41% of all fish live in freshwater. There are also some important fish which breed in rivers, and spend the rest of their life in the seas. Examples are salmon, trout, the sea lamprey, and three-spined stickleback. Some other fish are born in salt water, but live most of their adult lives in fresh water: for example the eels.
Species like these change their physiology to cope with the amount of salt in the water.
Fish swim by exerting force against the surrounding water. There are exceptions, but this is usually done by the fish contracting muscles on either side of its body in order to generate waves of flexion that travel the length of the body from nose to tail, generally getting larger as they go along. Most fishes generate thrust using lateral movements of their body & tail fin (caudal fin). However, there are also species which move mainly using their median and paired fins. The latter group profits from the gained manoeuvrability that is needed when living in coral reefs for example. But they can not swim as fast as fish using their bodies & caudal fins.
Fish can swim slowly for many hours using red muscle fibres. They also make short, fast bursts using white muscle. The two types of muscle have a fundamentally different physiology. The red fibres are usually alongside a much greater number of white fibres.
The white fibres get their energy by converting the carbohydrate glycogen to lactate (lactic acid). This is anaerobic metabolism, that is, it does not need oxygen. They are used for fast, short bursts. Once the lactic acid builds up in the muscles, they stop working, and it takes time for the lactate to be removed, and the glycogen replaced. Using their white fibres, fish can reach speeds of 10 lengths per second for short bursts.
Swimming for long periods needs oxygen for the red fibres. The oxygen supply has to be constant because these fibres only operate aerobically. They are red because they have a rich blood supply, and they contain myoglobin. Myoglobin transports the oxygen to the oxidising systems. Red muscle gets its energy by oxidising fat, which weight for weight has twice as much energy as carbohydrate or protein. Using their red fibres, fish can keep up a speed of 35 lengths per second for long periods.
Many fish swim in groups. Schools of fish can swim together for long distances, and may be chased by predators which also swim in schools. Casual groups are called 'shoals'.
The shape of the body of a fish is important to its swimming. This is because streamlined body shapes makes the water drag less. Here are some common fish shapes:-
The picture on the right shows a shark. This shark's shape is called "fusiform", and it is an ovoid shape where both ends of the fish are pointy. This is the best shape for going through water quickly. Fishes with fusiform shapes can chase prey and escape predators quickly. Many live in the open ocean and swim constantly, like marlins, swordfish, and tuna. Ichthyosaurs, porpoises, dolphins, killer whales all have similar shapes. This is an example of convergent evolution.
The long, ribbon-like shape of an eel's body shows another shape. This enables them to hide in cracks, springing out quickly to capture prey, then returning quickly to their hiding spot.
Flatfish live on the bottom of the ocean or lake. Most use camouflage: they change colors to match the ocean floor.
Fish with compressed shapes have flat, vertical bodies, with one eye on each side. They swim upright and can be very thin. They usually live in reefs where their flat bodies can slip in and out among the corals, sponges, and rocks, keeping hidden from predators. Angelfish, surgeonfish, and butterflyfish are all compressed fish.
Some people eat many different kinds of fish. These include carp, cod, herring, perch, sardines, sturgeon, tilapia, trout, tuna, and many others. A person who buys and sells fish for eating is called a fishmonger.
The word "to fish" is also used for the activity of catching fishes. People catch fish with small nets from the side of the water or from small boats, or with big nets from big boats. People can also catch fish with fishing poles and fishhooks with bait. This is often called angling. Anglers also different types of fishing lures.
Because people are catching too many fish for food or other uses, there are less and less fish in the sea. This is a problem known as Overfishing.
Selective breeding of carp made them into the domesticated koi in Japan, and goldfish in China. This breeding began over 2,000 years ago. The Chinese brought their goldfish indoors during the Song Dynasty. They kept them in large ceramic vessels. That we now do in glass fish tanks.
Foot
A foot (one "foot", two or more feet) is a body part on the end of a leg. It is used when walking. It is also important for balance: it helps people stand straight. People also use it to kick, in both fighting and sports, football being an example.
People's hands and feet have the same shape: they both have five "digits" (the fingers and toes). Many other animals with backbones also have five digits. The part of the foot which joins it to the leg is called the "heel". The bottom of the foot is called the "sole".
Most land vertebrates have feet, and there are many different sorts of foot. The feet of monkeys are much like the hands. The hard foot of an ungulate is a hoof. When an animal has soft feet, or feet with soft parts on the underside, we often call it a "paw". Many invertebrates also have feet.
Many people like to use footwear, especially outside. It has special names, for example "sandals", "shoes", and "boots". When people always wear footwear, especially in hot places or when they are very active, their feet can smell badly ("foot odour)". Wearing footwear that is too big or small can be bad for the feet. People who have foot, leg, and back problems can also get help from special shoes.
People have different traditions in different parts of the world for when to wear footwear. For example, in many countries, usually do not wear their shoes or boots in a home. In the United States people often wear shoes inside a home. In Japan, people do not wear shoes in homes, and floors are often made of very soft materials, too soft for shoes. In cultures where people always wear shoes, people sometimes think it is bad not to wear them. Not wearing shoes can be good for the feet, especially for children's feet.
Conditions like Athlete's foot affect the feet, causing the feet to feel dry and cracked. Doctors who work with people's feet are "podiatrists" or "chiropodists".
Half the bones in a human body are in the foot. There are 26 bones there. They are 14 phalanges (toes), 5 metatarsals (arch of the foot), and 7 tarsals (ankle bones).
France
France ( or ; ), officially the French Republic (, ), is a country whose metropolitan territory is in Western Europe and that also includes various overseas islands and territories in other continents. Metropolitan France extends from the Mediterranean Sea to the English Channel and the North Sea, and from the Rhine to the Atlantic Ocean. It is often referred to as "L’Hexagone" ("The Hexagon") because of the shape of its territory. France is a unitary semi-presidential republic with its main ideals expressed in the Declaration of the Rights of Man and of the Citizen.
Metropolitan France is bordered (clockwise from the north) by Belgium, Luxembourg, Germany, Switzerland, Italy, Monaco, Andorra, and Spain. The overseas departments and collectivities of France also share land borders with Brazil and Suriname (bordering French Guiana), and the Netherlands Antilles (bordering Saint-Martin). France is linked to the United Kingdom by the Channel Tunnel, which passes under the English Channel.