Ježovka Plochotrnná

Overview

Heterocentrotus trigonarius, commonly known as the slate pencil urchin, is a striking tropical sea urchin found throughout the Indo-Pacific region. It is easily recognized by its exceptionally thick, robust primary spines that provide both protection and stability in high-energy environments.

Distribution & habitat

This species is widely distributed from the Red Sea and East African coast across the Indian Ocean to the Pacific, reaching as far as the Marshall and Tuamotu Islands. It thrives in shallow, high-energy marine habitats, particularly in extreme surf zones, outer reef flats, and intertidal areas. These urchins are often found wedged into rocky crevices or near algal ridges, where they use their heavy spines to anchor themselves against strong wave action.

Appearance

H. trigonarius features a hard, oval-shaped test (shell) and primary spines that can reach up to 7 cm in length, with an overall diameter of up to 30 cm. The spines are notable for their pencil-like appearance and can vary from round to triangular in cross-section. Their coloration typically ranges from deep violet to dark purple or maroon. Smaller, flattened secondary spines are also present on the body, often with brown tips.

Biology & behavior

Primarily nocturnal, these urchins are well-adapted to turbulent waters. They are gonochoric, meaning individuals are either male or female, and they utilize external fertilization. Some populations exhibit brooding behavior, where eggs are retained near the mouth or anus until the larvae develop. While they are a common sight for divers, their sharp, sturdy spines can easily penetrate footwear and inflict painful wounds.