Ultrastructure of opisthosomal chaetae in Vestimentifera (Pogonophora, Obturata) and implications for phylogeny

The posterior segmented body region of Vestimentifera bears rows of uncini that function to anchor the animal within its tube. SEM studies of five ves timentiferan species reveal intraspecific and interspecific variation in th e number of chaetigerous segments and the arrangement of uncini within a gi ven segment. The portion of an uncinus that extends beyond the epidermis co mprises two opposing groups of teeth that probably correspond to the capiti um and subrostral process of polychaete uncini, and a distinct protuberance between them, interpreted as a rostrum. In Ridgeia piscesae, the uncini ar e formed by chaetal follicles, consisting of a chaetoblast, a follicle cell and an epidermis cell. The chaetal shaft is elongate and composed of up to 40 hollow cylinders that are invaded at their base by microvilli from the apical part of the chaetoblast. Opisthosomal chaetae in perviate Pogonophor a are usually restricted to four per segment and are of a rod-shaped type. It is hypothesized that the rod-shaped chaetae represent reduced hooked cha etae probably derived from a condition such as found in Monilifera. Uncini of Pogonophora, Sabellida,Terebellida and Oweniida are considered homologou s but details of chaetal design may be due to functional adaptations and th us do not represent reliable characters for phylogenetic studies on higher taxonomic levels than genera or potentially families.