Estimation and interpretation of egg provisioning in marine invertebrates

Per-offspring maternal investment is an integral part of life-history theory. To understand the evolution of per-offspring maternal investment in marine invertebrates, a number of mathematical models have been developed. These models examine how selection affects the proportion of maternally derived egg energy used to produce a newly metamorphosed juvenile (s) and make predictions about the distribution of s in nature. However, there are very few published values of s and therefore it is difficult to evaluate how well these models match nature. We present several equations to empirically estimate values of s for any group of marine invertebrate, and use data from echinoderms to compare the different equations. The calculations that directly estimate s require information on the amount of egg energy, juvenile energy, and energy metabolized during development. Currently, there are few data available for directly estimating s, and thus generating distributions of s derived from direct estimates is not possible. Furthermore, the direct estimations of s are informative for planktotrophy but not for lecithotrophy. We have developed an equation that can be used to directly estimate s for lecithotrophs. The calculations to indirectly estimate s only require egg energy or egg size for the species in question and the value of s and egg energy or size for a reference species. This reference species replaces the need to measure juvenile energy and energy metabolized during larval development. Because egg energy or size is currently available for many species, the indirect estimates will be useful for generating distributions of s, and will allow comparisons with models. Although these indirect methods are good for generating distributions of s, they do not provide reliable estimates of s for any particular species. Estimating values of s to compare models is a critical gap in our current evaluations of marine invertebrate life-history models.