Black Sea Urchin, Diadema antillarum

The black sea urchin (Diadema antillarum), also known as the lime sea urchin or the long-spined sea urchin, is a species that can be found in the Caribbean basin and the western waters of the Atlantic Ocean. It prefers to inhabit coral reefs in these areas and resides at depths of up to 32.8 feet. This species has a test, or outer shell, that is similar that of most species of sea urchin. However, this species has longer spines, a trait from which it derived one of its common names. These spines can grow to be an average of 3.9 to 4.7 inches, although lengths of 11.8 inches have been recorded in large individuals.

The black sea urchin typically situates itself within a coral crevice for optimum protection, but it can be found in areas that are more open if no suitable crevices can be found. This species feeds at night and will move as far as 3.2 feet each night in search of food. It prefers to feed on algae and sea grasses, but has been known to consume animal matter if no other food types are available.

In some areas of its range, the black sea urchin remains to be an abundant and vital species to the environment around it. Because it feeds primarily on algae, it provides a service to the reefs that it inhabits. Algae can smother reefs if it becomes too thick, so areas with high numbers of this species show that the reefs will most likely be safe from harm. This became evident in 1983, when the black sea urchin lost 97 percent of its population numbers in an area that extended from the Bahamas to South America. The coral reefs in this area have become overgrown with foliose macroalgae. Studies recently conducted in Discovery Bay, Jamaica and other areas have shown that higher numbers of sea urchins have helped the coral reefs to regenerate.

The sea urchin die off in 1983 is thought to have been caused by an unknown type of disease, and it caused a major growth in many types of algae. Because of the negative effects of this on the coral reefs, other species like fish were also affected, because they lost a major source of food and protection. This also caused several small countries to lose vital tourism, which strained the economies of those countries.

There are many factors which hinder the repopulation of sea urchins in areas where numbers are low. Although both males and females release their sperm and eggs at the same time into the water column, if there are not enough gametes, the spawning season for that year may fail. Other factors that hinder the growth of this species include harsh weather patterns, and predators, which feed on young sea urchins when they are in their larval form.

It is thought that humans can aid black sea urchins in recovering their population numbers with further research into possible conservation efforts. Although this research is still in its first stages, it has been found that reproducing the species in a laboratory for reintroduction is possible. Manmade reefs have also proven to be an efficient way to bolster sea urchin numbers. By placing concrete in the water, coral reefs have a higher chance of developing and many species like sea urchins and fish can have a temporary residence within the crevices of the concrete. Replacement of sea urchins from areas where numbers are high into less populated areas has also proven to be successful. It is though that these efforts could not only help to restore the black sea urchin to high numbers, but may also help coral reefs to dominate the coastal waters once more.

Image Caption: Diadema antillarum (black spiny Caribbean sea urchin). Credit: Dpbsmith/Wikipedia (CC BY-SA 3.0)