F-actin framework in Spirorbis cf. spirorbis (Annelida : Serpulidae): phalloidin staining investigated and reconstructed by cLSM

Abstract

Serpulidae encompasses polychaete species whose members have fused anterior ends bearing a tentacular crown, a heteronomous segmented body with a thorax and abdomen, and ``chaetal inversion'' between the two tagmata. The sessile filter-feeding organisms live in self-built, coiled, calcareous tubes on algae. The F-actin Muscular Subset of Spirorbis cf. spirorbis was stained with phalloidin and three-dimensionally reconstructed by means of cLSM, aiming to investigate (1) how the tentacular crown is organized and moved, (2) whether the internal structures, e.g., musculature, follow the thorax-abdomen inversion, and (3) whether circular muscles are present in serpulids. The third aim is by reason of recent investigations suggesting that lack of circular muscle fibers may be a common situation rather than a rare variation in polychaetes. In this manner, this article is part of a comparative evaluation of polychaete muscle systems. We found that longitudinal muscles of the body wall project into the tentacular crown, and that radioli and pinnulae possess three muscle types each, facilitating their great mobility. Operculum, collar, and a pair of unidentified organs possess distinct F-actin filaments. The trunk is mainly moved by five longitudinal muscle strands, most obvious in the abdomen: two dorsal, two ventral, and an unpaired ventromedian one, Out of which the dorsal ones are the strongest. In anterior regions, the two dorsal strands form a single continuous layer; the separated strands lessen posteriorly. Solitary transverse fibers are located ventrally in the middle of each segment, stretching between longitudinal muscles and coelomic lining laterally, where they end. Peripheral and central dorsoventral Muscles, two pairs per segment each, are present. Circular fibers as well as bracing muscles were not detected. The results indicate that the musculature does not follow the thorax-abdomen inversion and Serpulidae represents the 15th polychaete taxon in which circular fibers are totally missing.