Life-cycle variation of Musculium partumeium (Bivalvia: Sphaeriidae) from a temporary and a permanent pond in Minnesota

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Abstract
Variations in the life cycle of the fingernail clam Musculium partumeium were noted both within and between two ponds in Minnesota over a 34-month period. One of the ponds was permanent, and M. partumeium displayed a spring and a fall generation. There was little or no growth of the spring-born generation over the summer. The other population inhabited a pond that was normally ephemeral. In the years when the pond dried in the fall, only a spring generation was produced. In the odd year that the pond remained wet in the fall, a second generation was produced and the ephemeral-pond population switched to a "permanent-pond" life cycle. A transfer experiment between the two ponds was carried out and indicated that the differences in life cycle were phenotypic in nature. Clams transferred between the ponds displayed the life cycle of the pond to which they were transferred rather than of the pond from which they were removed. There were differences in population density between the ponds. The permanent-pond population apparently died out during the study period. Comparisons of the life-cycle traits with other published reports for M. partumeium indicate that there was more within-population than between-populations variation in life cycle, even when examined over a latitudinal gradient. The populations of M. partumeium from Minnesota, however, did appear to be more dense than more southerly populations of this species.
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