In this article we will discuss about Swim-Bladder:- 1. Introduction to Swim-Bladder 2. Development of Swim-Bladder 3. Basic Structure 4. Gas Composition 5. Types 6. Modifications 7. Shape and Size 8. Weberian Ossicles 9. Functions 10. Hydrostatic Organ 11. Adjustable Float 12. Maintains Proper Centre of Gravity 13. Respiration 14. Resonator.
- Introduction to Swim-Bladder
- Development of Swim-Bladder
- Basic Structure of Swim-Bladder
- Gas Composition of Swim-Bladder
- Types of Swim-Bladder
- Modifications in Swim-Bladder
- Shape and Size of Swim-Bladder
- Weberian Ossicles
- Functions of Swim-Bladder
- Hydrostatic Organ
- Swim-Bladder acts as Adjustable Float
- Maintains Proper Centre of Gravity
- Swim-Bladder helps in Respiration
- Swim-Bladder as Resonator
1. Introduction to Swim-Bladder:
In most of the fishes a characteristic saclike structure is present between the gut and the kidneys. This structure is called by various names, viz., swim-bladder, or gas-bladder, or air-bladder. In our present discussion, the name of the bladder is followed as the swim-bladder to avoid confusion.
The swim-bladder occupies- the same position as the lungs of higher vertebrates and is regarded as homologous to the lungs. It differs from the lungs of higher forms mainly in origin and blood supply.
The swim bladder arises from the dorsal wall of the gut and gets the blood supply usually from the dorsal aorta, while the vertebrate lung originates from the ventral wall of the pharynx and receives blood from the sixth aortic arch.
The swim-bladder is present in almost all the bony fishes and functions usually as a hydrostatic organ. Starting as a very insignificant cellular extension from the gut, the swim-bladder in fishes leads the whole group through an evolutionary channel.
2. Development of Swim-Bladder:
Opinions differ as regards the development of swim bladder in fishes. In teleosts, it originates as an unpaired dorsal or dorsolateral diverticulum of the oesophagus. It starts as a small pouch budded off from the oesophagus. The diverticulum with an opening in the oesophagus becomes subsequently divided into two halves.
Of these two, the left one often atrophies except in a few primitive forms. The right half becomes well-developed and take a median position. In dipnoans and Polypteridae, the swim-bladder is modified into the ‘lungs’ and originates as the down-growths from the floor of the pharynx.
These out-growths 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. Spengel advocates the view that the swim-bladder in fishes originates from the posterior pair of the gill-pouches, but definite embryological evidence in support of this idea is lacking.
3. Basic Structure of Swim-Bladder:
The swim-bladder in fishes varies greatly in structure, size and shape.
a. It is essentially a tough sac-like structure with an overlying capillary network.
b. Beneath the capillary system there is a connective tissue layer called tunica externa.
c. Below this layer lies the tunica interna consisting primarily of smooth muscle fibres and epithelial gas-gland.
d. The swim bladder lies below the kidneys, between the gonads and above the gut.
e. The connection with the oesophagus may be retained throughout life or may be lost in the adult.
4. Gas Composition of Swim-Bladder:
Biot (1807) and Morean (1876) have shown that the gas secreted by the swim-bladder is mostly oxygen. Nitrogen, and little quantity of carbon-dioxide are also present. Generally the gas composition varies in different species. In salmonids, the maximum amount of gas in the swim-bladder is Nitrogen. Again in many species the composition includes mostly a mixture of oxygen and carbon dioxide.
5. Types of Swim-Bladder:
Depending on the presence of the duct (ductus pneumaticus) between the swim-bladder and the oesophagus, the swim-bladder in fishes can be divided into two broad categories: Physostomous [Gk. physi = a bladder; stomata, mouth] and Physoclistous types [Gk. clistic = enclosed].
Depending on the condition of the swim-bladder, the teleosts are classified by older taxonomists into two groups Physostomi and Physoclisti. A transitional condition is observed in eels.
