Sniffing out new data and hypotheses on the form, function, and evolution of the echinopluteus post-oral vibratile lobe.

The performance requirements of ciliary band feeding explain the convoluted forms of many marine invertebrate larvae. Convolutions increase surface area and therefore feeding rates per unit body volume. We review recent advances in morphology, neural development, and behavior at settlement of the echinoid Lytechinus pictus and provide new ultrastructural and expression data on larvae of its congener, L. variegatus. Larvae of the echinometrid Colobocentrotus atratus contain neurons identified by their expression of nitric oxide synthase (NOS), indicating that this character is not unique to Lytechinus. We hypothesize that in some echinoids the convoluted shape of the post-oral vibratile lobe (POVL) covaries with the distribution of identified sensory neurons to enable olfaction during settlement. An analysis of variation in structural elaboration of the post-oral transverse ciliary band (PTB) within Echinoida and in feeding larvae of other echinoderm classes indicates that only echinoids, but not all echinoids, possess this novel character; larvae that do are distributed heterogeneously within the class. In recognition of this specialized function for the POVL and surrounding ectoderm, and because it is lobate and grows toward the mouth, we propose naming this structure the adoral lobe.