Development and phylogenetic significance of hooked setae in Arenicolidae (Polychaeta, Annelida)

Arenicolids have hooked setae aligned in a transverse row inside the neuropodial rim of each setigerous segment. Each seta consists of a large tooth (rostrum), several smaller spines (capitium) that partially cover the rostrum, and a shaft. Rostrum and shaft meet at an angle of about 100 degrees in adults and 70 degrees in recently settled specimens. The hooked setae of the latter stages also bear several hair-like structures, which arise from the shaft and surround the rostrum. These setae develop ventrally. Their structure is determined by modifications of the microvillar pattern on the surface of the basalmost cell of the setal follicle, the chaetoblast. In Arenicola marina, chaetogenesis starts with the rostrum, which is formed by a group of microvilli; each spine of the capitium is formed by a single stout microvillus. When both structures have been formed, the longitudinal axis of the anlage shifts due to a reorientation of actin filaments in the microvilli. The hairs of recently settled specimens are formed by long microvilli arising from the apex of the chaetoblast, which lies lateral to the setal anlage. All microvilli merge during the formation of the shaft. Hooked setae have been previously described for sabellidan, terebellidan, pogonophoran, arenicolid, maldanid, psammodrilid, and oweniid polychaetes. Similarities in form and development support homology of these setae, which characterize a monophyletic group within the Annelida. We propose that the ventral site of formation characterizes a sistergroup of arenicolids and maldanids.