Generally adult fishes depended chiefly on pharyngeal gills for aquatic respiration. However, other devices also occur to supplement or replace gill respiration. All such additional respiratory organs, other than gills, are known as accessory respiratory organs. The development of accessory respiratory organs is found mostly in freshwater fishes of tropical region and very rarely in marine fishes.

Sometimes the fishes of the tropical freshwaters and hill-streams develop accessory respiratory organs to meet extra demand for oxygen, because depletion of oxygen occurs during summers as the water level falls to a considerable degree. Accessory respiratory organs enable the fishes to live in oxygen-deficient water, to aestivate over prolonged droughts in dry summer, to take excursions on land or simply to meet extra demand for oxygen.

To overcome these adverse situations, accessory respiratory organs functionable in aquatic and/or aerial environment have been developed in fishes. So the development of such structures is essentially adaptive in native. Some accessory organs sub serve aquatic respiration, while others aerial respiration (Fig. 17.8).

Accessory Respiratory Organs

Several types of accessory respiratory organs have been evolved in different species of fishes. 


These accessory respiratory organs of fishes are as follows:

1. Skin or Integument:

In the eel, Anguilla anguilla, Amphipnous cuchia and in Periophthalmus and Boleophthalmus, the skin is highly vascular and serves for exchange of gases as in frog, when the fish is out of water. These fishes habitually leave the water and migrate from one place to another through damp vegetation. During this period, the moist skin serves as an important organ in respiration. They can respire cutaneously both in air and in water.


Since Amphipnous and Mastacembelus live in oxygen deficient stagnant water, the skin is of little use for respiration but it plays an important role in extracting oxygen from air, when the fishes are exposed in drying up muddy ponds, or when fish is moving out of water. The glandular secretions of the skin protect it from desiccation in the air.

Median fin folds of many fishes are supplied with numerous blood vessels that help in cutaneous respiration. Besides, the highly vascular opercular folds of Sturgeons and many cat fishes serve as accessory respiratory structures.

2. Bucco-Pharyngeal Epithelium:

In most of the fishes, the epithelial lining of buccal cavity and pharynx is usually highly vascular and permeable to gases in water. It may remain simple or may develop folds, pleats or tongues projecting into the buccal cavity and pharynx to make it an efficient respiratory organ.


But in mudskippers (Periophthalmus and Boleophthalmus) the highly vascularised buccopharyngeal epithelium helps in absorbing oxygen directly from the atmosphere. These tropical fishes leave water and spend most of the time skipping or walking about through dampy areas particularly round the roots of the mangroove trees. The old idea that the mudskippers use the vascular tail as the respiratory organ is not supported by recent ichthyologists.

3. Gut Epithelium:

In several fishes epithelial lining of certain parts of alimentary canal becomes vascular and modified to serve as a respiratory organ. It may be just behind stomach (Misgurus fossilis) or intestine (Lepidocephalus guntea, Gobitus (giant loach of Europe) or rectum (Callichthyes, Hypostomus and Doras).

Fresh air is drawn through mouth or anus and after gaseous exchange the gas is voided through the anus. In these fishes the wall of the gut is modified to perform the respiratory function. The walls of the gut in these areas become thin due to the reduction of muscular layers.

4. Outgrowths of Pelvic Fins:

In American lung fish, Lepidosiren, during breeding time, the pelvic fins of male become enlarged and grow filamentous vascular outgrowths which provide fresh oxygen to the guarded eggs.

5. Pharyngeal Diverticula:

Pharyngeal diverticula are a pair of simple sac-like outgrowths of pharynx, lined by thickened vascular epithelium and extending above the gills. In Channa (= Ophiocephalus), the accessory respiratory organs are relatively simpler and consist of a pair of air-chambers (Fig. 17.8).

These are developed from the pharynx and not from the branchial chambers as seen in others. The air-chambers are lined by thickened epithelium which is highly vascularised. The air-chambers are simple sac-like structures and do not contain any structure. These chambers function as the lung-like reservoirs. In Channa striatus, the vascular epithelium lining the chambers becomes folded to form some alveoli. The gill-filaments are greatly reduced in size.


In cuchia eel, Amphipnous cuchia, the accessory respiratory organs consist of a pair of vascular sac-like diverticula from the pharynx above the gills (Fig. 17.8). These diverticula open anteriorly into first gill-slit.

