Sea Cucumber
Wikipedia link: http://en.wikipedia.org/wiki/Sea_cucumber
They can be found in great numbers on the deep seafloor, where they often make up the majority of the animal biomass.[5] At depths deeper than 5.5 mi (8.8 km), sea cucumbers comprise 90% of the total mass of the macrofauna
more info: http://www.ouramazingplanet.com/1419-ocean-exploration-deep-sea-diving.html
From NOAA website: http://oceanexplorer.noaa.gov/explorations/07mexico/logs/june18/media/movies/...
"The deep seafloor is generally considered to be quite food poor away from the seep areas. This leads to a series of adaptations in the animals living there. Here we see a swimming sea cucumber evolved from benthic animals that feed upon the sediment. This one is capable of swimming over the seafloor using highly-modified feet that have turned into fins. If you look closely you can see the mud-filled gut inside the semi-transparent animal. When it finds a suitable spots it will drop to the bottom to feed on the sediment and then swim away again to find another suitable spot. Video courtesy of Bob Carney, Louisiana State University, NOAA-OE."