Larval budding, metamorphosis, and the evolution of life-history patterns in echinoderms

Abstract. The oral surface and mouth of juvenile asteroids and echinoids with indirect development forms on the lower left side of the larval body, thus establishing a new axis of body symmetry. In contrast, the juvenile mouth of ophiuroids and holothuroids develops from the larval one, and the larval and adult body axes roughly coincide. Explaining how two such disparate modes of development arose in evolution has been a perennial problem for echinoderm biologists, but recent observations on larval budding in asteroids may provide an answer. The juvenile mouth of asteroids forms near the base of the left posterolateral lobe. The posterolateral lobes are also the principal site of bud formation in asteroid larvae that propagate asexually, and buds form mouths. By accelerating the development of oral and ectodermal structures belonging to the bud, and combining these with internal organs derived from the parent larva, a composite individual could be constructed with the same orientation and positioning as the juvenile rudiment in asteroids. Whether this also explains the position of the juvenile rudiment in echinoids is a more complex question, depending in part on whether asexual propagation is derived, and restricted to asteroids and ophiuroids, or is more primitive and hence widespread among stem echinoderms.