The development of the enteropneust hemichordate Balanoglossus misakiensis KUWANO.

We describe development from fertilization to metamorphosis of the enteropneust hemichordate Balanoglossus misakiensis. This is the first report to describe the complete development of an indirect-developing hemichordate under laboratory conditions. Mature adults were induced to spawn by shifting the temperature of seawater from 23 to 28 degrees C. Eggs (200 microm diameter) were enclosed within a non-mucilaginous membrane, and dispersed readily in seawater. After artificial insemination, a fertilization envelope was elevated from the egg surface beneath the egg membrane; this was followed by the formation of the first and second polar bodies within the envelope. Zygotes cleaved at 20-min intervals to form blastulae, and gastrulation started 9 h after fertilization. Embryos hatched 1 day after fertilization to become typical feeding tornaria larvae. The larvae metamorphosed 7-10 days after fertilization without undergoing the first (Müller) or forth (Krohn) stage of indirect-developing hemichordate development. Larvae that were not fed failed to metamorphose. Juveniles completed adult body formation within a week of settling in sand at the bottom of the culture tube. We discuss heterochronical modifications of B. misakiensis development, and make the case for this species as a potential model organism for the investigation of indirect-developing hemichordates.