General Patterns of the Ontogenesis of Ambulacraria

The life cycle, involving two larvae (planktotrophic and reserve-trophic), is hypothesized to be a synapomorphy of Ambulacraria. Planktotrophic larvae have a complex ciliary band and use “up-stream” filtration for feeding. Planktotrophic larvae undergo metamorphosis and turn into non-feeding larvae with complete or incomplete ciliary rings. These larvae use the reserves accumulated at the previous planktotrophic stage rather than the yolk; therefore, they can be named reserve-trophic. In echinoderms, the holothurian life cycle can be considered plesiomorphic. It includes a planktotrophic larva (auricularia) and reserve-trophic larva (doliolaria). Most echinoderms have lost their reserve-trophic larvae, although rudimentary doliolaria are known in some sea urchins and brittle stars. Only doliolaria larvae occur in the majority of crinoids, whereas sea lilies have auricularia succeeded by doliolaria, but both larvae are lecithotrophic. In enteropneusts, Spengelidae and Ptychoderidae have planktotrophic tornaria succeeded by reserve-trophic Agassiz larvae. Species of the Harrimaniidae have lost tornaria but have retained the lecithotrophic Agassiz larvae. The giant planktotrophic larvae were detected both in echinoderms and hemichordates. The deep-sea holothurian Protankyra brychia has giant larva known by the name Auricularia nudibranchiata. Giant tornaria known by the name Planctosphaera pelagica is likely a larva of some species of deep-sea enteropneust.