Macellicephala longipalpa MicroCT-scans for 3d reconstruction

Annelids are predominantly found along the seafloor, but over time have colonised a vast diversity of habitats, such as the water column, where different modes of locomotion are necessary. Yet, little is known about their potential muscular adaptation to the continuously swimming required in the water column. The musculature and motility were examined for five scale worm species of Polynoidae (Aphroditiformia, Annelida) found in shallow waters, deep sea and caves that exhibit crawling, occasional swimming or continuous swimming, respectively. Their parapodial musculature was reconstructed using microCT and computational 3D analyses and the muscular functions interpreted from video recordings of their locomotion. Since most benthic annelids are able to swim for short distances using body and parapodial muscle movements, suitable musculature for swimming and a pelagic lifestyle is already present. Our results also indicate that rather than rearrangements or addition of muscles, a shift to a pelagic lifestyle is mainly accompanied by structural loss of muscle bundles and density, as well as elongation of extrinsic dorsal and ventral parapodial muscles. In addition, our study documents clear differences in locomotion and muscular arrangement among closely related annelids with different lifestyles as well as points to myoanatomical adaptations for accessing the water column.