Accessory respiratory organs in fishes

1. Skin or Integument:

• In the eel, Anguilla anguilla, Amphipnous cuchia, and in Periophthalmus and Boleophthalmus, the skin is highly vascular and serves for the 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.
• The glandular secretions of the skin protect it from desiccation in the air.

2. Bucco-Pharyngeal Epithelium:

• In most of the fishes, the epithelial lining of the 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 vascularized buccopharyngeal epithelium helps in absorbing oxygen directly from the atmosphere.

3. Gut Epithelium:

• In several instances, the epithelial lining of certain pa,rts the alimentary canal becomes vascular and modified to serve as a respiratory organ.
• It may be just behind the stomach (Misgurus fossilis) intestine (Lepidocephalus guntea, Gobitus (giant loach of Europe), or rectum (Callichthyes, Hypostomu,s, and Doras).
• Fresh air is drawn through h mouth or anus and after gaseous exchange, the gas is voided through the anus.

4. Outgrowths of Pelvic Fins:

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

5. 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 the exchange of gases. This is seen in Periophthalmus and Boleophthalmus.

6. 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.