The document discusses accessory respiratory organs in fishes. It describes 7 types of accessory respiratory organs that have developed in some fish species to help them breathe in low oxygen environments. These include the skin, bucco-pharyngeal epithelium, gut epithelium, outgrowths of pelvic fins, and modified gill structures. Examples are given of important modifications seen in the accessory respiratory organs of species like Heteropneustes fossilis, Anabas testudineus, and Clarias batrachus. The swim bladder acts as an accessory respiratory organ in some primitive fish species. Accessory organs allow fish to live in low oxygen water and breathe air when available, and their development is an adaptive response

1. Accessory Respiratory Organs in
Fishes
Dr. Balveer Singh Dhami
Assistant Professor
Department of Zoology
Bareilly College, Bareilly (U.P.)
E-mail: bsdhamimcm@gmail.com
Invited Lecture Series for Fisheries Students, HNBGU, SRT Campus, Dated: 03-12-2021

2. Contents
 Accessory respiratory organs
Types of accessory respiratory organs
Important modifications in some species
 Swim bladder
 Functions of accessory respiratory organs
Invited Lecture Series for Fisheries Students, HNBGU, SRT Campus, Dated: 03-12-2021

3. Accessory Respiratory organs
 Sometimes the fishes of the tropical freshwaters and hill-
streams develop accessory respiratory organs to meet extra
demand for oxygen.
 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 nature. Some accessory organs facilitate aquatic
respiration, while others aerial respiration.
Invited Lecture Series for Fisheries Students, HNBGU, SRT Campus, Dated: 03-12-2021

4. Types of Accessory Respiratory Organs
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.
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5. • 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.
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6. 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.
Invited Lecture Series for Fisheries Students, HNBGU, SRT Campus, Dated: 03-12-2021
 In mudskippers (Periophthalmus and
Boleophthalmus) the highly vascularised
bucco-pharyngeal 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 damp 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.

7. 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.
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8. 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. 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.
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9. 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.
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10. 7. 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.
 These are developed from the pharynx and not from the branchial chambers
as seen in others. The air-chambers are simple sac-like structures. 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. 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.
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12. Important Modifications in Some Species
(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,
supra-branchial 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.
Invited Lecture Series for Fisheries Students, HNBGU, SRT Campus, Dated: 03-12-2021

13. (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 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.
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14. (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 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 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.
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15. Swim Bladder/Air-Bladder
 Swim-bladder of 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.
 Similar to 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.
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17. Functions of Accessory Respiratory Organs
 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.
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18. THANKS !!
Invited Lecture Series for Fisheries Students, HNBGU, SRT Campus, Dated: 03-12-2021