Many branches, one lineage: museomic insights into the diversity and evolution of branching syllid worms

Branching syllids are unusual annelids that inhabit specific sponge hosts and possess a single anterior region giving rise to multiple branching posterior segments, an exceptional deviation from the typical unbranched annelid body plan. Although all known species live obligately inside sponges, they occupy contrasting ecological settings: deep-sea hexactinellids (Crateromorpha spp.) for Syllis ramosa and shallow-water demosponges (Petrosia spp.) for Ramisyllis spp. Genome-skimming of historical museum specimens and new field samples provided robust phylogenomic datasets that, together with morphology, resolved long-standing taxonomic uncertainties and revealed a well-supported monophyletic clade comprising two major lineages: Ramisyllis, including two described and at least seven undescribed species across the Indo-Pacific and Red Sea, and the branching Syllis, formerly represented only by S. ramosa, now reassigned to a new genus, Cladosyllis gen. nov., including four cryptic species. The ecological divergence between both genera appears to have been a major driver of diversification, while within each genus diversification is largely host-specific, suggesting fine-scale radiation associated with particular sponge hosts. Overall, our findings highlight the crucial contribution of museomics, together with integrated classical and modern taxonomical approaches, in uncovering hidden biodiversity and revealing the branching body as an evolutionary novelty in annelids.