This small decorator crab evidently thought he had found the perfect protective cover and may have not realized he had selected a large living nudibranch. A pair of the crab's posterior legs are designed to hold a chunk of sponge in place, and this unsuspecting Jorunna funebris must have come loose from the bottom just long enough for the crab to have firmly gripped its underside. It's doubtful this arrangement lasted for very long.
31 October 2018