I investigated life-history and parasitism in the salt marsh snail, Ce
rithidea californica Latitude and growing conditions were important fa
ctors determining maturation size. After accounting for environmental
variation, there was a negative association between the maturation siz
e of snails and the prevalence of parasitic castration by larval trema
todes. As predicted by life-history theory, this may represent an adap
tation against parasitism that is similar to previous observations of
life-history adaptations in species subject to predation or disturbanc
e. However, it was unclear whether this adaptation was due to phenotyp
ic plasticity or genetic differences among populations resulting from
natural selection so I conducted a reciprocal transplant between sites
with high and low prevalence and found source population differences
in maturation size. It appears, therefore, that the life-history diffe
rences between.these populations are at least partially genetic or may
represent an adaptive developmental switch that was initiate prior to
the transplant.