Some histological and physiological responses of Radix quadrasi von Moellendorf and Pomacea canaliculata Lamark (Mollusca: Gastropoda) to metaldehyde and niclosamide

Results of the study revealed that after the 3-day exposure period, the digestive gland and foot of some survivors were dissected out and processed for light microscopy. The others were transferred to aged tap water and kept here for a week to determine if the two organs could recover from the molluscicidal effects. The rest of the surviving R. quadrasi were maintained in aged tap water for one month and their reproductive capacity determined. Results showed that both metaldehyde and niclosamide altered the histological structure of the digestive gland and foot of the two snail species. The extent of damage was dose dependent. The common histopathological changes observed in the digestive gland of Pomacea canaliculata included the following: increased in the number of calcium cells, vacuolation of the digestive cells, presence of cytoplasmic materials and calcium pigments in the digestive tubule lumen, decreased in height of the columnar digestive cells and consequent increase in the size of the tubule lumen, disintegration of the digestive tubules, and widening of intertubular spaces. Almost similar effects were noted in Radix quadrasi. But instead of calcium cells, which are absent in this species, it was the goblet-like cells while became numerous, especially among those exposed to the lowest concentration. In addition, the occurrence of many dark-staining cells on the bases of digestive tubules were very noticeable in those snails which received the highest dose. The foot fo the treated snails exhibited the following changes: clumping of loss of cilia on the apical surface of the columnar epithelial cells of the epidermis, breaks in some areas of the epithelium, increased pigmentation of the sub-epidermal layer, absence of secretion in the mucous glands, and occurence of more amoebocytes. Only those snails which are exposed to lower dosages showed some signs of repair in their digestive glands and foot. Niclosamide and metaldehyde also significantly reduced the reproductive capacity of R. quadrasi. The number of egg masses/eggs laid, varied the treatment. However, the incubation period of the treated eggs was not statistically different from the treated ones. The results of the present study clearly show that both niclosamide and metaldehyde could affect not only the target organism, P. canaliculata but also the non-target snail species, R. quadrasi