Immunohistochemical investigations of the development of Scoloplos armiger (“intertidalis clade”) indicate a paedomorphic origin of Proscoloplos cygnochaetus (Annelida,Orbiniidae)

The embryology of Scoloplos armiger (“intertidalis clade”) was described in detail using light microscopy in a landmark paper by DT Anderson in 1959. To expand these investigations, we used immunohistochemical staining techniques (phalloidin, anti‐FMRFamide, anti‐serotonin, and anti‐α‐tubulin) coupled with confocal laser scanning microscopy to describe the early development of musculature and nervous system of this species. Moreover, we applied the same methods (and Azan staining) to adults of the putatively paedomorphic orbiniid species Proscoloplos cygnochaetus. Our results showed comparable patterns for stainings of the nervous and muscle system for juveniles of Scoloplos and adults of Proscoloplos. Both show scarce transverse musculature and only a few dorsoventral muscle fibers in the anterior body region. For the nervous system, the observed immunoreactive pattern is nearly superimposable for juveniles of Scoloplos armiger and adults of Proscoloplos. Moreover, the intraepidermal and basiepithelial position of the ventral nervous system only found in juvenile Scoloplos is comparable with the conditions exhibited in adults of Proscoloplos. In summary, our data are in congruence with the hypothesis of a paedomorphic origin for Proscoloplos as derived from molecular phylogenetic analyses.