Mitochondrial DNA phylogeny and rates of larval evolution in Macrophiothrix brittlestars.

Phylogenetic analysis has led to significant insights into the evolution of early life-history stages of marine invertebrates. Although echinoderms have been a major focus, developmental and phylogenetic information are relatively poor for ophiuroids, the most species-rich echinoderm class. We used DNA sequences from two mitochondrial genes to develop a phylogenetic hypothesis for 14 brittlestar species in the genus Macrophiothrix (Family Ophiotrichidae). Species are similar in adult form and ecology, but have diverse egg sizes and modes of larval development. In particular, two species have rare larval forms with characteristics that are intermediate between more common modes of feeding and non-feeding development. We use the phylogeny to address whether intermediate larval forms are rare because the evolution of a simplified morphology is rapid once food is no longer required for development. In support of this hypothesis, branch lengths for intermediate forms were short relative to those for species with highly derived non-feeding forms. The absolute rarity of such forms makes robust tests of the hypothesis difficult.