SUNFLOWER SEA STAR (Gold) - the largest sea star in the world [Pycnopodia helianthoides] by Paul Nicklen. This sea star is draped moplike over a seafloor rock off the British Columbia coast. Though commonly called starfish, sea stars are not fish but echinoderms, more closely related to sea urchins and sand dollars. Only the five-armed species really resemble stars—others may boast as many as 40 appendages.