Depending on the species, sea urchins range in size from 6 to 12 cm in diameter. The most distinctive part of the sea urchin are its spines which vary in length and are used to protect the sea urchin from predators. The length of these spines may range from 1 to 3 cm but some have spines as long as 10 to 30 cm. Depending on the species these spines may also contain irritant toxins.
In addition to providing your sea urchins with the proper tank environment you also need to ensure that they have an adequate supply of food. In the wild, sea urchins feed on various types of algae, including seaweed, kelp, laminaria and egregia. The preferred diet will depend on the species of sea urchin you select but you should be prepared to provide a variety of different types of algae to keep your sea urchin well-fed. Be careful not to overfeed your sea urchins, however, because any excess algae or vegetable matter in the tank could result in a build-up of harmful toxins like ammonia which could contribute to a decrease in water quality.
In addition to feeding your sea urchins you should also make sure to keep the tank clean. If you are adding a sea urchin to an existing saltwater tank you should already be performing routine water changes to keep the water quality in your tank high. As you perform your routine water changes, be careful not to disturb your sea urchins and be sure not to damage their spines when using an aquarium gravel vacuum.
You should also keep in mind that sea urchins may not be compatible with all of the species of fish you have in your saltwater tank. Certain species of fish like Triggerfish will prey on sea urchins and are thus not compatible. Starfish may also pose a danger to sea urchins.
It is important to note that there are approximately 800 sea urchin species worldwide and over 200 in Australia alone. As a result, it is important to ensure that you identify the species of sea urchin before you purchase it. The following is a list of sea urchin species commonly found in Australia; Long-spined sea urchin, Pin-cushion urchin, Pink Sea urchin, Purple Sea urchin and spiny sea urchin.