Excretory organs

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

Endocrine glands

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

The nervous system and sensory organs

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

Sight

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

Other senses (touch, pain, and special senses)

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

Agnatha: early jawless fishes

The earliest vertebrate fossils of certain relationships are fragments of dermal armour of jawless fishes (superclass Agnatha, order Heterostraci) from the Upper Ordovician Period in North America, about 450 million years in age. Early Ordovician toothlike fragments from the former Soviet Union are less certainly remains of agnathans. It is uncertain whether the North American jawless fishes inhabited shallow coastal marine waters, where their remains became fossilized, or were freshwater vertebrates washed into coastal deposits by stream action.

Jawless fishes probably arose from ancient, small, soft-bodied filter-feeding organisms much like and probably also ancestral to the modern sand-dwelling filter feeders, the Cephalochordata (Amphioxus and its relatives). The body in the ancestral animals was probably stiffened by a notochord. Although a vertebrate origin in fresh water is much debated by paleontologists, it is possible that mobility of the body and protection provided by dermal armour arose in response to streamflow in the freshwater environment and to the need to escape from and resist the clawed invertebrate eurypterids that lived in the same waters. Because of the marine distribution of the surviving primitive chordates, however, many paleontologists doubt that the vertebrates arose in fresh water.

Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the Silurian, about 416 million years ago, European heterostracan remains are found in what appear to be delta or coastal deposits. In the Late Silurian of the Baltic area, lagoon or freshwater deposits yield jawless fishes of the order Osteostraci. Somewhat later in the Silurian from the same region, layers contain fragments of jawed acanthodians, the earliest group of jawed vertebrates, and of jawless fishes. These layers lie between marine beds but appear to be washed out from fresh waters of a coastal region.

It is evident, therefore, that by the end of the Silurian both jawed and jawless vertebrates were well established and already must have had a long history of development. Yet paleontologists have remains only of specialized forms that cannot have been the ancestors of the placoderms and bony fishes that appear in the next period, the Devonian. No fossils are known of the more primitive ancestors of the agnathans and acanthodians. The extensive marine beds of the Silurian and those of the Ordovician are essentially void of vertebrate history. It is believed that the ancestors of fishlike vertebrates evolved in upland fresh waters, where whatever few and relatively small fossil beds were made probably have been long since eroded away. Remains of the earliest vertebrates may never be found.

By the close of the Silurian, all known orders of jawless vertebrates had evolved, except perhaps the modern cyclostomes, which are without the hard parts that ordinarily are preserved as fossils. Cyclostomes were unknown as fossils until 1968, when a lamprey of modern body structure was reported from the Middle Pennsylvanian of Illinois, in deposits more than 300 million years old. Fossil evidence of the four orders of armoured jawless vertebrates is absent from deposits later than the Devonian. Presumably, these vertebrates became extinct at that time, being replaced by the more efficient and probably more aggressive placoderms, acanthodians, selachians (sharks and relatives), and by early bony fishes. Cyclostomes survived probably because early on they evolved from anaspid agnathans and developed a rasping tonguelike structure and a sucking mouth, enabling them to prey on other fishes. With this way of life they apparently had no competition from other fish groups. Cyclostomes, the hagfishes and lampreys, were once thought to be closely related because of the similarity in their suctorial mouths, but it is now understood that the hagfishes, order Myxiniformes, are the most primitive living chordates, and they are classified separately from the lampreys, order Petromyzontiformes.

Early jawless vertebrates probably fed on tiny organisms by filter feeding, as do the larvae of their descendants, the modern lampreys. The gill cavity of the early agnathans was large. It is thought that small organisms taken from the bottom by a nibbling action of the mouth, or more certainly by a sucking action through the mouth, were passed into the gill cavity along with water for breathing. Small organisms then were strained out by the gill apparatus and directed to the food canal. The gill apparatus thus evolved as a feeding, as well as a breathing, structure. The head and gills in the agnathans were protected by a heavy dermal armour; the tail region was free, allowing motion for swimming.

