- 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.