A. Physostomous Condition:
The swim-bladder develops from the oesophagus. When the ductus pneumaticus is present between the swim-bladder and the oesophagus, the swim-bladder is called physostomous type (Fig. 6.85A).
A vessel emerging from the coeliacomesenteric artery supplies the swim bladder and the blood from it is conveyed to the heart through a vein joining the hepatic portal vein. This condition is observed in bony ganoid fishes, the dipnoans and soft-rayed teleosts.
B. Physoclistous Condition:
In this condition the ductus pneumaticus is either closed or atrophied (Fig. 6.85B). This type of swim bladder is observed in spiny-rayed fishes. In this type of swim-bladder, there lies an anteroventral secretory gas gland (containing retia mirabilia) and a posterodorsal gas absorbing region called the oval. The oval develops out of the degenerating ductus pneumaticus.
The rete mirabilis of the gas gland, the oval and the walls of the bladder are supplied by the coeliacomesenteric artery and also by arteries from the dorsal aorta. But the blood from the different parts of the swim bladder is returned by two routes.
The blood from the gas gland is returned to the heart by the hepatic portal vein, while from the rest of the bladder by the posterior cardinal veins. The bladder, specially the gas gland, gets the lateral branches from the vagus, while the oval is innervated by sympathetic nerves.
C. Transitional Condition:
In Eel (Anguilla), a transitional condition between the physostomous and physoclistous type is present. The swim-bladder retains the ductus pneumaticus which becomes enlarged to form a separate chamber containing the oval (Fig. 6.85C). The gas glands are also present.
The swim-bladder is supplied with the blood through a branch from the coeliacomesenteric artery while the blood is returned to the heart by a vessel joining the post cardinal vein. The condition represents an intermediate stage when a physostomous condition is on the verge of transformation into the physoclistous state.
6. Modifications in Swim-Bladder:
In fishes a great diversity in size, shape and function of the swim-bladder is observed. In elasmobranchs, bottom dwelling and deep-sea teleosts the swim-bladder is absent in an adult but a transitory rudiment during development may be present.
In flat fishes (Pleuronectidae) swim-bladder is present in the early life when the animals maintain a vertical position. As they tip over one side and assume the lazy adulthood, the swim-bladder becomes atrophied.
In elasmobranchs, the swim-bladder is represented by the transitory rudiment in the embryonic stages. Miklucho-Maclay (1867) has observed a rudimentary dorsal diverticulum from the foregut in the embryos of Squalus, Mustelus ana Caleus. In many fishes, viz., Heptranchias, Scyllium, Squatina, Pristiurus, Carcharius and many Rays, small pits are recorded in the oesophageal wall.
Wassnezow (1932) has observed one to six similar oesophageal pits in Pristiurus, Torpedo and Trygon. These pits are located posterior to the fifth pouch. In sharks the swim-bladder is absent in adults, but a hint of a rudimentary swim-bladder is observed during embryonic development. But almost all the teleosts possess the swim-bladder and extreme modifications of the same are encountered because of adaptation to the different modes of living.
Modifications of Physostomous Condition:
The typical physostomous pattern becomes modified in different fishes and the basic trends are:
(1) The formation of paired sacs and
(2) The gradual acquisition of two chambers— an anterior and a posterior.
The swim-bladder in Polypterus (bichir) (Fig. 6.87A, B) represents the primitive condition. It is a bilobed sac with two unequally developed lobes. The left lobe is shorter and the right lobe is longer. The bilobed sac opens on the floor of the pharynx through a slit-like glottis. The glottis is provided with muscular sphincter. The internal lining of the bladder is smooth and partly ciliated.
The lack of alveolar sacculations and the presence of muscular walls are the two noted feature in the swim-bladder of Polypterus. The walls of the bladder are highly vascular and are lined by two layers of striated muscle fibres.
The bladder is supplied by a pair of pulmonary arteries arising from the last pair of pulmonary arteries arising from the last pair of epibranchial arteries and the corresponding veins enter into the hepatic vein below the sinus venosus.
