Echinometra mathaei, commonly known as ‘burrowing sea urchin‘ or ‘spiny sea urchin‘ is a species of echinoderm belonging to the class Echinoidea and family Echinometridae. The genus name ‘Echinometra‘ comes from the Greek ‘echinos‘ meaning ‘hedgehog‘ and ‘metra‘ referring to the uterus or womb, probably alluding to the rounded and compact shape of these urchins. The specific epithet ‘mathaei‘ was assigned in honor of the French zoologist Émile-Marin Mathé. Echinometra mathaei is a widely distributed species in the Indian and Pacific Oceans, from the coasts of East Africa and the Red Sea to the islands of the central and western Pacific. It mainly inhabits coral reefs, both intertidal and subtidal, where it shelters in crevices and cavities it excavates in rock and dead coral.
Echinometra mathaei is a small to medium-sized sea urchin that can reach a diameter of up to 9 centimeters, not including spines. Its carapace, known as the testa, is rounded and slightly flattened at the top. The spines are of moderate length and can measure between 2 and 4 centimeters, being shorter and thicker compared to other sea urchin species. The spines are generally uniformly purple along the spines, or greenish with purple tips. A distinctive feature of this species compared to similar species is that it has a pale ring at the base of each spine. This feature differentiates Echinometra mathaei from similar species such as Echinometra oblonga.
As for its diet, Echinometra mathaei is mainly herbivorous and feeds on encrusting coralline algae, macroalgae and organic detritus found on hard reef surfaces. It uses its specialized mouth structure, known as Aristotle’s Lantern, to scrape algae from rocks and dead coral. This organ is composed of five calcareous teeth that move in a coordinated manner to tear and crush the food. In addition to algae, it occasionally consumes small amounts of animal material, such as sponges or hydrozoans. Its role in coral ecosystems is fundamental, as it contributes to the control of algal overgrowth, thus facilitating coral recovery.
Reproduction of Echinometra mathaei is sexual and occurs through external fertilization. During the reproductive season, which usually peaks during the warmer months, urchins release their gametes (eggs and sperm) directly into the water. Gamete release is synchronized between males and females to increase the likelihood of fertilization. Fertilized eggs develop into planktonic larvae called pluteus, that float in the water column for several weeks before settling on the substrate and metamorphosing into juvenile urchins. This planktonic phase allows the dispersal of the species to new areas, contributing to its wide geographic distribution.
A curiosity about Echinometra mathaei is its ability to burrow into coral rock, creating small cavities where it shelters during the day to protect itself from predators and sun exposure. This burrowing behavior can have a significant impact on the reef ecosystem, as it contributes to coral and rock biocorrosion. Although this species may be abundant in disturbed areas, such as degraded reef zones, it also plays an important role in reef dynamics by facilitating the creation of microhabitats for other species. In addition, in some cultures, sea urchins, including Echinometra mathaei, are considered a source of food, especially their gonads, known as ‘uni’ in Japan, although their commercial value is limited compared to other sea urchin species.
It is possible to observe a small shrimp (Athanas areteformi) in the specimens of Echinometra mathaei that takes refuge among the spines of these sea urchins seeking protection from possible predators.
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