Sea Urchins

Sea Urchins

Written by George J. Reclos Tuesday, 30 July 2002 00:00

PDF | Print | E-mail

          

Probably the most attractive sea urchin available to the aquarist. It grows to about 2-3 inches (although you can occasionally find specimens which are almost 4-5 inches in diameter – spines excluded) and has dark pink spines with white tips on a bright purple body. Rarely, the entire spines are white. Lives commonly with algae, corals and marine plants  at depths of up to 100 feet. Usually the juveniles live at greater depths and when grown they migrate in groups to shallower waters. In the course of this migration many of them are eaten by sea stars like Luidia ciliaris and Marthasterias glacialis. It does best in aquariums kept at temperatures of 21^oC (70^oF) or lower. A temperature of less than 18^oC for the winter months gradually increased to 21^oC for the summer months will help it to thrive while at the same time offering a “seasonal” fluctuation. It is a detritus feeder (especially in the juvenile stage) and requires fresh vegetable matter to survive in the home aquarium (from time to time). If your tank has algae growing on the rocks then you don’t need to do anything. The S. granularis will easily (and amazingly quickly) move around the tank grazing on the rocks and eating the algae very efficiently. In fact, so efficiently that it will leave the rock completely devoid of any algae at all. It will use its small thin “tentacles” (named pedicellariae) which have special suckers at the tips to move around its environment or stick on a rock (or even the glass of the tank if it covered with algae). It may carry a lot of things on its body (small stones and shells) both in its natural habitat as well as in the aquarium. This was misinterpreted in the past as an effort to protect itself from strong light. Collected in May 2002, at a depth of 5 meters.

          

As with most sea urchins this one is the marine equivalent of the freshwater plecostomus. It will eat anything but really prefers algae. It is capable of consuming a good amount of it on a daily basis leaving nothing but the rock behind (see photo above). Sea urchins will go to any place in the tank - at least those that are likely to have some algae. This species is relatively small, not growing more than 5 cm (the body) and an overall 8-11 cm (spines included). It is not an endemic Mediterranean species but it is found almost everywhere. The over-fishing of its natural enemies (basically Octopus vulgaris) allowed it to reach vast numbers. Still, finding a big specimen is quite rare. We had to scan a huge area of the sea bed till we came across two specimens that looked right. While collecting them take extra care to avoid their spines piercing you as well as you breaking them. Use a knife to remove the urchins carefully from their solid support. Often mistaken for Paracentrotus lividus although there are basic differences between the two species. The most important differences between those very common Mediterranean species are the following (again, after searching many sources and checking my specimens):

 

A very common sea urchin which has a body of 6 cm covered with spines which usually measure another 3 cm. Larger specimens reaching a size of 12 cm have been reported although very rarely. Indeed, after searching thousands of specimens, the larger we could find was about 9 cm in diameter (spines included). An excellent scavenger and profound algae eater. May attack coralline algae and (very rarely) sponges therefore it can’t be used as the algae eater of choice unless you don’t plan to keep sponges in your tank. If enough algae is present then your sponges can be regarded as safe. More beautiful than its black counterpart (Abracia lixula) but less beautiful than Sphaerechinus granularis which is far larger therefore not very suitable for small reef tanks. Along with the other sea urchins and the anemones they are the moving aquascape of reef tanks.

Read article on Starting a marine (Mediterranean) tank.