Heyland, A.*; Hodin, J.: Endogenous Thyroxine production in Sand Dollar larvae and its implication for the evolutionary loss of larval feeding

Studies on thyroid hormone [T3 and thyroxine (T4)] effects on metamorphosis in amphibians have begun to elucidate how changes in thyroid hormone production and cellular responses underlie alterations in life history patterns. Recently, work on several echinoderm species has demonstrated that thyroxine accelerates metamorphosis in this group as well. Interestingly, the feeding larvae of several species of sea urchins seem to obtain thyroxine through their diet of planktonic algae, while non-feeding larvae of the sand dollar Peronella Japonica endogenously produce thyroxine. In order to understand the role of thyroxine production in the evolutionary shift from feeding (planktotrophic) to non-feeding (lecithotrophic) larval development, endogenous thyroid hormone production in feeding larva has to be investigated more broadly in echinoderms. Here we examine the effects of thyroxine and a thyroxine synthesis inhibitor on the development of Dendraster excentricus, a sand dollar with a feeding larva. Thyroxine synthesis inhibitor treatments indicate that Dendraster excentricus larvae endogenously produce thyroxine in addition to the exogenous thyroxine source. Moreover, time to metamorphosis and juvenile size were significantly reduced in the thyroxine treatments, which emphasizes the important role of thyroxine in the development of this species. We also report that skeleton length was reduced and vestibule invagination progressed faster in the thyroxine treatments when compared to controls. Endogenous TH production might be a shared feature among sand dollars, which facilitates the evolution of lecithotrophy. Mounting evidence on the effects of thyroxine in echinoderm development suggests life history models need to incorporate the evolution of thyroxine production.