The prey reacts:
In this case, the predator is a Sunflower Star (Pycnopodia helianthoides) and the prey is a Purple Sea Urchin (Strongylocentrotus purpuratus), photographed on the Mendocino coast on 22 August 2013.
In the photo above, note that the sea urchin has flattened its spines and extended its pincer-like pedicellariae. The pedicellariae look like little white stars among the purple spines and tubefeet.
Although small, the pedicellariae can be very effective deterrents against larger predators such as Sunflower Stars. The pedicellariae pinch and they contain toxins! [The toxins in Purple Sea Urchin pedicellariae are not harmful to people.]
The next image shows a better view of the urchin's pedicellariae.
This urchin was in "full defense mode", which made it easy to photograph the exposed pedicellariae. In these images you can clearly see the three-parted pedicellariae spread wide open, ready to fend off the approaching sea star. In the still photo below, the pedicellariae look delicate and flower-like, but it's more appropriate to think of them as menacing and dangerous!