File:American malacological bulletin (1987) (17968739940).jpg

Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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60 AMER. MALAC. BULL. 5(1) (1987)
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TIME (DAYS) • Fig. 7. Growth curves (as increases in mean shell length) of Pisidium casertanum reared under various water hardnesses. Clams were taken from either Riopel Pond (RP) or Farriers Pond (FP). on Greenslade's model. In a predictable and favorable habitat (e.g. a lake) one finds long life span, an increased maximum shell length, an increased age at first reproduction and the production of only one brood per year. These traits are associated with "K-selection" and again are expected based on Greenslade's model. One important point of the principal component analysis is that strict covariation of life history traits is not found. Variable and stable habitats which are both favorable (e.g. a stream and a lake) may display similar ages at first reproduction, number of generations produced per year and maximum shell lengths attained (at least not separable based on principal component analysis) but they do differ considerably in life span (Fig. 9, Table 7). In addition to the variation in life history traits noted above, there are differences in physiological traits in these two populations. Hornbach (1985) has shown that metabolic rates of clams from FP may be as much as 11 times higher than for individuals from RP at comparable temperatures. The lowered overall metabolic rate of clams from RP can lead to a lowered amount of ingestion and assimilation and could result in the smaller shell lengths (Figs. 1 and 3) and reduced reproductive output (Fig. 2, Tables 2 and 4) noted for this population, again attesting to the harsh environmental con- ditions in RP. The question of interest is how much of the variation in life history traits that is noted interspecifically is due to genotypic differences in populations and how much of the variation is totally environmentally induced. Brown (1985a) has found that much of the intraspecific variability in popula- tions of pulmonate snails is environmentally induced and Russell-Hunter (1978) claims that much of the variation in life histories in freshwater snails is also due to phenotypic plastici- ty. Little work has been conducted on the importance of en- vironment vs. genotype in life history variation in freshwater clams. The data presented here provide some insight to these questions. The transfer experiments show that there are both en- vironmental influences on the expression of particular life history traits and potentially some genetic influences. For ex- ample, the increased reproductive output by individuals from RP transferred to FP (Table 4) shows an environmental effect, but the fact that the reproductive output does not reach the levels of those clams from FP indicate that the pond of origin (or differential genotype or developmental history) can also in- fluence this life history trait. It is also possible, however, that the increase in reproductive output was only due to increased embryonic survivorship and that the transfer experiments were too short to allow for the assessment of changes in fer- tility which could allow clams from FP to rival the fecundity of individuals from FP. If, however, the birth rates of transfers are representative of true phenotypic shifts and the differ- ences in birth rates noted for clams in their home ponds has a genetic component, then the changes in birth rate noted may be an example of cogradient selection where the

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