Eggs as energy: Revisiting the scaling of egg size and energetic content among echinoderms

摘要

Marine organisms exhibit substantial life-his-tory diversity, of which egg size is one fundamental param-eter. The size of an egg is generally assumed to reflect the amount of energy it contains and the amount of per-off-spring maternal investment. Egg size and energy are thought to scale isometrically. We investigated this relation-ship by updating published datasets for echinoderms, in-creasing the number of species over those in previous stud-ies by 62%. When we plotted egg energy versus egg size in the updated dataset we found that planktotrophs have a scaling factor significantly lower than 1, demonstrating an overall trend toward lower energy density in larger plank-totrophic eggs. By looking within three genera, Echino-metra, Strongylocentrotus, and Arbacia, we also found that the scaling exponent differed among taxa, and that in Echinometra, energy density was significantly lower in species with larger eggs. Theoretical models generally as-sume a strong tradeoff between egg size and fecundity that limits energetic investment and constrains life-history evo-lution. These data suggest that the evolution of egg size and egg energy content can be decoupled, possibly facilitating response to selective factors such as sperm limitation which could act on volume alone.