Phylum Echinodermata: Asteroidea

The asteroid echinoderms (Echinodermata: Asteroidea) display a diverse array of early life history strategies. Development can be indirect (with feeding or nonfeeding larval life history stages) or direct (the absence of larval stage(s)) and may occur within the water column or on the benthos (external or internal brooding). The bipinnaria is considered the primitive larval type and is, for species with feeding, indirect development, the first larva to form. In most groups, the feeding bipinnaria larva transforms into a brachiolaria larva. Both are morphologically complex larval forms that use two ciliary bands for locomotion and feeding; larval movements are also facilitated by contraction of muscles. The feeding bipinnaria and brachiolaria larvae of the world’s tropical oceans are among the most beautiful members of the meroplankton. Species with indirect, nonfeeding development bypass the bipinnaria-stage larva and develop into a morphologically simpler brachiolaria larva. For some species, planktonic feeding larvae reproduce asexually (i.e., >1 juvenile is potentially produced from a single fertilized egg). In the field, larval cloning occurs through at least two different processes resulting in the release of new, developmentally younger, embryos or larvae from the primary (parent) larva. At the end of larval life, brachiolariae settle to the sea floor and complete a metamorphosis that converts the bilaterally symmetric, planktonic larva into a radially symmetric, benthic juvenile.