Gain of palps within a lineage of ancestrally burrowing annelids (Scalibregmatidae)

Scalibregmatidae is a small annelid family of subsurface deposit feeders in sand or mud, which are generally well adapted to infaunal burrowing. The overall morphology of Scalibregmatidae is very similar, with thick bodies, small parapodia, and no prostomial appendages or short horns. The only exception is members of the genera Axiokebuita and Speleobregma that most frequently inhabit crevices or gravel and possess extensive ventral ciliated palps and globular adhesive pygidium. Character reconstruction using maximum likelihood and Bayesian methods show that ciliated palps and adhesive pygidium are synapomorphies of the Axiokebuita–Speleobregma clade. The most likely transformation series is from horns to ciliated palps, the origin of which correlates with the occurrence of Axiokebuita and Speleobregma in crevices or gravel. The wide spaces among rocks or granules yield high permeability and inertial water flow, preventing deposition of organic matter. Under these flow conditions that differ significantly from those of sand or mud bottom, ciliated palps aid to the collection of suspended particles and an adhesive pygidium provides attachment. With palps being a highly debated character in annelid evolution, it is remarkable that prominent ciliated palps are gained within a lineage of ancestrally nonpalpate annelids, most likely increasing their fitness when colonizing a new environment.