Red Pencil Urchin

Heterocentrotus mammillatus, also known as the slate pencil urchin, red slate pencil urchin, or red pencil urchin, is a species of tropical sea urchin found in the Indo-Pacific region.

This particular species of sea urchin is relatively large, with some specimens growing over 8 cm in diameter and having spikes up to 10 cm in length. While most individuals exhibit a bright red coloration, variations in brown and purple are also observed. The spines of the urchin may have a different color from its body, showing a white ring at their stem with alternating light and dark rings. The spines are rounded to triangular in cross-section and taper towards the tip. Interestingly, during the night, the red spines transition to a chalky pink hue.

While specimens from Hawai‘i typically showcase bright red spines, individuals from other parts of the Pacific may have yellowish or brown spines. Variations of Heterocentrotus mammillatus found on the Ogasawara Islands, for example, exhibit slimmer spines as opposed to the characteristic thick, broad spines. There has been debate as to whether these specific variants should be classified as Heterocentrotus mammillatus or categorized as Heterocentrotus trigonarius, another type of pencil urchin. However, it has been established that the Ogasawara type is indeed a form of Heterocentrotus mammillatus. Heterocentrotus trigonarius, on the other hand, is a similar species that has longer and darker spines, with a more angular appearance and a monochromatic coloration.

Heterocentrotus mammillatus can be found across the tropical waters of the Indo-Pacific region, ranging from the east coast of Africa to various Pacific archipelagos. It is particularly abundant in Hawai‘i and also occurs on the shores of the Sinai Peninsula. These sea urchins inhabit reefs in depths ranging from eight to 25 meters, mainly residing in the subtidal zones. They display a preference for burrowing into hard sediments such as limestone, coral, and basalt. Juvenile Heterocentrotus mammillatus can often be found hiding under rocks.

The primary diet of Heterocentrotus mammillatus consists of encrusting coralline algae, although small amounts of other algae such as Pterocladia and Ulva have also been observed in their diet. Sea urchins are typically marine grazers and consume algae in close proximity. Therefore, many species exhibit a more sedentary lifestyle. However, Heterocentrotus mammillatus displays a higher level of activity compared to other urchins like Echinothrix calamaris, Echinometra mathaei, and Echinometra oblonga. This species has been observed to travel distances of up to 600 cm and move an average of 96 cm per day.

Heterocentrotus mammillatus faces similar predators to other tropical sea urchins within its habitat. Fish belonging to the families Balistidae, Labridae, and Lethrinidae are known to prey on sea urchins. While Heterocentrotus mammillatus is occasionally consumed by humans, it is not as commonly targeted as the Tripneustes gratilla species.

The thick, rounded spines of Heterocentrotus mammillatus allow it to bore into hard substrates and defend itself against predators, wave drag, and pressure. These spines consist of magnesium calcite organized into a porous network, enabling the passage of fluids and organic matter. The spines have a dense base and become more porous toward the tip. Notably, Heterocentrotus mammillatus possesses additional layers of dense material within its spines, sandwiching the more delicate porous layers. This unique layered structure allows for graceful failure, preserving the lower layers of the spine farthest from the point of pressure. The layered constitution also facilitates crack deflection.

In living organisms, bilateral symmetry establishes an anterior and posterior end. The anterior end undergoes cephalization, forming a "head" containing sensory organs and the mouth. Typically, this head becomes the locomotor anterior from which the organism orients itself, moving forward with the head leading the way. However, Heterocentrotus mammillatus, despite being bilaterally symmetrical, does not possess a fixed locomotor anterior like many other organisms. Instead, it displays temporary locomotor anteriors when moving, orienting itself in the direction it initially moves. As a result, Heterocentrotus mammillatus tends to persist in its original direction, even if disrupted and placed in a different orientation.

Similar to other sea urchins, Heterocentrotus mammillatus undergoes periodic gamete development and release for fertilization, followed by a resting period to restart the cycle. However, the timing and duration of these reproductive cycles vary among individuals. For instance, Heterocentrotus mammillatus near the Gulf of Aqaba and 🌊 Red Sea follows an annual reproductive cycle. Its resting period lasts from October to January, during which it accumulates nutrients in its gonads for gametogenesis. Gametogenesis begins in January and concludes in May, during which the gonads become almost entirely filled with gametes. Spawning typically occurs between May and June, although it can extend until October for all urchins to complete the release of eggs and sperm. The start and end times of these reproductive periods may vary by a few months, suggesting a lunar or semilunar cycle in spawning and gametogenesis. While spawning does not consistently occur during the same lunar phase, other factors such as the tidal cycle may influence the timing of reproductive stages in Heterocentrotus mammillatus.