Most important for the evolution of fishes and vertebrates in general was the early appearance of bone, cartilage, and enamel-like substance. These materials became modified in later fishes, enabling them to adapt to many aquatic environments and finally even to land. Other basic organs and tissues of the vertebrates—such as the central nervous system, heart, liver, digestive tract, kidney, and circulatory system— undoubtedly were present in the ancestors of the agnathans. In many ways, bone, both external and internal, was the key to vertebrate evolution.

Acanthodii: early jawed fishes

The next class of fishes to appear was the Acanthodii, containing the earliest known jawed vertebrates, which arose in the Late Silurian, more than 416 million years ago. The acanthodians declined after the Devonian but lasted into the Early Permian, a little less than 280 million years ago. The first complete specimens appear in Lower Devonian freshwater deposits, but later in the Devonian and Permian some members appear to have been marine. Most were small fishes, not more than 75 cm (approximately 30 inches) in length.

We know nothing of the ancestors of the acanthodians. They must have arisen from some jawless vertebrate, probably in fresh water. They appear to have been active swimmers with almost no head armour but with large eyes, indicating that they depended heavily on vision. Perhaps they preyed on invertebrates. The rows of spines and spinelike fins between the pectoral and pelvic fins give some credence to the idea that paired fins arose from “fin folds” along the body sides.

The relationships of the acanthodians to other jawed vertebrates are obscure. They possess features found in both sharks and bony fishes. They are like early bony fishes in possessing ganoidlike scales and a partially ossified internal skeleton. Certain aspects of the jaw appear to be more like those of bony fishes than sharks, but the bony fin spines and certain aspects of the gill apparatus would seem to favour relationships with early sharks. Acanthodians do not seem particularly close to the Placodermi, although, like the placoderms, they apparently possessed less efficient tooth replacement and tooth structure than the sharks and the bony fishes, possibly one reason for their subsequent extinction.

Placodermi: plate-skin fishes

The first record of the jawed Placodermi is from the Early Devonian, about 400 million years ago. The placoderms flourished for about 60 million years and were almost gone at the end of the Devonian. Nothing is known of their ancestors, who must have existed in the Silurian. The evolution of several other, better-adapted fish groups soon followed the appearance of the placoderms, and this apparently led to their early extinction. Their greatest period of success was approximately during the middle of the Devonian, when some of them became marine. As their name indicates (placoderm meaning “plate skin”), most of these fishes had heavy coats of bony armour, especially about the head and anterior part of the body. The tail remained free and heterocercal (that is, the upper lobe long, the lower one small or lacking). Most placoderms remained small, 30 cm (12 inches) or less in length, but one group, the arthrodires, had a few marine members that reached 10 metres (about 33 feet) in length.

Important evolutionary advances of the placoderms were in the jaws (which usually were amphistylic—that is, involving the hyoid and quadrate bones) and development of fins, especially the paired fins with well-formed basal or radial elements. The jaws tended to be of single elements with strongly attached toothlike structures. These were too specialized to be considered ancestral to the more adaptable jaws of subsequent bony fish groups. It has been proposed that sharks arose from some group of placoderms near the Stensioelliformes and that the chimaera line (class Holocephali) arose from certain arthrodires; this suggestion, however, is uncertain.

A peculiar 5-cm (2-inch) fossilized fish, Palaeospondylus, from Middle Devonian rocks in Scotland, is probably not a placoderm, although it is sometimes classed with placoderms. Various suggestions that its relationships are with the agnathans, placoderms, acanthodians, sharks, and even lungfishes and amphibians are unconvincing, and its relationships remain completely unknown.

Actinopterygii: ray-finned fishes

The Actinopterygii, or ray-finned fishes, are the largest class of fishes. In existence for about 400 million years, since the Early Devonian, it consists of some 42 orders containing more than 480 families, at least 80 of which are known only from fossils. The class contains the great majority of known living and fossil fishes, with about 26,900 living species. The history of actinopterygians can be divided into three basic stages or evolutionary radiations, each representing a different level of structural organization and efficiency.

