A sea anemone attaches by a pedal disc, a muscular base that grips substrate through adhesion and suction. Contraction waves passing through that disc let the animal inch along, typically a few centimetres per hour, slow enough that movement is only obvious in time-lapse or between visits.
Several species do more. Some detach entirely and let the current carry them. Stomphia and a handful of others perform an escape response, releasing their hold and swimming with lashing bends of the column when touched by a predatory sea star or nudibranch. Others somersault, bending the column over to plant tentacles and lift the disc.
The frame that they are sessile plants is doubly wrong: they are animals and they are not fixed. Anemones also fight for space. Species such as Anthopleura elegantissima carry acrorhagi, specialised stinging structures used to attack neighbouring clones, and colonies of genetically distinct anemones on a rock separate along visible bare bands where the fighting happens.