This sea cucumber, Enypniastes eximia, spends most of its time on the seafloor, feeding off of surface sediments; it can, however, swim if it wants to get somewhere more quickly or evade a predator. Common names for these holothurians include Spanish dancer and headless chicken monster. This one was spotted during the Gulf of Mexico 2017 expedition.
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Video courtesy of NOAA Ocean Exploration, Gulf of Mexico 2017.