Fish

The classes Actinopterygii and Sarcopterygii, or bony fishes, in the superclass Osteichthyes, constitute a large and diverse superclass of vertebrates (some scientists no longer use Osteichthyes to group these fish and consider Actinopterygii and Sarcopterygii superclasses). Like the jawless fishes (Agnatha) and cartilaginous fishes (Chondrichthyes), they are characterized by gills, fins, and a dependence on water as a medium in which to live. Unlike those fishes, however, they typically possess a skeleton made of bone. Additional features of most bony fishes include a lateral line system, scales, osmoregulation (salt balance) by means of salt retention or secretion, and a bony operculum (gill cover) over the gill openings.

The fossil record of bony fishes began around 420 million years ago in the early Devonian geological period, mostly in freshwater deposits. Thus, there is reason to believe that bony fishes originated in freshwater habitats. Living bony fish species inhabit both freshwater habitats (42 percent of species) and marine habitats (58 percent). Of these, about 4 percent can live in saltwater or freshwater, moving between the two environments on a regular basis. This distribution does not reflect the relative proportions of these environments since 97 percent of Earth’s water is in the oceans, and only 0.001 percent is in freshwater lakes, rivers, and streams (the rest is ice, groundwater, and atmospheric water). Rather, the high diversity of freshwater species reflects the ease with which freshwater populations become isolated and evolve into new species.

Fish Subclasses

Bony fish belong to the superclass Osteichthyes, which includes ray-finned and lobe-finned fish. There are six classes of bony fishes: Dipnoi, Coelacanth, Chondrostei, Cladistia, Holostei, and Teleostei. The first two of these include a total of only eight primitive living species. The class Actinopterygii (ray-finned fish) includes all others and makes up over half the vertebrate species on Earth. The Dipnoi, or lungfish, are named for their possession of lungs, an ancestral characteristic suggesting that the earliest steps of bony fish evolution took place in tropical freshwaters subject to stagnation. Modern lungfishes (six species) cope with such conditions by swallowing air and exchanging respiratory gases (oxygen and carbon dioxide) in the lungs. Once considered closely related to terrestrial vertebrates, they are now believed to share certain similarities merely because of convergence (independent evolution of characteristics that appear similar).

The Crossopterygii, or fringe-finned fishes, were the dominant freshwater predators of the Devonian period. One fossil subgroup, the rhipidistians, had many features intermediate between fishes and ancestral amphibians, including tooth structure, lobed fins, and a jaw connected directly to the skull. Therefore, they are believed to represent a link between fishes and higher vertebrates. The group Coelacanth was also believed to be extinct (for 70 million years) until a coelacanth, Latimeria chalumnae, was taken from deep water off South Africa in 1938. This species and L. menadoensis are of great interest as “living fossils.”

The class Cladistia (bichirs, birchers, lobed-finned pike, and reed fishes) includes fourteen living species known from swamps and rivers in tropical Africa. Though they share some characteristics with the other bony fish classes, they have some distinct features that warrant placing them in a separate class. One such feature is a dorsal fin consisting of many separate finlets, each supported by a single spine.

The class Actinopterygii, or ray-finned fishes, comprise the classes Cladistia (bichirs and reedfish) and Actinopteri. Actinopteri is composed of several subclasses, orders, and families, including Chondrostei (paddlefishes and sturgeons), Teleostei (most living fishes), Lepisosteiformes (gars), and Amiiformes (bowfin). Over thirty species of chondrosteans in the class Chondrostei, which have reverted to a largely cartilaginous skeleton, include the sturgeons and paddlefishes. One species, the beluga or great sturgeon (Huso huso), a source of the famous Russian caviar, may be the largest living Actinopterygii species. It is known to achieve a length of 8.5 meters (27.75 feet) and a weight of nearly 1,300 kilograms (2,860 pounds).

Holosteans, close relatives of the teleosts, include eight species: seven gar and one bowfin species, all known from North America. These freshwater piscivores (fish predators) are characterized by a skeleton made entirely of bone, but they do have certain other features in common with their more primitive ancestors, such as the ability to breathe air (with the swim bladder) and ganoid scales (also found in sturgeons).

