Terrestrial polychaetes - models for the evolution of the Clitellata (Annelida)?

Abstract

The origin and the position of the Clitellata in the phylogenetic system are the object of vigorous scientific debate. Since most extant Clitellata are either terrestrial or limnetic, the question arises which characters are specific adaptations to these environments and which are plesiomorphies taken from the annelid stem species. A promising approach is to study whether clitellate characters are present in terrestrial polychaetes as well. In fact, the terrestrial polychaetes Hrabeiella periglandulata Pizl & Chalupsky, 1984 and Parergodrilus heideri Reisinger, 1925 have many features in common with the Clitellata, as does Stygocapitella subterranea Knollner, 1934, which inhabits sandy beaches at the transition between the marine and terrestrial realms. These features include a prostomium lacking appendages, chaetae that are simple and short, an absence of parapodia, an epidermis without kinocilia, sensory cells with cilia that project only slightly beyond the cuticle or not at all, direct development, direct sperm transfer and eggs laid in cocoons. The nuchal organs so characteristic of polychaetes are either displaced into the body and reduced (S. subterranea, H. periglandulata) or absent (P. heideri). In H. periglandulata, the nervous system and the foregut also closely resemble those of the oligochaetes. These polychaetes are certainly not closely related and clearly do not belong to the Clitellata, as shown by their lack of a clitellum, the different structure and position of the reproductive organs and ultrastructural differences in the sperm. Hence their similarities to clitellates have most probably arisen by convergence. A hypothesis is presented explaining the evolution of the terrestrial polychaetes as a model for that of the Clitellata, documenting a primary terrestrial origin of the latter.