Care
Several Diadema species are encountered in the fishkeeping hobby; these are usually jet black, but sometimes violet, blue-black, or grey varieties are also seen, occasionally with banded patterns on the spines. Regardless of colour, the care requirements for all Diadema species are the same. A spacious, mature reef tank is necessary, with plenty of coralline and filamentous algae for the urchin to graze on. The specific gravity of the water should be stable, in the region 1.027-1.028, so be sure that other livestock is happy to live in this slightly higher than average salinity range. It is of utmost importance that sea urchins are acclimatised very, very carefully, ideally using the drip method "“ too rapid a change in salinity can be fatal. Never expose the urchin to air. Excellent water quality is required at all times, so powerful filtration, water movement and protein skimming should be considered essential. Sea urchins do not take well to even the smallest changes in water chemistry. The spherical body is surrounded by long, thin, sharp spines, which are longer on the upper surface than the lower; this allows the mouth parts to reach down to the substrate to feed. The spines are not venomous, but they can inflict a painful wound (particularly if a small barb breaks off), so always take care when handling or carrying out maintenance in the aquarium. There should be plenty of live rock for the urchin to manoeuvre over and graze upon; these are creatures that are accustomed to moving over and holding on to hard surfaces amid quite forceful tidal action in the wild. Observe these largely nocturnal animals carefully to ensure that your urchin can move freely in all areas of the aquarium and that there are no areas where it could become trapped. Some consideration must be given to tankmates, as although the spines are a fearsome deterrent, some larger, belligerent fish species may not be able to resist a nibble just to see whether it is palatable (e.g. some large tangs, large angelfish, large wrasse, parrotfish, puffers, triggers etc). Crabs should also be avoided and there should not be too many direct food competitors housed in the same aquarium. As with other invertebrates, do not use copper-based medications if you have sea urchins in the tank. Sea urchins do lose the occasional spine from time to time, but excessive spine dropping is not a good sign and is usually an indicator of stress/illness due to poor water quality, too rapid a change in water chemistry, or nutritional issues.
Feeding
Primarily herbivorous, but will take meaty foods and will constantly scavenge for a variety of natural foodstuffs within the aquarium. A large mature system is required as this will provide an ongoing source of various types of algae (macro and micro), and this can be supplemented with green-based foods (Caulerpa and Nori are particularly relished by many urchins) and occasional bits of meaty fare.
Breeding
Successful reproduction is unlikely in the home aquarium. In the wild, the males and females simultaneously release their gametes into the water column, where they mix and are fertilised. Shortly after fertilization, the blastula (early stage of embryonic development) is free-swimming. This then develops into a gastrula which quickly progresses into a planktonic larval stage, known as echinopluteus, with a laterally compressed shape, bearing 4-6 pairs of arms supported by calcareous spicules. After a few weeks, the echinopluteus sinks to the substrate where it rapidly metamorphoses into a juvenile urchin.
