Harlequins are easy to tell if they are mated or not...if not they won't tolerate a second in their territory let alone eat the same star as a second. Plus females have a fatter tale than the males do (for holding the eggs). They will eat "almost" any kind of star. They prefer Linkias and will not touch serpents/ brittles. Also, and I know this seems crazy but harlequins are actually wired to flip stars over and eat them legs first before devouring the disc. This insures that the starfish stays alive for the extent of time it takes to be consumed. They don't need you to precut their meals for them. Tossing a small chocolate chip in their tank once every week or two is enough to keep them going.