Reef Tank Janitors - Part 2

Did you know that:

• Sea urchins are used as indicator organisms in public aquariums to determine if the system is functioning properly? Because these animals are so "picky" about water quality and cleanliness in an aquarium, they are the first to show signs of stress, seen when their spines are laying down or falling off.
• There are numerous varieties found in both tropical and colder water oceans around the world?
• Urchins are generally referred to as Wana (sounds like vawna) in Hawai'i?
• It is not unusual for some urchins to house tiny, species associated shrimps within their spines?
• Triggerfishes and Puffers will pick the spines off sea urchins, turn them over, then break open their shells to eat them?
• Prior to the early 1970's, sea urchins in California were considered nothing more than pests, but now the export value of California's sea urchin fishery is a multi-million dollar industry?
• Many species enjoy eating coralline algae? This is not necessarily bad, unless you place too many in number and/or of large size in a tank where their appetite can exceed the amount of coralline growth available for them to feed on. If this is a concern, you might think about placing a few urchins in the sump to help remove excess coralline, micro and macro algae growth there, like John Rice did with some Variegated Urchins (Lytechinus variegatus).
• Some species are particularly predatory, and many do not eat algae at all?
• Long-spined members of the Family Diadematidae, such as Diadema and Echinothrix species, have venomous stinging spines?
• Large growing species can be cumbersome and act like little bulldozers, causing damage and the rearrangement or destabilization of rock and coral scapes?

Now, with so many similar or closely related species found world wide, urchins can be difficult to identify, and even though a lot of these animals can be algae eating machines, as you can see some do not have positive traits. The bottom line about picking out sea urchins is to be careful of what kind you are really getting. Being too large in number to try and cover all the varieties here, we are going to profile some of the more common species.

Pencil Urchins

These urchins are so named for the pencil shape of their spines.

• Some Pencil species, such as the Caribbean species Eucidaris tribuloides, are meat-eating animals, says Ronald Shimek in his There's No Reason To Be Spineless article. He states that, "Before I found this out in my own system, I watched a Pencil Urchin (Eucidaris tribuloides) catch and eat a scarlet cleaner shrimp".
• A member of the Family Echinomertridae, the Hawaiian Pencil Slate or Slate Pencil Urchin (Heterocentrotus mammillatus) is one of our favorites because of its beautiful smooth, blunt ended, brightly red colored spines. Whether or not this species is completely reef safe is questionable, but we know that it is not an easy urchin to care for, not living long in captivity unless a plentiful supply of algae is provided.
• Members of the Family Cidaridae are often referred to as Pencil Urchins, but these species have blunt, well-spaced primary spines, ringed at the base by smaller secondary spines, and the primary spines are often covered with algae and detritus. The Hawaiian Rough-Spined Urchin (Chondrocidaris gigantea) grows quite large to four inches in diameter with six inch long spines, while the Ten-Lined or Sputnik Urchin (Eucidaris metularia) grows to a diameter of four to five inches, and the Thomas' Urchin (Actinocidaris thomasi) to only about one inch.

Collector Urchins

These urchins get their name from the trait they have of picking up and "collecting" objects, such as leaves, small rocks or pebbles, coral rubble, plant matter, and just about anything else they can find to camouflage themselves with. They come in an array of colors from light pastels to black.

• The Black Collector Urchin (Tripneustes gratilla) is commonly seen in Hawai'i. It spends its time on the reef slowing cruising around rocks and corals searching for algae to eat, and grows to about five inches in size. This urchins will cruise around on the glass and roam the aquarium in search of algae to eat.
• Mentioned earlier, the Variegated Urchin (Lytechinus variegatus) is yet another type of collector urchin.
• Once again referring to Ronald Shimek's There's No Reason To Be Spineless article, he states that one of his personal favorites is the Blue Tuxedo or Royal Urchin (Mespilia globulus) found in Palau that seldom exceeds 3 cm in skeletal diameter. John Rice states that it is an excellent herbivore, is small and does not disturb other corals or other inhabitants. It likes to cruise the rock work eating microalgae and some coralline. Besides, with its beautiful royal blue color, its quite an eye catching urchin.

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