9. AIR BREATHING IN FISHES
365
fixed material revealed a surface area of the mouth respiratory organ
about 15% of the total body surface (Johansen et al., 1968). This is considerably less than for either gills in purely aquatic fishes or lungs of
higher vertebrates. Yet the mouth organ in Electrophorus functions well
enough to have permitted the gills of adult fishes to become almost
vestigial (Fig. 1).
The next step in a progressive structural adjustment to air breathing
includes the formation of an air chamber extending from the buccal or
pharyngeal cavities. In the Ophiocephalidae with several air-breathing
species in tropical marshes of Asia and Africa, the air chamber lies dorsal
to the branchial area and becomes partly enclosed within the cavity
of the skull ( Munshi, 1962).
In Amphipnous cuchia, another air-breathing fish living in India, the
paired air chambers referred to as pharyngeal lungs (Das, 1940) are
more free and extend several centimeters behind the head. The internal
walls of the air chamber are very vascular and may show surface expanding structures in the form of rosettelike arborizations protruding into the
lumen. In others, internal trabeculation may give the air chamber a
resemblance to an amphibian lung.
Another structural variation is seen in fishes where the opercular
cavities have become richly vascularized and serve as receptacles for air.
When air is lodged in such chambers they stand out in baIloonlike
fashion. The opercular walls have become thin and elastic and their
bone structures have regressed. The opercular openings are either fused
to one small channel ventrally or have otherwise become reduced in size.
These modifications and others involving the intrinsic muscles of the
opercular walls permit ventilatory movements and renewal of air inside
the opercular chambers. Examples of such specialization are seen in
Hypopomus breuirostris, hlonopterus sp., and several species of gobiid
fishes (e.g., Pseudapocryptes lanceolutus) (Wu and Kung, 1940).
More elaborate use of the opercular cavities in air breathing results
when specialized diverticula develop from their dorsal side commonly
between the hyoid and the first branchial arches. Two distinctly different
types of such opercular air chambers or lungs are found. In some cases
they remain free and extend posteriorly. Internally they possess ridges
and trabeculations or other protrusions expanding the highly vascular
internal walls. Often these air sacs extend half the length of the animal
and lie embedded within the myotomes of the body wall, a fact which
undoubtedly bears significance in the filling and emptying of the sacs.
Indian fishes of the genus Saccobranchus are well-known examples of
this type of structural adaptation ( Rauther, 1910).
The other variety of opercular air chamber results when the opercular
diverticulum extends inside the skull to take the form of a labyrinthlike
365
fixed material revealed a surface area of the mouth respiratory organ
about 15% of the total body surface (Johansen et al., 1968). This is considerably less than for either gills in purely aquatic fishes or lungs of
higher vertebrates. Yet the mouth organ in Electrophorus functions well
enough to have permitted the gills of adult fishes to become almost
vestigial (Fig. 1).
The next step in a progressive structural adjustment to air breathing
includes the formation of an air chamber extending from the buccal or
pharyngeal cavities. In the Ophiocephalidae with several air-breathing
species in tropical marshes of Asia and Africa, the air chamber lies dorsal
to the branchial area and becomes partly enclosed within the cavity
of the skull ( Munshi, 1962).
In Amphipnous cuchia, another air-breathing fish living in India, the
paired air chambers referred to as pharyngeal lungs (Das, 1940) are
more free and extend several centimeters behind the head. The internal
walls of the air chamber are very vascular and may show surface expanding structures in the form of rosettelike arborizations protruding into the
lumen. In others, internal trabeculation may give the air chamber a
resemblance to an amphibian lung.
Another structural variation is seen in fishes where the opercular
cavities have become richly vascularized and serve as receptacles for air.
When air is lodged in such chambers they stand out in baIloonlike
fashion. The opercular walls have become thin and elastic and their
bone structures have regressed. The opercular openings are either fused
to one small channel ventrally or have otherwise become reduced in size.
These modifications and others involving the intrinsic muscles of the
opercular walls permit ventilatory movements and renewal of air inside
the opercular chambers. Examples of such specialization are seen in
Hypopomus breuirostris, hlonopterus sp., and several species of gobiid
fishes (e.g., Pseudapocryptes lanceolutus) (Wu and Kung, 1940).
More elaborate use of the opercular cavities in air breathing results
when specialized diverticula develop from their dorsal side commonly
between the hyoid and the first branchial arches. Two distinctly different
types of such opercular air chambers or lungs are found. In some cases
they remain free and extend posteriorly. Internally they possess ridges
and trabeculations or other protrusions expanding the highly vascular
internal walls. Often these air sacs extend half the length of the animal
and lie embedded within the myotomes of the body wall, a fact which
undoubtedly bears significance in the filling and emptying of the sacs.
Indian fishes of the genus Saccobranchus are well-known examples of
this type of structural adaptation ( Rauther, 1910).
The other variety of opercular air chamber results when the opercular
diverticulum extends inside the skull to take the form of a labyrinthlike