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
The earliest fish were jawless a feature still retained by some of today’s aquatic vertebrates like the hagfish and lampreys Fish having true jaws the acanthodians did not evolve for another 40 to 50 million years and it is from them that the present-day fish have arisen with numerous extinctions along the way Among these extinctions are the acanthodians themselves which disappeared after about 150 million years Another group the antiarchs had a shorter history surviving for about 70 million years having first emerged 400 million years ago The fossil record is incomplete of course so some of our interpretations have to be based on partial information Nonetheless the record is sufficiently wide ranging and representative for some major conclusions to be drawn For instance we can say that bichirs family Polypteridae have been around for 135 million years or so Likewise lungfish families Ceratodontidae and Lepidosirenidae have long histories stretching back to Devonian times 400 to 350 million years Most famous of all these so-called living fossils is the coelacanth pp 8 9 the first genuine remains of which occur in Devonian rocks and the last in 70 million-year-old sediments After that time no more fossil coelacanths are found so the line was assumed to have become 1 extinct Then in 1938 a living specimen was raised by fishermen off the Comoro Islands in the Indian Ocean Others have followed over the years leading to increased knowledge of this fascinating fish which appears to have undergone no major changes when compared to the youngest of the known fossils Why are Fish at Risk Few places on our planet appear to be free of human influences although sometimes they do not cause problems for the continued survival of a particular species In fact there are instances of both animals and plants that actually benefit from such situations Sparrows and European blackbirds are just two of many species that might not be as numerous as they are today if they did not live in close proximity to humans and human-influenced environments Broadly speaking though human influences often place species under greater pressure than they would normally experience Fish also have to cope with the consequences of habitat alteration as other animals do In the case of migratory species like salmon sturgeon pp 12 13 and others dams reservoirs river channelization and other changes can present insurmountable hurdles as the fish try to reach their spawning grounds upriver In the worst cases local populations or even species can be driven to extinction as a result Hunters prey and scavengers existed in prehistoric times just as they do today Here a large Glyptolepis 1 hunts a shoal of small Paleospondylis 2 while two Pterichthyodes 3 scavenge along the bottom 6 2 3
WHAT IS A FISH cichlids pp 16 17 gold sawfin goodeid and Lake Wanam rainbowfish Overfishing is also a major factor affecting abundance with some species being placed under such survival pressures that only drastic action including but not restricted to total or partial bans can hope to save them from extinction Species as diverse as tuna cod and shark all fall within this category but there are many many more The Situation Today The IUCN 2000 Red List concluded that there was evidence of an extremely serious deterioration especially in the status of riverine species An increase of 18 species on the Red List from 1996 to 2000 did not at first sight appear too drastic see table However betwen 2000 and 2010 there was a dramatic increase in the number of endangered species in part because of the problems faced by freshwater fish More than 35 percent of North American freshwater species for example are now threatened And the IUCN’s forecast in 2000 that It is highly probable that increased attention to these species by the Species Survival Commission over the next three years will confirm a worldwide global crisis in freshwater fish was confirmed Even allowing for the possibility that some of the increases may be the result of more accurate recording than in the past we are still facing a grave situation we need to do all that we can to reverse the trend Numbers of Threatened Species of Fish Critically Endangered Endangered Vulnerable TOTAL 1996 157 134 443 734 2000 156 144 452 752 2010 376 400 1,075 1,851 From 2000 to 2010 see above there was an increase of 1,099 threatened species of fish 7 Rainbow trout killed by an oil seepage into the water Pollution of the waters poses serious threats to the survival of fish species around the world In many other instances it is not changes in the quality of the water or the migratory routes that pose the greatest risks but the restricted distribution of the species concerned When this restriction is extreme as is the case for the devil’s hole pupfish and bandula barb pp 30 31 for instance the threat is so serious that stringent measures are required to ensure the survival of the species Ironically one of the biggest threats that some fish have to face is posed by other fish There are countless instances of fish being introduced into habitats outside their own natural range It may be done to cater to a demand for sport or angling species as in the case of trout or as a means of providing a new source of animal protein for local communities as in the case of the Nile perch introduction into Lake Victoria or even to combat disease as in the case of the mosquito fish which has been widely introduced for biological control of malarial mosquitoes In many of these cases the introduced exotic species places the native species under threat It could be through direct predation on adults or young or eggs competition for space competition for food or spawning sites or quite simply through having a higher reproductive rate than the endemic species There are several examples of fish that face such situations such as mojarra caracolera Lake Victoria
