Diversity and Evolution of Branching Worms (Annelida: Syllidae)

Branching syllids are extraordinary marine annelids that live inside specific sponge hosts and display a unique body plan: a single head region giving rise to multiple branching posterior segments, an exceptional departure from the typical unbranched worm form.
These worms occupy strikingly different environments, from deep-sea glass sponges to shallow-water demosponges. By combining museomics (genomic analyses of historical museum specimens) with new field sampling and detailed morphological studies, we have reconstructed their systematics, biogeography, and evolutionary history.
Our work reveals that branching syllids form a distinct evolutionary lineage that has diversified through ecological specialization and cryptic radiation within their sponge hosts. This research highlights how the interplay of ecological divergence and hidden species diversity has driven the evolution of this remarkable branching body plan.