Once, late in the day, I had an opportunity to witness an amazing scene.
A gross ophiura climbed from under the stone with an urchin on it and tried to creep under a different stone, where another two brittle stars have been settled already. Both of them actively suppressed it's claims for the place by shoving aside intruder's tentacles.
The action unfolded right under the belly of Gobius cobitis watching the clash.