Jyoti S.D. Munshi

Microanatomy and Cytochemistry of the Gastro-Respiratory Tract of an Air-Breathing Cobitidid Fish, Lepidocephalichthys guntea

Gastro-respiratory tract of the loach,Lepidocephalichthys guntea has been studied with special reference to the nature of its mucus secreting epithelia. The mucous cells are strongly PAS-positive and their number per unit area (mm2) in the mucosal layers of oesophagus, intestinal bulb, intestine and rectum are 733, 531, 223 and 540, respectively. The air-breathing segment of the gut is completely devoid of neutral mucosubstances, and there is a predominance of acidic mucosubstances over the neutral ones throughout the digestive tube. The air-blood pathway of the accessory respiratory organ is about 2.6 μm which is higher than the values of air-breathing organs of other fishes.

Vascular organization of the head and respiratory organs of the air-breathing catfish, Heteropneustes fossilis

Light and scanning electron microscopy of vascular replicas from the facultative air‐breathing fish Heteropneustes fossilis show modifications in the macrocirculation of the respiratory organs and systemic circulation, whereas, gill microcirculation is similar to that found in typical water‐breathing fish. Three and sometimes four ventral aortae arise directly from the bulbus. The most ventral vessel supplies the first pair of arches. Dorsal to this another aorta supplies the second gill arches, and a third, dorsal to, and larger than the other two, supplies the third and fourth arches and the air sacs. Occasionally a small vessel that may be the remnant of a primitive aortic arch arises from the first ventral aorta and proceeds directly to the mandibular region without perfusing gill tissue. The air sac is perfused by a large‐diameter extension of the afferent branchial artery of the fourth gill arch and its circulation is in parallel with the gill arches. Blood drains from the air sac into the fourth arch epibranchial artery. A number of arteries also provide direct communication between the efferent air sac artery and the dorsal aorta. All four gill arches are well developed and contain respiratory (lamellar) and nonrespiratory (interlamellar and nutrient) networks common to gills of water‐breathing fish. Air sac lamellae are reduced in size. The outer 30% of the air sac lamellar sinusoids are organized into thoroughfare channels; the remaining vasculature, normally embedded in the air sac parenchyma, is discontinuous. A gill‐type interlamellar vasculature is lacking in the air sac circulation. Despite the elaborate development of the ventral aortae, there is little other anatomical evidence to suggest that gill and air sac outflow are separated and that dorsal aortic oxygen tensions are maintained when the gills are in a hypoxic environment. Physiological adjustments to hypoxic water conditions probably include temporal regulation of gill and air sac perfusion to be effective, if indeed they are so.

Vasculature of the head and respiratory organs in an obligate air‐breathing fish, the swamp eel Monopterus (=Amphipnous) cuchia

Methyl methacrylate vascular corrosion replicas were used to examine the macrocirculation in the head region and the microcirculation of respiratory vessels in the air‐breathing swamp eel Monopterus cuchia. Fixed respiratory tissue was also examined by SEM to verify capillary orientation. The respiratory and systemic circulations are only partially separated, presumably resulting in supply of mixed oxygenated and venous blood to the tissues. A long ventral aorta gives rise directly to the coronary and hypobranchial arteries. Two large shunt vessels connect the ventral aorta to the dorsal aorta, whereas the remaining ventral aortic flow goes to the respiratory islets and gills. Only two pairs of vestigial gill arches remain, equivalent to the second and third arches, yet five pairs of aortic arches were identified. Most aortic arches supply the respiratory islets. Respiratory islet capillaries are tightly coiled spirals with only a fraction of their total length in contact with the respiratory epithelium. Valve‐like endothelial cells delimit the capillary spirals and are unlike endothelial cells in other vertebrates. The gills are highly modified in that the lamellae are reduced to a single‐channel capillary with a characteristic three‐dimensional zig‐zag pathway. There are no arterio‐arterial lamellar shunts, although the afferent branchial artery supplying the gill arches also supplies respiratory islets distally. A modified interlamellar filamental vasculature is present in gill tissue but absent or greatly reduced in the respiratory islets. The macro‐ and micro‐circulatory systems of M. cuchia have been considerably modified presumably to accommodate aerial respiration. Some of these modifications involve retention of primitive vessel types, whereas others, especially in the microcirculation, incorporate new architectural designs some of whose functions are not readily apparent.

