A phylogeny reconstruction of substrate selection by tube-dwelling worms within the genus Spirobranchus (Polychaeta: Serpulidae)

Large tube-dwelling polychaetes of the genus Spirobranchus, popularly known as Christmas tree worms, are predominantly known as coral associates on tropical shallow reefs. In temperate seas, species are generally smaller and mostly occur on hard substrates, as solitary individuals or colonial aggregations. Coral-associated Spirobranchus species have been recorded from a broad host range and occasionally from non-coral substrates, indicating a generalist strategy in terms of host selection. Other ones display preferences for abiotic substrates or scleractinian host species. This raises the question whether their substrate preferences are phylogenetically determined and if there is a biogeographical component herein. This study infers phylogenetic relationships from 165 newly sequenced specimens analysed with ITS2 and cytochrome b markers, complemented by publicly available sequences, and integrates substrate types and host-coral taxa by projecting them onto the resulting trees alongside their geographic records. This represents one of the largest phylogenetic datasets available for Spirobranchus. Using a family-level framework, coral association vs. abiotic substrate is evaluated as a sessile strategy in an evolutionary context. Two principal Spirobranchus clades were recovered, both represented by Atlantic and Indo-Pacific species, with coral association occurring as a polyphyletic trait next to alternative substrate preferences. The Indo-Pacific S. corniculatus and the Atlantic S. giganteus occupy broad ranges of host species, with S. giganteus appearing to be the most generalist of the two. The most common species on Caribbean reefs, S. giganteus, reveals minimal genetic variation, whereas S. corniculatus, which is dominant on Indo-Pacific reefs, comprises multiple lineages that correspond to their geographic localities.