The air-bladder or swim-bladder is more or less a sac-like structure lying between the alimentary canal and the kidneys. It is a characteristic organ of Osteichthyes (bony fishes). It is a gas-filled pneumatic sac, called air-bladder or swim-bladder. Air-bladder does not occur in elasmobranchs.
However, it is found in all Osteichthyes (bony fishes) except a few bottom dwellers (Lophius, Pleuronectes, etc.). It is vestigial in Latimeria, the only living crossopterygian. Air-bladder shows a number of structural modifications in various groups of bony fishes.
Origin of Air-Bladder:
The air-bladder is generally considered to be homologous with the lungs of Dipnoi, Osteichthyes and Tetrapods. Opinions differ as regard the development of air- bladder in fishes. In teleosts, it originates as an unpaired dorsal and dorso-lateral diverticulum of the oesophagus. The diverticulum with an opening in the oesophagus becomes subsequently divided into two halves.
The left half often atrophies and the right half becomes well developed and takes a median position. In dipnoans and Polypteridae (Polypterus), the air-bladder is modified into lungs and originates as the down growths from the floor of the pharynx. These outgrowths have been rotated around the right side of the alimentary canal to occupy the dorsal position. As a consequence of shifting of the position, the original right lung becomes the left one.
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But the air-bladder arises from the dorsal wall of the foregut, whereas the lungs develop from the ventral wall of the pharynx. They differ in blood supply and functions also. Goodrich (1930), concludes that both these structures have developed from the posterior pair of gill- pouches.
According to Jones (1957), air-bladder of teleosts may have evolved independently of the lungs. Spengel advocates the view that the air-bladder in fishes originates from the posterior pair of gill-pouches, but definite embryo-logical evidence in support of this idea is lacking.
Generally speaking, the air-bladder arises as an outgrowth from the oesophageal region of the alimentary canal. It shows a great diversity in mode of development, structure and function in different fishes. It lies ventral to alimentary canal in Polyptems, laterally in Dipnoi and dorsally or dorsoventrally in teleosts (Fig. 17.6).
Basic Structure of Air- Bladder:
The air-bladder in fishes varies greatly in structure, shape and size. It is essentially a tough sac-like structure with an overlying capillary network. Beneath the capillary system there is a connective tissue layer called tunica externa. Below this layer lies the tunica interna consisting primarily of smooth muscle fibres and epithelial gas-gland.
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The air-bladder may be composed of one or two or many chambers. In teleosts, the air-bladder lies below the kidneys between the gonads and above the gut. The connection with the oesophagus may be retained throughout life or may be lost in the adult. Blot (1807) and Morean (1876) have shown that the gas secreted by the air-bladder is mostly oxygen. Nitrogen and little quantity of carbon dioxide are also present.
The air-bladder receives its blood from branches of the coeliaco-mesenteric artery or directly from the posterior branches of dorsal aorta. The venous blood is then drained into a vessel that joins the hepatic portal system, while in some species the air-bladder vein joins the posterior cardinal vein. The air-bladder also shows differences in its degree of vascularity in various teleosts and in the formation of ‘red bodies’ or ‘red glands’.
In some species (Clupeidae and Salmonidae), the capillaries are uniformly distributed all over the surface of the air-bladder and do not form a ‘retia mirabilia’, while in other Physostomes as the carps (Cyprinus, Labeo, Tortor) the blood vessels are arranged in a fan-like manner and are concentrated at one or more points on the inner surface of the air-bladder, forming red masses of various shapes, called the ‘red bodies’.
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These are essentially ‘retia mirabilia’ consisting of numerous arterial and venous capillaries, running parallel to one another and carrying blood to and from the gas gland. These constitute a ‘wonder net’ of capillaries which do not communicate until they reach the postcardinal vein epithelium of the air-bladder. In the physostomous fishes, this structure is more primitive, being covered with a simple flat epithelium and is called the ‘red body’.
In the physoclistous fishes, the capillary are covered with a thick glandular folded epithelium and is called the ‘red gland’. Among the Physostomi, the eels (Anguilla anguilla) shows a close resemblance to the condition found in physoclisti.
Gases in the air-bladder come from blood secreted by the red gland. The posterior chamber is thin-walled and forms the oval gland which permits reabsorption of gases by the blood. Secretion and resorption are under the control of autonomic nervous system. In physostome fishes with air-bladder connected to pharynx by a duct, air can also be gulped or bubbled through the mouth (Fig. 17.7).
Types of Air-Bladder:
Depending on the presence of the duct (ductus pneumaticus) between the air-bladder and the oesophagus, the air-bladder in fishes can be divided into two broad categories- physostomus and physoclistous types. Depending on the conditions of air-bladder, the teleosts are classified by older taxonomists into two groups- Physostomi and Physoclisti. A transitional condition is observed in eels.
Condition in Ganoids and Dipnoi:
In most primitive fishes, the air-bladder serves as an accessory respiratory organ or lung which seems to have been its original function. In Polypterus (Chondrostei), one of the most primitive bony fishes living today, it is a smooth walled bilobed sac with a short left and a long right lobe, opening on the floor of the pharynx below the gill- slits.
The two sacs are fused into one at the proximal end and the opening (glottis) is provided with a muscular sphincter. In Acipenser, the air-bladder is short and oval in shape, its walls are smooth and the glottis is a wide opening into the oesophagus. In the holosteans Amia and Lepidosteus, the air-bladder is a single large and highly vascular lobe, lying dorsal to oesophagus and its pneumatic duct also opening dorsally into oesophagus.
