J. S. Datta Munshi’s research while affiliated with University of Bristol and other places

The accessory organs of this swamp-living fish have been studied using SEM and light microscopy. Accessory organs are found in the suprabranchial chambers, labyrinthine plates on the first epibranchial, and roof of the buccopharynx. In these regions, there are nodules of respiratory islets each consisting of many vascular papillae. Non-respiratory ‘lanes’ separate the islets and are covered with microridged epithelial cells. The dome-shaped papillae have a smooth surface with band-like structures running over them. Microvilli are present only at the base of the papillae. The vascular papillae of the buccopharynx are lodged in cup-like receptacles into which they can retract. The intraepithelial capillaries contain unicellular valves which project into the lumen of the papillae. Capillaries supplying the papillae have an undulatory path which represents a second stage in the evolution of this type of structure.

A comparative study of the skin, based on the micro-anatomical investigation of skin fragments taken from a specific region of the body, has been made of three air-breathing fishes, namely, Heteropneustes fossilis, Amphipnous cuchia and Mastacembelus pancalus. On the basis of their structure and histochemical nature, five types of skin glands have been distinguished in the epidermis of these fishes. The relative thickness of the epidermis (A. cuchia–119 μm (average value), H. fossilis– 98 μm (average value), M. pancalus–34 μm (average value)) and its vascularization has been considered and compared with other fishes and amphibians. The possibilities of cutaneous respiration in these air-breathing fishes has been discussed. The presence of a well-defined lymphatic system, comprising a series of lymph spaces containing small lymphocytes, in the stratum germinativum layer of the epidermis of these fishes has been established. The stratum laxum layer of the dermis in Amphipnous is characterized by the presence of definite areas containing “Substantia amorpha” having acidic mucopolysaccharides which may be related with the amphibious habit of the fishes. This is an adaptation against desiccation similar to that found in the Anura. In Mastacembelus elliptical areas of the stratum laxum penetrate into the epidermis thus making these areas of the epidermis considerably thin (about 7 μm) for cutaneous respiration. There is an inverse relationship between the thickness of the stratum compactum and squamation.

The surface area of the gills, air sacs and skin have been measured in specimens of different body size and their relationship to body weight fits the equation: area=aWb. The slopes (b) of the double logarithmic plots are 0.746 (gills), 0.662 (air sacs) and 0.684 (skin). The gills are poorly developed and their average weight specific area is less than figures obtained for sluggish marine fishes. The skin has an area about 70% of the total respiratory surfaces (gills+air sac+skin). Nevertheless the greater thickness of the skin leads to a smaller diffusing capacity of the tissue barrier (Dt) as compared with the gills and air sac. The air sac area for each ml of air that it contains is about 10.5 cm2 which is much lower than figures obtained for lungs of other air-breathing fish and for tetrapods.

The structure of the air-breathing organs of the Indian fishes Channa punctatus, Channa Striatus, Amphipnous cuchia, Clarias batrachus and Saccobranchus fossilis has been investigated using electron microscopy. In all species the barrier separating the air from the blood consists of three main layers (epithelium, basal lamina and endothelium). The total thickness ranging from 0.78 μm in C. punctatus 1.6 μm in S. fossilis. In Clarias and Saccobranchus the presence of pillar cells characteristic of gill secondary lamellae confirms evidence for the origin of these organs by modification of a typical gill structure. In Amphipnous and two species of Channa, however, the evidence suggests that the accessory organs represent modified gills. The presence of valve-like structures between the afferent and efferent blood spaces of the vascular papillae gave the appearance of pillar cells under the light microscope. The structure of these organs is correlated with physiological studies on the degree of their importance in the life of the animal and the degree of gill development

Measurements have been made of the surface area of the gills and accessory respiratory organs of Anabas in the weight range 1–120 g, and the data analysed with respect to body weight using logarithmic transformations. The slope of the regression line for total gill area (0–615) is less than that found in most fish, the number of secondary lamellae/mm decreased more rapidly with body weight than for most water-breathing species (h = -0.152). The gill area of Anabas is relatively small but when the area of the accessory organs is added, the total respiratory area is of the same order as inactive water-breathing fish. The regression coefficient for combined areas of labyrinthine organs and lining of the suprabranchial chambers (0.713) exceeds that for the gills and together with other evidence (including estimates of diffusing capacity from morphological measurements), indicates an increasing importance of air-breathing of larger specimens. The average surface area of the accessory organs available for 1 ml of air within the suprabranchial chambers was found to be 2226 mm2.

Oxygen uptake through gills and skin has been measured in juvenile and adult Saccobranchus fossilis and its relationship represented by the equation Vo2=aWb. When air-breathing is allowed the O2 uptake via the gills and skin together increases by powers of 1 -084, 0–986 and 1–328 in juvenile, adult and juvenile+adult respectively. When air-breathing is prevented the slope (b) for O2 uptake via the gills appear to be less in juveniles (0–765) than in adults (0–784) and juveniles+adults together (0–814). Under the same experimental condition, the slope for O2 uptake via the gills+skin is also less in juveniles (0–478) than in adults (0–799) and juveniles+adults (0–755). Further decrease in the exponent value is found for the oxygen uptake of skin in relation to body weight under surfacing-prevented conditions (b= 0–542). Different exponent values for juvenile and adult fishes may be due to their different growth pattern and physiology.

