So the researchers mimicked the crabs in two ways:
1. make a robocrab arm to wave to the ladies at different sizes and speeds
2. lie to the ladies, as male fiddler crabs are apt to do, by making them think they are legitimate mates and not just some flailing piece of machinery (read another example of fiddler crab dishonesty here).
Are you a man or a machine?
Actually, you look like a pretty cute male U. mjoebergi!