Stichopus tremulus, a sea cucumber. Known as "Red sausage", when translated directly from Norwegian. A peaceful critter that slowly creeps along, munching algae and detritus from the sea floor. Rather popular for aquariums, as they keep any and all growth of algae down on zero by eating everything that settles.

Also shown in the random debris it's resting on the shell of a snail, an oyster, and a king scallop in the lower-right corner.