In this article we will discuss about:- 1. Types of Accessory Respiratory Organs 2. Functions of Accessory Respiratory Organs 3. Significance.
Types of Accessory Respiratory Organs:
1. Supra-branchial organ:
The supra-branchial organ is a specialised type of respiratory structure encountered in Clarias (Fig. 2.36A).
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It has a complex structural organisation and consists of:
(i) The supra branchial chamber,
(ii) Two rosettes or air trees,
(iii) The fans and
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(iv) The respiratory membrane.
The supra-branchial chamber lies above the gills and is divided into two cup-like compartments, and is lined by a highly vascular respiratory membrane. Two elaborate tree-like structures grow from the upper end of the second and fourth gill-arches of either side.
These dendritic organ or rosettes are composed of numerous terminal knobs; each has a core of cartilage covered by vascular membrane. Each exhibits eight folds which suggest that one such knob is formed by the coalescence of eight gill-filaments.
The supra-branchial chamber has inhalant and exhalant apertures. The entrance of the supra-branchial chamber is guarded by ‘fan’-like structures which are developed by the fusion of the adjacent gill-filaments of the dorsal side of the gill-arches.
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The supra-branchial organs, like the gills, are lined by a thin outer epithelial layer with intercellular spaces separated by the pilaster cells. The organs and the supra-branchial chambers are supplied by afferent and efferent blood vessels from the gill-arches.
The supra-branchial organs help to breathe in air. These fishes come to the surface of the water and gulp air into the supra-branchial organs. Atmospheric air from the pharyngeal cavity is taken into the supra-branchial chamber by an inhalant aperture located between the second and third gill-arches.
After gaseous exchange the air from the said chamber is expelled into the opercular cavity by the gill-slit lying between the third and fourth gill-arches. The fan-like structures present in the second and third gill-arches help to intake the air while the expulsion of the air from the supra-branchial chamber is caused by the contraction of its wall.
2. Branchial Outgrowths:
In climbing perch (Anabas testudineus) there are two spacious sac-like outgrowths from the dorsal side of the branchial chambers (Fig. 2.36B). The epithelium lining of these outgrowths is highly vascular and becomes folded to increase the respiratory area. Each chamber contains a characteristic rosette-like labyrinthine organ. This organ develops from the first epibranchial bone and consists of three concentrically arranged bony plates.
The margins of the plates are wavy and the plates are covered with vascular gill-like epithelium. Each branchial out-growth communicates freely not only with the opercular cavity but also with the buccopharyngeal cavity. Air enters into the out-growth by way of the buccopharyngeal opening and goes out through the external gill-slits. The entrance is controlled by valves.
Anabas can breathe in air by the help of these organs. These fishes have the habit of migration from one pond to the other. Their overland progression is peculiar and is assisted by the operculum and the fins. In Trichogaster fasciatus the accessory respiratory organs are similar to that of Anabas and consist of supra-branchial chamber, labyrinthine organ and respiratory membrane (Fig. 2.36F).
The labyrinthine organ is simpler in construction in comparison to that of Anabas. Each organ assumes a spiral configuration with two leaf-like expansions. Each of these two expansions is composed of loose connective tissue which is covered by highly vascular epithelium.
3. Pharyngeal Diverticula:
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In the Snake- headed fishes and Cuchia eels, the accessory respiratory organs are relatively simplified. These fishes can survive prolong drought and their air breathing habit enables them to remain out of water for some time. In both the group of fishes, from the pharynx evaginate a pair of saclike diverticula for gaseous exchange.
In Channa, the accessory respiratory organs are relatively simpler and consist of a pair of air-chambers (Fig. 2.36D). These supra branchial cavities are developed from the roof of the bucco-pharynx and not from the branchial chamber as seen in others.
The air-chambers are lined by thickened epithelium which is highly vascularized. 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 the cuchia eel (Amphipnus cuchia) the gills are reduced and gill-lamellae are present only on the second gill-arch, while the third arch bears a fleshy vascular membrane (Fig. 2.36E). The air breathing organs are in the form of a pair of sacs situated on the lateral sides of the head.
These are diverticulae of the pharynx and are lined with vascular epithelium thrown into folds or ridges. Each sac is provided with an inhalant and exhalant aperture. The respiratory epithelium of the air sac consists of vascular areas in the form of islets bearing numerous rosettes. The isletes are covered by a thin epithelium with numerous capillaries ending in it.
Contraction of bucco-pharyngeal muscles closes the mouth and the air is expelled from the air sac into the opercular chamber through the exhalant aperture and goes out through external branchial opening. Relaxation of the muscles opened the month and air rushes in to fill the vacuum in the air sacs.
