Figure 1

The three scenarios show the origin of the pelagic larval body plan, indicated by red arrows. The presence of pelagic forms is indicated by blue lines and that of benthic forms by brown lines. A single-coloured line indicates a monophasic life cycle that would be pelagic in scenario B and benthic in scenario C. Double lines (blue and brown) indicate a biphasic, pelago-benthic life cycle (with pelagic larval and benthic adult forms). Note that the biphasic life cycle is assumed to have evolved multiple times independently in scenario C. (A) The classical view implies homology of both ciliated larvae and benthic adults that, once evolved, have remained part of the eumetazoan life cycle [17]. (B) Nielsen [16] modified this view to propose that the holopelagic neuralian ancestors persisted beyond the initial divergence of the major neuralian clades, and that the biphasic life cycle with benthic adults arose independently in the cnidarians and once or twice in the bilaterians. (C) In stark contrast, other authors assume that today’s ciliated larvae arose convergently many times by the repeated intercalation of a pelagic dispersal larva into primarily monophasic, holobenthic life cycles and are thus evolutionarily unrelated. This view implies that the characteristics of today’s swimming larvae such as apical organs and equatorial ciliary bands evolved convergently [18].