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...