Ecophysiology of strains A and C of Potamophyrgus jenkinsi
The life history parameters growth, survivorship and fecundity were investigated in strains A and C of the clonal apomictic snail Potamopyrgus jenkinsi. Both strains were obtained from separate habitats. Strain A from an inland freshwater stream and strain C from a boating pond which receives seawater and is drained every winter. When reared from birth in constant laboratory conditions the two strains were shown to differ in life history strategy. Strain C showed rapid growth, matured early and at a relatively small size. Hence it had greater reproductive effort. Strain A delayed growth, matured relatively late in its life history and hence at a larger size. The results in this work suggests that strain A and C are distinct clones, which support recent electrophoretic studies. In manipulated environments of food stress and increased salinity the two strains continued to show significant differences. Both strains were plastic in their growth, reducing growth in response to decreasing food availability and high salinities. However at 5% and 10%SW the two strains showed an increase in growth from that of the control. Their reproductive output also decreased with decreasing food supply and increasing salinity. However, strain A was more sensitive to food and salinity stress, ceasing to release young at low ration and starved environments and at salinities of 20%SW and above. Strain C continued to release young in all environments, albeit at a reduced rate and at a cost to survivorship. A yearlong field study on the size-frequency distribution of the two strains in their natural habitat showed the life history of strain A to approximate `IC'-selection and strain C to approximate `e-selection.