20,000 species fish but bearing in mind the numerous criteria these creatures were very different from their modern-day equivalents Nonetheless they form the basal stock from which not just fish It is possible albeit with difficulty and but all other vertebrates evolved numerous exceptions to list the characteristics that in combination allow us to recognize a fish as such Fish The seahorse with its forward-curled tail and horselike head may not look like a fish but it is 5