The Chondrostei may have first arisen as early as the Early Devonian, increased in numbers and complexity until about the Permian, and thereafter declined, becoming almost extinct by the middle of the Cretaceous, 100 million years ago. The chondrostean order Palaeonisciformes is the basal actinopterygian stock from which all other chondrosteans and the holosteans evolved. They were the most common fishes of their time, relatively small and typically like later fishes in appearance. In comparison with today’s fishes, they had peculiar-looking jaws and tails. Their tails were heterocercal. On their bodies were thick ganoid scales that abutted each other, rather than overlapping as in most modern fishes. Palaeonisciformes often had large eyes placed far forward, long mouths with the upper jaw firmly bound to the fully armoured cheek, and a relatively weak lower jaw muscle. They gave rise to a great variety of types, with elongate bodies and jaws, bottom-living types that fed on microorganisms, deep-bodied marine reef fishes, and coral-eating reef fishes. Almost all of these were replaced by modern teleosts. Surviving Chondrostei are the bottom-feeding marine and freshwater sturgeons, the strange plankton-feeding paddlefishes of the Mississippi River of North America and the Yangtze River (Chang Jiang) of China, and the freshwater bichirs and reedfishes (family Polypteridae) of Africa. The relationship of the polypterids is in some doubt, and that group has sometimes been placed in the class Sarcopterygii.

Several of the chondrostean orders developed characteristics that approached the holostean level of anatomic organization and are sometimes called subholosteans. One of these orders, the Parasemionotiformes, evolved from the Palaeonisciformes in the Early Triassic and may have given rise to at least some of the holosteans. This evolutionary line leads to the Pholidophoriformes, which gave rise to modern bony fishes, or teleosts.

The holosteans are thought to be of mixed origin and represent a stage in the evolution of a group of chondrostean orders. Therefore, the infraclass or division Holostei does not represent a single lineage. Important holostean characteristics are the approach of the tail toward the homocercal condition and the equal number of fin rays and basal elements of the fin rays. Both of these conditions make the holostean a more efficient swimmer than the chondrostean, as does thinning of the holostean body scales. Another important advance of holosteans was the freeing of the upper jaw from the preopercular bone of the cheek, allowing greater movement of the gill chamber and jaws, with more powerful development of the lower jaw muscle.

Five orders of holosteans are known, with their greatest evolutionary radiation occurring during the Triassic, Jurassic, and Cretaceous periods, when the chondrosteans were declining and the teleosts just beginning to expand. Two holostean groups survive today: the bowfin, Amia calva, and several species of gars, Lepisosteus, all found in North America. The current understanding of bony fish evolution recognizes the Amiiformes as the closest living relatives of the teleosts.

The modern bony fishes, infraclass or division Teleostei, include the great majority of living fishes. They first appear in the fossil record about 200 million years ago (as the family Leptolepididae), with their homocercal caudal fin and caudal skeleton already fully developed. They arose from an order of holosteans now extinct, the Pholidophoriformes. This group was intermediate in character between the chondrosteans and the teleosts. Teleosts reached their fullest extent within the last 50 million years and represent a distinct functional advance over their holostean ancestors. They have greater swimming ability, due to the improvement in the tail structure, and have a still more efficient feeding and gill-ventilating apparatus.

The bony fishes represent the culmination of a long evolution toward a body plan with maximum swimming efficiency. Particularly important in this evolution have been changes in fins and in the tail. Some authorities believe that the paired fins arose from a single continuous tail and anal fin that was divided at the vent and extended forward along each side to the head. Later the sections between the pectoral, pelvic, anal, and caudal fins were lost. The fin rays of sharks and rays are of a horny material, but those of many primitive fossil fishes are of bone. The bony fin rays of sarcopterygians and actinopterygians probably arose from scales lying in the fin folds. Modern teleost fishes have flexible fin rays (called soft rays) of jointed segments of bone, or spiny rays, each of solid continuous bone. The first dorsal fin of acanthopterygian fishes is of the spiny type.

