Subclass Irregularia: Sand Dollars and Heart Urchins

Order Spatangoida: Heart Urchins

Order Clypeasteroida: Sand Dollars

Selection: General to Specific

Let's talk about, okay strike that, let's read about the two dangers of dealing with urchins, chemical and physical: Their spines are very sharp , hollow and brittle and many are accompanied with a painful irritant. Allow me to relate an anecdote to you regarding these physical injuries. One friend had a small tip of a spine break off in his hand. These spines in turn have smaller recurved  scales if you will, otherwise processes for working their way, "ratcheting" into the wound. This piece got walled-off (thank goodness for leukocytes!) and "surfaced" on the other side of his hand two years later. I helped him tease it out! Much interesting and profitable work is done in night diving. Let the diver beware (Cave divum?). See my photo of Lytechinus, close to my least favorite genus, coming out at sunset to scour the reef and poke the uninitiated. If you believe you would never cut into yourself for any purpose, you've just haven't been jabbed but good.

If the mechanical injuries weren't bad enough, many species possess very venomous poison sacs. Is this coming through clear enough? Handles these suckers carefully...with nets!

Oh, one more time for emphasis, you don't have to be punctured to be envenomed, just touched, okay?

One other doggie downer re the sale and attempted keeping of cold water specimens in tropical systems. Strongylocentrotus purpuratus and S. franciscanus from off the west coast of the U.S. are ofttimes peddled off as "tropicals". They're not, will kick and possibly take the rest of your livestock with them. If you don't have provisions for cool-water organisms (a chiller...) don't...

An image to show you what cooler water Urchins can/do look like. The larger, redder individual in the upper part of the frame is the larger species, Strongylocentrotus sanfriscanus, the ones in the foreground are the more common, smaller Strongylocentrotus purpuratus. Both have their eggs used in the sushi bar business as uni.

Environmental: Conditions

Sea Urchins are adapted for living on rocks and other types of hard substrates with spines and podia for movement and secure placement to resist tidal and wave action. They need similar materials in their systems to live comfortably.

Be aware that most are active only at night, so even if your Urchins look like they're able to negotiate the free space in your/their system by day, such may not be the case in the evening when they're active.

Chemical/Physical

Urchins are best placed into systems that are well broken into/cycled with some good growth of Green Algaes as a bio-indicator. They are sensitive to very small changes in water quality. A well-functioning protein skimmer should be a functional part of all marine captive systems; with urchins it is demanded.

Biology/Other

Just a few neat things, I promise. Urchin body walls have a ciliated epidermis under which lies a nerve layer, then the skeleton (= test). The skeleton is composed of rows of ossicles, flattened into plates and fused together into a solid test. Podia (tube feet for locomotion) penetrate the ossicles. And you thought they only made for spiffy night lights covers!

Filtration:

Excrete ammonia through their five pair of gills and solid feces through an aboral anus. Depending on the temperature of the system, food availability, ... may produce copious amounts of wastes. Under some circumstances, virtual feeding-frenzies can scrape up most all overnight!

Display

Corral those suckers! If you have a specific purpose in mind, isolate your specimens to that task. They can and will move otherwise to whatever area interests them. Be aware too that they may burrow into or under things and require extrication. Don't pull on them! Instead, remove the decor, live rock from on top, on the side of your Urchins.

 Behavior

Introduction/Acclimation

Urchins are a notable exception to freshwater or any type of preventative dip or bath. They should be "water-mixed" slowly and deliberately and not exposed to the air. Many wave-bearing species have thick and tough tests, some will collapse if raised from the water.

Predator/Prey Relations

For the most part, due to prickly- and poison- ness (is this English?) Urchins virtually lack predators under ordinary circumstances; but since when are aquariums ordinary? Some Triggers and their relatives the various Puffers, will try out Urchins (heaters, rocks, bubbles...), as will some Sea Stars, large Butterflies, Parrotfishes and Surgeonfishes and Crabs; otherwise most all organisms will leave them alone and vice versa.

Reproduction:

All echinoids are of separate sexes with external fertilization. There is no general structural difference between the sexes (sexual dimorphism for you scientific types). In brooding species the eggs are either retained in external body cavities or between spines. The feeding larvae are planktonic until the skeleton begins to form (about one millimeter!).

Locomotion:

Tube feet are used as in Sea Stars and their spines may be used for pushing. Burrowing and grinding into substrates is accomplished with their chewing apparatus and rotating spines. If turned over, the podia are employed group by group, attaching and releasing toward the mouth to right the animal.

Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes

Sea Urchins feed on algae, sessile benthic animals and animal remains. Deep sea and burrowing forms use the ooze (hey, that's fun to say!). In captivity they are ready eaters of most everything and anything. Keep your eyes on them!

All Sea Urchins have a celebrated chewing apparatus called Aristotle's Lantern composed of five large calcareous plates, a number of smaller rod-like pies and related special muscles. A real spiffy contraption but not very fast; consumption of a bunch of seaweed may take weeks.

Wilkens mentions an apparent need for supplying urchins with adequate sources of lime (calcium carbonate) for test & spine growth/strength (shades of osteoporosis!). Crushed shells, substrate work well. He also alludes to the unquenchable appetite his specimen showed for Caulerpa. They will eat anything and everything.

One more gratuitous endorsement, Tetra Tips are accepted with relish (or without) by many urchins.

Disease: Infectious, Parasitic

Not much of practical worth is known that can be easily elucidated. When purchasing specimens, look for damage to the spines (broken), tears around the mouth (peristome). Pick out individuals that are firmly attached with suction feet and podia active to some degree. Ask for a decided response to stimulation through food offering.

Questions/Close:

Bibliography/Further Reading:

Barnes, Robert D. 1974. Invertebrate Zoology, Third Ed. W. B. Saunders, Philadelphia.

Brusca, Richard C. 1973. A Handbook to the Common Intertidal Invertebrates of the Gulf of California. University of Arizona Press, Tucson.

Burgess, Warren E. 1977. Salts From the Seven Seas: Sea Urchins. TFH 1/77.

Erhardt, Harry & Horst Moosleitner. 1998. Marine Atlas, v.3 Invertebrates. MERGUS, Germany. 1,326pp.

Fenner, Robert M. 1998. The Conscientious Marine Aquarist. Microcosm, VT. 432pp.

Fox, Gregory A. 1993. Buried treasure. The Sand Dollar is sure to provide great commercial and aesthetic value for years to come. FAMA 3/93.

Herwig, Nelson. 1980. Starfish, Sea Urchins and Their Kin. by FAMA, A Division of RCM Publications, CA.

Humann, Paul. 1992. Reef Creature Identification; Florida, Caribbean, Bahamas. New World Publications. Florida. 320pp.

Hoover, John P. 1998. Hawai'i's Sea Creatures; A Guide To Hawai'i's Marine Invertebrates. Mutual Publishing, Honolulu. 366pp.

Wilkens, Peter. 1973. Translated by U. Erich Friese. Sea Urchins. Marine Aquarist Magazine. 4(5):73.

Wilkens, Peter. 1975. Tripneustes gratilla, A Glutton Among Sea Urchins. Aquarium Digest Intl. 3:4, 1975.

To: Part 1, Part 2, Part 3, Part 4, Part 5,