The first third of a ball worm’s body is spherical while the last two-thirds form a tail. They are scavengers of anything that will fit in their mouths. They range from microscopic to three centimeters long. Unable to hide as easily, the larger ones are toxic and advertise this by changing color rapidly.
Bridge crabs are crab-like false crustaceans with very wide, short bodies. Their legs attach to the outer lobes which are joined by a bridge-like structure. For protection, they place color-changing poison ball worms onto their backs to warn away predators. To entice the worms to keep from breaking loose and swimming away, the bridge crabs pre-chew food for them and dump the crumbs onto their backs.