Interspecific variations in the surface ultrastructure of the gills of freshwater mullets.

SEM studies were made on the gills of freshwater mullets,Rhinomugil corsula andSicamugil cascasia, to correlate surface ultrastructure of various gill units with their probable functions. Two types of lamellated gill rakers of the former fish are suited for plankton feeding and the short, stumpy and transversely beaded gill rakers of the latter reflect the varied food and feeding habit of the fish.R. corsula has numerous mucous glands on the epithelium covering the gill arch and gill filaments,S. cascasia has fewer. In accordance with the differences in the density and distribution of the mucous glands, the microridged epithelial cells also show variations in their architectural plan. In both species the epithelium of the secondary lamellae is smooth, probably an adaptation for better gaseous exchange.

Bimodal Oxygen Uptake in a Freshwater Air-breathing Fish, Notopterus chitala

Measurements of bimodal oxygen uptake have been made in a freshwater air-breathing fish,Notopterus chitala at 29.0±1(S.D.)°C. xhe mean oxygen uptake from continuously flowing water without any access to air, was found to be 3.58±0.37 (S.E.) ml O2 · h−1 and 56.84+4.29 (S.E.) ml O2 · kg−1 · h−1 for a fish weighing 66.92 + 11.27 (S.E.) g body weight. In still water with access to air, the mean oxygen uptake through the gills were recorded to be 2.49 ± 0.31 (S.E.) ml O2 · h−1 and 38.78 ± 1.92 (S.E.) ml O2 · kg−1 · h−1 and through the accessory respiratory organs (swim-bladder) 6.04±0.87 (S.E.) ml O2 · h−1 and 92.32±2.91 (S.E.) ml O2 · kg−1 · h−1 for a fish averaging 66.92±11.27 (S.E.) g. Out of the total oxygen uptake (131.10 ml O2 · kg−1 · h−1), about 70% was obtained through the aerial route and the remainder 30% through the gills.

Bimodal oxygen uptake in juveniles and adults amphibious fish, Channa (= Ophiocephalus) marulius

Oxygen uptake of Channa marulius was studied under water with and without access to air. There was a significant increase in the oxygen uptake through the gills when access to air was prevented. However, this value (0.863 ± 0.058 mlO2/indiv./h) was quite low in comparison to the total bimodal oxygen uptake (2.04 ± 0.14 mlO2/indiv./h) in juveniles. In adult fish the oxygen uptake per unit time increased appreciably (4.673 ± 0.404 mlO2/indiv./h). In juveniles as well as in adults the air breathing dominated over aquatic breathing. This fish showed a definite circadian rhythm in the bimodal oxygen uptake during different hours of the day.

Structure of the respiratory islets of accessory respiratory organs and their relationship with the gills in the climbing perch, Anabas testudineus (Teleostei, Perciformes)

The epithelial and sub‐epithelial organization of the accessory respiratory organs of Anabas testudineus has been compared with that of gills by using light and transmission electron microscopy. The details of vascular supply of respiratory islets (RI) and gill filaments and the presence of venous sinusoids in the two systems suggest that the RI have been derived from gill filaments and lamellae. The biserial arrangement of transverse capillaries (TC) in the respiratory islets (RI) is evident under the scanning electron microscope and their homology with the gill filaments and their secondary lamellae has been established. The two sets of transverse capillaries of respiratory islets have been derived either from embryonic transverse or marginal channels of two sets of lamellae of a gill filament. These capillaries with their endothelial septate valves and tongue‐like processes offer resistance to blood flow.

Scanning electron microscopy of the post-embryonic stages of the climbing perch,Anabas testudineus

Different developmental stages, fertilized eggs through hatchlings, of the climbing perch,Anabas testudineus, have been studied by scanning electron microscopy. The surface specialization of eggs and hatchlings reveals that while the egg surface is reticulate in appearance, the hatchlings are covered with microridges. Vitelline arteries are seen at the pharyngeal and abdominal regions. They supply nutrients directly from the yolk sac to the developing embryo. Three pairs of such arteries are distinctly seen in the pharyngeal region. Mucous glands are discernible at places over the entire body surface of the embryo before the formation of scales. The skin seems to be helpful in gaseous exchange till the gills and accessory respiratory organs develop and become functional.