Life history variations in two populations of Sphaerium rhomboideum (Bivalvia: Pisidiidae)

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Abstract
Distinctly different life history patterns of S. rhomboideum were observed in 2 permanent ponds within 1 km of each other near Guelph, Ontario [Canada]. Compared with the population from the large pond, the population from the small pond usually had larger numbers of newborn (40 vs. 20%) that did not grow immediately after birth, 3 birth periods (peaks in late May, July-August and Oct.-Nov.) instead of 1 (peak in Sept.), significantly smaller newborn (3.8-4.2 mm in length depending on birth period vs. 4.5 mm in length), significantly faster average summer growth rates (0.147 vs. 0.062 mm .cntdot. day-1), shorter lengevities (12 vs. 14 mo.), both semlparous and iteroparous reproduction (depending on birth period) as opposed to only iteroparous reproduction in the other population, and less mortality of larvae (58 vs. 72%) resulting in annual ratios of selection of 3.21 for the semelparous form and 4.39 for the interoparous form as opposed to 5.08 for the iteroparous form in the other population. Neither r, K, nor stochastic theories of life history tactics appear to predict the differences in life history patterns of these 2 populations of S. rhomboideum.