Brood reduction: A reproductive strategy of the freshwater clam Sphaerium striatinum.

摘要

The principal benefit of brooding lies in its overall effect on growth, survivorship, and eventual breeding success of offspring. However, the benefits of brooding may be opposed by substantial costs. Often, parents produce more offspring than they can successfully brood to independence. As a result, parents may need to reduce brood size. The assumption that offspring mortality represents a loss in parental fitness assumes that initial number of offspring parents produce size is correct. Surplus offspring may act as energy reserves which assist both parents and siblings during fluctuations in food availability. Furthermore, surplus offspring insure parents some minimal level of reproductive success if some offspring fail to develop. Thus, brood reduction may be an advantageous strategy in environments where food availability and offspring survivorship are highly variable.; Here I investigate the occurrence of brood reduction in the freshwater clam Sphaerium striatinum. S. striatinum is a sequential brooder and produces small cohorts of offspring throughout the year. The offspring are brooded in marsupial sacs located within the inner demibranch. However, up to 97.5% of the offspring parents initially produce fail to reach independence. Such high brood mortality is indicative of severe constraints on brood size. The results indicate that adult size limits brood capacity. Adults produce far more offspring than they are capable of rearing to independence. The evolution of sequential brooding and marsupial sacs represent the primary constraints on brood size in this species. Variation in food availability has an effect on initial fecundity, brood size, and number of reproductive attempts. However, increased food availability results in increased rates of brood reduction. Thus resource availability plays a small role in brood reduction in S. striatinum. Rearing experiments indicate brood reduction occurs through sibling competition for nutrients and space. Additionally, surplus offspring act as insurance against defective embryos. Finally, field experiments indicate offspring size at independence plays a key role in regulating brood reduction. The evolution of brood reduction in Sphaerium striatinum allows parents to adjust brood size to variation in food availability, disturbance frequency, and offspring survivorship within the upper limit set on brood capacity by adult size.