Chaetal type diversity increases during evolution of Eunicida (Annelida)

Annelid chaetae are a superior diagnostic character on species and supraspecific levels, because of their structural variety and taxon specificity. A certain chaetal type, once evolved, must be passed on to descendants, to become characteristic for supraspecific taxa. Therefore, one would expect that chaetal diversity increases within a monophyletic group and that additional chaetae types largely result from transformation of plesiomorphic chaetae. In order to test these hypotheses and to explain potential losses of diversity, we take up a systematic approach in this paper and investigate chaetation in Eunicida. As a backbone for our analysis, we used a three-gene (COI, 16S, 18S) molecular phylogeny of the studied eunicidan species. This phylogeny largely corresponds to previous assessments of the phylogeny of Eunicida. Presence or absence of chaetal types was coded for each species included into the molecular analysis and transformations for these characters were then estimated using the mK1 likelihood model. Our results show that chaetal type diversity does indeed increase within eunicids and provide possible explanations for the homology, convergence, and loss of chaetal types in eunicidan subtaxa.