Anyone who has been barefoot in the sea and has stepped accidentally on a sharp spined sea urchin is likely to remember the experience. The spines are often needle sharp (there are variations) and some are equipped with poison glands.
Sea urchins are a good addition to reef aquariums, and also to fish only systems provided there aren’t any fish, such as triggers and puffers, that will do them harm. There should not be large hermit or ordinary crabs, lobsters or predatory snails in the aquarium either. It is not necessary to keep just one specimen of urchin, or the same type, though space restrictions and food supply mean usually one, or maybe two in a large system. They are considered safe with ordinary snails, shrimps and the like, and also corals. Sea stars and brittle stars are normally safe also, though some care has to be taken that the stars are not predatory. Urchins tend generally to move about at night, and during the day find a crevice or hole which affords them good protection. Having a sea urchin makes the overall picture more natural.
Sea urchins require good seawater quality or they may lose their spines and eventually die. They also require good seawater movement. Good seawater quality and movement is a requirement for the marine aquarium anyway so this should not be a problem.
Having enough food for the urchin is usually the main concern. For most algae is needed and in a marine aquarium this can be in short supply, in fact the aquarist probably ensures a low algae presence by if necessary having snails etc present which keep it low. Without algae the urchin will last a while but its demise is inevitable. The choice is to provide algae or not have an urchin. Some aquarists meet the problem by having a few marine safe flat stones available. They allow algae to develop on these stones in a very shallow small separate tank and then they are placed in the aquarium for the urchin. If the stones are used half and half then a continuous supply of food should be available. The other alternative is to reduce or not have any competition for algae in the aquarium though a watch is still needed to ensure there is enough.
There is clearly a requirement to check the space available in the aquarium taking into account the length of the urchin’s spines and the well-being of other livestock, for example the needed swimming space for fish. This usually will not present a problem.
When an urchin is purchased great care is required that seawater is not lost during the journey home because of a punctured bag. When being acclimatized to the home seawater, as with shrimps and sea stars, a fairly long period is needed preferably using the drip method. When the urchin is released it should not be permitted to come into contact with the air.
Urchins come in different sizes with short blunt spines to long sharp ones. The urchins I kept were black long-spined ones, generally known as ‘hatpin’ urchins. Though the retailer couldn’t give the proper name for them, research strongly suggested that they were Diadema setosum.
As these urchins have stinging spines care had to be taken when carrying out maintenance with a hand in the aquarium, but nothing untoward occurred because unlike say a lion fish, the urchins were in one place. They certainly added interest, increased the diversity of the livestock, and at the same time controlled algae (unfortunately it is reported that they are unlikely to touch green hair algae). I no longer have an aquarium large enough to allow an urchin to be present, or there would be one.
The link will give some idea of the colour and diversity of sea urchins.