Swimming sea cucumbers

Swimming, once considered a surprising and unusual feat for sea cucumbers, which are representatives of the phylum Echinodermata typically characterized by a semi-rigid calcareous skeleton, is emerging as a relatively common feature in species living in the deep sea. Swimming is typical for members of several families in the order Elasipodida (Pelagothuriidae, Elpidiidae, and Psychropotidae) and the family Synallactidae. Moreover, the pelagothuriid Pelagothuria natatrix is the only known species of sea cucumber with no benthic life stage at all, living and feeding exclusively in the water column. Other swimming species are benthopelagic, i.e. swimmers spending a variable amount of time in the water column, but feeding on the seafloor. Swimming appears to be an adaptation to the deep-sea environment, allowing energy-effective travel over far distances above vast abyssal plains to explore feeding grounds using near-bottom currents. In the mid-ocean ridge and seamount settings, swimming helps sea cucumbers exploit habitats that are difficult to access, such as narrow sedimented terraces on steep rocky slopes. These and other aspects of swimming are discussed in this chapter, including patterns of swimming locomotion, distribution of swimmers in the water column, and morphological and behavioral adaptations to swimming.