Video recordings of polynoid (Annelida) swimming and crawling

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.