Regeneration and Caudal Segmentation in Phragmatopoma californica (Annelida: Sabellariidae)

The abilities of sabellariid annelids to regenerate missing anterior or posterior body regions are not well understood. We described regeneration ability in the sabellariid Phragmatopoma californica, carrying out amputations in three locations (the opercular region, the middle of the abdomen, and the posterior of the abdomen) and observing survivorship and regeneration in the resulting body fragments for 6 weeks. Both anterior and posterior regeneration were possible, with fragments missing the head and part of the opercular region regenerating missing anterior structures and fragments missing the posterior part of the abdomen and the cauda regenerating missing posterior structures. The posterior-most body region of sabellariids, the cauda, lacks parapodia and chaetae and is typically described as “apparently unsegmented,” which raises questions about where new segments are added during growth. Using light and scanning electron microscopy, we show that caudae of P. californica contain serially repeated septa and lateral blood vessels, suggesting strongly that they are segmented, albeit cryptically. These results clarify previous ambiguity in the literature about the regeneration abilities of sabellariids, which will improve future reconstructions of the evolutionary history of regeneration ability in annelids and help focus new questions on how and where sabellariids add new segments to abdominal and caudal body regions.