Most ray-finned fishes, hence, of bony fishes—and indeed nearly half of all living vertebrates—belong to the infraclass Teleostei. It includes more than 30,000 species. Among the features characteristic of teleosts are cycloid or ctenoid scales (though some are scaleless), a swim bladder (lost in many bottom fishes), highly maneuverable fins, and a homocercal tail (meaning that its upper and lower lobes are symmetrical). Teleosts are represented by an amazing range of body sizes and shapes. Many species are quite small, enabling them to occupy niches (ways of living) unavailable to other fishes. One of the smallest known fish is a goby from the Indian Ocean, Trimmatom nanus, which matures at 8 to 10 millimeters in length.

Fish Shapes and Habits

There are several common body shape categories among teleosts that relate strongly to the fish's habits. “Rover-predators” have the fusiform (streamlined) body shape that is perhaps most typically fishlike. Fins are distributed evenly around the body, and the mouth is terminal (at the end of the snout). This category includes minnows, basses, tunas, and others that typically are constantly moving—searching for and pursuing prey. “Lie-in-wait predators” tend to be more elongated, with the unpaired fins far back on the body, favoring a sudden lunge for their prey. The pike, barracuda, and needlefish typify this category.

“Bottom fishes” include a wide variety of shapes. Some are flattened for lying in close contact with the bottom (as are flatfishes such as flounder), some have flattened heads and sensory barbels (filaments with taste buds) near the mouth (as do catfishes), and some have fleshy lips for sucking food from the bottom sediment (as do suckers). A number of bottom-fish species have structures, usually modified pelvic fins, that enable them to cling to the bottom in areas with strong currents (sculpins and clingfishes have these).

“Surface-oriented fishes” tend to be small, with upward-pointing mouths, heads flattened from top to bottom, and large eyes. The mosquitofish, killifish, and flying fish belong to this category. “Deep-bodied fishes” are laterally flattened and have pectoral fins high on the body, with pelvic fins immediately below. This arrangement favors maneuverability in tight quarters such as coral reefs, thick plant beds, or dense schools of their own species. Examples include angelfishes, surgeonfishes, and freshwater sunfishes. “Eel-like fishes” have highly elongated bodies, tapering or rounded tails, and small, embedded scales (or no scales at all). They are adapted for maneuvering through crevices and holes in reefs and rocks and for burrowing in sediments. Eels, loaches, and gunnels typify this category.

Teleosts occupy habitats ranging from torrential streams high in the Himalayas to the bottom of the deepest oceanic trenches. They are found in the world’s highest large lake (Lake Titicaca) and deepest lake (Lake Baikal). Some blind species live in the total darkness of underground caves. One Tilapia species lives in hot soda lakes in Africa at 44 degrees Celsius (111 degrees Fahrenheit), while the Antarctic icefish Trematomus lives at -2 degrees Celsius (28 degrees Fahrenheit)

Most teleost fishes, both marine and freshwater, are tropical. Southeast Asia contains the greatest number of freshwater fish species, but the Amazon and its tributaries contain almost as many (and perhaps many hundreds more, still undiscovered). Marine teleosts are most diverse in the Indo-Pacific region, especially in the area from New Guinea to Queensland, Australia. A single collection made in the Great Barrier Reef off northeastern Australia may contain one hundred or more species. Some marine teleost species have a nearly worldwide distribution, while certain freshwater species have highly restricted ranges. The Devil’s Hole pupfish (Cyprinodon diabolis), for example, is found only in one small spring in Nevada.

Many teleosts have highly specialized associations, called symbioses, with other organisms. Some live among the stinging tentacles of sea anemones (the clownfish Amphiprion), within the gut of sea cucumbers (the pearlfish Carapus), within the mantle cavity of giant snails (the conchfish Astrapogon), or among the stinging tentacles of the Portuguese man-of-war jellyfish (the man-of-war fish Nomeus).

