Brandon here. Many aquarists that come through our fish room will ask what we recommend as algae eaters for their reef. The answer is usually the same: snails, hermit crabs, or maybe a lawnmower blenny. One of the most efficient algae eaters found on the reef is usually overlooked, the sea urchin.
Sea urchins are relatives of starfish and sea cucumbers, belonging to the phylum Echinodermata. They are generally covered in hard spines for protection, little clawed arms called pedicellariae which are used to remove debris and detritus from the urchins’ skin and can also aid in protection, and tube like feet used for moving across the substrate. Their mouths are surrounded by a structure called Aristotle’s Lantern, which is used for scraping rock and breaking food apart. The Aristotle’s Lantern is what makes some urchins such efficient algae eaters.
While not all sea urchins eat algae, and not all that eat algae are desirable for a reef tank, there are a few that would make a great addition to the aquarium. The Variegated Green Urchin, Lytechinus variegates, stays relatively small and clears live rock of virtually all types of algae. One urchin that we use in some of our display tanks here at the store is the Tuxedo Urchin, Mespilia globulus. These urchins also remain relatively small and do a number on different types of algae. Another extremely efficient, algae-eating urchin is the Longspine Urchin, Diadema setosum. These can grow very large, and have spines capable of puncturing skin and leaving a painful injury.
Here are some urchins to avoid in the reef tank. Rock-boring Urchins, Echinometra lucunter, do little to clean up algae. They spend most of their time chewing holes in live rock. Priest Hat Urchins, Tripneustes gratilla, are generally considered reef safe, and will even do some scavenging, but they can also grab immobile fish and inverts for dinner. One urchin that we do not carry here at That Fish Place, the Flower Urchin, Toxopneustes pileolus, is very beautiful but can inflict a potentially deadly sting. It is covered in what appears to be little flowers, but are actually pedicellariae. These specialized pedicellariae have three jaws on the tip, each of which is hollow and filled with venom. Upon contact they snap shut and inject venom into the skin, which causes extreme pain and even muscle paralysis, which could drown an unsuspecting diver.
Urchins are not for everyone. While they will clean up most types of undesirable algae, they can also scrape coralline algae from the rock work, leaving it white and bare. They also require good water quality in respect to temperature, salinity, and other factors. Always be sure of the urchins’ specific requirements and adult size before purchasing. Whether you have a reef tank or not, urchins can make very interesting additions to the aquarium.