Sea Urchin

Sea Urchin

Sea urchins are globe-shaped, spiny marine echinoderms belonging to the class Echinoidea. Found in benthic environments worldwide, these ancient organisms feature a rigid internal shell known as a test, composed of fused calcium carbonate plates called stereom. Adult sea urchins possess five-fold radial symmetry (pentamerism), which is arranged in segments radiating around a central axis like an orange. The mouth is situated on the flat underside, or oral surface, while the anus is positioned at the top aboral surface. Body sizes typically range between 3 and 10 centimetres across, although the largest recorded species can measure up to 36 centimetres. Taxonomically, sea urchins are categorised into two primary subclasses: Cidaroidea and Euechinoidea. Subclass Cidaroidea, or slate-pencil urchins, represents a primitive lineage dating back approximately 268 million years, identifiable by heavy, blunt spines that frequently host epibiotic algae and sponges. Modern sea urchins belong to Euechinoidea, which includes regular, symmetrical forms as well as irregular echinoids like sand dollars and heart urchins. Irregular urchins have evolved flattened body plans enabling them to burrow through soft sandy sediments. Lacking a central brain, sea urchins navigate the seabed using hydraulic pressure within hundreds of tiny, adhesive tube feet, occasionally using their movable spines to push themselves forward. They are crucial ecological engineers in marine ecosystems. By grazing heavily on algae and seaweed, urchins prevent fast-growing turf and macroalgae from smothering hard corals and rocky substrate. However, if their natural predators—such as triggerfish, puffers, otters, and starfish—are depleted, unchecked urchin populations can overgraze habitats, creating completely barren marine zones. For underwater naturalists and scuba divers, sea urchins provide endless fascination. They range from visually striking species like the needle-spined Diadema to delicate, flower-like varieties. Divers frequently observe them clutching bits of shells or gravel over their bodies for shade and protection. Their intricate anatomy, ancient evolutionary history dating back 450 million years, and vital role in maintaining reef health make sea urchins an indispensable highlight of any benthic dive.