These diverticula function physiologically as the lungs. The gills are greatly reduced and a few rudimentary gill-filaments are present on the second of the three remaining gill-arches. The third gill-arch is found to bear fleshy vascular (respiratory) epithelium.

In Periophthalmus also, a small, shallow pharyngeal diverticulum lined with respiratory epithelium (vascular epithelium) is present on each side of the roof of the pharynx.

6. Opercular Chamber Modified for Aerial Respiration:

In some species, the inhaled air is passed through the gill-slits into the opercular chamber where it is stored for some time. The opercular chamber becomes bulged out in the form of two little balloons in the hinder region of the head and after sometimes its walls collapse and the air is passed out through the small external branchial opening. The membrane lining the opercular chamber becomes thin and highly vascular to allow exchange of gases. This is seen in Periophthalmus and Boleophthalmus.

7. Branchial Diverticula:

The outgrowths from gill-chambers form more complicated aerial accessory respiratory organs than the simpler pharyngeal outgrowths in other fishes. Such air breathing organs are present in Heteropneustes, Clarias, Anabas, Trichogaster, Macropodus, Betta, etc.

Important modifications in some of these species are described below:

(a) Heteropneustes Fossilis (= Saccobranchus):

This Indian catfish has a pair of long, tubular and dorsally situated air-sacs, arising posteriorly from gill-chambers and extending almost up to the tail. They are highly vascular. The air is drawn in and expelled out through pharynx.

(b) Anabas Testudineus:

The Indian climbing perch has two, spacious, suprabranchial cavities as dorsal outgrowths of the two gill-chambers. Each cavity contains a special labyrinthine organ formed of much folded, concentric bony plates developed from the first epibranchial bone and covered with thin vascular mucous membrane. Margins of these plates are wavy and the plates are covered with vascular gill-like epithelium.

Each branchial outgrowth communicates freely not only with the opercular cavity but also with buccopharyngeal cavity. Air is drawn through mouth into suprabranchial cavities and expelled through opercular opening. The fish is so dependent on atmospheric oxygen that it will drown if denied access to surface to gulp air.

(c) Trichogaster Fasciatus:

The accessory respiratory organs in this species consist of a suprabranchial chamber, a labyrinthine organ and the respiratory membrane. The suprabranchial chamber is situated above the gills on either side as in Anabas, communicates with the pharynx by means of inhalent aperture and with the exterior through the opercular chamber by means of an exhalent aperture.

The labyrinthine organ develops from the epibranchial of the first gill-arch and is simpler in structure than that of Anabas. It is in the form of a spiral organ possessing two leaf-like expansions and is composed of loose connective tissue covered by a vascular epithelium.

The respiratory membrane lining the air-chamber and covering the labyrinthine organs consist of vascular and non-vascular areas, of which the former possesses a large number of ‘islets’ containing parallel blood capillaries. The islets are believed to be derived from the secondary lamellae of a typical gill-filament.

(d) Clarias Batrachus:

The Indian cat fish, Clarias batrachus has the most complicated accessory respiratory organs.

The accessory air-breathing organs of this fish consist of:

(i) The suprabranchial cavity or chamber,

(ii) The two beautiful ‘rosettes’ or air-trees or arborescent organs or dendritic organs,

(iii) The ‘fans’ and

(iv) The respiratory membrane.

The suprabranchial chamber lies above the gills and is divided into two cup-like compartments and is lined by a highly vascular respiratory membrane.

Two beautiful ‘rosettes’ or dendritic organs are present on each side and are supported by epibranchials of the second and the fourth branchial arches. The first of these is smaller in size and lies in the anterior compartment. Each is a highly branched tree-like structure supported by cartilaginous internal skeleton. The terminal knobs or bulbs of each dendritic organ consist of a core of cartilage covered by vascular epithelium showing eight folds in it.

According to Datta Munshi, each knob represents eight abbreviated and fused gill-filaments. Some of the primary gill-lamellae of each gill-arch are fused so as to form a ‘fan’ or gill-plate. Hence, there are ‘four ‘fans’ on either side and each consists of vascular and non- vascular areas. The respiratory membrane lining the suprabranchial chamber is also composed of vascular and non-vascular areas, of which the former show a large number of ‘islets’.

