Blog Post Twenty-Three
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This is a Banded Sea Urchin, Echinothrix calamaris. This urchin is also sometimes referred to as the double spined urchin. If you look closely at the photo, you will see a set of shorter dark spines grouped together to form a star shape, called iridophores. These spines are venomous and will deliver a nasty sting. The other spines you notice are longer, thicker, and banded with lighter and darker colors. They are called radiolas. Fun fact, they can shoot these long spines, filled with venom, a short distance. The balloon shaped circle in the center of the urchin is it’s anal sac. This sac is circled with a ring of visual receptors. This enables the urchin to have some basic rudimentary vision.
This is the most common long spine sea urchin found in Hawaii. The urchin can be found anywhere from 5 feet of water to 200 feet of water. It is not uncommon to see a triggerfish or wrasse nibbling on the spines and eventually go for the fleshy underside. This urchin is usually out feeding on algae in the evening. Like most urchins, they move very slowly and scrape algae from the coral and rocks.