Bony Fishes

The term bony fishes strictly refers to a large grouping of jawed vertebrates, which together compose the Osteichthyes. Among other features, osteichthyans are characterized by the presence of true endochondral bone—that is, the bones of their endoskeleton ossify internally. The members of this huge group include the lobe-finned fishes and the four-limbed vertebrates (the sarcopterygians), and the ray-finned fishes (the actinopterygians).

The remains of sarcopterygians date from the Early Devonian, but phylogenetic data indicate that the origin of the group probably long predates that and may have been as early as the Silurian. Living members of the Sar-copterygii are the lungfishes (Dipnoi), coela-canths (Actinistia), and the four-legged vertebrates (Tetrapoda). A characteristic of all sarcopterygians, fossil and living, is the possession of a so-called monobasal (single-based) articulation of the paired fins with a fin musculature extending in a fleshy lobe. The largest of the sarcopterygian subgroups, the Tetrapoda, will be dealt with elsewhere, while the "fishlike" members (lungfishes and coelacanths) will be briefly dealt with here, followed by a consideration of the bulk of the Osteichthyes, the Actinopterygii or ray-finned fishes.

The earliest known actinistians (coela-canths: coel = "hollow"; acanth = "spine") are Late Devonian in age, and the group is represented by an extensive fossil record to the end of the Cretaceous, after which no further fossils are known; the lineage was presumed to have gone extinct. The discovery of a living coelacanth in the late 1930s (Smith, 1939) was thus a great surprise and generated a strong interest in coelacanth biology and their phy-logenetic relations to other sarcopterygian groups. Today, coelacanths are represented by two known living species: Latimeria chalumnae, from the Comores Islands in the southwestern Indian Ocean, and the recently discovered Latimeria menadoensis, from off Manado, Sulawesi, Indonesia (Holder et al., 1999). The coelacanth body plan is remarkably conservative, and these two species are regarded as "living fossils" strikingly similar to their long extinct relatives. They possess a typically actin-istian caudal fin in which the dorsal and ventral webs are equal in size and separated by a horizontal prolongation containing the posterior extent of the notochord and ending in a small, rounded tuft. Coelacanths have a unique rostral organ in the snout that is believed to have an electrosensory function, and they share numerous additional features of the skull and paired fin skeleton that are unique to the group. The braincase of adult Latimeria is noteworthy in that it is made up of two parts articulated together by an intracranial joint. However, this remarkable feature is thought probably to be a primitive sar-copterygian character that has subsequently been lost in lungfishes and tetrapods.

The earliest known lungfishes (Dipnoi) are also Late Devonian in age and are today represented by three freshwater genera: the African Protopterus (four species); the South American Lepidosiren (one species); and what is thought to represent the most primitive of living forms, Neoceratodus (one species) from Australia. Unlike the coelacanths, living lung-fishes are not very similar to their early fossil relatives. For example, the skeleton of living lungfishes is mostly cartilaginous, while that of their early relatives was fully ossified, and in living forms the dorsal fin is continuous with the caudal fin, although separate in fossil lungfishes. Despite the specializations of the living taxa, all lungfishes, living and fossil, share many characteristics. For example, all possess massive crushing tooth plates that rest on the palate and inside of the lower jaw. As their name suggests (dipnoi = "two lunged"), lungfishes have well-developed lungs (one in Neoceratodus, and two in Lepidosiren and Protopterus). The latter two breathe exclusively with their lungs, while Neoceratodus must respire with its gills. Pharyngeal anatomy of fossil forms suggests that obligate air-breathing probably arose rather late within the lungfish lineage, and that early lungfishes were probably exclusively gill breathers.

The discovery of living lungfishes in the early nineteenth century, and their striking resemblance in many features of their soft anatomy to tetrapods, engendered a debate as to the precise relationships of lungfishes to the land-dwelling forms. Aspects of their lungs, internal nostrils, heart structure, and paired fins were thought to indicate close affinity to (if not identity with) tetrapods. However, recent consensus suggests that the immediate relationships of the Tetrapoda lies with various Palaeozoic fossil sarcopterygians rather than with either the lungfishes or coelacanths. However, controversy still exists as to which of these two represents the closest living relatives to the tetrapods (Janvier, 1996).

