Evo‐devo in the ophiuroid family Ophiocomidae

Abstract

The Ophiuroidea, the most diverse echinoderm class, exhibits an array of life history patterns and larval forms, the evolution of which is poorly understood. With a robust phylogeny available for the major family, the Ophiocomidae, we undertook an assessment of the evolution of development in these ophiuroids. We spawned 15 species and documented larval development in 14 of these. In total, data on egg size or larval development were available for 27 species across the four recognized genera (Breviturma, Ophiocoma, Ophiocomella, and Ophiomastix). Ophiocomids have a bimodal egg size distribution. Species with small eggs (x̄ = 82 μm diameter, range 55–100 μm diameter) and large eggs (x̄ = 424 μm diameter, range 335–550 μm diameter) have planktotrophic ophioplutei and lecithotrophic vitellariae, respectively. The advanced ophiopluteus transforms into an armless vitellaria prior to metamorphosis through resorption of the larval arms and rearrangement of the ciliary tracts into series of transverse bands. Thus, the Type II pattern of development (which includes both the ophiopluteus and vitellaria) may be characteristic of the Ophiocomidae. This is not seen in other ophiuroid families. Evolution of nonfeeding (lecithotrophic) development through a vitellaria larva appears to have occurred only once in this family, in the genus Ophiomastix. Several traits appear to be specific to some species or genera, including an ornate fertilization envelope (Ophiocoma species), the presence of an early barrel‐shaped larva (Breviturma species), and the presence of vibratile lobes in advanced ophioplutei (Breviturma, Ophiocomella). In species with lecithotrophic development, the vitellaria is distinct, with prominent ciliated lobes around the developing juvenile rudiment. Larval form may assist with identification of ophiocomid larvae in plankton samples.