ELECTRORECEPTORS IN INDIAN CATFISH TELEOSTS

C.B.L. Srivastava and M. Seal,     Department of Zoology, University of Allahabad, Allahabad, India

SUMMARY

Four Indian freshwater catfishes, viz. Clarias batrachus, Heteropneustes fossilis, Rita rita and Mystus vittatus, have been investigated for detailed structure of their ampullary organs. The ontogenetical development of these organs was followed in one of these catfishes, namely H. fossilis, using light microscopy. It was found that the organs of these catfishes essentially resemble those of freshwater catfishes of other tropical parts of the world and of tropical freshwater weakly electric teleosts of S. America and S. Africa. The ecological conditions of the water and nocturnal habit of the fishes in the case of the Indian species parallels those of fishes of other countries. All these facts suggest an electroreception function for the ampullary organs of Indian species as well. Developmental studies show that in H. fossilis, fully formed ampullary organs are present in the larval stage, indicating that these may be functional at this stage of life as well as assisting in the location of food and the detection of enemies.In the ontogeny of the ampullary organ, histogenesis shows a primordial stage very similar to that found in the development of ordinary lateral line organs. This feature, which we report here for the first time, is the first developmental evidence in favour of homology between these two categories of organs. It has also been brought to light that the canal of the ampullary organ has a developmental origin independent of neuromastic origin of the sensory epithelium of the ampulla. A constant accompaniment of the ampullary organs of freshwater catfishes seems to be a compact collagen layer in the dermis.

INTRODUCTION

In India a number of catfishes inhabit tropical waters (muddy bottoms of ponds and rivers, swamps and paddy fields) in which visibility is poor owing to increased turbidity. Still other catfishes live in clear water of rivers, but the nocturnal habit of these catfishes renders eyes useless. These fishes, eyes notwithstanding, may, thus, be thought to depend on a sense other than vision for normal perception of the surroundings. The electroreception mechanism, known to occur in a number of tropical freshwater teleosts including some catfishes inhabiting conditions of poor visibility (Lissmann and Machin, 1958), is also a likely candidate for Indian catfishes. Histological demonstration of ampullary organs (small pit organs) in these fishes would be the first requirement for such a suggestion. Preliminary reports have indicated the occurrence of such electroreceptor organs in Indian catfishes (Mittal, 1968; Lahiri and Kapoor, 1975; Srivastava et al., 1978; Seal and Srivastava, 1978). The present paper aims at a detailed investigation of the structure of the ampullary organs in four Indian catfishes: Clarias batrachus, Heteropneustes fossilis, Rita rita and Mystus vittatus, and of the development of the ampullary organs in one of these, namely H. fossilis. It may be pointed out that development of ampullary organs is not known, except for a brief report on Parasilurus (Sato, 1956), owing to the difficulty in procuring the developmental stages of electric and nonelectric electroreceptive teleosts.

MATERIALS AND METHODS

The catfishes were procured from local markets or collected from fishing sites on rivers. Pieces of skin from head and trunk, especially from the dorsal surface, were excised and fixed in Bouin’s fluid by immersion. Material was then processed for paraffin microtomy and 6 to 8 μm thick sections were cut. Staining was done with Haematoxylene-eosin. Developmental stages of H. fossilis consisted of spawn from induced breeding*, which was raised in the laboratory. Entire larvae were processed as above and serially sectioned. Ampullary organs were first located in an advanced stage and then these were traced back through intermediate developmental stages to their earliest recognisable stage.

OBSERVATIONS

I Structure of the ampullary organs

a) Clarias batrachus (Linn.) (Figs. 1 and 2)

Each ampullary organ has a fairly long, narrow intraepidermal canal, c. 100 um in length, leading to a single ampulla or to two or three ampullae, each of which rests on the basement membrane. The ampulla is lined with a sensory epithelium. The canal wall is composed of very compactly packed two to three layers of flat cells. These cells which line the canal lumen up to where it opens on the surface are continuous with the surface layer of the epidermis. The cells are well demarcated from ordinary cells of the middle layer and surface layer of the epidermis in size, shape and orientation. Some gland cells occur in the canal wall especially at the junction between the canal and the ampulla. The sensory epithelium shows two distinct categories of cells: the smaller but more prominent sensory cells, and the larger and more numerous supporting cells. The sensory cells are exposed to the lumen of the ampulla by an appreciable apical surface. No hair-like process is present on the apical surface. No cupula is seen in the lumen.

Fig. 1 Transverse section of skin of C. batrachus showing an ampullary organ in vertical section; inset shows a magnified view of the ampullary organ. Note the canal wall, the sensory cells and the thick collagen layer in the dermis. × 30.

Fig. 2 Tangential section of the epidermis of C. batrachus showing the canal in transverse section. Note the canal wall and the lumen. × 300.

The skin has a moderately thick epidermis, c. 150 μm in thickness. The dermis shows a thick layer of densely set collagen bundles just beneath the basement membrane.

b) Heteropneustes fossilis (Bloch) (Figs 3 – 5)

The structure of the ampullary organ is similar to that described for C. batrachus. The canal wall is very distinct and clearly distinguishable from the adjacent epidermal cells. The canal wall is continuous with the surface layer of the epidermis at the opening of the ampullary organ. The canal length measures c. 80 μm. The skin is provided with a very thick layer of compact collagen bundles, lying next to the basement membrane. The epidermis measures c. 120 μm in thickness.

Fig. 3 An ampullary organ of H. fossilis in vertical section. Note the canal wall. × 260.

Fig. 4 Tangential section of the epidermis of H. fossilis showing the canal in transverse section. Note the canal wall and the lumen. × 550.

Fig. 5 Transverse section of the skin of H. fossilis. Note the thick collagen layer in dermis. × 220.

c) Rita rita (Ham.) (Figs. 6 – 8)

The skin has a very thick epidermis measuring c. 400 μm in thickness. The canal of the ampullary organ, however, is not more than c. 40 um in length. A dermal papilla supports the ampullary organ high up in the epidermis, compensating the shortness of canal length relative to thickness of epidermis. The canal wall is very prominent. Dermis has a very conspicuous and exceptionally thick layer of compact collagen bundles just below the basement membrane.

Fig. 6 An ampullary organ of R. rita in vertical section. Note the canal wall. × 400.

Fig. 7 Tangential section of the epidermis of R. rita showing the canal in transverse section. Note the canal wall and the lumen. × 400.

Fig. 8 Transverse section of the skin of R. rita. Note the very thick collagen layer in dermis. × 70.

d) Mystus vittatus (Bloch) (Figs 9 and 10)

The ampullary organ in this case differs from that of the above three catfishes in having a very short (c. 20 μm) and broad canal which distorts the ampullary profile into a wide-mouthed pit. However, the canal wall is still very clearly distinguishable as composed of specialised flat cells. The sensory epithelium is very prominent. The epidermis appears thin, measuring c. 50 μm in thickness. But in the dermis a thick layer of densely packed collagen bundles is present below the basement membrane.

Fig. 9 An ampullary organ of M. vittatus in vertical section. Note the canal wall. × 530.

Fig. 10 Transverse section of the skin of M. vittatus. Note the collagen layer in the dermis. × 230.

II Development of the ampullary organ in H....