Toxicopathological impact of cadmium chloride on the accessory respiratory organ of the air-breathing catfish heteropneustes fossilis
The air-breathing catfish Heteropneustes fossil/s (Order: Siluriformes; Family: Heteropneustidae) is a cherished table fish in India and is distributed throughout the Indian sub continent in various fresh water ecosystems including muddy, marshy and derelicts ponds having low levels of water and dissolved oxygen. They are seen even in contaminated water bodies also. In fact, the presence the accessory respiratory organ (ARO) keeps the fish fit enough to survive in these oxygen deficient water bodies by means of aerial mode of respiration. The presence of ARO also enables the fish to temporarily stay out of water for hours together through aerial mode of respiration. This ability is being exploited in marketing the fish in live condition. ARO has a common embryological origin with gills and also has histological similarity with the latter but it essentially differs from gills in having no direct contact with the aquatic medium (Munshi, 1962). Through ARO, H. fossil/s respires aerially by gulping in air above the water surface. Cadmium is a non-essential, non-biodegradable element with no known biological ifinction and is reported to be a major contaminant of aquatic ecosystems causing adverse effects on aquatic organisms (Hollis et al., 1999). It is released from diverse sources such as electroplating, paper, PVC plastic, pigments and ceramic industries, battery, mining and smoldering units and many other modem industries (Gupta et aL, 2003). Many workers have reported the manifestation of toxic effects of cadmium on the gills of fishes (Randi et aL, 1996; Hollis et al., 1999). Even though the ARO plays a pivotal role in the survival of the species, almost no data is available on the toxicity of cadmium on this vital organ system of H. fossil/s.