Echinoderms: Sea-Urchins and Brittle-Stars

Abstract

Adult sea-urchins, with their rounded, rigid bodies and spacious coeloms, look very different from adult brittle-stars, with their snake-like arms and restricted coeloms, but the majority of species in both classes have a unique form of larva, the pluteus. Its uniqueness lies in its internal skeleton of slender calcareous rods that supports its ciliated arms. Hyman (1955, p. 700), in her volume on The Echinodermata, thought it impossible to account for the occurrence of pluteus larvae with similar skeletal rods in both echinomorphs and ophiuromorphs “except on the basis of some community of ancestry,” and a similar view was expressed by Jägersten (1972) in his book Evolution of the Metazoan Life Cycle. MacBride (1914, p. 511), in his Text-Book of Embryology, went so far as to say that the differences between echinoplutei and ophioplutei were “of minor taxonomic importance, and would be such as one would expect to find separating the larvae of two families.”

Sea-urchins and brittle-stars very different as adults, very similar as pluteus larvae—Echinoplutei and ophioplutei only larvae with calcareous skeletal rods—Convergent evolution rejected as explanation—Postulated that echinomorph acquired larva from asteromorph then evolved skeletal rods (first pluteus); ophiuromorph acquired pluteus from echinomorph—Some biochemical similarities between adult sea-urchins and brittle-stars attributable to growth of early juveniles within pluteus larvae—Three pairs of species of echinoplutei show marked similarities but adults assigned to different orders—Different types of pluteus larvae within genus Lytechinus (Echinomorpha) and within genus Ophiura (Ophiuromorpha)—Some brittle-stars have doliolaria larvae—These examples attributed comparatively recent genetic transfers affecting larval form.