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WHAT IS A FISH?

Ahh, that is not an easy question to answer. What do we normally think of as a fish? Well, they have scales, they live in water and they have fins.

These three things however, are not true of all fish. Some fish such as catfish don't have scales. Some eels do not have fins. Lungfish can spend a considerable amount of time out of water, breathing gaseous air instead of out of water, at times..

So what do all fish have in common? 1. Fish have a backbone or a notochord and 2. all fish breathe oxygen using gills. However, a lot of animals have a notochord (including us humans). Some animals that are not fish, such as the axolotls also breathe using gills. However axolotls have fully formed limbs that are lacking in fishes.

T. M. Berra (2001) states that "If we allow room for these and other exceptions, we can define a fish as a poikilothermic, aquatic chordate with appendages (when present) developed as fins, whose chief respiratory organs are gills and whose body is usually covered with scales." [Berra, T.M. 2001. Freshwater Fish Distribution. Academic Press. Pp. 604.]



People often get confused when to use the term fish or fishes. Here are the general rules:

1 fish of a given species is called a fish

a group of fish of the same species are called fish

1 fish or a group of one species with 1 fish or a group of other fish are called fishes.

A fairly large to very large group of fish of the same species is referred to as a school of fish.

If 1 fish of a given species swims with 1, a group or a school of another species, they are referred to as fishes.

Sometimes when fishing, if you catch several fish, vulgate language at least may refer to this as a "mess" of fish (the term probably derived from the housewife who had to clean the fish)

For example if you catch a "mess" of trout of the same species, you would say you caught some fish or a mess of fish.

However, if you caught some bass and a catfish, you would say I caught some fishes.

Now is that as clear as mud?

Fish are often divided into three main groups, totaling about 25,000 species of fish.
These are the Jawless fish; the Cartilagenous fish; and the bony fish, called Teleost.


Jawless fish are the last survivors of the world's first vertebrate animals. Vertebrate means that the animal has a "back-bone". Jawless fish lack both scales and jaws. They are very ancient and date back from over 500 million years ago. The hagfish and lampreys are the only two which remain.


Cartilaginous fish developed about 400 million years ago, and they were the ancestors of today's sharks and rays. Cartilage makes up the skeleton of these fish, which is softer and more pliable than bone. Cartilaginous fish have jaws, as well as teeth which are usually hard and sharp. Most Cartilaginous fish have rows of teeth in their mouths, so that when a tooth is lost another is already in place to roll up and forward and thereby replace the lost tooth. Their bodies are covered with denticles, which are somewhat like small hard scales, which is why shark skins are used to make shoes and other very durable leather. Shark fins are eaten, especially in the Orient and also are used in medicine in the Orient.

Male Cartilaginous fish fertilize the eggs of the female internal, often using what are called claspers on the underside back part of the male. In some species the eggs hatch internally so that the babies are born from the mother. Others lay the fertilized eggs in pouches to hatch later.

Cartilaginous fish have what is called the ampullae of Lorenzini, an electricity detecting organ, which are pores. These help the fish locate prey.


Bony fish have bony skeletons, as most other vertebrates, instead of cartilage skeletons, as the Cartilaginous fish. Bony fish first appeared at about the same time as the Cartilaginous fish. The Bony fish are the largest group of fish, with about 20,000 species. The Bony fish have an organ called a swim bladder, which gives the animal buoyancy, i.e. the ability to float.

Female Bony fish lay their eggs in the water and the male deposits milt into the water as well, which fertilizes some of the eggs. Because of the chance fertilization, many more eggs and milt (sperm) are produced than in the Cartilaginous fish, to assure a reasonable number of the eggs are fertilized. Some Bony fish then guard the eggs and some just leave the eggs unprotected, with the safety of numbers to resolve that some do hatch.


Fish have a sense organ which we do not have. It is a lineal line running from the front of the body toward the tail, called the

All fish live in water and breathe with gills. Fish are cold-blooded, which means their internal body temperature changes as the surrounding temperature changes. Fish live in fresh water, salt water, and brackish water. They vary widely in size, food, color, appearance, methods of predation and methods of hiding and survival. Some are egg layers and some are live baby deliverers. Fish habitats vary extremely in temperature, salinity, clear or muddy water and many other factors. They are found worldwide, in oceans, rivers, streams, lakes and ponds.



Fish are divided into four Classes as follows with some examples of members of that Class:

Class Cephalaspidomorphi--Lampreys

Class Myxini--Hagfish

Class Chondrichthyes--sharks, skates, rays, ratfishes

Class Osteichthyes--i.e. Teleost--Bony fish including lungfishes, lobe-finned fishes, and ray-finned fishes, and many others.



Another important division is between living fish and fossil fish. Both are found virtually worldwide, as fossil fish spread worldwide, just as living modern fish do today. Interestingly some of the most ancient fish still survive today.



Click here, for information on the living fossil fish called the Coelacanths (a descendent of the extinct Crossopterigian amphibian/fish).



At present we invite you to visit some pages on this Website, which pertain to fish:

Crossopterigian and Coelacanth--(same link as above)
Fossil Butte National Monument--for information on Wyoming fossil fish.
Fossils, then click on Fossil Fish--for information on various fossil fish.
Marine Life--for living and fossil links.
Plecostomus--an interesting, modern but prehistoric appearing, catfish
Prof. Robert Cross' Biographical Page--for fossil fish along with other of his fossils
Warfield Quarry--for information on fossil fish at Kemmerer, Wyoming.



This page is still under construction as are some of the above pages listed. Thank you for your patience as we build this website.