In the dipnoans, the swim-bladder is called the lung and the inner walls are produced into numerous alveoli. The swim-bladder resembles the tetrapod lungs both structurally as well as functionally. In Neoceratodus it is single- lobed, while in Protopterus and Lepidosiren it is bilobed (Fig. 6.87C, D, E).
Other details regarding the structural construction, blood’ and nerve supplies have already been dealt in the biology of the lung-fishes.
In Sturgeons (Acipenser), the swim-bladder is short and oval in shape. The ductus pneumaticus enters the bladder ventrally and it opens into the gut posterior to the pharynx. The glottis is lacking and the opening into the oesophagus is closed by the simple constriction of the ductus pneumaticus.
The walls of the bladder are fibrous and thick but the inner walls are smooth (Fig. 6.87H). In Acipenser, both the left and right lobes develop from the dorsal side of the oesophagus in the embryonic stage, but the left one becomes completely obliterated and right one gives rise to the adult swim-bladder.
In Amia and Lepisosteus, the swim bladder is an unpaired sac extending nearly the entire length of the body cavity. In both the cases rudiment of the left lobe appears during development but persists only for a short time. The ductus pneumatics opens into the oesophagus posterior to the pharynx through a dorsal slit-like glottis.
The walls are highly vascular and exhibit sacculations resembling the pulmonary alveoli (Fig. 6.87G). The sacculations or the respiratory pouches are arranged in two lateral rows. As regards the development of sacculations the swim-bladder of Lepisosteus is more advanced than that of Amia. There are some more minor differences regarding the supply of blood.
The swim-bladder in Amia gets arterial blood from the pulmonary arteries, while that of Lepisosteus gets arterial branches from the dorsal aorta. The blood from the bladder is returned by the left ductus Cuvieri in Amia and by the right post-cardinal in Lepisosteus.
Gymnarchus presents an intermediate stage where the efferent branchial arteries from the third and fourth gill-arches join to form a common root for the emergence of the pulmonary and coeliacomesenteric arteries (Fig. 6.87F). Amongst the dipnoans, the swim-bladder of Neocertatodus resembles that of Lepisosteus. The walls are sacculated and act as the lung’.
In Clupea harengus, the ductus pneumaticus opens into “the fundus of the stomach and there is a second duct from the posterior part of the swim-bladder opening to the exterior near the anus (Fig. 6.87 I). Similar posterior opening is present in Pellona, Caranx, Sardinella.
Modifications of Physoclistous Condition:
The swim-bladder in all teleosts begins as a physostomous type but in an adult condition the ductus pneumaticus gets degenerated to become a physoclistous type. A typical physoclistous swim-bladder consists of a closed sac having two compartments—an anterior and a posterior. These two compartments are intercommunicated through an aperture called ductus communicans.
The opening and closure of this aperture is regulated by circular and radiating muscles which act as the sphicter. The anterior chamber is formed by circular and radiating muscles which act as the sphincter. The anterior chamber is formed by the enlargement and forward growth of the budding swim-bladder, while the posterior chamber develops as an enlargement of the ductus pneumaticus.
This typical structural plan is modified in certain forms. The posterior chamber with retia Mirabella becomes flattened almost to the point of obliteration and is designated ‘oval’ as seen in the families like Myctophidae, Percidae, Mugilidae.
The oval is a thin-walled highly muscular area specialised for the reabsorption of gases (see Fig. 6.86D). The opening of the oval is guarded by circular and longitudinal muscles. This device is of great significance for the fishes undergoing rapid vertical movements.
The morphological modifications of the swim-bladder are accompanied by histological modifications in different fishes, the swim-bladder acts as a hydrostatic organ. It helps fishes to sink or ascend to various depths by altering the gas content in the bladder. In fishes having open ductus pneumaticus, the volume of gas content in the bladder can be changed by swallowing or removing air from the bladder.
But in some physostomous and all physoclistous fishes this process of gas transference is done directly from the blood stream. Inside the bladder there is an oxygen-producing device and an oxygen-absorbing device. The swim bladder is a vascular structure but the degree of vascularization varies in different teleosts.
