COWART, J.D.*; MINER, B.G.: A Re-evaluation of Egg Energetics for the Facultative Planktotroph Clypeaster rosaceus (Echinodermata: Echinoidea)

The sand dollar, Clypeaster rosaceus, has an unusual reproductive strategy (facultative planktotrophy) that is intermediate between the two common types of echinoid larval development: planktotrophy and lecithotrophy. The larvae of C. rosaceus can complete development and metamorphosis entirely on maternally provisioned egg energy, even though they still possess the ability to capture and metabolize exogenous food. Clypeaster rosaceus is representative of a level of egg provisioning close to, but greater than, the minimum necessary for lecithotrophic development. Any lower levels of egg provisioning results in larvae that are obligate planktotrophs, with development and larval survivorship limited by the availability of planktonic food. Although rare, facultative planktotrophs may represent an important transition in the evolution of marine invertebrate life-history strategies. The original value of C. rosaceus egg energy content reported by Emlet (1986) seems low when compared to species with similar egg sizes. Considering the importance of C. rosaceus in estimating values of s in marine invertebrate life-history models, we reexamined its egg energy content. Our measurement of egg energy content was five times greater than Emlet’s (0.11 joules egg^-1 vs. 0.020 joules egg^-1, respectively) with no difference in egg diameter (274.2 �m vs. 280.3 �m, respectively). It is unlikely that this result is due to temporal or spatial variation among C. rosaceus populations. Given the two techniques used to measure egg energy content and a closer match with predicted values from the regressions of McEdward and Morgan (2001) we conclude that our estimate is more accurate.