Molecular and Morphological Phylogenies of Spirorbinae (Serpulidae, Polychaeta, Annelida) and the Evolution of Brooding Modes

Abstract

Spirorbinae, a ubiquitous group of marine calcareous tubeworms with a small body size as adults, have a fascinating diversity of brooding modes that form the basis for their taxonomic division into six tribes (traditionally subfamilies): in-tube incubation, with varying degrees of attachment to adult structures (four tribes), and external incubation in a modified radiole (opercular brood chambers; two tribes). We investigated the evolutionary transitions among these brooding modes. Phylogenetic reconstruction with molecular (28s and 18s rDNA) and morphological data (83 characters) among 36 taxa (32 ingroup spirorbins; 4 filogranin outgroups) of the combined data set, using maximum parsimony, maximum likelihood, and Bayesian analyses, inferred Spirorbinae to be monophyletic, with strong support for the monophyly for five tribes (Circeini, Januini, Romanchellini, Paralaeospirini and Spirorbini), but non-monophyly for Pileolariini. However, deeper relationships among some tribes remain unresolved. Neomicrorbis was found to be the sistergroup to all other Spirorbinae. Alternative coding strategies for assessing the ancestral state reconstruction for the reproductive mode allowed for a range of conclusions as to the evolution of tube and opercular brooding in Spirorbinae. Two of the transformations suggest that opercular brooding may be ancestral for Spirorbinae, and the tube-incubating tribes may have been derived independently from opercular-brooding ancestors.