Within the phylum of echinoderms and the Echinoidea class, we find the Arbacioida order and the Arbaciidae family to which the black sea urchin or Arbacia lixula belongs. It is a species that inhabits the tropical and subtropical waters of the Atlantic Ocean and the Mediterranean Sea. In the Atlantic Ocean this species is present in the Azores, Madeira and Canary Islands, and is rather scarce on the African coasts. In the western Atlantic, it is only present on the Atlantic coasts of Brazil. It has a preference for shallow waters in rocky environments with depths not exceeding 50 meters, although in the first 10 meters of depth, especially in the intertidal zone, is where they are more frequent. Arbacia lixula has a great resistance to waves. As a curiosity it takes a force of about 5 kilos to release the specimen from the substrate in which they live. This strong fixation is due to the force exerted by the different ambulacral feet of the specimen that hold it firmly to the substrate. It is very common to find specimens of Arbacia lixula together with specimens of the species Paracentrotus lividus, the latter characterized by a smaller number of spikes and by being arranged in a much more disordered way.
The body of Arbacia lixula is hemispherical in shape, with a flattened basal part. They can reach 5-8 cm in diameter, and the figure can reach up to 12 cm in diameter if we include the barbs. Continuing with the barbs, they are all approximately the same length (~6 cm) and are always erect, arranged in a dense and uniform manner. The ventral part of the body has no spines and the mouth is surrounded by a smooth dark greenish skin. As for coloration, the specimens of Arbacia lixula are of a very intense black color. It has been observed that those specimens placed in very poorly lit environments, can change the color from the characteristic black to a very dark brown.
Arbacia lixula is a herbivorous species that feeds mainly on calcareous algae that grow on rocky surfaces, as well as small filamentous algae.
During the months of December and January is when the specimens of Arbacia lixula reproduce. This reproduction is sexual, and occurs through the release of the gonads of the male and female specimens into the water column, where fertilization occurs and planktonic larvae are formed that will be spread thanks to the currents. Curiously, the gonads of Arbacia lixula contain very high levels of a carotenoid pigment called astaxanthin. This compound is of interest from a biomedical point of view as it has been extensively described for its ability to prevent neurodegenerative diseases.
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