PODOLSKY, Robert; ALLEN, Jonathan; University of North Carolina, Chapel Hill; University of North Carolina, Chapel Hill: Egg size and the evolutionary transition to non-feeding development in Macrophiothrix brittlestars

Marine invertebrates exhibit broad diversity in developmental form and life-history mode. Efforts to understand the origins of this diversity have focused on processes that underlie transitions between modes. One widespread transition has involved the evolution of a morphologically simplified, non-feeding larval form from a morphologically complex feeding larva. This transition has been inferred in diverse taxa where both modes of development are known from closely-related species. There are, however, few examples of clades that include both modes of development as well as functionally-intermediate stages in the transition between them. The brittlestar genus Macrophiothrix is species-rich and common in certain Indo-west Pacific coral reef habitats. Twelve species at Lizard Island, Australia are similar as adults but vary more than 60-fold in average egg volume, and egg size is correlated with a suite of traits involving size, form, growth, and timing. In addition, early life-history stages in the genus include simple non-feeding larvae, complex feeding larvae, and two larval forms that are functionally intermediate. Diversity of larval form and function in Macrophiothrix provides evidence of a life-history transition involving sequential changes in morphology, feeding necessity, and feeding capacity that are resolvable in evolutionary time. Estimates of times to metamorphosis also allow a comparison of the ecological conditions under which different egg sizes, and their associated life-history modes, would be favored by selection.