Regrowing the growth zone: metamorphosis kickstarts regeneration in the annelid <i>Capitella teleta</i>

The ability to regenerate can vary across the life history of an animal. We previously showed that Capitella teleta, an annelid worm, gains regenerative ability with age. Although larvae do not replace lost structures, juveniles and adults regenerate posteriorly following metamorphosis. To determine whether metamorphosis enables juveniles to regrow structures lost as larvae, we amputated C. teleta larvae, removing posterior segments, hindgut, posterior growth zone, anus and pygidium. Metamorphosis was then induced in these amputated larvae and they were reared as juveniles for 3, 7 or 14 days. New growth in juveniles was assessed by confocal microscopy, 5-ethynyl-2'-deoxyuridine incorporation, immunohistochemistry and nuclear staining. A posterior growth zone and new segments were observed by 7 days post-metamorphosis. Remodeling of the digestive system in pre-existing tissue was also observed. Additionally, re-amputation of juveniles resulted in regeneration of the posterior growth zone, segments and hindgut. Our results demonstrate that amputated C. teleta larvae can metamorphose into functional juveniles capable of growing new segments, suggesting that metamorphosis acts as a switch to enable regeneration of structures essential for growth. This study highlights the impact of metamorphosis on changes in developmental plasticity.