Bumblebees need between 5 and 10 to do well. One just does not last long, which, unfortunately, is the only amount of stock my petco ever seems to get off them.
They take the starfish apart, especially the little ones. They're better than a harlequin because they don't have to eat starfish only. You can get away with meaty food scavenging if you run out of stars. But in order to take care of stars, they have to work as a team.