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Some Histological and Physiological Responses of Radix Quadrasi and Pomacea Canaliculata (Mollusca: Gastropoda) to Metaldehyde and Niclosamide.
Thesis Abstract:
A semi-static acute toxicity test was conducted for 72 hours to determine some of the physiological and histological responses of Radix quadrasi and Pomacea canaliculata to molluscides, metaldehyde and niclosamide. P. canaliculata was exposed to 0.065, 0.1 and 0.125 mg/L niclosamide, and 0.75, 1.0 and 1.25 mg/L metaldehyde. On the other hand, R. quadrasi was exposed to lower concentrations of niclosamide: 0.045, 0.058 and 0.075 mg/L, and higher dosages of metaldehyde: 100, 112 and 130 mg/L. For the control, aged tap water was used. Thirty-five snails were used per treatment with five replicates each.
After the three-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 there for a week to determine if the two organs could recover from the molluscicidal effect. The rest of the surviving R. quadrasi was 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 P. canaliculata included the following: (1) increase in the number of calcium cells, (2) vacuolation of the digestive cells, (3) presence of cytoplasmic materials and calcium pigments in the digestive tubule lumen, (4) decrease in height of the columnar digestive cells and consequent increase in the size of the tubule lumen, (5) disintegration of the digestive tubules, and (6) widening of intertubular spaces. Almost similar effects were noted in R. quadrasi. But instead of calcium cells, which were absent in this species, it was the goblet-like cells, which became numerous, especially those exposed to the lowest concentration. In addition, the occurrence of many dark-staining cells on the bases of digestive tubules was very noticeable in those snails which received the highest dose.
The foot of the treated snails exhibited the following changes: (1) clumping and loss of cilia on the apical surface of the columnar epithelial cells of the epidermis, (2) breaks in some areas of the epithelium, (3) increased pigmentation of the subepidermal layer, (4) absence of secretion in the mucous glands, and (5) occurrence of more amoebacytes. Only those snails, which were 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 with treatment. However, the incubation period of the treated eggs was not statistically different from the treated ones.
The results of the study clearly showed that both niclosamide and metaldehyde could affect not only the target organism, P. canaliculata but also the non-target snail species, R. quadrasi.