Measurements of the dimensions of the different gills and the suprabranchial chambers have been made and the data analysed with respect to body weight using logarithmic transformations (Y = aWb). The slope (b) for area of the total gill surface is 0–592 and for the supra-branchial chamber 0–696, and their combined respiratory surface: 0–623. The slope values for the surface areas of the 1st, 2nd, 3rd and the 4th gill arches were 0–595,0–578,0–614 and 0–572 respectively. The slope for secondary lamellae/mm is –0138 and that for the bilateral surface area of an average-sized lamella 0–304. These results indicate differences in growth patterns for the dimensions of the different gills. The growth-related decrease in the number of secondary lamellae/mm and size of an average secondary lamella together with evidence from “drowning” experiments and diffusing capacity calculation, suggest that this fish is better adapted for aquatic respiration than Anabas or Saccobranchus. The slopes for the total respiratory surface area and gill area seem to be comparatively low in this species.

The micro-circulatory system of the lamellae is in the form of a network of more or less parallel blood channels. A row of pillar cells separates two contiguous blood channels. All the pillar cells are situated in the same plane between the upper and lower basement membranes to which they remain fixed in position by means of columns. Histochemical investigations show that the basement membrane as well as the columns are collagenous in nature. Though both basement membranes and the columns are PAS positive (magenta colour), reticulin is not present as they do not respond to silver techniques. The development of new blood channels in the micro-circulatory system of Channa striatus has been studied. They arise as buds from the wall of the pre-existing vessels. There is some evidence to show the possible transformation of the smooth muscle cells of the tunica media of blood vessels into the pillar cells of the micro-circulatory system. Various aspects of the physiology of the micro-circulatory system of the gills have been discussed.

A detailed morphometric study has been made of the air-sacs of this air-breathing catfish using whole mounts, light and electron microscopy, of six specimens, body weight 40±2 g. Measurements of surface areas of the gas exchange and non-respiratory surfaces have taken into account foldings of the surface at macro and ultramicroscopic levels. Area of the gas exchange surface was estimated as 23.915cm2 (=0.598cm2/g) which is 67%of the total surface area of the two air-sacs. Significant differences were found in some morphometric parameters which were related to the three antero-posterior regions into which air-sacs were divided. Harmonic mean thickness of the tissue component of the air/blood barrier was estimated for the whole air-sac as 0.342 μm. These and other measurements enabled the diffusing capacity for the air-sacs to be calculated as 0.0638 m1O2/min/mmHg/kg. These results show that Heteropneustes has an air-breathing organ which is superior to that of Amphipnous cuchia, similar to that of Lepidosiren, but less well developed than that of Protopterus. In addition, Heteropneustes is well adapted to obtain oxygen directly from water by means of its gills and skin as indicated by both morphometric and physiological measurements which also correlate with its life in ponds and streams which are Iiable to dry up.

SUMMARY1The accessory respiratory organs of Anabas testudineus consists of the following structures:(i) the supra-branchial chamber,(ii) the labyrinthine organ, and(iii) the respiratory membrane.2The incomplete division of the supra-branchial chamber into outer and inner recesses, the structure and action of the shutter, the manner of communication between the two recesses and communication with the pharynx and opercular chamber by means of inhalent and exhalent apertures are described.3The labyrinthine organ, made up of several saucer-shaped plates, is composed of and derived from modified, abbreviated and fused primary and secondary gill lamellae. The surface of the labyrinthine organ is divided into respiratory ‘islets’, and injected preparations have disclosed the presence of capillaries resembling those of a typical gill lamella.4The respiratory membrane lining the supra-branchial chamber and the labyrinthine organ consists of vascular and non-vascular areas. The vascular part of the membrane comprises small and large ‘islets’ distributed over the surface. The presence of pilaster cells and the specialized character of the blood capillaries strongly suggests that the ‘islets’ are derived from the histological elements of the gill lamellae. The islets make intriguing configurations on account of the fusion of neighbouring primary gill lamellae and the flattening of the secondary lamellae. Associated with each islet are two efferent and one median afferent vessel, and several transverse channels interconnecting them. The non-vascular part of the membrane is represented by the irregular ‘lanes’ present between the islets. This represents the interlamellar areas between contiguous gill lamellae.5The branchial, hyoid and mandibular muscles have become modified in connexion with the change-over from aquatic to aerial respiration. The adductor arcus palatini, a muscle of the hyoid segment, is modified to form the contractile floor of the anterior part of the respiratory air chamber. The branchial muscles associated with the first four gill arches and the respiratory sac have been studied and a list of them is given.