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Home , Actinopterygii, Osteichthyes, Permit (fish), Polypterus, Sarcopterygii, Swim bladder

BONY (: AND CLASS ) MODULE 8

MODULE 8 BONY FISHES (Osteichthyes: Class Sarcopterygii and Class Actinopterygii)

Unit 1

General Characteristics and Classification Sub-Unit 1: General Characteristics 1. The is more or less bony, may persist in part and the tail is usually homocercal. 2. has mucous glands and is embedded with dermal scales which are usually of three types: ganoid, cycloid or ctenoid scales. Some are without scales. 3. They possess both median and paired , with rays of or . 4. is usually terminal with or without teeth; jaws are present; olfactory sacs are paired and may or may not open into the mouth. 5. Respiration is by supported by bony arches and covered by a common . 6. often present with or without duct connected to the . 7. The circulatory system consists of a two-chambered ; arterial and venous systems, and characteristically four pairs of aortic arches; blood contains nucleated red cells. 8. Nervous system consists of a with small olfactory lobes and ; large optic lobes and . There are ten pairs of cranial nerves and three pairs of semi- circular canals. 9. Sexes are separate although there is sex reversal in some. Gonads are paired. Fertilization is usually external.

Sub-unit 2: Classification The Osteichthyes is divided into two classes: 1. Class Sarcopterygii (lobe finned fishes) 2. Class Actinopterygii (ray-finned fishes) a. Subclass () b. Subclass (Gr. chondros, cartilage + osteon, bone) e.g. and

AOE 1

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

c. Subclass (Gr. holos, entire + osteon, bone) e.g. , and .

Unit 2

Class Sarcopterygii Members of the Sarcopterygii (G. sarkos, flesh + pteryx, fin or ) includes the extinct rhipidistians, coelocanths and the , which are collectively referred to as the lobe-finned fishes. The rhipidistians have a fusiform shape, two dorsal fins and a heterocercal tail. The paired fins bore sharp resemblance to . They are believed to have been ancestors of ancient . The coelocanths have only one living with two : the West Indian coelocanth, chalumnae (named after Majorie Courtenay- Latimer, South African Museum Director) and the Indonesian coelocanth, Latimeria menadoensis. The last were believed to have become extinct 70 million ago. However, in 1938, people in deep water off the coast of South brought up fishes that were identified as coelocanths. Since then, numerous other specimens have been caught in deep water around the Comoro Islands off Madagascar, where it was learned, native Comoran fishermen occasionally caught them with hand lines at great depths. Latimeria is large, up to 80kg, and has heavy scales. The tail is diphycercal but possesses a small lobe between the upper and lower caudal lobes, producing a three-pronged structure. Coelocanths show some degenerative features, such as more cartilaginous parts and a swim Figure 1: Latimeria chalumnae bladder that was either calcified or else persisted as a mere vestige. They also lack internal nostrils. Ancient coelocanths lived in freshwater lakes and ; therefore, the ancestors of Latimeria must have moved from freshwater to the .

AOE 2

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

The fishes and are represented today as three genera. All live in regions where seasonal droughts are common. The least specialized is , the living Australian (from freshwaters of Queensland, ), which may which Figure 3: Neoceratodus may attain a length of 1.5m. This lungfish is able to survive in stagnant, -poor water by coming out and gulping air into its single lung, but it cannot live out of water. The other two genera are found in freshwater rivers and lakes of tropical Africa, , and tropical , Lepidosiren. They have lost the use of gills for gas exchange and can survive when rivers and lakes are dry Figure 2: Lepidosiren by burrowing into the mud. Protopterus lives in African streams and rivers that run completely dry during the dry season (the is called Protopterus annectens). The burrows down at the approach of the dry season and secretes a copious slime that is mixed with mud to form a hard cocoon in which it aestivates until the rains return. They then emerge from their burrows to feed Figure 4: Protopterus and reproduce.

