Studies on Tricula Aperta (Temcharoen) and Related Taxa (Gastropoda: Hydrobiidae), the Molluscan Intermediate Hosts of Schistosoma Mekongi.

Abstract

The geographical distribution, habitats, methods of collecting, maintaining and culturing, and biological relationships of the so-called alpha, beta and gamma races of Tricula aperta, two other unnamed phenotypic taxa (pigmented and non-pigmented) of T. aperta, and Manningiella (? = Tricula) conica were studied. The alpha and gamma races were found in the Mekong River, ranging from Ban Sabouxai, Savannakhet Province, Laos, to Ban Dan, Ubon Province, Thail and ; the gamma race extended to Sompamit Falls in southern Laos as well, and possibly to Kratie in Cambodia; the beta race of T. aperta occurred exclusively in the Mune River (a tributary of the Mekong) at Phibun, Thail and . The two unnamed phenotypes of T. aperta were found only in the Mekong River above Sompamit Falls near the Laos-Cambodia border. M. conica was found in the Mekong River, mainly in front of Moung Khong Hospital, Khong Isl and , Laos. All of these snails preferred to live only on solid substrates. They may live at various depths, and occur in either quiet or swiftly flowing water. To collect the snails, a h and -picking technique was found to be the best. Small glass or plastic bottles with air tight lids and plastic thermos bottle were found to be excellent for transportation, but they must contain washed rocks, conditioned water and diatoms. Raising the snails to maximum size in the laboratory was quite difficult. Although, small bottles and crystallizing dish vivaria were successful, a prototype stream aquarium was found to be one of the best ways to raise them. Medium organic-rich mud, small rocks and diatoms are essential for culturing and maintaining these snails. Susceptibility of the six groups of snails to infection with Schistosoma mekongi was determined. All of the populations tested were found to be susceptible to S. mekongi, but to different degrees. Chromosome cytology and isozymes were studied in the alpha, beta and gamma races of Tricula aperta. Haploid chromosome numbers in males ranged from 13-17 in the alpha and 14-17 in both the beta and gamma races. Females in the alpha and gamma races had the haploid chromosome numbers of 16 or 17, and 17 in the beta race. Variation in chromosome pairing at male meiosis occurred. The basic haploid chromosome numbers appeared to be 13 in the alpha race and 14 in the beta and gamma races. The chromosome numbers of 2n + 1 were seen in the males only. These extra chromosomes of all three races of T. aperta may be related to sex determination. Males would have an XO chromosomal sex determinating mechanism, and females would have XX. Diploid chromosome numbers also showed variation. Beta males had 2n = 31 and 33, while the alpha and gamma males had 2n = 29, 31 and 33. Beta females contained 2n = 34, whereas both the alpha and gamma races had both 2n = 32 and 34. Karyotypes of spermatogonial mitotic metaphase of these three races showed distinct morphological differences. Genotypic differences between the three races of Tricula aperta were observed in the electrophoretically separated isozymes alkaline phosphatase, esterase and glutamate-oxaloacetate transaminase. The differences between the "alpha", "beta" and "gamma" races of Tricula aperta shown in the present study are of special importance, because they indicate that probably these three races are distinct species.