Bioenergetics of age-related versus size-related reproductive tactics in female Viviparus georgianus

Abstract

Individual Viviparus georgianus from a New York population are iteroparous, maturing in their second year. Ovoviviparous females have a maximum lifespan 1 year longer than that of males (4 + versus 3+ years), though individuals of both sexes can be reproductively active for three consecutive years. Embryophores are carried over winter and released each spring. Female survivorship is positively size related, as is incremental growth within any year. Reproductive effort is age related, increasing from 5.3% for modal 2-year females to 79.7% for modal 4-year females. Reproduction is size limited in 2-year females, no females below 16 mm shell length were found containing broods. The lack of observable survivorship penalties related to reproduction may result from reproductive females have greater biomass and hence greater survivorship. Spat size is positively correlated with female age irrespective of female size, though brood numbers increase with maternal size and growth rates. Caged experiments have shown spat born early to older females grow fastest. Data presented indicate that the young of older females have greater expected lifetime fecundity and fitness. In this population, 3-year females produce approximately 50.0% of each new cohort, with 2-year and 4-year females producing 21.0% and 26.0% respectively. Matrix models of the natural population and modified hypothetical populations emphasize the importance of survivorship and early reproduction in such stable, age-structured populations.