The ray-finned, or actinopterygian, fishes form the largest and most diverse of all vertebrate groups. In total, the number of actinopterygian species exceeds that of all other vertebrate groups added together. The notion that life in an aquatic medium is somehow a dead end is clearly without merit, as evidenced by the enormous success of the contemporary actinopterygian radiations. Although the earliest indisputable record of actinopterygian remains dates back to the Devonian, around 400 million years ago, the origin of the group probably predates those fossil finds. The actinopterygian fossil record is rich, but unlike most other vertebrate groups, there are far more living forms than fossil. Most of the living diversity of ray-finned fishes is found in one group, the teleosts, which first appear in the fossil record around 200 million years ago. The term teleost ("perfect bone") refers to their position as the most advanced of all bony fishes. Compared with the teleosts, the other living actinopterygian groups are small and relictual. They include the bichirs and ropefish (about eleven species), the sturgeons and paddlefishes (about twenty-six species), the gars (eight species), and the bowfin (one species). These will be considered briefly before going on to discuss the great bulk of actinopterygian diversity, the Teleostei.

Bichirs and ropefish (Cladistia) are represented today by only two genera (Polypterus and Erpetoichthyes) with a total of about eleven species in the freshwaters of West and Central Africa. These bizarre, elongate, predatory fishes possess an intriguing combination of primitive, derived, and unique features. Since their discovery some 200 years ago, the phy-logenetic position of the cladistians has proven problematic, although today most authorities agree that they represent a relict of the basal actinopterygian lineage. They possess strange, lobelike pectoral fins, a thick jacket of interconnected ganoid scales, and a distinctive series of ten to eighteen dorsal finlets. Cladis-tians are obligate air breathers and will drown if unable to access atmospheric oxygen. Respiration is mediated almost entirely by highly vascularized lungs, which are aspirated via elastic recoil of the encasing ganoid scale jacket.

Sturgeons and paddlefishes (Chondrostei), like the cladistians, are another relictual group possessing a mixture of primitive and derived actinopterygian features. Chondrosteans are represented today by two families, the Acipenseridae (sturgeons), with about twenty-four species restricted to fresh and coastal waters of the Northern Hemisphere, and the Polyodontidae (paddlefishes), with one species in the freshwaters of North America and another in China. Squamation is reduced to five rows of bony scutes in sturgeons and is absent in paddlefishes, except for a single row of scales along the upper margin of the caudal fin. Internally their skeletons are cartilagi-nous—but secondarily so, as the skeletons of fossil chondrosteans were fully endochon-drally ossified. They retain strongly hetero-cercal tails and a spiral valve in the intestine. Perhaps as a result of their marked longevity, migratory spawning runs, and high market value (for caviar and meat), nearly all chon-

drosteans are today highly endangered, threatened, or already extinct. No other group of fishes has been so affected by the dam-building, habitat degradation, and overexploitation that have taken place throughout their range in the past century.

The gars (Ginglymodi) are represented by a small group of seven species arrayed in two genera, Lepisosteus and Atractosteus. They typically inhabit backwater areas of lakes and rivers in North and Central America. Like the cladistians, they retain a jacket of interlocking ganoid scales and have a fully ossified internal skeleton; internally their tail is het-erocercal, but that is not always evident externally. The centra of gar vertebrae are characteristically opisthocoelous—that is, they are concave on their posterior surface and convex on the anterior, allowing for a ball-and-socket articulation, a configuration that is almost unique among actinopterygians.

Bowfins (Halecomorphi) are today represented by a single species, Amia calva, widely distributed in the freshwaters of eastern North America. However, the group has a rich fossil record dating back to the Early Triassic, some 240 million years ago, and during the Mesozoic it was taxonomically diversified into upward of eleven genera. Generally considered to be the closest living relative to the teleostean fishes, the living bowfin has many characteristics in common with them. Among the functionally most important of such characteristics may be the presence of a hinged maxilla, which facilitates efficient, high-velocity suction feeding. In waters of low oxygen pressure, bowfins utilize a highly vascularized air bladder to extract atmospheric oxygen.

