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Tag Archives: Sea Urchins

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Sea Urchins: Unusual Algae Eaters

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.

Green Variagated UrchineWhile 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.

Brittle Stars, Sea Stars and Sea Urchins – an Introduction to Some Popular Echinoderms

Frank Indiviglio here with an introduction to Echinoderms.

Sea stars, or starfishes, are perhaps the most familiar of the Echinoderms (a phylum containing over 7,000 marine species), and many adapt well to aquarium life.  Most people are quite surprised to realize that they are active, interesting predators that routinely exhibit a wide variety of behaviors in the aquarium.  Many are also useful scavengers, but all are predatory in nature and, depending upon the species, will consume mollusks, coral polyps and other sedentary invertebrates.

Red-knobbed Sea Star, Protoreaster lincki
Red-knobbed Sea StarWhen picturing a sea star, many people think of the simple reddish-orange animal so often seen as a dried curio in beachfront shops.  However, many are fantastic in appearance and coloration.  The Red-knobbed Sea Star, with brick-red dorsal spines set off against a dazzling white background, is a case in point.

Native to the Indo-Pacific region, this perennial aquarium favorite reaches a length of 12 inches and is capable of consuming quite large mollusks.  It is best fed by placing a piece of clam, scallop or mussel directly below the body, although it is quite active and capable of finding food on its own.

Although sea stars are quite adept at sensing and locating food, they respond more slowly than do most fish.  Therefore, they will usually remain hungry in a mixed-species tank unless care is taken to see that food is placed directly below each animal.

Brittle Stars
Black Brittle StarBrittle stars bring the word “bizarre” to mind instantly, even to those well acquainted with the sea’s curiosities.  They react very quickly to the scent of food, and their long, slender arms thrash wildly about as they begin to explore.  It is quite a sight to see a tank housing several of these normally sessile creatures suddenly come to life – the many sinuous arms seem to take on a life of their own, yet the animals glide unerringly toward the source of the odor that aroused them.

Brittle stars are harmless to most other creatures and are extremely valuable scavengers.  Perpetually hungry, their thin arms can get into the tiniest of crevices between coral heads and other places where bits of uneaten food might otherwise go unnoticed.

Sea Urchins
These slow-moving, spiny invertebrates are often encountered in tide pools, and are worldwide in distribution.  The spines of all are effective weapons, and many secrete venoms that are as yet not well-studied.  Hot-water baths seem to assist in alleviating the sting caused by most species, but handle all with extreme care.

With over 800 species identified to date, urchin enthusiasts have much to celebrate.  Many unusual species are commercially available, including the Long-spined Sea Urchin, Diadem antillarum and the Pencil Urchin, Heterocentrotus mammillatus. Both feed primarily upon algae, but will also consume bits of fish and shrimp.  The Long-spined Urchin is armed with extremely sharp spines, much to the chagrin of bathers in tropical waters.  The Pencil Urchin is well named – its spines, less numerous than those of other urchins, are very thick and blunt-ended.

Wave your hand above a captive or wild sea urchin and you will likely be surprised at how quickly the seemingly inert beast responds.  A shadow or object passing overhead is viewed by an urchin as a predator, and all the spines are oriented to face the threat.  Although parrot fish, sea otters and wolf fish are adept at clipping off urchin spines or turning the animals over to expose the soft underbody, the defense is, in general, foolproof.

Despite their slow-moving ways, sea urchins are quite active and seem bent on getting into every possible nook and cranny in their aquarium.  Be sure to check that they do not wedge themselves too tightly into small corners, or tumble backwards into coral and become stuck.

I’ll cover individual species in depth in future articles.  Until then, please write in with your questions and comments.  Thanks, Frank.

You can read more about sea urchins, sea stars and their relatives at: