Drifting in the abyss: an in-situ observation of swimming in Psychropotes (Psychropotidae, Elasipodida, Holothuroidea)

Holothuroids are conspicuous members of megabenthic assemblages, particularly in the deep sea. Some are capable of swimming and while this behaviour was once considered uncommon, deep-sea exploration and technological advances have increased the number of such observations. Swimming or drifting is now regarded to be a potentially common adaptation to deep-sea life that enables long distance dispersal at a low energetic cost and perhaps the ability to track ephemeral food sources. Swimming is well documented for some species, as well as the specialised structures such as brims, swimming lobes, modified podia, and other adaptations like gelatinous consistency, ossicle reduction and flattened bodies. Some species of Psychropotes Théel, 1882 have a long ‘tail’ (sail-like unpaired dorsal appendage) that has been suggested to facilitate drifting in near-seabed currents. Yet, to date, swimming had been mainly attributed to early life stages for these species, as these had been collected above the seafloor. While there is evidence that adults can also use this sail-like lobe to drift using bottom currents, the purpose of this dorsal appendage itself is still inconclusive. Here, we document a remotely operated vehicle observation of a long ‘tailed’ adult Psychropotes cf. semperiana Théel, 1882 actively swimming with the aid of anterior and posterior webbed podia fanning in synchrony, combined with dorso-ventral body flexing, while using the sail-like dorsal appendage to drift in the near-bottom water column.