Reprodução e parasitismo em Melanoides tuberculata (Müller, 1774) (Gastropoda, Thiaridae) da Vila do Abraão, Ilha Grande, Angra dos Reis, RJ

Abstract

Melanoides tuberculata (Müller, 1774), an exotic freshwater snail of Afro-Asiatic origin, was first recorded to Vila do Abraão, Ilha Grande, Angra dos Reis, Rio de Janeiro, in 2005. It acts as first intermediate host for trematodes of medical importance. The populations are mostly composed by parthenogenetic females that have a brood-pouch, where occurs the development of the juveniles. The objectives were to verify the presence of males; to check if there are fluctuations in the production of eggs and juveniles along time, to determine when sexual maturity is reached and to check for parasites and relate their presence with the number of eggs and juveniles. To count eggs and juveniles, we used females collected bimonthly from September/07 to October 07/10, separated into four classes according to the diameter of the shell: Class I: < 3mm; Class II: 3 to 5.99mm; Class III: 6 to 8.99mm; Class IV: > 9mm. We selected five females/class, totaling 20 females/collect, except for the samples of September/07 (16 specimens), April/09 (19 specimens) and April/10 (13 specimens), totaling 348 females. To observe parasites, eggs and juveniles, the molluscs had the roof of the cavity and brood-pouch dissected. The juveniles were separated into classes according to the number of shell whorls: less than two whorls, with two to four whorls, and more than four whorls. The data was analyzed using an Excel spreadsheet and the statistic package SYSTAT 12. For histology of gonad and brood-pouch, five specimens of each of the four size classes, collected monthly from September/12 to February/13 were used, totaling 20 specimens/collect except to the sample from October/12, with 16 specimens since only one specimen from Class IV was found. The specimens were fixed in Carson s Millonig formalin, decalcified with EDTA, embedded in paraffin and stained with hematoxylin and eosin. We found 16 males. We count 24.694 eggs and 31.474 youngs in total, showing the following averages: Class I: 0 eggs and 0.31 juveniles, Class II: 24.19 eggs and 47.56 juveniles, Class III: 114.84 eggs and 155.78 juveniles, Class IV: 146.76 eggs and 158.41 juveniles. The Kruskal-Wallis test showed that there are significant variation in the number of eggs and juveniles along the year (p<0.01 for eggs and juveniles). It was found a significant difference between the number of eggs and juveniles according to different shell size classes (p<0.01 for both). We count 27.877 juveniles with less than two whorls, 3.251 with two to four whorls and 346 juveniles with more than four whorls. We found 111 females parasitized by Centrocestus formosanus (Nishigori, 1924) out of 348 females (32% of total). The Mann-Whitney test showed that the parasitized females presented a smaller number of eggs and juveniles compared to the nonparasitized (p<0.01 for both).The Mann-Whitney test excluding females from Class 1 show the same result with p<0.01 for both. We conclude that the population of M. tuberculata from Vila do Abraão isn t composed only by females; the females reproduce during all the year, because eggs and juveniles were found in all the field surveys, and larger females have more eggs and, on average, more juveniles in the brood-pouch; the reproductive maturity should begin with approximately 3mm shell diameter, because we observed late vitelogenic oocytes in the ovary and that parasitism affects negatively M. tuberculata, reducing the number of eggs and juveniles formed