Morhological variation analysis and allozyme study of viviparid snall genus Mekongia Crosse & Fischer, 1876 in Thailand

Abstract

The viviparid snail genus Mekongia crosse & Fischer, 1876 is classified in order Mesogastropoda family Viviparidae. The snail is widely consumed in Thailand and neighboring countries, and also plays important roles in medical and veterinary science. It acts as intermediate hosts of many parasitic worms such as flukes. Snail usually lives on the bottom of fresh water habitat such as river, canals, and streams. In this study, shell morphological variations, reproductive system and allozyme genetic variation were analysed. Morphological variation revealed that shell height ranged from 10-30 mm to more than 30 mm in larger size, shell dextral with corneous operculum. The shell is subglobose, ovate-conic and elongate. Fourteen variables of shell measurements were used to discriminate the group of snails. The results from Canonical Discriminant Analysis showing the effective variables with order by size of correlation within function were shell height; shell width, aperture height, aperture width, body whorl width, spire height, penultimate whorl height, shell height/shell width, shell height/spire height, shell height / penultimate whorl height, shell width / aperture width, aperture height / aperture width, aperture width /spire height, and body whorl width / aperture width. The results indicated that measurement characters of shell measurement can be divided snails into eight groups congruent to Brandt’s (1974) classification. Moreover, the snails, M. swainsoni braueri revealed sexual size dimorphism, which shell of female larger than male. The comperative reproductive system revealed no significant differences between these organ systems in all taxa and population. Snail is dioecious, which sexes are separate and, in male the right tentacle is enlarged and it used for sperm transfer to the male. In addition, on dissecting snails for reproductive system study, water mite species, Unionicola sp. were found on gill surfaces. Nine enzymatic proteins used to detect genetic variations were esterase, glucose-6-phosphate isomerase, malate dehydrogenase, mannose-6-phosphate isomerase, peptidase (glycyl-L-luecine and luecyl-glycyl-glycine), 6-phosphogluconate dehydrogenase, phosphoglucomutase, and sorbitol dehydrogenase. Based on the data obtained for 13 allozyme loci, the allele frequencies, genetic variability, and F-statistics values were determined. The genetic diversity and genetic differences were estimated from allele frequency. The mean heterozygosities, mean allele number per locus, and the percentage of loci polymorphic were 0.281, 1.30, and 9.8, respectively. According to the Nei’s genetic distances, indices of pairwise comparison between two populations in this study ranged from 0.010 to 1.125. The genetic distances among of species ranged from 0.071 to 1.125 and among subspecies 0.089 to 1.125. Genetic distance of M. sphaericula spiralis comprised with other subspecies ranged from 0.907 1.125. These values are much greater than conspecific level. This is indicated the different identity of M. sphaericula spiralis from the recognized species. As from the above results, I have proposed the new classification of Mekongia as follows; M. pongensis, M. lamarcki, M. spiralis, M. swainsoni swainsoni, M. swainsoni braueri, M. swainsoni kmeriana, M. sphaericula sphaericula, and M. sphaericula extensa.