Thus, in Amia and Lepidosteus, the air-bladder serves as both a respiratory organ (lung) and a hydrostatic organ. In the Dipnoi (lung-fishes), the air-bladder resembles closely with the lung of amphibians in structure. In Protoptenis and Lepidosiren, the condition is similar but the two lobes are equal in size and have thick, vascular walls with alveoli and septa.
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In the Australian lung fish Neoceratodus, air-bladder consists of a single lobe lying dorsal to the gut, but still connected ventrally to the oesophagus by a narrow pneumatic duct, passing down along the right side of the gut.
Condition in Teleosts:
The air-bladder, although present in many teleosts, is completely absent in several groups of fishes as the flat fishes (Pleuronectiformes), Giganturiformes, Saccophyringiformes, Pegasiformes, Echeineiformes, Gobeisociformes and Symbranchiformes. The air-bladder assumes various shapes in different teleosts and may be tubular, fusiform, oval, heart-shaped, dumb-bell shaped or horse-shoe shaped.
The air-bladder in higher teleosts or bony fishes is the most specialised, playing little or no part in respiration, and primarily serving as a hydrostatic organ. Two types of air-bladders are known. In the more generalised groups of teleosts, (salmon, eel) the air bladder retains connection with the gut via a pneumatic duct, just as in ganoids and dipnoi. Such an open air-bladder is called physostomous.
In the teleost Erythrinus the air-bladder has a lateral attachment to gut. In the more specialised teleosts (perch, cod) the duct becomes atrophied or lost. Such a closed or ductless air-bladder is called physoclistous.
Functions of Air-Bladder:
The air-bladder or swim-bladder in fishes performs a number of functions, the more important of which are discussed below:
1. Hydrostatic Organ:
The air-bladder serves as an important hydrostatic organ in fishes and helps to keep the weight of the body equal to the volume of the water the fish displaces. It also serves to equilibrate the body in relation to the surrounding medium by increasing or decreasing the volume of gas content. Secretion of more gases means lower specific gravity so that the fish rises in water. Resorption of gases means increased specific gravity and the fish sinks.
Thus, the fish is able to rise or sink and maintain its equilibrium or position in water without any muscular effort. In the physostomous fishes the expulsion of the gas from the air-bladder is caused by the pneumatic duct, but in the physoclistous fishes where the pneumatic duct is absent the superfluous gas is removed by diffusion.
2. Respiration:
The original function of the air-bladder was probably respiratory. The respiratory function of the air-bladder is quite significant. A cellular lung-like air-bladder is present in the teleostomes such as Polypterus, Amia and Lepidosteus in which it is primarily respiratory in function. In Dipnoi, the air-bladder resembles closely with an amphibian lung.
In certain teleosts as Megalops, Chirocentrus, Gymnarchus, Erythrinus, Umbra and some cyprinoids, the air-bladder works like a lung. These fishes live in swamps or pools where the carbon dioxide tension is high and that of oxygen is low.
These teleosts come to the surface, swallow air and pass it back to the air-bladder which is highly vascular. It appears that the lung-like function has been reacquired in these teleosts as a special adaptation for living in foul water.
Air-bladder can also be used for storing oxygen to be used in emergency. This has been demonstrated both in physoclistous and physostomous fishes as Perca fluviatilis. Tinea tinea, Opsanus tau, Cyprinous carpio, Carassius auratus, etc. Although it has been shown that in many species the amount of oxygen stored in the bladder would enable the fish to survive for a few minutes only, the air-bladder may be more important in deep water fishes where the amount of oxygen stored is much higher.
Studies have shown that in fishes living nearer the surface the gas in air-bladder is much like the air, while in those living at greater depths, oxygen forms a major component, extending up to 75% in some species.
3. Sound Production:
The air-bladder plays an important role in sound production. Some fishes are able to produce sounds with the gases inside air-bladder by the use of special muscles attached to the air-bladder. But the actual mechanism is not understood. Many fishes as Doras, Platystoma, Malapterurus, Trigla can produce grunting or hissing or drumming sound.
The circulation of air inside the air-bladder causes the vibrations of the incomplete septa, which in turn, produce sound. The sound may also be produced by compression of extrinsic and intrinsic musculature of air-bladder.
Polypterus, Protopterus and Lepidosiren can produce sound by compression and forceful expulsion of gases in the air-bladder. In Cynoseion male, the compression of the air-bladder is achieved by contraction of specialised muscles, musculus sonorificus. The sound production serves to startle the enemies or to attract mates.
4. Auditory Function:
Air-bladder serves to transmit sound waves to the ear especially in the Ostariophysi, more efficiently than in the species in which a connection with the ear is missing. In Cypriniformes, a series of small bones, the Weberian ossicles, connect the air-bladder and perilymph cavity containing internal ear. Low frequency vibrations of the contained gas, induced by noises in water, are transmitted by the ossicles to the membranous labyrinth. Thus, these fishes can hear.
5. Sensory Function:
When the fish is subjected to pressure changes by moving into different depths of water, compression of the wall of the air-bladder which functions as a pressure receptor like a manometer, barometer or a hydrophone. It has been suggested that the air-bladder ac mg as a sense organ enables a fish to maintain a steady depth.
If the fish moves above or below a certain depth at which it is in equilibrium, changes in the tension in the bladder wall lead to compensatory swimming movements and the fish returns to the original level. However, there is not much evidence in support of this hypothesis.
In Cypriniformes (Ostariophysi), the connection of the air-bladder (gas bladder) with the membranous labyrinth through the Weberian ossicles, increases the sensitivity of the fish to volume changes in the air-bladder and the fish is able to control the escape of gas via the pneumatic duct.