The original tail fin of primitive fishes was not an effective swimming organ, because of its asymmetry. The steady improvement in tail shape over 400 million years is one of the prominent features of fish evolution. In primitive fishes the tail (vertebral) axis turned upward (heterocercal) or downward (hypocercal), and a lobe of flesh projected from it. This form of tail cannot provide a powerful driving mechanism, because the driving force is unevenly distributed relative to the body axis. With an asymmetrical tail, the fish swims by an undulating motion of the body and tail. In some fishes with a diphycercal tail (with the axis of the vertebrae extending down the middle of the fin lobe), developed in both modern and ancient fishes, the tail remains relatively ineffective because it has remained too rigid for proper propulsive action. The development of a true homocercal tail fin, in which powerful muscles move strong fin rays with a very flexible basal joint and in which the upper and lower lobes are about equal, is a development exclusive to teleost fishes.

As suggested by the existence of more than 400 families, teleosts are extremely varied in anatomical form and in the habitat occupied. They can be divided into about 12 superorders or subdivisions, each with distinct evolutionary significance. The Leptolepidimorpha, an extinct, relatively primitive group, has uncertain relationships with other teleosts and is as yet poorly understood. The second group, the superorder Osteoglossomorpha, consists of relatively primitive teleosts, most of which are now extinct. The few surviving members are mostly tropical and worldwide in distribution but adapted to restricted habitats. The third group, the Elopomorpha, retains some relatively primitive living members, such as the tarpons, but is mostly represented by the large variety of specialized true eels. The Clupeomorpha includes the herrings and anchovies, relatively primitive fishes, mostly specialized for existence near the surface of the open ocean. A few species are anadromous, breeding in freshwater environments but spending most of their lives in the sea. The Protacanthopterygii is a varied collection of relatively primitive orders, marine, deep-sea, and freshwater in distribution; trouts, smelts, and argentines are examples. The Ostariophysi are an important group of primarily freshwater fishes, including the characins, carps, minnows, loaches, suckers, and catfishes.

The remaining groups have a complex fossil history and are not yet fully understood, but all seem to possess similar evolutionary trends. Each group shows a tendency to develop spiny fin rays in the dorsal and anal fins (reduced in some) and a shelf of bone under the eye. There is a tendency for the pelvic fins to move forward on the body, with a reorganization of swimming methods and a slight gain in maneuverability. All three groups probably are related and presumably arose from some early protacanthopterygian-like ancestor. The Scopelomorpha include a wide variety of deep-sea open-ocean plankton feeders and predators, some of which bear light organs. The Paracanthopterygii are a rather miscellaneous collection of fishes, the most important to humans being the cods. The final superorder, the Acanthopterygii, is the result of the great radiation of modern spiny-rayed fishes and contains the dominant fishes in marine shore habitats, tropical, temperate, and Arctic. They also live in the freshwater environment, especially in lakes, slow-moving streams, and ponds. The superorder has some important open-ocean members, such as tunas. The key to the successful acanthopterygian radiation probably has been their mobile, protractile mouth.

Distinguishing taxonomic features

In forming hypotheses about the evolution of fishes and in establishing classifications based on these hypotheses, ichthyologists place special emphasis on the comparative study of the skeleton. There are two primary advantages of this approach. First, direct comparison between extant and fossil groups is possible, the latter usually represented only by bony remains. The second advantage is that the bones of living fishes are relatively easy to observe and to study, compared with other body structures. Proper preservation and special preparation of the nervous system, for example, are difficult and expensive when the fishes being compared are from the far ends of the Earth. In the study of the relationships of species within a group, major use has been made of similarities and differences in the dimensions of external features, such as head and body length, and of counts of external characters, such as teeth, fin rays, and scales. Colour pattern is also important. In recent years, valuable data on classification of fishes have been obtained from studies of comparative behaviour, physiology, genetics and functional anatomy.