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- 134 -
THE RESr'IRATORY SYSTEM
The four species selected for the present study
make use of atmospheric oxygen, either wholly or partly
for their respiration and so they are capable of surviv
ing out of water for varying lengths of time. A.bicolor
and P.boro can be kept alive for four to five hours, if
covered with moist earth or wet cloth. Under the same
conditions S.bengalensis md A.fossorius can be kept alive
for several days. This capacity enables S.bengalensis
and A.fossorius to tide over adverse conqitions. As al
ready stated, they ar$ often found in shallow ponds and
ditches. In summler when these water bodies become partly
or completely dry, S.bengalensis and ~fos@orius burrow
deep into the moist subsoil and aestivate for about eight
to ten weeks till the rains set in.
Under normal conditions, these fishes rise up to
the surface at intervals to take in atmospheric air, But,
even if they are kept in small aquaria and prevented from
rising up to the surface, they can survive without showing
any signs of discomfort for twenty four to thirty six hours,
indicating thereby that their oxygen requirement is very
low. Tait (1952) observes that "compared with land
animals, fishes demand surprisingly little oxygen to sus
tain life" and in the case of deep water fishes, often less
than 1% and in some cases Il even 1/3 or 1% of oxygen by
volume" is sufficient to support life. According to Fry
(1957) the oxygen requirement for the.metabolic activities
of all fishes is more or less the same. The adoption of
- 135 -
air-breathing habit by some fishes is therefore not in
response to excessive metabolic requirements, but is only
a means of supplementing the low oxygen content of the
water in which such fishes live. The enclosed water bodies
inrhich these fishes occur are extremely deficient in oxygen
owing to the accumulation of decaying organic material and
rise of temperature during summer. Respiration under such
conditions will be possible only by taking in atmospheric
air and this has resulted in the progressive adaptation to
'air-breathing', the most advanced condition of which is
seen in A.fossorius, which spends most of its time in
deep burrows. •
The earliest notice of the capacity of fishes to
"breathe" atmospheric air was that of Taylor (1831) and
Hyrtl (1858). Day (1868) and Dobson (1874), who also
recorded some fishes with air-breathing habits, called
them "compound-breathers". Since then, numerous papers.
have been published on the accessory respiratory organs,.
and the process of respiration in a number offresh water
fishes. The most important of these are the works of
Rauther (1910), Carter and Beadle (1931), Carter (1935),
Bader (1937), Das (1927) and Hora (1935). However, a
perusal of the earlier literature shows that the four
species selected for the present study have not so far
received attention.
Although the structure and functions of the
resp~ratory organs in these fishes are more or less
si~lar to those of other air-breathing fishes in essen
tial respects, still, there are interesting minor varia-
- 136 -
tions. In A.bicolor and P.boro the gills are normally
developed and there are no accessory respiratory structures.
It is therefore evident that they are mainly dependent on
oxygen dissolved in water. Nevertheless, both these spe
cies come to the surface. of water at frequent intervals
and take in atmospheric air, when the water body in which
they live is deficient in oxygen. In this process, the
snout is protruded above water level with the body in a
vertical position. If undisturbed, they remain this
position for several minutes at a stretch, with their
branchial chambers distended with ai~. Then they return
to the bottom and in doing so, bubbles of air are ex
pelled at intervals through the external branchial
openings.
Though air is drawn into the branchial char-lbers,
this region does not possess any respiratory surface.
Detailed histological study of the lining of the branchial
chambers and the buccal cavity shows that it is formed
o~ly of ordinary mucous membrane. Allover this area, the
cells are short, cuboidal and deeply staining, unlike the
vascular epithelial layer commonly associated with res
piratory function.
In both these species, there are four pairs of
gills. The first gill-opening is the largest and the
succeeding ones are progressively smaller. A pseudo
branch is absent. The structure of the gills (Figs~22&
223) and the disposition of the blood vessels both in
the gill-arches and in the gill-filaments are the same as
- 137 -in other teleosts. It is therefore eVident that they
normally respire like other teleosts. Carter (1957)
observes that the use 01gillS for air-breathing is rane
among air-breathing fishes. According to him, the gills
are ill-adapted for aerial respiration. However, Hynopo
~ and probably Synbranchus appear to use their gills
for air-breathing. In A.bicolor and P.boro, since the
branchial chambers do not seem to be capable of any res-
piratory function, it has to be presumed that respira
tion takes place only through the gills. When these.t~
fishes lift their snout aboveAwater surface to take in
atmospheric air, the gills are surrounded by ~thin film
of water or probably mucus. This water or mucus is de-
ficient in oxygen, So when the fish takes in air into the
branchial chambers, the oxygen in the air is absorbed by
the film of water or mucus which in turn supplies it to
the gills. This process of diffusion 1s continuous till
the oxygen in the enclosed air is completely exhausted.,
From this it may be inferred that the branchial chambers
only serve as temporary rece~tacles in which air is re
tained until the oxygen which it contains is absorbed by
the film of water or mucus.