Today, it is teleost fishes that dominate the fresh and marine waters of the planet, and they are undoubtedly the dominant actinopterygian group. Current estimates of the number of living species vary, but most authors agree that there are at least 23,500. However, for the past ten years about 200 new species have been described each year. The uniformity of this figure probably reflects a fixed number of taxonomists working on teleost species descriptions rather than any cap to the actual number of undescribed species still to be encountered; a final tally of 28,000 to 30,000 species seems reasonable.

Teleost fishes occupy almost every conceivable aquatic habitat, from high-elevation mountain springs more than 5,000 m above sea level to the ocean abyss some 8,000 m below. Perhaps not surprisingly, in view of this remarkable elevational span, the variety in manner of life, anatomy, physiology, and behavior is unsurpassed among vertebrates. A few examples serve to illustrate the extraordinary range. Some fish live for less than one year, whereas others may live for more than 150 years. Some fish live their entire lives within meters of their natal site; others migrate more than 3,000 km between spawning and feeding grounds. Some will spawn once in their lifetime, whereas others may spawn many times a year over a period of many years. Some display parental care; some are viviparous. Most are gonochoristic (that is, ovaries and testes are present in different individuals), but many are hermaphrodite (one individual has both ovarian and testicular tissue, or is sequentially male then female, or vice versa); some are even capable of self-fertilization. Some produce light, venom, and electricity, and many produce sound. Some are parasitic on other species or their own. Most are ectotherms (that is, rely on external heat sources), but some have evolved endothermy (they generate and retain their own heat). Some fishes can live in almost pure water of 0.01 parts per thousand (ppt), whereas others can live in water of up to 100 ppt (seawater usually ranges from 34 to 36 ppt). Some can withstand temperatures as high as 44 degrees centigrade; others, which inhabit frigid polar seas, produce antifreeze proteins that depress their blood's freezing point to 2 degrees below zero centigrade.

Patterns of global distribution are also of interest. Although it is noteworthy that nearly all of the living nonteleostean actinoptery-gians are freshwater inhabitants, among teleosts a little more than half (58 percent) of all species are found in marine habitats; about 41 percent are confined to freshwaters; and a little less than 1 percent are diadromous (that is, they migrate between fresh and salt waters). The high number of freshwater-restricted teleost species is noteworthy, as less than 0.01 percent of the earth's water is fresh (occupying only 0.8 percent of the planet's surface). However, it should also be noted that the majority of marine fish species are restricted to the relatively narrow region of the continental shelves (representing only 5.4 percent of the planet's surface), and the richness of marine species declines markedly away from those coastal areas.

The remarkable success of the teleostean fishes has resulted in a staggeringly diverse radiation, and it is difficult to summarize the extent and complexity of forms and life styles in such a short entry. In terms of their classification, there are four major teleostean lineages currently recognized: the Osteoglosso-morpha (the bonytongues, mooneyes, knifefishes, and elephantfishes); the Elopo-morpha (the ladyfish, tarpons, deep sea spiny eels, and true eels); the Clupeomorpha (the herrings and sardines); and the Euteleostei (a massive grouping of some 22,260 species, including such diverse members as the carps and catfishes, salmons and smelts, bristle-mouths, lizardfishes, lanternfishes, cods, gup-pies, sticklebacks, sculpins, gobies, flatfishes, cichlids, and seabass, among many others). The interrelationships among these many fish groups are a topic of much ongoing research, and we are still far from a final consensus as to the details of the evolutionary histories of these animals.

