Is Hrabeiella periglandulata (Annelida, "Polychaeta") the sister group of Clitellata? Evidence from an ultrastructural analysis of the dorsal pharynx in H. periglandulata and Enchytraeus minutus (Annelida, Clitellata)

Abstract

The terrestrial polychaete species Hrabeiella periglandulata has many characters in common with Clitellata and with Parergodrilidae, the latter representing another taxon of "Polychaeta" comprising a terrestrial species. Absence of a typical clitellum, different structure and position of the genital organs, different ultrastructure of the spermatozoa and chaetae as well as the presence of nuchal organs, although internal and highly modified, precluded inclusion of H. periglandulata within Clitellata. Consequently, convergent evolution of the clitellate-like features in these groups due to similar selection pressure was generally assumed. Similarities between H. periglandulata and Clitellata are greater than between both of these and Parergodrilidae and include not only reductions and absences but comparatively complex structures as well, among them the dorsally positioned pharynx, so far only known for Clitellata. However, the possibility of homology and thus synapomorphy between Hrabeiella and Clitellata has never seriously been discussed, a view seemingly supported by molecular data. In the present study an ultrastructural investigation of the dorsal pharynx was undertaken in H. periglandulata and in the clitellate species Enchytraeus minutus. The structures in common include a dense ciliation of the dorsal epithelium with short and immobile cilia, a prominent rootlet system which is connected to the musculature, numerous openings of pharyngeal gland cells between the ciliated cells, a cuticular fold surrounding the cilia, an unciliated buccal cavity, and cell bodies of the pharyngeal glands situated outside the pharyngeal epithelium. This makes the general assumption of convergence at least questionable. Likelihood of homology of this type of pharynx is further supported by the corresponding structure of cerebral sense organs and the central nervous system. Thus, these characters may likewise and more parsimoniously be explained as synapomorphies, making a sister group relationship of H. periglandulata and Clitellata conceivable.