Definition: The additional organ which are concerned with respiration other than gills are known as Accessory Respiratory Organ in fish.
These organs developed mostly in tropical fresh water fishes to meet the extra demand of O₂ during summer to aestivate over prolonged drought.
The accessory respiratory organs in fish are:-
1.) Skin/Integument: Cartain eels like Anguila sp. & Amphipnous sp. and mud skippers like Periopthalmus of frequently leave water and move from one stream to another through damp vegetation.
The skin of these fishes is serves as a respiratory surface effecting exchange of gases.
2.) Pharyngeal Diverticula
These are a pair of simple sac like out growth of pharynx lined by thickened vascular epithelium and extending above the gills.
Pharyngeal diverticula are very small in Periopthalmus sp., small and smooth in Amphipnous sp., and folded in Channa (Ophiocephalus sp.).
Those fishes have poorly developed gill-filaments, these accessory respiratory organ/chambers serve to breathe atmospheric air during aestivation.
3.) Bucco Pharyngeal Epithelium:
In the mouth portion of certain fishes, the bucco-pharyngeal epithelium becomes vascular and may become folded to serve the respiratory function.
4.) Branchial Outgrowth:
In Climbing perch (Anabas sp.), the branchial outgrowth are the spacious air chamber on either side of skull situated between 1st gill arch & hyomandibular.
These chambers are highly vascular and becomes folded contains rosette like labyrinthine organ. It consists of 3 concentrically arranged plates which are wavy (covered with epithelium).
5.) Pneumatic Sacs:
In Heteropneustes sp., Saccobranchus sp. fishes, a pair of simple tubular pneumatic sacs serve as the accessory respiratory organs.
They developed as out growths from the branchial chamber and extended up to the tail portion.
6.) Suprabranchial Organ:
The Indian cat fish, Clarius batracus has most complicated accessory respiratory organ. These are in the form of two branched tree like dendritic organs developed inside the suprabranchial cavities.
The cavities and the organ are covered by vascular, mucous epithelium (functioning as lungs).
The fish periodically reaches water surface to renew air.
7.) Swim Bladder modified as Lung:
In some fishes, the swim/ air bladder modified lung like structure for purpose of aerial respiration.
Example: Amia sp.; Lepisosteus sp. Polypterus sp. & Dipnoi sp.