SCHULZE, Anja; RICE, Mary E.; Harvard University; Smithsonian Marine Station at Fort Pierce: Muscle development in larvae and juveniles of four sipunculan species

The three main muscle systems in Sipuncula � body wall musculature, introvert retractor muscles and intestinal fasteners � are important taxonomic characters for within-clade relationships of the Sipuncula, although homoplasy seems to be common. In adults, the body wall musculature may be a continuous sheet of circular and longitudinal musculature and the number of retractor muscles ranges from a single column to four. Here we examine body wall and retractor muscle development in the larvae and juveniles of four sipunculan species with different developmental modes, using fluorescence staining of F-actin and confocal laser scanning microscopy. In all species examined, both the circular and the longitudinal body wall musculature first develop in distinct bands which later fuse to varying degrees. All pelagosphera larvae and the juveniles of Phascolion cryptum and Themiste lageniformes have four retractor muscles that are later reduced to two in Nephasoma pellucidum and fuse to form a single retractor column in Phascolion cryptum. These observations suggest that a reduced number of introvert retractor muscles and continuous body wall musculature are apomorphic features in sipunculans.