Size and Organic Content of Eggs of Marine Annelids, and the Underestimation of Egg Energy Content by Dichromate Oxidation

Dichromate oxidation is a simple technique that is often used to estimate the energy content of eggs in studies of marine invertebrate life histories (1). We used this method to measure the energy contents of the eggs of 12 species of marine annelids. In combination with measures of egg ash-free dry weight (AFDW), these data yielded estimates of AFDW-specific energy density that were mostly lower than the average weight-specific energy density of carbohydrates. This seemed unlikely to be correct, as invertebrate eggs typically contain little carbohydrate and instead are composed primarily of energy-dense protein and lipid (1, 2). After validating our methods (by using them to estimate energy content and AFDW of the eggs of a previously studied echinoderm) and reexamining published data on the energy contents of echinoderm eggs, we conclude that dichromate oxidation often underestimates the energy contents of small eggs of marine invertebrates. This systematic error, which is likely related to the tendency of the assay to incompletely oxidize proteins, can only be corrected with substantial independent data on egg biochemical composition. We thus suggest that dichromate oxidation should not be used for routine measurement of the total energy content of marine invertebrate eggs. [PUBLICATION ABSTRACT]