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Sea Cucumber
It's a flying, or er...swimming sea pig! Seen during the Océano Profundo 2015: Exploring Puerto Rico’s Seamounts, Trenches, and Troughs expedition on April 28, 2015, while exploring off St. Croix at 2,400 meters (1.5 miles) depth.
Credit

NOAA Ocean Exploration, Exploring Puerto Rico’s Seamounts, Trenches, and Troughs