Genetic and Life History Characteristics of Venustaconcha Ellipsiformis and Venustaconcha Pleasii (Bivalvia, Unionidae) in the Ozark Plateaus Region
Genetic, morphological, and life history characteristics were compared among 7 populations of two closely related taxa of freshwater mussels in river systems of the Ozark Plateaus. Venustaconcha pleasii (White River system) exhibited fixed electrophoretic allelic differences from V. ellipsiformis (Osage and Meramec basins) at 3 of 11 loci, and a 3.9% sequence divergence (mtCOI gene, 330 bp). Shell size and shape also differed between these species. These results support the classification of V. pleasii as a species, rather than a subspecies. Venustaconcha from the Spring River (Neosho basin) were previously classified as either V. pleasii or V. ellipsiformis. When compared to Osage basin and Meramec basin V. ellipsiformis, these mussels had no fixed allelic differences and a sequence divergence of only 0.9-1.5%, but differed by 3.1% from V. pleasii. This result supports classification of the Spring River mussels as V. ellipsiformis. Rainbow darters (Etheostoma caeruleum) were an important natural host for V. pleasii in the James River (White system), but not for V. ellipsiformis, which utilized orange throat darters (E. spectabile) in the Spring River. In laboratory tests, 31% of V. pleasii glochidia transformed on sympatric rainbow darters but only 6% and 3% of Spring River and Meramec River V. ellipsiformis transformed on these fish. Transformation success varied among individual fish from zero to over 80%. These results demonstrate that host relations differ among closely related unionid species. Length at age of 426 individual Venustaconcha suggesting that sexual maturity usually occurs at age 4. Females grew more slowly than males after age 4. Growth rates for 1993 were significantly depressed in 4 of 7 populations examined, and this result is attributed to unusually heavy preciptitation in that year. The appearance of this signal at the same inferred age in widely separated sites validates the annual nature of external growth lines in this species.
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