Heterocentrotus mamillatus, commonly known as ‘red pencil sea urchin‘ or ‘pencil urchin‘ is a species of sea urchin belonging to the family Echinometridae. The genus name ‘Heterocentrotus‘ comes from the Greek ‘heteros‘ meaning ‘different‘ and ‘kentron‘ meaning ‘sting‘ or ‘spine‘ referring to the distinctive spines possessed by urchins of this genus. The epithet ‘mamillatus‘ refers to the protuberances or ‘mamelons‘ found on the carapace (testa) of this hedgehog. Heterocentrotus mamillatus is found in coral reefs and rocky areas of the tropical regions of the Indian and Pacific Oceans, from the coasts of East Africa, the Red Sea, to the Pacific islands, including Hawaii, the Great Barrier Reef in Australia, and other central Pacific archipelagos. Its preferred habitat is intertidal and shallow subtidal zones, where it adheres firmly to rocky and coral surfaces.
This sea urchin is distinguished by its medium to large size, reaching diameters of up to 8-10 centimeters excluding spines. The spines of Heterocentrotus mamillatus are remarkably thick, strong and cylindrical in shape, giving them the common name ‘pencil urchin.’ These spines can reach up to 12 centimeters in length and have a very striking coloration, ranging from bright red, orange, purple to dark brown, depending on the individual and the region in which it is found. The thicker, more robust spines are used primarily for defense and locomotion, while the thinner, shorter spines, which cover the carapace, aid in substrate manipulation. Heterocentrotus mamillatus can be differentiated from its close relative, Heterocentrotus trigonarius, by its thicker and more rounded spines in cross section, while the spines of Heterocentrotus trigonarius are more triangular and sharp.
As for its diet, Heterocentrotus mamillatus is a generalist herbivore. It feeds mainly on encrusting coralline algae, macroalgae and organic detritus that it scrapes from the substrate using its mouth structure known as Aristotle’s Lantern. This structure, composed of five calcareous teeth, allows the urchin to efficiently scrape and grind its food. Like other sea urchins, its role in the ecosystem is critical, as it controls the growth of algae on coral reefs, thus allowing corals and other sessile organisms to thrive. However, in areas where sea urchin populations are excessive due to overfishing of their natural predators, Heterocentrotus mamillatus may contribute to reef erosion by consuming large quantities of coralline algae.
Reproduction of Heterocentrotus mamillatus is sexual and occurs through external fertilization. During the reproductive season, which generally occurs during the warmer months of the year, males and females release their gametes (eggs and sperm) synchronously into the water. The simultaneous release of gametes increases the chances of fertilization in the water column. Fertilized eggs develop into planktonic larvae called ‘pluteus‘ which float freely in the water column for several weeks before settling to the seafloor and transforming into juvenile urchins. This planktonic phase allows wide dispersal of the species through ocean currents, facilitating its distribution on coral reefs.
A curiosity about Heterocentrotus mamillatus is its ability to withstand strong waves and ocean currents, thanks to its thick and robust spines that allow it to anchor firmly to rocky substrates. This species is known for its longevity, being able to live for several years, and for its ability to regenerate damaged spines, which gives it an advantage against predators. In addition, the spines of Heterocentrotus mamillatus are highly valued in some cultures as materials for the manufacture of handicrafts and jewelry. Ecologically, this sea urchin is considered a bioindicator of reef health, since its presence in large numbers can indicate imbalances in the ecosystem, such as a decrease in natural predators.
Among the spines of Heterocentrotus mamillatus specimens from the Hawaiian region, we can sometimes observe the small shrimp Levicaris mammillata. This shrimp establishes a relationship by which it cleans Heterocentrotus mamillatus of potential parasites, obtaining as a reward the security that comes from being among the spines of the urchin, thus scaring away possible predators.
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