Can the Movement of the Deep-Sea Bivalve Acesta excavata Lead to a Dynamic Habitat?

Acesta excavata is one of the largest and ecologically relevant bivalves along continental margins and is often associated with cold-water coral assemblages of the upper bathyal zone. Like other habitat-forming species, A. excavata contributes to increasing the secondary substrata and provides opportunities for the colonization and feeding of other sessile and mobile organisms. Despite most of the bivalves producing byssus being thought to be sessile or sedentary throughout their adult life stages, some species are known to be able to displace. Here we investigated, in mesocosm conditions, the ability of this deep-sea species to move/displace and compared its mobility with that of other shallow-water species. We report here for the first time that A. excavata moves almost continuously, with a maximum speed of 6.5 cm day⁻¹ (maximum weekly displacement of ca 28 cm), with average speeds of approximately 0.3–1.3 cm per day. This speed is the highest value reported so far for byssus-attached bivalves (including Mytilus spp. and Pictada imbricata radiata). The movement of these bivalves, apparently due to the search for optimal feeding and substratum characteristics, can displace the habitat they create in response to changes in environmental and ecological conditions. These findings offer new opportunities for using this species in restoration protocols of deep-sea habitats and change our view of deep-sea hard bottoms from static to dynamic entities.