THE RELATIONSHIP OF BODY SIZE AND GROWTH TO EGG PRODUCTION IN THE HERMAPHRODITIC FRESHWATER SNAIL, HELISOMA TRIVOLVIS

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Abstract

Among several other factors, body size has been found to influence egg production in several species of hermaphroditic snail. We tested whether this relationship between body size and egg production exists in Helisoma trivolvis, a freshwater hermaphroditic species. We isolated 50 H. trivolvis from a laboratory population, measured shell diameter, and monitored egg production for seven weeks. We found a positive relationship between body size and total number of eggs produced, as well as body size and number of eggs per egg mass. When body size and egg production are linked, it should be adaptive for larger individuals to act as females and smaller individuals as males. Since body size is related to female fecundity in this species, the relative size of snails should determine, at least in part, which individual acts as male and which as female during copulation. However, the relationship between body size and egg production is not nearly as strong as it is in other snail genera. Other factors such as age, genotype and previous experience may be important in determining egg-laying capacity and therefore gender choice in this species. In addition, we found a negative relationship between growth during this period and egg production. This relationship has been found in other pulmonates, and is evidence of resource allocation tradeoffs.

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