The Red Knob Star, Echinaster echinophorus, also known as the Red Thorny Sea Star, has a thick grey body with red tubercles (knobs) going up each of its 5 arms. This star requires a large aquarium with plenty of live rock to graze on. When small it will graze on algae, however, it will not longer be reef compatible as it will begin to eat soft corals, sponges, tubeworms, starfish, clams, starfish, and other invertebrates. Like many stars the Red Knobby is very sensitive to sudden changes in salinity, pH, and oxygen levels. It is also intolerant to any copper based medications.