The other group of fish which retain bone in its skeleton, also had to overcome weight problems in the water. Early fish with heavy bone-based scales, colonized shallow lagoons and swamps which had warm, poorly oxygenated water. The bichir (Polypterus)(order Polypteriformes), a heavy scaled fish occurring in Africa indicates how these early fish overcame such problems. These animals rise regularly to the surface and take a gulp of air which goes into a pouch leading off the top part of the gut. A concentration of capillaries in the walls of the pouch absorb the gaseous oxygen. These air-filled pouches which were the first lungs also provided buoyancy and the ability to float without using the tail and eventually evolved into swimbladders. With the ability to absorb gas from the blood systems there was no need to collect air from the surface and the connecting tube to the throat became no more than a solid thread. The diffusion of gases into and the expelling of air out of the swimbladder would permit a precise means of vertical control in the water. The pectoral fins would provide refinement to this control. However, swimming skills were improved still further with increased tapering of the twin-bladed symmetrical tail that is driven by banks of muscles on either side of the backbone. Streamlining was enhanced with reduction of heavy scales into smaller tightly fitting ones that overlap like tiles of a roof and are covered by slippery mucous, and pectoral and pelvic fins being able to fold back into depressions in the lateral sides of the fish. The respiration using gills was further refined with the development of a movable, bony operculum which by inducing negative pressure forces water over the gills and improves respiration.
The bichirs are a family (Polypteridae) of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes. They have thick bonelike scales and a series of dorsal finlets instead of a single fin. Their jaw structure more closely resembles that of the tetrapods than that of the teleost fishes. All 16 recognized species occur in freshwater habitats in Africa. They are popular subjects of public and large hobby aquaria.
In some Asian cultures, the swim bladder (otherwise known as fish maw) is considered a food delicacy. Usually served braised or in stews, it is rather tasteless by itself, but it is enjoyed more for its slightly rubbery and crunchy texture.
The diversity of morphological forms is testimony to the success of the group. One group, the flying fish (order Atheriniformes) leap out of the water and glide hundreds of metres in the air using the elongated pectoral fins. This may be an anti-predator tactic. Garfish (order Lepisosteiformes) have pectoral fins that have become filmy skulls rotating slowly back and forth which permits them to hover in water. Dragonfish (order Pegasiformes) have lateral fins modified into defensive mechanisms with each ray barbed with poison.
Down the flanks and around the head of fish runs a series of pores, connected by a canal running just below the surface. This is called a lateral line and enables the fish to detect differences of pressure in water. As a fish swims, it creates a pressure wave ahead of it, when this wave meets another surface the fish can detect pressure changes created by this surface. It is this ability that permits them to detect other fish and to polarize themselves into swimming in shoals. Vulnerability to predators is thought to be reduced by shoaling.
Fish also have an acute sense of smell and detect minute changes in the chemical composition of water. This sense of smell may guide fish to food. Fish also detect sound with the addition of a third canal (in a horizontal plane and below the sac) which supplements the two semicircular canals that are found on either side of the skull of the lamprey. All three canals and the sac have very sensitive linings and contain small calcium particles which move and vibrate. Sound waves, which travel better in water, penetrate the semicircular canals without the need for passages which are required by terrestrial animals.
The eyespot of the lamprey is primitive compared with the bony fishes. The eye of the bony fish and higher vertebrates is a closed chamber with a transparent window and a lens in front and a photosensitive lining at the back (retina). The photosensitive lining contains two kinds of cells, rods for distinguishing light and dark and cones which are sensitive to colour. Sharks and rays lack cones and are unable to perceive colour; this may reflect the lack of highly coloured examples within the group. Bony fish have both types of cells in their retina, and are also characterized by vivid colours and striking patterns. The Butterfly fish (Family Chaetodontidae) showing particularly diverse colours and patterns which permits species recognition. Colour is also an important asset in male fish during spawning. Such displays serve to chase other male fishes away, and to attract female fish. Pigment granules diffuse within the skin as the fish become excited and fights other rivals or to stimulate a female fish to lay her eggs.
Butterflyfish are named for their brightly coloured and strikingly patterned bodies in shades of black, white, blue, red, orange and yellow (though some species are dull in colour). Many have eyespots on their flanks and dark bands across their eyes, not unlike the patterns seen on butterfly wings. Their deep, laterally compressed bodies are easily noticed through the profusion of reef life, leading most to believe the conspicuous coloration of butterflyfish is intended for interspecies communication. Butterflyfish have uninterrupted dorsal fins with tail fins that may be rounded or truncated, but are never forked.
Eyes of fish have become adapted in various ways to vision below and above water. The archer fish (Toxotes jaculator) squirts fluid at an insect above the water and knocks them into the water where they can be eaten. This required compensation since light bends as it passes from water to air due to differences in density. Anableps has a horizontal division across its pupils which effectively gives it four eyes, the two lower halves for underwater use and the two upper halves for above water. Since fish can occur at great depths (below 750 m) where there is no light, they may posses modified cells producing luminescent chemicals which are activated rhythmically and may represent some form of communication to the rest of the shoal. The whiskery angler fish (Antennarius scaber) (order Lophiiformes) has a modified dorsal fin spine with an elongated thread at the end of which are cells producing luminescence. This is used to entice other fish to explore the light and be consumed.
Anglerfish is the common name for the 200+ species that comprise the bony fish orderLophiiformes. They are for the most part a deep-sea fish, although there are some anglerfish families that have shallow-water representatives, and one family, the frogfishes (familyAntennariidae), occurs only in shallow water. Examples of other anglerfish families that have some shallow water species are the monkfish or goosefish, (family Lophiidae) and the batfishes (Family Ogcocephalidae). These families also have deep water representatives. The deep-sea mid-water anglerfishes belong to the suborder Ceratioidei and are usually referred to as ceratioids.
Water that is covered with floating mats of vegetation is also turbid, and in such an environment some fish have generated electricity from modified muscles in their flanks. Electrical signals are transmitted almost continuously creating flow patterns of current in the immediate vicinity. Any object encountered disrupts these flow patterns and the fish perceives these changes through receptor pores located over the body. The electric eel of South AmericaElectrophorus electricus, although not a true eel, has additional body tissues that produces a massive shock of waves with which it kills or stuns prey items.
From the jawless armour-laden prototype fish have evolved some 30 000 different forms to occupy seas, lakes and rivers of the world.