In Periophthalmus, a pair of very small pharyngeal diverticula is present which are lined by vascular epithelium.
4. Pneumatic Sacs:
In Heteropneustes fossilis, pneumatic sacs are present as accessory respiratory organ along with others (Fig. 2.36C).
The accessory respiratory organs include:
(i) Fans or the expanded gill plates,
(ii) The air or pneumatic sac and
(iii) The respiratory membrane.
In this fish, four pairs of gills with reduced gill lamellae are present.
Four pairs of ‘fans’ develop on these gill-arches. The inner gill-filaments of the first gill-arch joined laterally to form first fan. The second fan is umbrella-like and well developed. The third fan is formed by the fusion of outer filament of the third gill-arch. Similarly, fourth fan is formed.
In addition to these fans, a pair of simple sac-like structures extend posteriorly from the supra-branchial chamber, up to the middle of the body. These air sacs or pneumatic sacs are thin walled, tubular structure. Their wall is highly vascular. The air sacs receive blood from the fourth branchial vessels of its own side.
The respiratory membrane lining the air sac is thrown into folds and ridges. These ridges are composed of both vascular and non-vascular areas.
The air enters the supra branchial chamber through the inhalant aperture into the air-sac. The inhalant aperture is guarded by the second and third fans. In Sacco-branchus, similar tubular lung-like outgrowths of the branchial chamber extend back into the body musculature.
5. Buccopharyngeal epithelium:
The vascular membrane of buccopharyngeal region in almost all the fishes helps in absorbing oxygen from water. But in mudskippers Periophthalmus, Boleophthalmus, Monopterus and Electrophorus, folds, pleats or tongues develop projecting into the buccal cavity and pharynx to increase its efficiency.
The highly vascularized 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 mangrove trees.
6. Integument:
Eels are recorded to make considerable journey through damp vegetation. The common eel, Anguilla Anguilla, can respire through the integument both in air and in water. In Amphipnus cuchia and mudskippers, the moist skin sub-serves respiration.
Many embryos and larvae of fishes respire through the skin before the emergence of the gills. The median fin-fold of many larval fishes is supplied with numerous blood vessels and helps in breathing. The highly vascular opercular fold of Sturgeon and many cat-fishes serves as the accessory respiratory structure.
7. Gut Epithelium:
The inner epithelium of the gut essentially helps in digestive process. But in many fishes the gut becomes modified to sub serve respiratory function. Cobitis (giant loach of Europe) comes above the water-level and swallows a certain volume of air which passes back along the stomach and intestine. In Misgurnus fossilis, a bulge just behind the stomach is produced which is lined by fine blood vessels.
The bulge acts as the reservoir of air and functions as the accessory respiratory organ. After the gaseous exchange, the gas is voided through the anus. In certain other fishes, Callichthyes, Hypostomus and Doras the highly vascular rectum acts as the respiratory organ by sucking in and giving out water through the anus alternately. In these fishes the wall of the gut becomes modified. The wall becomes thin due to the reduction of the muscular layers.
8. Swim-bladder acts as lung:
Swim-bladder is essentially a hydrostatic organ but in some fishes it functions as the ‘lung’. In Amia and Lepisosteus, 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.
Functions of Accessory Respiratory Organs:
The accessory respiratory organs contain a high 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.
Significance of Accessory Respiratory Organs:
The cause of emergence of the accessory respiratory structures in fishes in addition to the primary respiratory organ is very difficult to interprete. There are two contrasting views regarding the origin of the aerial accessory respiratory structures.
First view emphasises that some fishes have the natural instinct to make short excursion to the land from the primary aquatic home. To remain out of water, the developments of certain devices to breathe in air become necessary.
Second view holds that the fishes are forced to ascend to the land when the oxygen content of water falls to a considerable extent. The fishes in that particular condition of life gulp in atmospheric air from the land and pass it into the accessory respiratory structures. If they are prevented by mechanical barriers to come to surface, the fishes will die of suffocation.
This habit of swallowing bubbles of air is observed in many bony fishes, especially those living in shallow water which dries up periodically or becomes foul by the decomposition of aquatic vegetation.
As a consequence of the air-breathing habit for a considerable span of time, the fishes have developed specialised accessory respiratory organs in addition to the gills. Most of such structures encountered in the fishes assume the shape of reservoir of air and originate either from the pharyngeal or branchial cavities.
In extreme cases, the reservoir may house special structure for gaseous exchange. However, the development of such accessory respiratory organs is essentially adaptive in nature to meet the respiratory need and thus enables the fishes to tolerate oxygen depletion in water or to live on land over a varying period of time. The development of the accessory respiratory organs depends directly on the ability to remain out of the water.