HEYLAND, A.; REITZEL, A.M.; HODIN, J.: Facultative feeding in an obligatorily feeding sand dollar larva: The role of thyroid hormones in echinoderm life history evolution.

Lecithotrophic development has evolved from planktotrophic development numerous times in echinoderms and other marine invertebrates. Lecithotrophic larvae develop from relatively large yolky eggs that provide the organism with sufficient energy to develop to metamorphosis without exogenous food; planktotrophic larvae develop from smaller eggs and require exogenous food to develop to metamorphosis. Some echinoid and gastropod larvae have the capacity to feed facultatively (facultative planktotrophy). These larvae can develop to metamorphosis without food although functional feeding structures are present. Here we experimentally induce facultative feeding in obligatorily feeding larvae of the sand dollar Leodia sexiesperforata with the vertebrate thyroid hormone thyroxine (T4). In other words, Leodia sexiesperforata larvae treated with thyroxine can develop to metamorphosis in the absence of food. We also show that larvae of the sand dollars Dendraster excentricus and Melitta quinquiesperforata, which develop from much smaller eggs than those of Leodia sexiesperforata, do not have the capacity to develop to metamorphosis without food when treated with thyroxine. Juveniles of Leodia sexiesperforata from the experimental treatment were significantly smaller than juveniles developing with exogenous food, while adult features such as spines and skeletons appeared normal. Based on the present data we hypothesize that thyroxine plays a major role in echinoderm metamorphosis and the evolution of life history transitions in this group. Moreover, we suggest that juvenile size might have a much stronger influence on adult fitness in echinoids than had been previously recognized.