In some species of the families Clupeidae and Salmonidae the capillaries are uniformly present all over the swim-bladder, but in most cases these highly vascular interlacing and tightly packed capillaries form a mass called rete mirabilis. The anterior chamber of swim bladder shows the tendency to become differentiated into oxygen-producing area called red body.
The oxygen is produced by the reduction of the oxyhaemoglobin in the erythrocytes when brought into close contact with the secreting epithelial cells of the gas gland. The red body consists of internal oxygen-secreting cells (gas gland) and supplied by the blood vessels from the retia Mirabella (sing, rete mirabilis).
It forms a complicated structure where the arterial and venous capillaries communicate only after reaching the gas gland. The most primitive condition is observed in Pickerel where the gland is covered by thick glandular epithelium which is thrown into a number of folds. In eels and some other fishes, the red bodies are non- glandular in nature but serve the same physiological function.
The red gland is supplied with blood from the coeliac artery and is returned to the portal vein. The activity of the red gland is controlled by the vagus nerve. In the fishes with functional ductus pneumaticus the gas glands are absent but in eels this function is taken up by the red gland.
In the physoclistous fishes, the anterior region is modified for gas production and the posterior region or chamber is specialised for the absorption of gas into the blood. The posterior chamber becomes excessively thin- walled to facilitate gas diffusion.
Beneath the walls, the gas is absorbed directly into the blood. The formation of the oval in some fishes, is a special development for the absorption of gas. The wall of the oval is very thin and highly vascular. Through this epithelial lining oxygen can easily pass to the network of vessels. This gas absorbing region receives blood supply from the dorsal aorta and the blood is returned to the post cardinal vein. The activities are governed by the sympathetic nerves.
The histological differentiation for the gas production and gas absorption is a very significant achievement in fishes. The gas produced by the red body is mostly oxygen and this oxygen is readily absorbed or diffused from the swim-bladder directly into the capillaries. The oval is modified for gas absorption in many fishes.
By the alternate process of gas production and gas absorption, the internal pressure and volume of the gas content inside the swim-bladder can be increased or decreased. The red body is usually confined to the anterior chamber, but in fishes where the anterior chamber becomes secondarily associated with the auditory function, the gas gland may be confined to the posterior chamber.
7. Shape and Size of Swim-Bladder:
The swim-bladder varies extensively in shape and size. In Umbrina (Fig. 6.88A), it is oval shaped and without any appendage. In Atractoscion (Fig. 6.88B), it gives off only one pair of simple diverticula that extends from the anterior side. In Kathala (Fig. 6.88C), the swim-bladder develops a pair of appendage extending in front of transverse septum into head.
In some forms it gives off many branched diverticula. In many fishes, the anterior prolongations of the swim-bladder come into close contact with the wall of the space containing the internal ear. In Clupea, the narrow anterior end of the swim-bladder enters into a canal in the basioccipital of the skull and divides into two slender branches.
The anterior end of each branch dilates to form a round swelling and lies in close contact with the internal ear. A more or less similar condition is observed in Tenualosa ilisha. In many fishes finger-like diverticula develop from the swim-bladder.
In Gadus, a pair of diverticula originating from the anterior part of the bladder project into the head region. In Otolithus, each anterolateral end of the swim-bladder gives rise to an outgrowth which sends one anterior and a posterior horn.
In Otolithoides (Fig. 6.88D), the appendages attached to posterior end of bladder and at least the main part lying parallel to the bladder. In Corvina lobata, many such branched diverticula develop from the lateral walls of the swim-bladder. In Johnius (Fig. 6.88E), it is hammer-shaped with 12 to 15 pairs arborescent appendages, the first branching in the head and the posterior tip are highly pointed.
Usually in most cases, the swim-bladder is divided transversely into an anterior and a posterior chamber as seen in cyprinoids (Fig. 6.87K), Esox (Fig. 6.87J), Catostomus, Pangassius, Corvina, etc. But the longitudinal division of the swim-bladder is rare.