Unit 3

Class Actinopterygii They are referred to as ray-finned fishes because their fins lack muscular lobes. They usually possess swim bladders, gas-filled sacs along the dorsal wall of the body cavity that regulate . A modern classification system divides the Actinopterygii into three subclasses; these are the Cladistia (bichirs), Chondrostei, (sturgeons and paddlefishes); and the Neopterygii (gars, bowfin and teleosts). Members of the subclass Cladistia (bichirs) have long, slender bodies that are covered by rhombic ganoid scales; examples include bichirs of Nigerian freshwaters such as senegalus and P. . Polypterus has a lung-like swim bladder. The sturgeons and

AOE 3

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

paddlefishes have cartilaginous . Chondrosteans have a tail with a large upper lobe. Sturgeons are large (up to 1,000kg), and bony plates cover the anterior portion of the body, heavy scales cover the tail. The mouth is small, and jaws are weak. They feed on using their snout and they are valued for their caviar (). Paddlefishes are large freshwater chondrosteans. They have large, paddle-like rostrum that is innervated with sensory organs believed to detect weak electrical fields. They swim through the water with their large open, Figure 5: (a) Bichir (b) Sturgeon filtering and small fishes. (c) The Neopterygii comprise the bowfin, gars and teleosts. The bowfin, (Greek name of - like fish) of shallow weedy waters of the Great Lakes and Mississippi Valley; the gars, (garpike), of eastern North America; and the Figure 6: Bowfin teleosts or modern fishes. The teleosts have evolved into a variety of body forms today. They have a symmetrical caudal fin and a swim bladder (for hydrostatic functions) that Figure 7: Garpike has lost its connection to the digestive tract.

Figure 8: Teleosts (a) (b) (c) (d) African pike (e) Mormyrid (f)

AOE 4

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

Unit 4

Success of Bony Fishes Bony fishes have adapted themselves to nearly every available aquatic . The remarkable success of bony fishes has resulted from a series of significant adaptations that have enabled them to dominate life in water. These include the swim bladder, system and cover. 1. Swim bladder: possession of swim bladder keeps bony fishes buoyant despite the fact that are heavier than cartilaginous skeleton. The swim bladder is a gas-filled sac that allows them to regulate their buoyant density and so remain suspended in water effortlessly at any depth. In most of today’s bony fishes, the swim bladder is an independent that is filled and drained of gases, mostly and oxygen internally. It turns out that the gases are released from their blood. The gas flow is regulated by lactic acid, the acidity of which drives nitrogen out of the blood. The lower pH also alters the shape of the haemoglobin so that it is less able to bind oxygen. 2. Lateral line system: bony fishes possess a fully developed lateral line system, which consist of a series of sensory organs that project into a canal beneath the surface of the skin. Movement of water past the fish forces water through the canal. The sensory organs are deflected by the slightest movement of water over them. Nerve impulses from these sensory organs the fish to assess its rate of movement through water, sensing the movement as waves against its lateral line. The lateral line enables a fish to detect motionless objects at a distance by the movement of water reflected off the object. In a very real , this is the fish equivalent of hearing. 3. Gill cover: most bony fishes have a hard plate, called the operculum, covering the gills on each side of the head. Flexing the operculum permits bony fishes to pump water over gills. When the mouth is open, the operculum is closed thereby sealing off the exit. This increases the volume of the mouth cavity so that the water is drawn into the mouth. When the mouth is closed, the operculum opens thereby decreasing the volume of the mouth cavity, forcing water past the gills to the outside.

