This is a sea turtle I saw while at the Shedd Aquarium in Chicago.  The story went that he was rescued after being hurt in a collision with a boat propeller.  His hind legs were paralyzed in the accident, and apparently while he was being operated on the surgeons found a nickel embedded in his hide, and so they named it after that.  He definitely didn’t use his hind legs at all when I saw him, and seemed to like hanging out in this particular spot, which must have been very comfy.