Reproduction

Reproduction among teleosts is incredibly varied. Most species are egg-layers, often producing an enormous number of eggs. The female ocean sunfish Mola mola may produce up to 300 million eggs, making it the most fecund vertebrate of all. Some species are livebearers, such as the platyfishes, swordtails, and surfperches. Some species are oral brooders, incubating the eggs in the mouth of the male (as in many cardinal fishes) or of the female (as in many cichlids). In one South American cichlid species, Symphysodon discus, the female “nurses” its young with a whitish milklike substance secreted by the skin.

Many teleost species are hermaphroditic. A few of these are synchronous hermaphrodites (functioning as male and female at the same time), such as the hamlet Hypoplectrus, but many more are sequential hermaphrodites (first one sex, then the other), such as the sea bass Serranus. In some coral reef fishes in the wrasse family, dominant males mate with a harem of females. If this male is removed, the largest female becomes male and takes over the missing male’s behavioral and reproductive function.

In a few species, all individuals are female, as in the Amazon molly Poecilia formosa. It has been shown that this species is a “sexual parasite” of two related “host” species. Sperm from host males are required to activate the development of Amazon molly eggs, but male and female chromosomes (genetic material) do not join, and the offspring are all genetically uniform females.

Ichthyology

The scientific study of bony fish dates to Aristotle, who was the first to note, for example, that the sea bass is hermaphroditic. The “father of ichthyology” in more recent times was Peter Artedi (1705–1735), whose classification system was used by Carolus Linnaeus (1707–1778) in his Systema Naturae, which became the basis for all future classification systems.

Bony fish classification depends on the study of taxonomic features, or characters, which vary from one species, or group of species, to another. Useful characters include countable features (meristic characters), such as the number of fin supports (rays) or the number of scales in the lateral line, and measurable features (morphometric characters), such as the relative lengths of body parts. Such studies are typically done on museum specimens that are preserved in alcohol solution after fixation in formaldehyde solution. Dissecting tools, microscopes, and even X-ray machines are used for revealing meristic and morphometric characters. For studying bones, dry skeletons are sometimes prepared, or (especially for small species) specimens are “cleared and stained.” This latter technique involves clearing the flesh with potassium hydroxide and staining the bones with Alizarin red stain.

Other techniques use samples of living tissue for finding taxonomic characters. Karyotyping (analysis of the chromosomes) and enzyme electrophoresis (using an electric field to separate similar proteins) are also important sources of taxonomic information.

Specimens for taxonomic studies are collected by means of netting, trapping, catching with hook and line, and spearing. Specialized techniques include electrofishing (use of an electric shocking device) for stream fishes, and ichthyocide (fish poison such as rotenone) for coral reef fishes.

Understanding the evolutionary history and classification of the Osteichthyes also depends on paleontological studies (the study of fossils). Bony fishes are well represented in the fossil record because of the superior fossilizing nature of their bony skeletons. Many fish biologists are concerned with matters other than taxonomy. Because of the economic importance of both marine and freshwater bony fishes, the science of fisheriesbiology (concerned with the management and exploitation of fish populations) is of great significance. Fish populations are often studied with “age and growth” techniques. Age (determined by scale analysis), length, and weight data can be used to calculate growth and mortality rates, age at maturity, and life span. Other techniques for studying fish populations involve tagging individuals (useful for making estimates of population size) and even using tiny radio transmitters that can be followed by aircraft (useful for studying fish migrations).

Ecologists and ethologists (behavioral biologists) are also active in fish studies, particularly since the invention of scuba diving, which allows direct observation of fishes in their natural habitat. An example of an important discovery made possible by scuba diving is cleaning symbiosis, which is common in coral reef areas. This symbiosis (an association involving members of two different species) involves a “cleaner” species (often a goby or wrasse), which feeds on the external parasites and diseased tissue of a host (“cleanee”) species, which visits the cleaner for this service.

Questions Still to Be Answered

Bony fishes are by far the most numerous of all vertebrates. They are also arguably the most diverse in terms of body form, reproductive habits, symbiotic relationships, and other characteristics. Yet, much remains to be learned. Virtually every ichthyological expedition into the Amazon region, for example, returns with specimens of previously unknown species. Some ichthyologists estimate that perhaps five or ten thousand undiscovered teleosts remain in unexplored streams and remote coral reefs.

