Lecture notes on extrabranchial respiration in fishes

Fishes are aquatic, gill breathing vertebrates. They possess gills for branchial mode of respiration. However, in many fishes, either due to poorly developed branchial respiration or due to environmental pressures, additional respiratory structures have been developed. Such extrabranchial organs that supplement gills in respiration are collectively referred to as accessory respiratory organs. 

Several reasons have been suggested for the existence of accessory respiratory organs:

1. To meet the environmental oxygen deficiency.

2. To compensate the degenerate gills.

3. To maintain life during aestivation (summer sleep).

4. To counter balance the intemal oxygendeficiency.

5. Air breathing may be obligatory (habtual) in some fishes.

These take different form and structure that suit the habit and habitat of the fish. Some accessory respiratory organs commonly found in fishes are skin, buccopharyngeal epithelium, gut epithelium, pelvic fins, pharyngeal diverticula, branchial diverticula and air bladder (swim bladder).

1.Skin:The skin of eels and many other fishes is richly supplied with blood vessels and serves as accessory respiratory organ. lt performs the respiratory function both in water and on land. Anguilla anguilla and Amphiopnous cuchia (eels) often move on land through wet vegetation. This feature is especially useful when the eels undertake extensive migratory journeys. In Periophtha/mus (mud skipper) too skin functions as an accessory respiratory organ.

2.Buccopharyngeal Epithelium:The buccal cavity and pharynx of Periophthalmus and Symbranchus are lined with highly vascular epithelium. Fresh air is gulped into these cavities and exchange of respiratory gases occurs through the epithelium.

3.Gut Epithelium: In a few fishes, certain regions of the alimentary canal are highly vascular and become modified for aerial respiration. ln Misgurus fossilis, for example, the region behind stomach serves as an accessory respiratory organ. The intestine of Lepidosiren and the rectum of Callichthyes also function as accessory respiratory organs. In these cases, fresh air enters the gut either through mouth or anus. The expiratory air goes out through anus.

3.Pelvic Fins:In the American lung fish Lepidosiren, during breeding season, the pelvic fins of the male fish enlarge in size, become highly vascular and form filamentous outgrowths. These fin; besides serving as accessory respiratory organs supply oxygen to the eggs guarded by them.

4.Pharyngeal Diverticula:The pharynx of the fishes Periophthalmus, Amphiopnous , Channa (Ophiocephalus), etc. forms sac-like outgrowth called diverticulum, which is lined by vascular epithelium. Sometimes the sac extends above the gill pouch.Air is drawn into the diverticulum, and oxygen and carbon dioxide are exchanged.Amphiopnous possesses a small and smooth diverticulum,which opens through midventral gill slits . The diverticulum of Channa is folded . In all these cases, the diverticula function as accessory respiratory organs during aestivation or when the oxygen content of the water is too low.

5.Branchial Diverticula : In many fishes the outgrowths or diverticula formed from gill  (branchial) chambers contain complex accessory respiratory organs for  aerial respiration. These include tubular diverticula, labyrinthine organs  and dendriti¢ er-arborescent organs. 

a.Tubular Diverticula: in Heteropneustes (Sacchobranchus) a pair of  long tubular air sacs arises from the gill chamber one on either side and  extends up to the tail (Fig. 5.14). Exchange of respiratory gases takes place  through the highly vascular wall of the diverticulum.

b. Labyrinthine organs: Anabas (lnclian climbing perch) often comes  to water gurface and gulp down fresh air for aerial respiration. lt has twospacious suprabranchial cavities as dorsal outgrowths of the gill chambers. Each of these cavities contains an accessory respiratory organ called labyrinthine organ . It is formed by much folded concentric bony plates, which develop from the first epibranchial bone. This organ is covered by vascular mucous membrane. Fresh air reaches the suprabranchial chamber through the mouth and after gas exchange the expiratory air is expelled through the opercular opening. InTrichogaster fasciates, a similar but simple a similar but simpler labyrinthine organ is present

c.Dendritic(Arborescent) Organs: In Clarias (Indian cat fish), the wall of the gill chamber on either side evaginates to form a pair of suprabranchial cavities. These cavities contain highly branched and tree-like accessory air breathing organs  called dendritic or arborescent organ.Exchange of respiratory gases occurs through the highly vascularized mucous membrane covering these organs.

6.Air bladder (Swim Bladder): Air bladder or swim bladder is found in all bony fishes. ln teleosts, it functions as a hydrostatic organ. However, in lower bony fishes such as Dipnoans, ganoids, etc. gills are poorly developed and the air bladder functions as an accessory respiratory organ. lt is vascular and contains many alveolus-like structures. ln Amia and Lepisosteus, single air bladder is present, which opens dorsally into pharynx. in Lepidosiren and Protopterus, the air bladder is bilobed, ventral and opens ventrally into pharynx.

7. Oral Papilla In electric eel (Electrophorus), the mucous membrane of the mouth is raised into oral papillae. They are well vascularized and they   help in aenal respiration.

8.Lungs:PLungs are present in Dipnoi. Eg.Protopterus, Lepidosiren, etc. They have apair of lungs. They open into the oesophagus by a common opening called glottis The lungs contain alveoli and are well vascularized. In Dipnoi, lungs are used for aerial respiration during aestivatiom