Branching out: a remarkable new branching syllid (Annelida) living in a Petrosia sponge (Porifera: Demospongiae)

We describe the morphology and biology of a previously unknown form of branching annelid, Ramisyllis multicaudata  gen. et sp. nov. , an endosymbiont of shallow‐water marine sponges ( Petrosia  sp., Demospongiae) in northern Australia. It belongs to the polychaete family Syllidae, as does Syllis ramosa McIntosh, 1879, the only other named branching annelid, which was collected from deep‐water hexactinellid sponges during the 1875 Challenger expedition. It differs from S. ramosa in parapodial and chaetal morphology. Ramisyllis multicaudata  gen. et sp. nov. has segments of several types, including specialized posterior segments on the emergent portions of the worm, and simplified elongate segments that bridge larger cavities in the sponge interior. Aside from the obvious branching form, the new annelid is similar to Parahaplosyllis , differing from it in lacking pharyngeal armature and in the details of the parapodial chaetae and dorsal cirri. Molecular evidence from 16S and 18S rDNA supports a sister‐group relationship with Parahaplosyllis , with both being sister to Trypanosyllis and Eurysyllis . The phylogenetic position of R. multicaudata  gen. et sp. nov. indicates that branching has evolved independently in Ramisyllis  gen. nov. and Syllis . This is supported by differences in the branching process between the two taxa: in S. ramosa branching is initiated by segment addition at the parapodium, whereas in R. multicaudata  gen. et sp. nov. segments are added from a region between parapodia. A model for branching in R. multicaudata  gen. et sp. nov. is proposed and possible developmental processes underlying branching in Annelida, and body symmetry comparisons with other invertebrates, are also discussed. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society , 2012, 164 , 481–497.