In the face of such a diverse array of fishes, perhaps a key to understanding their radiation is the recognition that life in water is fundamentally different from life on land. Water is a dense, viscous medium that, in comparison with air, places a premium on effective generation of suction for acquiring food and efficient fluid propulsion mechanisms for locomotion. The heads of most teleosts are capable of quite remarkable kinesis and suction generation, with more than thirty movable bony parts controlled by more than fifty individual muscles. Some teleosts are capable of increasing their mouth volume by as much as forty-fold in milliseconds, and of generating negative pressures of up to -800 cm H2O (0.7 atmosphere), a figure approaching the physical limits imposed by fluid mechanics. The dense aquatic medium, combined with an unrivaled suction generation capability, offers an unparalleled array of prey capture opportunities for teleosts that are unavailable to their terrestrial counterparts. Because density and drag are considerably higher in water than in air, locomotion is relatively more energetically expensive in the aquatic realm. In addition to hydrodynamic streamlining, perfection of caudal locomotion has been cited as the second major attribute of the teleostean radiation, and indeed much of the evolutionary transformation of the group can be seen in a series of modifications and refinements of their locomotor systems. Powered by a swimming musculature that makes up between 40 and 65 percent of their body weight, teleostean vertebral columns have a lateral flexibility and compressional rigidity capable of powering a caudal propulsion mechanism of unrivaled efficiency. Although teleostean swim ming usually involves alternating contraction and relaxation of the swimming musculature, many specialized swimming modes have also evolved. For example, some species can "walk" along the bottom, climb vertical rock walls, glide on the water's surface, or even fly for extended distances. It is in the context of the two basic functions of aquatic feeding by suction generation and caudal propulsion locomotion that the tremendous success of the teleostean radiation is perhaps best understood.

Despite the remarkable potential of the aquatic medium to support life, there are also particular challenges. This is nowhere more starkly evident than in the arena of contemporary biodiversity loss. The past fifty years have seen an accelerated loss in aquatic systems whereby human activity is placing increasing pressure on fish populations, particularly in freshwaters. As in terrestrial systems, the three major sources of human-induced stress are habitat degradation (both within stream and land-based), introduction of exotic species, and overexploitation. Human dependencies upon and benefits from the world's fish species are many, and some are critical—such as for food and as indicators of water quality. For example, marine teleosts provide a primary source of protein for more than 1 billion people, and worldwide provide more people with animal protein than pork or beef. A full half of the world's growing human population is coastal, and another quarter lives within 60 km of the coast. Inland water and coastal ecosystems are among the most endangered in the world, and once perturbed deteriorate at a faster rate and with a poorer recovery prognosis than their terrestrial counterparts. The cumulative impacts in freshwaters have been profound; human appropriation of the planet's accessible runoff is now more than 50 percent, and dammed reservoirs hold five times as much water as is in rivers. Worldwide, it has been estimated that some 20 to 30 percent of freshwater teleost species are already extinct or in serious decline. Figures for marine fishes are harder to come by, but already more than 50 percent of the world's mangrove habitats have been lost; neuston (communities living on or just under the surface film of water) are impacted by the 3.25 million metric tons of petroleum products that enter the sea yearly from ships, accidents, and run-off from land; and upward of 75 percent of the world's major fisheries are considered overfished, with population declines now commonplace.

—Melanie Stiassny

See also: Chondrichthyes; Freshwater; Hap-lochromine Cichlids of Lake Victoria; Oceans

Bibliography

Eldredge, Niles, and Steven M. Stanley. 1984. Living Fossils. New York: Springer-Verlag; Forey, Peter L. 1991. "Latimeria Chalumnae and Its Pedigree." Environmental Biology of Fishes. 32: 75-97; Holder, Mark T., Mark V. Erdmann, Thomas P. Wilcox, Roy L. Caldwell, and David M. Hillis. 1999. "Two Living Species of Coelacanths?" PNAS: Proceedings of the National Academy of Sciences of the United States of America. 96, no. 22: 12616-12620; Janvier, Philippe. 1996. Early Vertebrates. Oxford Monographs on Geology and Geophysics No. 33. Oxford: Clarendon; Nelson, Joseph S. 1994. Fishes of the World. New York: John Wiley; Smith, James L. B. 1939. "A Living Fish of Mesozoic Type." Nature 143: 455^56.

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