In Arius the swim-bladder is splitted longitudinally. In Notopterus, a longitudinal septum divides the swim-bladder into two lateral chambers. Due to the presence of septum or septa, the internal cavity of the swim-bladder is either completely or partially divided.
8. Weberian Ossicles:
The perilymphatic sac and the anterior end of the swim-bladder are connected by a series of four ossicles (Fig. 6.89), which are articulated as a conducting chain.
Of the four, the tripus, intercalarium and scaphium actually form the chain, while the fourth one, claustrum lies dorsal to the scaphium and lies in the wall of posterior prolongation of the perilymphatic sac. The function of these ossicles is controversial.
It is regarded that the Weberian ossicles either help to intensify sound vibrations and convey these waves to the internal ear of help to understand the state of tension of air pressure in the bladder and transit changes of such pressure to the perilymph to set up a reflex action. There are various views regarding the actual process of derivation of these ossicles.
De Beer (1937) and Watson (1939) regarded that these are detached or modified processes of the first three anterior vertebrae. As regards the actual mode of origin of the four ossicles there are differences of opinion.
The claustrum is regarded to be modified interspinous ossicle or modified spine of first vertebra or modified neural arch of first vertebra or modified intercalated cartilage or modified neural process of first cartilage.
The scaphium is considered to be the modified neural arch of the first vertebra or modified rib of the first vertebra or derived from the neural arch of the first vertebra and also from the mesenchyme.
The intercalarium is derived from the neural arch and transverse process of the second vertebra or from the neural arch of the second vertebra and also from the ossified ligament or from the neural arch of the second vertebra only.
The tripus is formed from the rib of the third vertebra and the ossified ligament or from the transverse process of the third vertebra along with ossified wall of the swim-bladder or from the transverse process of the third vertebra and the ribs of third and fourth vertebrae.
9. Functions of Swim-Bladder:
The swim-bladder in fishes performs a variety of functions.
10. Hydrostatic Organ:
It is primarily a hydrostatic organ 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.
In the physostomous fishes the expulsion of the gas from the swim-bladder is caused by way of the ductus pneumaticus, but in the physoclistous fishes where the ductus pneumaticus is absent the superfluous gas is removed by diffusion.
11. Swim-Bladder acts as Adjustable Float:
The swim-bladder also acts as an adjustable float to enable the fishes to swim at any depth with the least effort. When a fish likes to sink, the specific gravity of the body is increased. When it ascends the swim-bladder is distended and the specific gravity is diminished. By such adjustment, a fish can maintain equilibrium at any level.
12. Swim-Bladder Maintains Proper Centre of Gravity:
The swim bladder helps to maintain the proper centre of gravity by shifting the contained gas from one part of it to the other and this facilitates in exhibiting a variety of movement.
13. Swim-Bladder helps in Respiration:
The respiratory function of the swim-bladder is quite significant. In many fishes living in water in which oxygen content is considerably low, the oxygen produced in the bladder may serve as a source of oxygen. In a few fishes, specially in the dipnoans, the swim bladder becomes modified into the ‘lung’. The ‘lung’ is capable of taking atmospheric air.
14. Swim-Bladder as Resonator:
The swim-bladder is regarded to act as a resonator. It intensifies the vibrations of sound and transmits these to the ear through the Weberian ossicles.
Production of sound:
The swim-bladder helps in the production of sound. Many fishes, Doras, Platystoma, Malapterurus, Trigla can produce grunting or hissing or drumming sound. The circulation of the contained air inside the swim-bladder causes the vibration of the incomplete septa.
The sound is produced as the consequence of vibration of the incomplete septa present on the inner wall of the swim-bladder. The vibrations are caused by the movement of the contained air of the swim-bladder.
Sound may also be produced by the compression of the extrinsic and intrinsic musculature of the swim-bladder. Polypterus, Protopterus and Lepidosiren can produce sound by compression and forceful expulsion of the contained gas in the swimbladder. In Cynoscion male, the musculus sonorificus probably helps in compression.