Are the feeding larvae of the commonest Antarctic asteroid really demersal?

Although many high latitude marine invertebrates brood their young, some of the most abundant and conspicuous species broadcast-spawn and have feeding larvae. The asteroid Odontaster validus is a prime example of the latter type; it occurs at high densities in shallow water around the antarctic continent, and it broadcast-spawns. In 1969 Pearse described the development of O. validus to the early bipinnaria stage, and he proposed that the embryos and larvae usually are demersal. We now have reared the larvae through metamorphosis; they pass through a typical brachiolaria stage and metamorphose after about 5.5 months. In cultures that were unstirred, the larvae usually swam near the bottom, but when stirred the larvae remained suspended in the water. A total of 14 larvae of O. validus were collected in plankton samples in McMurdo Sound, all taken near the surface; considering the large number of adults in the area that had spawned 1 to 5 months before the plankton collections were made, this is not a large number of larvae. However, no embryos or larvae of this species have been taken in plankton samples made near the bottom, or in bottom sediment samples. Whether the larvae are usually demersal or planktonic, and how they survive such a long period in the plankton if they are planktonic, remain unresolved.