Chaetogenesis in Maldanidae

Chaetae are important structures to facilitate locomotion in annelids. Being at the interface between the organisms and its environment, chaetae are supposed to underlie strong functional constraints to optimize the relation between structure and function. As such chaetae are potentially susceptible for convergent evolution. On the other hand, chaetae gained enormous taxonomic importance due to their conservative structure in species and supraspecific taxa which reasonably can only be explained by strong evolutionary constrains that conserve their structure. In this paper, we study the chaetation and chaetogenesis in two species of Maldanidae, Clymenura clypeata Saint-Joseph 1894 and Johnstonia clymenoides Quatrefages 1866 to unravel conservative traits in their structure and development. In a literature survey across maldanids, we address questions on the ontogenetic variation, on homology and on the phylogenetic significance especially of the bearded hooked neurochaetae. We provide evidence that functionally constraint ontogenetic variation overlies historically (phylogenetically) constraint expression of structural information and can show that within maldanids a variety of different chaetal types must be homologous due to their ontogenetic continuity. Furthermore, we use chaetation and chaetal characters to discuss the subgroup relationships within Maldanomorpha in the light of recent cladistics analyses based on morphological and molecular data. This study shows that functional considerations need to use phylogenies as backbone.