According to Datta Munshi (1961), the respiratory membrane has developed by the abbreviation and fusion of the primary gill-lamellae and shortening of secondary gill-lamellae. Well-developed exhalent and inhalent apertures are present for the suprabranchial chamber. The fish rises to the surface of water and gulps in air, which from the pharynx enters into the suprabranchial chamber through the inhalent aperture.

When the air enters the opercular cavity, it is directed into the suprabranchial chamber by the action of the two fans on the second and third gill-arches. The exhalation of the air is effected by the contraction of suprabranchial chamber and the movement of the fans. This creates a partial vacuum in the suprabranchial chamber. The mouth is opened and the buccopharyngeal cavity is enlarged to inhale air.

8. Air-Bladders:

Swim-bladder of higher bony fishes (teleosts) is essentially a hydrostatic organ. But in lower bony fishes (dipnoans and ganoids), the air-bladder acts like a lung to breathe air and is truly an accessory respiratory organ. The wall of bladder is vascular and sacculated with alveoli. In Amia and Lepidosteus, the wall of the swim-bladder is sacculated and resembles lung.

In Polypterus, the swim-bladder is more lung-like and gets a pair of pulmonary arteries arising from the last pair of epibranchial arteries. The swim-bladder in dipnoans resembles strikingly the tetrapod lung in structure as well as in function. In Neoceratodus, it is single but in Protopterus and Lepidosiren it is bilobed.

The inner surface of the ‘lung’ is increased by spongy alveolar structures. In these fishes, the lung is mainly respiratory in function during aestivation because the gills become useless during this period.

Like that of Polypterus, the ‘lung’ in dipnoans gets the pulmonary arteries from the last epibranchial arteries. The swim-bladder of feather tail, Notopterus notopterus has a wide pneumatic duct and a network of blood capillaries covered by a thin epithelium in its wall. This helps in exchange of gases.

Functions of Accessory Respiratory Organs:

The accessory respiratory organs contain a higher percentage of oxygen. The fishes possessing such respiratory organs are capable of living in water where oxygen concentration is very low. Under this condition these fishes come to the surface of water to gulp in air for transmission to the accessory respiratory organs. If these fishes are prevented from coming to the surface, they will die due to asphyxiation for want of oxygen. So the acquisition of accessory respiratory organs in fishes is an adaptive feature.

Further it has been observed that the rate of absorption of oxygen in such organs is much higher than the rate of elimination of carbon dioxide. Hence, it is natural that the gills excrete most of the carbon dioxide. Absorption of oxygen appears to be the primary function of the accessory respiratory organs.

Origin and Significance of Accessory Respiratory Organs:

During development, the fifth gill-arch does not develop gill-lamellae, and its embryonic gill material forms rudiments of the gill-arch, and aggregates to form a structure called the ‘gill-mass’. The air-breathing organs or accessory respiratory organs develop from gill-mass. In some species, the gill- arches other than the fifth arch, also take part in the formation of accessory respiratory organs.

The gill-lamellae which normally develop on gill-arches for aquatic respiration, become modified to form the respiratory epithelium of the suprabranchial chamber, dendritic organs and the air- sacs for aerial respiration. According to Singh (1993), the air-sacs have evolved from the same basic material which has given rise to the gill in teleosteam fishes.

The accessory respiratory organs of Heteropneustes and Clarias, are modifications of the gills. In these species, swim-bladder is either absent or greatly reduced. During the Tertiary and Quaternary period of the Cenozoic era, the oxygen level of the atmosphere and of water was reduced. Due to depletion of oxygen in rivers and swamps, the gills were unable to cope with the requirements of the body. Hence, several teleostean species developed accessory respiratory organs to absorb oxygen from air.

Most of the fishes possessing air-breathing organs or accessory respiratory organs are capable of living in highly deoxygenated water of the swamps and muddy ponds infested with weeds. They have been observed to gulp in air from the surface and to pass it to the accessory respiratory organs. If prevented from reaching the surface, these fishes die due to asphyxiation.

This shows that the accessory respiratory organs are capable of maintaining life of the fish in oxygen deficient water. It has been shown that the absorption of oxygen in the accessory respiratory organs is much greater than the excretion of carbon dioxide. Hence, most of the carbon dioxide is excreted in the gills and the chief function of the accessory respiratory organs is the absorption of oxygen required for sustenance of life.

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