Banded Sea Urchin

Echinothrix calamaris, commonly known as the banded sea urchin or double spined urchin, is a sea urchin species belonging to the family Diadematidae.

The banded sea urchin has an oval-shaped shell, or test, with a diameter of approximately 5 cm. It exhibits a distinguishing characteristic shared by most members of the Diadematidae family – two types of spines. The shorter and slender spines range in color from yellow to dark brown and can deliver a painful sting. Conversely, the longer and thicker spines often display alternating light and dark bands, although they can also be uniformly dark or white. These spines can reach lengths of 10 to 15 cm and possess a hollow structure. The spines are arranged in such a way that five bare areas, known as "iridophores," are visible in a star pattern on the central part of the shell. These iridophores can exhibit various colors, frequently appearing blue.

On the aboral face (opposite to the mouth) of the sea urchin, a large and somewhat translucent anal papilla is evident. This papilla typically appears whitish and is embellished with black and white spots. Surrounding the papilla is a ring of visual receptors, providing it with rudimentary visual capabilities.

In its darkest forms, the banded sea urchin can be easily mistaken for Diadema setosum or Diadema savignyi, both of which can retain banded spines in adulthood. However, these other species are larger, uniformly black, possess less noticeable secondary spines, and display five distinct white points on the top of their shells, accompanied by an orange circle around the anus. The most frequently encountered confusion arises between the banded sea urchin and the closely related species Echinothrix diadema, which can have banded spines when young. However, Echinothrix diadema exhibits a very small black anal papilla, spines with a blue sheen, and more fragile primary spines that can be crushed between two fingers – a characteristic not shared by Echinothrix calamaris.

A dark variant of the banded sea urchin has been documented in the 🌊 Indian Ocean, particularly in the 🌊 Red Sea, Mascarene Islands, 🇸🇨 Seychelles, and the 🇲🇻 Maldives. In this variant, the radioles (modified spines used for respiration and feeding) are very dark, often unbanded, and tend to display a green sheen. The test may present a reddish hue, occasionally appearing completely red, especially in the 🌊 Red Sea. The anal papilla is black, occasionally punctuated with white, but sometimes solely displaying black markings. These distinguishing features make it difficult to differentiate this variant from Echinothrix diadema, particularly due to the absence of iridophores. Furthermore, the spines of this variant are sharper and more solid, unable to be crushed between two fingers. It is possible that this variant represents a subspecies or even a distinct species, especially considering its slightly different ecological niche.

The banded sea urchin is distributed across tropical waters within the Indo-Pacific region, spanning from the eastern coast of Africa to 🇵🇫 French Polynesia, including Hawaii (🇺🇸 United States) and the 🌊 Red Sea. It can be found at depths ranging from the surface to 70 meters and inhabits lagoons, outer reef slopes, and channels.

This sea urchin species is primarily active at night, seeking shelter in crevices or under rocks during the daytime. While the thinner spines possess venom, they are not considered dangerous to humans.