Development and Metamorphosis of the Brittle Star Ophiocoma pumila: Evolutionary and Ecological Implications

The development to metamorphosis of the shallow-water tropical-Atlantic ophiuroid Ophiocoma pumila is described for the first time. This species possesses an eight-armed planktotrophic ophiopluteus larva that has a potential pelagic existence of three months (26-27.5°C), considerably longer than other brittle star species so far studied. This likely accounts for the amphi-Atlantic distribution of O. pumila. During metamorphosis, all of the larval arms are resorbed into the developing ophiuroid rudiment although the right antero-lateral arm disappears last. The metamorphosing larva transforms into a highly mobile vitellaria possessing transverse ciliary bands and tube feet. Ophiuroid vitellariae have hitherto been described only for species with abbreviated lecithotrophic development. The vitellaria of O. pumila can delay settlement for at least a week and probably functions as a pre-settling exploratory stage. The discovery of both an ophiopluteus and a vitellaria in the ontogeny of a brittle star strongly supports the proposal that ophiuroid vitellariae are related to ophioplutei and are not a divergent larval series. Abbreviated development in brittle stars with vitellariae may be the outcome of heterochrony caused by an acceleration of metamorphosis. Little is known about ophiuroid metamorphosis so it is possible that the ophioplutei of many other species pass through a vitellaria stage towards the end of their pelagic existence.