Morphology and vascular anatomy of the accessory respiratory organs of the air‐breathing climbing perch, Anabas testudineus (Bloch)

Abstract

The vascular organization and endothelial cell specialization of the air-breathing organs of Anabas testudineus were examined by light and scanning electron microscopy of fixed tissue and vascular corrosion replicas. The vessels supplying blood to the lining of paired suprabranchial chambers and the plicated labyrinthine organs within the chambers are tripartite, having a median artery and paired, lateral veins. Hundreds of respiratory islets, the functional units of gas exchange, cover the surfaces of both the chamber and labyrinthine organ. A median islet artery supplies the central aspect of each islet and gives rise to numerous short arterioles from which the transverse channels are formed. Transverse channels are parallel capillary-sized vessels that extend in two rows away from the medial arterioles and drain laterally into one of two lateral islet veins. Basally situated single rows of endothelial cells lining the transverse channels form thick, evaginated, tongue-like cytoplasmic processes that project freely into the lumen from the tissue side of the channel. Other thin, septate, cytoplasmic extensions of the same cells form valve-like septa that extend across the channel. Both the septa and tongue-like processes appear to direct the red blood cells to the epithelial side of the channel and thus decrease the diffusion distance between the air and red cell. A large sinusoidal space lies under the transverse channels and may support the channels and even elevate them during increased oxygen demand. The epithelium covering the transverse channels is smooth, which enhances air convection and minimizes unstirred layer effects. The epithelium between the channels contains microvilli that may serve to trap bacteria or particulates and to humidify the air chambers.

Gill microcirculation of the air‐breathing climbing perch, Anabas testudineus (Bloch): Relationships with the accessory respiratory organs and systemic circulation

“…The physiological significance of this separation depends upon the ability of the heart and central vessels to minimize the mixing of venous return from the systemic and accessory respiratory organs. (A detailed account of the vascular connections between the heart, gills, and accessory air-breathing organs can be found in Munshi et al, 1986). …”

“…Oxygenated blood from the air-breathing organs returns directly to the heart through short, largebore jugular veins that empty into the ductus Cuvieri. There appears to be relatively little opportunity for mixing of venous blood from the air-breathing organs with the systemic venous blood in the jugular veins (Munshi et al, 1986).…”

“…The present study examines two aspects of the circulatory system of Anabas: 1) modifications of the macrocirculation that allow the addition of a n accessory respiratory organ, and 2) adaptations of the gill microcirculation that minimize oxygen loss to a hypoxic environment yet do not greatly compromise the transbranchial exchange of ions and other molecules. The microqirculation of the accessory air-breathing organ is described in a subsequent communication (Munshi et al, 1986).…”

Microcirculation of gills and accessory respiratory organs of the walking catfish Clarias batrachus

“…Interestingly, Hughes and Munshi (1979) observed that lamellae of Heteropneustes fossilis were also covered with microvilli, whereas Rajbanshi (1977) reported that this fish has microridges on its lamellar surface. The respiratory surfaces of other bimodal breathing fish are also modified; microridges and microvilli are completely absent from the smooth ARO respiratory epithelium of Anabas testudineus and Monopterus cuchia (Munshi et al, 1986. While the transition from microridges to a microvillous or smooth respiratory epithelium may be correlated with some property of epithelial function, it is doubtful if this is solely a n adaptation for bimodal breathing.…”

Active ammonia transport and excretory nitrogen metabolism in the climbing perch,Anabas testudineus, during 4 days of emersion or 10 minutes of forced exercise on land

“…It possesses a special accessory breathing organ (ABO), situated just above the gills in a large extension on the upper part of each gill chamber, which facilitates the utilization of atmospheric air (Graham, 1997). The ABO is composed of lamellae which are covered with an extremely vascular layer of skin and convoluted into numerous folds to maximize the surface area for respiration (Hughes and Munshi, 1973;Munshi et al, 1986). Because the ABO structure resembles a complicated maze, it has been dubbed the labyrinth organ.…”