Phylogenetic relations among spirorbid subgenera and the evolution of opercular brooding

Spirorbid polychaetes are a common and often conspicuous element of marine, hard-bottom environments worldwide. Because they exhibit a surprising variety of brooding modes, they offer an opportunity to study the evolution of alternative life-history strategies. A preliminary investigation of their phylogenetic relationships was conducted on morphological data amalgamated from published species descriptions and light microscopy. Cladistic analysis was parsimony-based and was done at the subgenus level. All 27 spirorbid genera and subgenera are represented, along with 7 serpulid outgroups. The phylogenetic hypotheses were used to explore the origin of opercular brood chambers in the Pileolariinae and the Januinae. Both weighted and unweighted analyses strongly supported the monophyly of the Spirorbidae, and yielded three and a single most parsimonious tree respectively. All spirorbid subfamilies were monophyletic except two: the Pileolariinae, from which the Januinae seem to have arisen, and the Romanchellinae, which were polyphyletic in the unweighted analysis and paraphyletic with Paralaeospira in the weighted analysis. However, support for the hypothesized relationships among spirorbid subfamilies was generally weak. A weakly supported clade of opercular brooders occupied the most derived position on the tree, and the strongly supported monophyletic Januinae comprise a derived clade within the Pileolariinae. This analysis suggests that the evolutionary novelty of opercular brooding arose only once, and the simple cylindrical januin brood chamber is a simplification of the more complex pileolariin form.