Structural changes induced by metaldehyde on the brackishwater pond snail, Cerithidea cingulata

After determining the 24, 48, 72, and 96-hr LC50 (6.1, 3.9, 2.8, 2.0 ppm, respectively), the snails were exposed to each corresponding LC50 concentration for each exposure time plus the control. The experiment was conducted in plastic basins (50 cm diameter) filled with brackish water (20-25 ppt) maintained at 5 cm depth. One hundred twenty fully acclimated snails (1-1.8 g and 15-30 mm shell length) were placed into each basin. There were four basins used per concentration. At exact time intervals (24, 48, 72, and 96 h), 10 snail samples from each treatment were processed for light microscopy. The foot, gut and digestive gland of some specimens were fixed in Bovin's solution (paraffin method) while others were fixed in 3% glutaraldehyde and 1% osmium tetroxide (transmission electron microscopy). Microscopic observation revealed that the epithelium of the foot became disorganized and swollen. The cilia that are normally present disappeared. The muscle tissue was also damaged, with larger spaces between fibers. A number of dark-staining cells, the amoebocyte had invaded the sub-epithelial layer. The degree of damage was greater among the 24-h LC50-treated snail. In contrast to the control, the epithelium of the 24-h treated digestive glands decreased in height. This consequently increased the size of the tubule lumen. Boundaries between cells were no longer discernible and cytoplasmic materials were seen in the lumen. The 72-h treated digestive glands became more pigmented due to the increase in the number of calcium cells. Loss of cellular details was also observed in some tubules. Results further showed that the mucosal epithelium of the treated gut became distorted. There was shrinkage of cells and its exudates were observed in the gut lumen. The presence of many amoebocytes in the intestine was highly remarkable. Based on the above findings, metaldehyde is a potent molluscicide against brackish water pond snail, C. cingulata. The histological damage observed can perhaps lead to the eventual death of the animal