Close-up of a Sunburst Anemone (Anthopleura sola) with its acrorhagi extended. The acrorhagi are the white-tipped structures along the perimeter of the anemone. Normally, the acrorhagi are hidden beneath the ring of feeding tentacles. You only see them occasionally, when the anemone is "feeling defensive."
Acrorhagi contain dense concentrations of nematocysts — their toxins are capable of damaging neighboring anemones. To maintain space around itself, an anemone will lean towards a neighbor, touching it with the tips of its acrorhagi, attempting to force the neighbor to move further away.
Here's most of the anemone with its ring of defensive acrorhagi:
If you'd like to see anemones "fighting" with their acrorhagi, watch this short video clip from The Shape of Life.