Growth, reproduction and longevity in the hermaphroditic freshwater snail Helisoma trivolvis

Life-history evolution often results in trade-offs between reproduction, growth and longevity. We investigated the relationships among these traits in a hermaphroditic freshwater snail, Helisoma trivolvis, by manipulating opportunities for reproduction and measuring body size and egg production throughout the lifespan. Snails were placed in one of four treatment groups: snails isolated for their entire lives, snails housed with a partner for 1 week at 18 weeks, snails housed with a partner for 1 week at 26 weeks and snails afforded 1 week of mating opportunities six times between 18 and 97 weeks. We monitored egg production and shell diameter regularly throughout the experiment until all snails had died. Isolated snails laid almost no eggs, confirming a low level of self-fertilization in this species. For both groups of snails with only one mating opportunity, the average duration of egg production was 16.8 weeks. Snails in the multiply-mated group continued to lay about 1 egg mass per day with an average of 19-25 eggs per mass for 48 weeks and then egg production decreased. Although all of the snails grew consistently throughout the experiment, when snails were actively laying eggs their growth slowed relative to those no longer producing eggs. There were no significant differences in longevity among snails that were isolated and never mated, those that mated once (either early or later in life) and those that had multiple mating opportunities and continued to lay eggs throughout their lifetimes. These overall patterns of growth demonstrate that costs of reproduction may result in trade-offs in the short term, but not in the long term.