Stage-dependent thyroxine effects on sea urchin development

Thyroxine, one of the iodinated hormones produced by vertebrate thyroids, h as been reported to accelerate late larval development in several sea urchi ns (Chino et al. 1994) and in the crown-of-thorns starfish (Johnson & Cartw right 1996), but thyroxine effects on earlier portions of echinoderm develo pment have not been reported. I investigated thyroxine effects on developme ntal rates during several periods spanning development from early cleavage to metamorphosis in the New Zealand sea urchin Evechinus chloroticus (Valen ciennes). Thyroxine treatment slowed development between the eight-cell sta ge and assembly of the four-armed pluteus and mid-larval development betwee n the four-armed and six-armed stages. Thyroxine treatment accelerated prog ress of eight-armed plutei toward settling, but did not alter the final per centages of larvae that settled and metamorphosed to juvenile urchins. Acce leration of late larval echinoderm development by thyroxine may indicate a relatively ancient evolutionary origin of thyroxine's effects on developmen tal processes (Johnson 1997).