AOE 5

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

Unit 5

Biology of Bony Fish Teleosts have internal skeleton almost completely ossified. They have developed thin cycloid scales from the thick, ganoid type of earlier fishes. There is also the absence of spiracles. Locomotion The streamlined shape of a fish and the mucoid secretions that lubricates its body surface reduce friction between the fish and the water. The buoyant properties of water also contribute to the efficiency of a fish swimming through the water. The propulsive mechanism of a fish is its trunk and tail musculature. The musculature is composed of zig-zag muscle bands (myotomes). The slimy surface of a fish reduces water friction by at least 66% as compared with the same surface from which the slime has been removed. Nutrition and the Digestive System Most fishes are that prey on various foods from zooplankton and insect larvae to large . The eat flowering plants, algae and grasses. Although the herbivores are few in number, they are crucial intermediates in the aquatic food chain. The feed on both plants and . The kinds of food that one fish eats at different times in its life varies. The filter feeders strain both phytoplankton and zooplankton from the water with a sieve-like device, the gill rakers. There are also scavengers and parasites. The fish digestive tract is similar to that of vertebrates and digestion follows the plan. Food proceeds from the into the tubular intestine. The stomach is an enlargement that stores large often infrequent meals. The small intestine is the primary site for enzyme secretion and food digestion. Digestion and absorption proceeds simultaneously in the intestine. The intestine is short in carnivores but may be extremely long and coiled in herbivorous forms. A long intestine is an adaptation for lengthy digestion required for plant carbohydrates. An important feature of ray-finned fishes, especially teleosts, is the presence of numerous pyloric caeca, found in no other vertebrate group. Their primary function appears to be fat absorption, although all classes of enzymes are secreted there. Circulation Blood flows in the venous system through the sinus venosus, the atrium, the ventricle and conus arteriosus and into the ventral . Five afferent vessels carry blood to the gills. Blood is collected by efferent vessels, delivered to the dorsal aorta, and distributed to the body.

AOE 6

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

Respiration Fishes live in an environment that contains less than 2.5% of the oxygen present in air, therefore they must pass large quantities of water across the gill surfaces to maintain adequate levels of oxygen in the bloodstream. Fish gills are composed of gill arches (for gill support) and thin filaments which include vascular folds of thin epidermal membrane called lamellae. These are richly supplied with blood vessels. The gills are located inside the pharyngeal cavity and are covered by a movable flap, the operculum (protects the delicate gill filaments, streamlines the body, and makes possible a muscular pumping system of water into the mouth, across the gills and out of the operculum). Gas exchange occurs as blood and water move in opposite directions on either side of the lamellar epithelium. This mechanism provides efficient gas exchange and is the best arrangement for extracting the greatest possible amount of oxygen from water. Some fishes such as the lungfish, Protopterus, can breathe in air with . Freshwater often make overland excursions during rainy weather using the skin as a major respiratory surface. The bowfin, Amia, utilizes both gills and a swim bladder. The Indian climbing perch spends its life almost entirely on land, breathing air through special chambers called pneumatic sacs, above the much reduced gills. Buoyancy regulation Fishes maintain their vertical position in a column of water in four ways: one way is to incorporate low density compounds (buoyant oils) into their tissues, e.g. . A second way is to use fins to provide lift; the third adaptation is the reduction of heavy tissues in fishes as the bones of fishes are generally less dense than that of terrestrial vertebrates. The fourth adaptation is the swim bladder; a fish regulates buoyancy by precisely controlling the volume of gas in its swim bladder. Fishes adjust gas volume in swim bladders in two ways. The less specialized fishes have a pneumatic duct that connects the swim bladder to the oesophagus. More specialized teleosts have lost the pneumatic duct. In these fishes, the gas must originate in the blood and be secreted into the swim bladder. Gas exchange depends on two highly specialized areas; a gas gland that secretes gas into the bladder and a resorptive area that can remove gas from the bladder. The gas gland contains a remarkable network of blood capillaries called the (“marvelous net”) that acts to transfer gas, especially oxygen from the blood to the swim bladder. The gas gland secretes lactic acid, which enters the blood, causing a localized high acidity in the rete mirabile that forces haemoglobin to release its load of oxygen. The released oxygen accumulates in the rete and finally diffuses into the swim bladder.

