Ultrastructural observations in gills and hepatopancreas of prawn Macrobrachium malcolmsonii exposed to mercury.

The juveniles of M. malcolmsonii were exposed to 24.1 microg l(-1) of Hg fora period of 21 days. The gills and hepatopancreas of test prawns were sampled and processed for electron microscopic observations. Mitochondria are the organelle most affected in the gills of test prawns. The number of mitochondria and the electron-density of the matrix were found to be less in test prawns. The in-folding of cell membrane associated with mitochondria was absent in test prawns. This suggests that operation of the mitochondrial pumps was affected in the gills of test prawns. Vacuoles with crystalline granular inclusions were noted in the gills of test prawns. These are suggestive of metal-rich inorganic deposits or granules representing detoxified dumps of Hg. In the hepatopancreas of test prawns, the tubules exhibit vacuoles with granular inclusion and the cell cytoplasm contains electron-dense granules, which indicate a storage detoxification of Hg. The mitochondria were shrunken in the hepatopancreas of test prawns. This suggests attenuation of its function. The rough endoplasmic reticulum appeared vesiculated and dilated. These reactions denote the hyperactivity of the rough endoplasmic reticulum. Membranous whorl-like structures with myelin fibers and residual bodies were seen in the hepatopancreas of test prawns. Such structures indicate the involvement of lysosomal breakdown in detoxification process. The ultrastructural alterations are suggestive of the operation of compensatory mechanisms within the test prawns to enable it to tolerate Hg toxicity. However these alterations would have an impact on the cellular integrity of the gills and hepatopancreas and such alterations can be taken as 'biomarkers' for assessing Hg pollution in the aquatic environment.