Spindly and sturdy, sea urchins in the wild are found on ocean floors across the globe. These animals are part of the Echinodermata phylum, a group that also includes starfish, sea cucumbers and sand dollars. Sea urchins typically sport spines ranging from just over one-third inch to nearly 1 1/2 inches in length. These spines, although painful if you step on one or suffer a puncture wound while diving, are not poisonous to humans.
As aquarium pets, sea urchins function as a habitat barometer of sorts, as they are highly sensitive to changes in water quality.
No Wooing Involved
Sea urchins reproduce sexually, but they have no courtship or mating ritual, and they are impossible to sex before mating, as the external features of males and females are identical. Triggered by a chemical or temperature change, spawning occurs when the males release sperm and the females release millions of eggs simultaneously.
Fertilized eggs initially form into larvae, which migrate from the ocean floor to warmer surface waters to feed and grow. For weeks they eat phytoplankton that they catch with long, hairlike arms. Eventually, the larvae develop features such as the tube feet that sea urchins use to move, and their larval anatomy is re-absorbed back into the body. The now juvenile sea urchins sink back down to the ocean floor, and continue developing their adult form.
Do not house sea urchins with trigger fish or puffer fish, as these fish can pick off spines, break shells and eventually eat your urchins.
High-maintenance House Guests
Sea urchins require quite a bit of space in home saltwater aquariums. Allow at least one gallon of space per inch of the urchin's predicted adult size. This is not because sea urchins move very often, or quickly, but because they do add to the bioload, or biological load, of the tank. The bioload correlates to the amount of waste generated by all of the living species in one tank, including invertebrates such as sea urchins. Crowding a tank lowers the oxygen levels necessary for the inhabitants to sustain life.
Because sea urchins have a reputation for finickiness, it's imperative that their housing be adequately lit and filtered. The water temperature for most urchins ranges between 75 and 82 degrees Fahrenheit, and weekly water tests are highly encouraged. Sea urchins, like many other aquatic creatures, become stressed in crowded tanks. To help make your sea urchin comfortable, consider adding natural accouterments that the urchins surround themselves with in the wild: exposed rocks, coral, plenty of sand, sea grass and seaweed.
If possible, maintain a temperature and light cycle that mimics the peak season for your sea urchin. This consists of putting lights on a timer and adjusting heat to replicate winter -- short days and colder temps -- and summer -- long light periods and warmer temps.
How About a Nice Salad?
Sea urchins are omnivores but will be quite content with a purely vegetarian diet, as meats of any variety tend to dirty a tank quickly. Your urchin will consume algae, kelp and detritus in tanks. Algae pellets and other supplements such as blanched greens help keep your spiny pet well-fed and happy. It's important not to overfeed your sea urchins, however, as rotting food builds excess ammonia, and this negatively impacts the water quality of the tank.
Underfeeding sea urchins will prompt them to feed on anything else available, including each other.
Although it ranges by species, the life span of some sea urchins -- such as the purple sea urchin -- can last up to 30 years, if cared for properly.