P J C Honkoop | PubMed Health

PubMed2004

A general solution for optimal egg size during external fertilization, extended scope for intermediate optimal egg size and the introduction of Don Ottavio 'tango'.

Egg sizes of marine invertebrates vary greatly, both within and between species. Among the proposed causes of this are a trade-off between egg size, egg number and survival probability of offspring, and a selection pressure exerted by sperm limitation during external fertilization. Although larger eggs are indeed a larger target for sperm, producing larger eggs also implies making fewer of them. There has been discussion about whether sperm limitation can (theoretically) and does (in nature) select for larger egg size than under ad libitum sperm. In one specific model, based on a particular fertilization kinetics model and an empirically derived mortality function, the theoretical possibility of a negative shift in optimal egg size with sperm concentration was demonstrated. Here we present a generalized analytical model to explore the effects of survival and fertilization probabilities on optimal egg size. It is demonstrated that incorporating fertilization kinetics greatly increases the scope for intermediate optimal egg size, as opposed to eggs of minimal or maximal size. Second, we present a general analytical qualitative solution to the question whether optimal egg size depends on sperm concentration. It is shown that, under the condition that an intermediate optimal egg size exists, this qualitative outcome of the model (positive, negative or no relation between optimal egg size and sperm limitation) depends on the structure of the fertilization kinetics part of the model. Finally, we evaluate fertilization kinetics models with respect to the general solution, using two previously published kinetics models ('Don Giovanni' and 'Don Ottavio') and a novel alteration of one of them in which sperm concentration covaries with egg concentration (Don Ottavio 'tango'). For all three models the relationship between optimal egg size and sperm concentration is shown to be always negative. This paper thus shows how biologically realistic relationships between egg size on the one hand and survival and fertilization probability on the other hand predict optimal egg size to be intermediate, and that this optimum is in general expected to increase when sperm become more limiting.

Animals↗

PubMed2003

Physiological costs of reproduction in the Sydney rock oyster Saccostrea glomerata. How expensive is reproduction?

In this study, triploid Sydney rock oysters Saccostrea glomerata, which do not reproduce and have only limited gonadal development, were used to calculate the cost of producing and maintaining somatic tissues. The consumption of oxygen was measured and converted to units of energy expended. The consumption of oxygen of diploid oysters, in different stages of the reproductive cycle, was also measured. Knowing the costs of producing and maintaining somatic tissues (obtained from the triploid oysters), it was possible to calculate the energy demand of somatic and reproductive tissues of diploid oysters. The focus of this study was to test whether this method would work, to investigate if this method would give results in accordance with modern life-history theory and to test hypotheses about costs of reproduction in oysters. It was found that in diploid oysters, 27% of the consumed oxygen was needed for reproductive processes. It was also found that the costs of production and maintenance of reproductive tissues were on average 84% of those of somatic tissues. Costs for the production and maintenance of somatic tissues decreased over time. Costs for reproduction also decreased, but were dependent on the stage of gonadal development. If the relative mass of gametes in the gonads was large, the costs were relatively small; if the mass was relatively small, the costs were large. Differences between traits of males and females were never significant, suggesting that reproductive effort and costs were similar in males and females. It was estimated that if diploid oysters did not reproduce, they could gain 64% more somatic ash-free dry mass. Thus, in terms of growth, reproduction is an expensive activity.

Animals↗