Many people ask about a snail or slug with a half-shell on its back. This one is a little different, as it is a black Stomatella, a fast moving slug. This one is pictured with a few spirorbid worms on its back, making it a little more unusual. (This is the more common Stomatella.) Their shell is similar to a fingernail in shape, which covers the head and torso of the creature. These are found moving about the rockwork only at night, never seen during the daylight hours, and are an excellent part of the clean-up crew. They do breed and you may see smaller ones, but I've never seen them reach plague proportions. Reef-safe.