Diadema antillarum

The sea urchin Diadema antillarum, which belongs to the echinoderm family like sea stars or sea cucumbers, is present on both sides of the Atlantic Ocean. It is also known as lime urchin, black sea urchin or long-spined sea urchin. It can reach between two to 12 cm diameter and it has a black colour with large fine spines, easily breakable and mobile. The Diadem sea urchin is present in Macaronesia long time before the human beings. It is named Diadema antillarum-b (DNA tests).





This sea urchin is preferably herbivorous but it can turn into a terrible omnivorous predator according to food resource availability. It owns a five teeth mouth placed on the posterior part of its body and it can feed from everything: algae, sponges, but also organic pieces or even rubbish. The size on the sea urchin depends on the quantity of food available. If there are many nutrients available, it reaches its larger size (vertical diameter of 10 /12 cm) and its size decreases proportionally with the lake of food. But that does not mean that the number of sea urchin decreases, all the opposite. With many nutrients, they are large and in small number, but with few nutrients, the number of sea urchin per square metre dramatically increases.

In the water, we can not differentiate the males from the females. The sea urchin reproduces all year long, with a reproduction peak in the Canary Islands during spring. Fecundation is coordinated by the full Moon and takes place in water. The planktonic larvae can drift during two months before landing on the sea bed and change themselves into “baby urchin”.

The sea urchins move upon any surface, including vertical walls or sandy bottom. During day light, they appear motionless but they are very active at night.



The sea urchin Diadema antillarum is a species which forms vast groups on the rock formations, by seeking shelter in caves or cracks. They are not only trying to protect themselves from predators but also from water movements. A sea urchin which shifts is an opportunity for a puffer fish or a parrot fish… Studies shows that this specie can move up to six metres away from its shelter and generally goes back there at dawn.






If lime sea urchins are surprised by the day light on a sandy bottom, they gather as the Roman troops from the Antiquity. They form blocks of sea urchins, impenetrable and resistant to the currents. When we approach them at half - metre distance, they identify our presence thanks to photosensitive sensors, and answer each change of light by a frantic dance of their spines as a warning sign.


The same phenomenon occurs when a predator approaches them. A sea urchin can flee at full speed when a sea star stays at finger’s range. We know little about the Canarian predators of the Diadema sea urchin. Its long spines, the round shape of its body, his night lifestyle reduce the list considerably. In the Canaries, there is not only one species which can control its distribution on its own, but indeed a set of several species. The lake of natural predators is one of the reasons why the Diadema sea urchin proliferates on our coasts.




Studies made by the University of Las Palmas from Gran Canaria showed a clear relation between the lake of big size carnivorous fishes with high fishing value and the actual huge density of sea urchins. The direct consequence is that the blanquizales (rocky area devastated by the sea urchins), increase their surface dramatically. The actual density of sea urchins is so big that it breaks the food chain. Impossible for Mother Nature to go back to normal. The few algae (at the beginning of the food chain) which grow during the day are systematically devoured at night.




Several actions are taken to try to control the number of sea urchins in specific zones, but it is a titanic work, since only one tenth of the quantity of sea urchins present in a blanquizal can keep it like it is… A draconian control of fishing activities is thus necessary to leave a chance to the nature to produce new predators, and in great numbers.



One of the reasons of the expansion of the blanquizal is the massive use of fish traps (like bow nets). This non selective fishing way leads to the overexploitation of the Canarian littoral. According to the study, around 70% the Canarian artisanal fleet practises this kind of fishing. Personally, I have never seen in diving a bow net carrying its registration number (obligatory according to the law…). And moreover, we do not have luck since the minimal depth to place fish traps is brought back to 12 meters in Tenerife, instead of 18 meters in the other Canarian islands. Logic should not be to prohibit fishing with this kind of huge fish traps at shallow depth, precisely where the sea urchins live, until complete environmental recovery?