Abstract
Reverse development, or the ability to rejuvenate by morphological reorganization into the preceding life cycle stage is thought to be restricted to a few species within Cnidaria. To date, Turritopsis dohrnii is the only known species capable of undergoing reverse development after the onset of sexual reproduction. Here, we demonstrate that the ctenophore Mnemiopsis leidyi is capable of reversal from mature lobate to early cydippid when fed following a period of stress. Our findings illuminate central aspects of ctenophore development, ecology, and evolution and show the high potential of M. leidyi as a unique model system to study reverse development and rejuvenation. Besides shedding light on the plasticity of developmental programs, these results raise fundamental questions about early animal development, body plans, and life cycles.