Analysis of the Central Nervous System and Sense Organs in Potamodrilus fluviatilis (Annelida: Potamodrilidae)

Abstract

The Potamodrilidae are small limnetic annelids of apparently simple organization and uncertain phylogenetic position. This group, which was regarded either as a subtaxon of the Aeolosomatidae or united with the Aeolosomatidae as Aphanoneura, has been placed in various positions within the Annelida: (1) at the base of the Clitellata, (2) as a highly derived clitellate taxon closely related to the Naididae, or (3) excluded from the Clitellata because of fundamental differences in the reproductive system and the lack of a clitellum. Moreover, a sister-group relationship between the Aeolosomatidae and Potamodrilidae has been questioned as well. The results of an immunohistochemical (cLSM) and ultrastructural analysis of the central nervous system in Potamodrilus fluviatilis support an exclusion from the Clitellata. The circumoesophageal connectives enter the brain through dorsal and ventral roots, a situation unknown in Clitellata but generally occurring in polychaetes. The ciliated pits, located in front of the brain, are not nuchal organs as previously thought but different sensory structures. However, modified nuchal organs are present behind the brain close to the ventral roots of the circumoesophageal connectives in a position corresponding to that of the nuchal organs found in Aeolosoma spp. Such organs are characteristic for polychaetes and absent in clitellates without exception. These morphological results clearly support the removal of the Potamodrilidae from the Clitellata. Furthermore, the characters present in the nervous system and sense organs show a general correspondence between the Potamodrilidae and the Aeolosomatidae.