MAY 11,2012 UPDATE:
Steven Haddock, a scientist for the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., says that the mysterious creature is a Deepstaria enigmatica jellyfish, much to the chagrin of some Reddit users who thought it was a whale placenta.
"This bag-like jelly is not that rare, but is large, so rarely seen intact," Haddock said on his "JellyWatch" Facebook page. "In the video, the swirling from the sub makes the medusa appear to undulate and it even turns inside-out."
This type of jellyfish is usually found in the south Atlantic Ocean, some 5,000 feet below. According to the Marine Species Identification Portal, the jellyfish has "oral arms terminating in curious hook-shaped organ."