S.bengalensis and A.fossorius live in ponds and
tanks which are deficient in dissolved oxygen. As already
observed, they burrow into the soft mud along the banks,
close to the edge of water bodies and the burrow is so
made that the fish can come out of it and lift their
snout above water to take in'atmospheric air.
- 138 -
In A.fossorius the gill-filaments are complete
ly atrophied. The lining epithelium of the gill-arches is
continuous with the lining of the branchial chambers and
the buccal cavity. In S.bengalensis the epithelial lining
of the buccal cavity shows the same structure as in other,
fishes, but in the branchial ,chambers, it i~thrown into
numerous, parallel folds, formed of a single layer of
cuboid cells (Fig. 238). Beneath this layer of vascular
epithelium, there is a layer of connective tissue which
also extends into these folds. A few, small, scattered
flask-shaped mucous cells are also found in the vascular
epithelium and very often projecting slightly into the
connective tissue layer beneath.
In S.bengalensis all the four gills are more or
less of the same structure (Fig. 224~ as in other teleosts
and so it is to be presumed that the gills function nor
mally for absorbing oxygen from water. So the branchial
chamber is eVidently a supplementary organ which functions
only when oxygen content of the water is low. In the eels,
as already stated, even though air is store~ in the br~n
chial chambers, respiration takes place only through the
gills, the oxygen from the air being absorbed into the
thin film of water or mucus surrounding the gills. On
the other hand, in S.bengalensis, the lining of the bran
chial chambers is highly vascular and so it i~ossible
,that oxygen is absorbed directly from the air through
the vascular epithelium and also through the thin film
of water or mucus surrounding the gill-filaments as ob-
served by Carten (1957).
- 139 -
In A.fo~sorius the most noteworthy feature is
that the gill-lamellae are completely atrophied. The
fourth gill-arch is very much reduceq and the first is
a smooth rod-like structUl~e. All tfue gill-arches are
covered by a lining of vascular epithelium of varying
thickness, which is continuous with the epithelial lin
ing of the branchial chambers. On the gill-arches, the
vascular epithelium is throvm into small, irregular lobes
(Figs.225 & 226) and is formed of two to three layers of
cuboid cells and numerous blood capillaries on the sur
face (Fig. 227). Beneath the epithelial layer is a thick
layer of spongy connective tissue, which also extends
into the folds. This connective tissue layer carries a
number of small blood vessels. In the branchial chamber,
the vascular epithelial layer (Fig.229) is very thick and
is formed of a number of layers of cuboid cells. The
folds are more prominent and solid without an axial ex
tension of connective tissue. The olood capillaries on
the ~ surface of the epithelimn are numerous and pro
minent. The cOlli1ective tissue layer which lies immediate
ly beneath the vascular layer is thin and spongy and con
tains a few small blood vessels. In the buccal cavity,
the folds are much more prominent and like the vascular
lining of the branchial chamber, carries numerous small
capillaries on the surface~(Fig.230). The main difference
between these folds and the folds of the branchial chamber
is that in the former the connective tissue layer ex
tends into the folds and the mucous cells are more
numerous and comparatively large.
- 140 -
The reduction of the 3ill-arches and the atrophy
of the gill-filaments indicate that this fish does not
depend very much on aquatic respiration. This is further
supported by the disposit.ion of the blood vascular system
in the anterior region. As already stated, the afferent
branchials pass through the 3ill-arches carrying the
blood directly to the branchial ohambers. The efferent
branohials are absent. So the oxygenated blood from the
branchial ohamber is oolleoted by an opercular vein which
opens into the internal jugular. It is therefore evident
that respiration in this species is mainly through the
vascular epithelium lining the branohia1 chambers and the
gill arohes, the oxygen neoessary for respiration being
obtained direotly from the atmospherio air. This is fur
ther proved by the fact that if a speoimen of A.fossorius
be forced to remain under wate~~~~inuously for two or
three days without access to atmospheric air, i~beoomes
very weak and unless it is allowed to ooms up to the sur-
face to take in atmosperio air, it dies after some time.
So unlike other fishes in which accessory respiration is
only a supplementary method of obtaining oxygen, in this
species, it is the main source and this is no doubt asso
ciated with its habit or burro~lng int~Ud.
In A.cuchia, according to Hyrtl (1858) the
accessory respiratory organs are of the nature of a pair
of pharyngeal diverticula and receive their venous supply
from the pseudobranchial and hyoidean veins and the
afferent branchials. The oxygenated blood is drained
into the anterior oardinal (internal jugular). The gi11-
- 141 -
lamellae are reduced except on the second gill-arch. So
respiration in A.cuchia is through the lining of the
pharyngeal diverticula and the lamellae of the second
gill-arch. This species has been regarded as a 'proto
amphibian'. In A.fossorius, the reduct!onpf the g111~
lamellae is complete and the pharyngeal diverticula are
substituted by the branchial chambers. So A.fossoriu8
may be regarded as more advanced than A.cucb1a in its
adaptation to aerial respiration.