AOE 7

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

Nervous system The central nervous system of fishes consists of a brain and a spinal cord. Sensory receptors, e.g. for touch, temperature, olfaction, vision, hearing, equilibrium, balance and for detecting water movements are widely distributed over the body. Receptors for equilibrium, balance and hearing are in the inner ears of fishes, and their functions are similar to those of other vertebrates. The lateral line system runs along each side and branches over the head of most fishes. It consist of sensory pits in the epidermis of skin that connects to canals that run just below the epidermis. Lateral lines are used to detect water currents, or a predator or a prey that may be causing water movements in the vicinity of the fish. Excretion and Osmoregulation Osmoregulation is a major function of the kidneys and gills of fishes. Up to 90% of nitrogenous wastes in fishes are eliminated as ammonia across the gill surface, the remaining 10% are excreted as urea, creatine and creatinine. These wastes are produced in the liver and excreted via the kidney. Freshwater fishes live in an environment containing few dissolved substances. Water, therefore, tends to enter their bodies (gills, orally and intestinal surfaces) osmotically and the loss of essential ions by excretion and defaecation are constant. Freshwater fishes never drink and only take in water when feeding. To control excess water build-up and ion loss, water is pumped out by the mesonephric kidney which is capable of forming very dilute urine (nephrons of freshwater fishes frequently possess large glomeruli and relatively short tubule systems, therefore, little water is reabsorbed following filtration). Also, special salt absorbing cells, located in the gill epithelium actively move salt ions, principally sodium and chloride, from the water to the blood. Freshwater fishes also get some salts in their food to compensate for this ion loss. Marine bony fishes face the opposite problem, they are hyperosmotic regulators, having much lower blood concentration (0.3-0.4M) than sea water (about 1M), and they tend to loose water and gain salt. To compensate for water loss, the marine drinks water, however, this is unfortunately accompanied by a great deal of salt. Unwanted salt is disposed of in two ways; first, the major sea salt ions (sodium chloride and potassium) are carried by blood to the gills where they are secreted outwards across the gill surface by special salt secretory cells (active transport). Secondly, the remaining ions (magnesium, sulphate and calcium) are left in the intestine and voided with the faeces, a significant fraction of this residual salt (divalent ions) penetrate the intestinal mucosa and enters the bloodstream which are excreted by the kidney.

AOE 8

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

The excretory structures in the kidneys are the nephrons. Nephrons filter blood borne nitrogenous wastes, ions, water and small organic compounds across a network of capillaries called glomerulus. The filterate is then passed into a tubule system, where essential components may be reabsorbed into the blood, the filterate remaining in the tubule system is then excreted. The nephrons of marine fishes frequently possess small glomeruli and long tubule systems. Much less blood is filtered than in freshwater fishes, and water is efficiently, although not entirely, reabsorbed from the nephron. Reproduction and Development The vast majority of fishes are dioecious and oviparous, i.e., and external development of the eggs and embryos. The testes of male are elongate, whitish organs divided into lobules that contain cysts of maturing sperm cells in the same stage of development. The lobules open into a spermatic duct that runs the length of the abdominal cavity and are made up of many ovarian follicles supported by connective tissue. The eggs develop within the individual follicles. When the eggs are mature, they are shed either directly into the body cavity or into . Internal development of viviparous bony fishes usually occur in ovarian follicles, rather than in the . In (Lebistes), eggs are retained in the ovary and fertilization and early development occur there. Embryos are then released into a cavity within the ovary and development continues, with nourishment coming partly from ovarian secretions. This is ovoviviparous form of reproduction. In viviparous forms, for example, the dwarf top-, , the young develop some type of placental attachment with the follicle, ovarian wall or specialized uterus. Most fishes care little, if any, for young after hatching. Some fishes however, construct and tend nests, and some carry embryos during development. Some brood embryos in the mouth. The engage in long term care.

Conclusion Bony fishes are the most successful of all vertebrates. The bony fishes evolved in freshwater unlike . Most bony fishes have highly mobile fins, very thin scales, and completely symmetrical tails. Bony fishes care categorized as lobe-finned fishes and ray-finned fishes. The lobe-finned fishes include the coelocanths and lungfishes while the ray-finned fished are the bichirs, chondrosteans and neopterygians. The remarkable success of bony fishes have enabled them to dominate life in water through a series of significant adaptations such as the development of swim bladder, lateral line system and gill cover. A countercurrent exchange

AOE 9

BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8

mechanism (blood and water moving in opposite directions on either side of the lamellar epithelium) is employed to provide efficient gas exchange between blood and water. The gills and kidneys of fishes function as osmoregulatory organs. The vast majority of fishes are dioecious and oviparous; showing little parental care except for some cichlids.

References/Further Reading Raven PH, Johnson GB (1999). Biology. WCB McGraw-Hill. 1284 pp. Segun AO (2013). Tropical Zoology, 3rd Edition.

AOE 10