Description of a new syllid species as a model for evolutionary research of reproduction and regeneration in annelids

Abstract

Syllids are one of the most speciose annelid taxa and characterized by their variety of reproductive modes. We provide the description of a new species of Syllidae (Annelida, Phyllodocida), Typosyllis antoni n. sp., which is characterized by its distinct color pattern consisting of transversal red lines on the dorsum of anterior segments; long antennae and dorsal cirri with strong alternation in length; bidentate chaetae falciger like with long spinulation on edge, one tiny and thin acicula appearing in posterior segments in addition to thicker and pointed one, and a long proventricle. A phylogenetic analysis of Syllinae based on three genes supports that T. antoni n. sp. is sister species to Typosyllis heronislandensis. This sister group relationship may indicate a common ancestor from the Pacific. Moreover, we recommend several steps to unify the taxonomy with phylogenetic knowledge of this group. Using immunocytochemistry coupled with confocal laser scanning microscopy (cLSM), we describe the internal morphology of this species. The body wall is composed of two dorsal and two ventral longitudinal muscle bundles that form a distinct inner layer. The outer or “circular layer” of body wall musculature is represented by prominent transverse muscle fibers that exhibit a semicircular arrangement. The musculature of the uniramous parapodia is characterized by distinct parapodial retractor muscles, acicular protractor muscles, as well as prominent acicular and chaetal flexor muscle bundles. T. antoni n. sp. reproduces by schizogamic scissiparity producing dicerous stolons. This species is able to regenerate the anterior end, including the prostomium, the first chaetae-less segment with all appendages, and some additional chaetigers, depending on the dissection side. Regeneration of the proventricle, ventricle, caeca, or pharyngeal tooth is not detectable. In contrast, regeneration of the posterior end appears to be complete. The available data makes T. antoni n. sp. to be one of the best investigated syllids, emphasizing its potential as model for the whole group. Our analysis establishes a framework for future studies on the evolution of reproductive modes in Syllidae, and we outline research questions how they are related to regeneration and development.