Many biological mysteries remain about even some of the most familiar species. A good example is the American eel, Anguilla rostrata. This predatory species spends most of its life in the rivers, streams, and lakes of eastern North America, where it is often one of the dominant species. After six to twelve years in these habitats, the adult eels swim to the ocean and apparently migrate more than 5,000 kilometers (around 3,100 miles) to spawn in deep water in the Sargasso Sea (an area in the western Atlantic south of Bermuda).

This general location of eel spawning has been inferred from the appearance there of the tiniest eel larvae (called leptocephali); these were once considered a separate species). The larvae become larger and larger as they drift in the Gulf Stream toward the North American coast. This much has been known since 1922. The adult migration has never actually been followed, however, and no one knows exactly where, at what depth, or how they mate and spawn, nor is it known what then happens to the adults.

Despite many advances in scientific knowledge, much remains to be learned about the interrelationships, ecology, behavior, and fishery potential of the world’s bony fish species.

Principal Terms

Ctenoid Scales: Thin, flat, bony scales with tiny spines on the exposed rear edge, found on sunfish, perch, sea bass, and other advanced teleosts

Cycloid Scales: Thin, flat, bony scales with a smooth surface; rounded in shape, found on herrings, minnows, trout, and other primitive teleosts

Ganoid Scales: Thick, diamond-shaped, bony scales that are covered with ganoine, a hard inorganic substance; found on bichirs, gars, and other primitive bony fishes

Osteichthyes: The taxonomic superclass in which the bony fishes were placed, contained species related to the ancestors of higher vertebrates, now known as two superclasses, Actinopterygii and Sarcopterygii.

Pectoral Fins: Paired fins found near the head end of the fish body; related to the forelimbs of higher vertebrates

Pelvic Fins: Paired fins found either near the tail end of the fish body or below the pectoral fins; related to the hindlimbs of higher vertebrates

Swim Bladder: The hydrostatic (buoyancy) organ of teleost fishes derived from the lung of more primitive bony fishes

Teleosts: Members of the class Teleostei, the most advanced of the ray-finned fishes; they compose the vast majority of living bony fish species

Bibliography

Bales, Rebecca. "Fish: Different Types, Definitions, Photos, and More." A-Z Animals, 3 Jan. 2023, a-z-animals.com/animals/fish. Accessed 10 Sept. 2024.

Bond, C. E. Biology of Fishes. 2nd ed., Philadelphia, W. B. Saunders, 1996.

Cailliet, G. M., M. S. Love, and A. W. Ebeling. Fishes: A Field and Laboratory Manual on Their Structure, Identification, and Natural History. Belmont, Wadsworth, 1986.

Chiasson, Robert B. Laboratory Anatomy of the Perch. 4th ed. New York City, McGraw-Hill, 1991.

Friedman, Matt, and Martin D. Brazeau. "Paleontology: A Jaw-Dropping Fossil Fish." Nature, vol. 502 no. 7470, 2013, pp. 175–77.

McClane, A. J. McClane’s Field Guide to Freshwater Fishes of North America. New York City, Henry Holt, 1978.

Mills, D., and G. Vevers. The Golden Encyclopedia of Freshwater Tropical Aquarium Fishes. Racine, Golden Press, 1982.

Moyle, P. B., and J. J. Cech. Fishes: An Introduction to Ichthyology. 4th ed. Saddle River, Prentice Hall, 2000.

“Osteichthyes.” ITIS, Washington, DC, Integrated Taxonomic Information System, 2017, www.itis.gov/servlet/SingleRpt/SingleRpt?search‗topic=TSN&search‗value=161030#null. Accessed 5 Feb. 2018.

Paxton, John R., and William N. Eschmeyer. Encyclopedia of Fishes. 2nd ed., San Diego, Academic Press, 1998.

Pough, F. H., J. B. Heiser, and W. N. McFarland. Vertebrate Life. 11th ed. Saddle River, Prentice Hall, 2023.

Seaman, William Jr., ed. Artificial Habitats for Marine and Freshwater Fisheries. San Diego, Academic Press, 2013.