Invertebrates
Sea Urchin
Echinoidea · also called sea urchins, urchins
A sea urchin is a starfish folded into a ball. Pull a starfish’s arms up over its back and fuse them into a dome, keep the mouth underneath, and run the rows of tube feet up and over the outside between a forest of movable spines — that is an urchin. It grazes seaweed with a five-toothed jaw, and where its predators are gone it can strip an entire kelp forest to bare rock.
Overview
Sea urchins are the class Echinoidea, about 950 species, in every ocean from the intertidal to the deep sea and the tropics to the poles. They are echinoderms, kin to starfish and sea cucumbers, built on the same five-fold radial plan. Grazing species live on rocky reefs and kelp beds; others burrow in sand, and the flattened sand dollars lie in soft sediment.
Aristotle’s lantern
The underside carries a five-toothed chewing structure of about forty skeletal pieces, first described by Aristotle. The teeth sharpen themselves as they wear and grow continuously from the base — an arrangement materials scientists have studied for tools that never go blunt. Some urchins use it to grind burrows into solid rock.
Urchin barrens
Where sea otters, wolf eels, sheephead, or lobsters that eat urchins are removed, urchin numbers explode and the herd mows down the kelp forest faster than it regrows, leaving a bare pink expanse of coralline rock — an urchin barren — that can persist for decades. The loss of the long-spined Diadema urchin to disease in the 1980s Caribbean helped tip reefs from coral to algae.
Status
Sea urchins as a class are Not Evaluated and in no broad danger — several species are abundant enough to be a management problem. But heavily fished food species have been overharvested locally, and disease die-offs of key grazers like Diadema have had large knock-on effects on reefs. The seafood uni is the five gonads inside the shell.
Sea Urchin: common questions
Is a sea urchin related to a starfish?
Yes, closely. Both are echinoderms, the group that also includes brittle stars, sea cucumbers, and feather stars, and all of them are built on a five-part radial body plan with a water-vascular system that powers rows of tube feet. A sea urchin is essentially a starfish whose arms have been drawn up and fused into a rigid dome, with the mouth still on the underside and the five rows of tube feet now running up and over the globe between the spines. The spines, which are the urchin's most obvious feature, are movable skeletal rods mounted on ball-and-socket joints, and they are used for walking, wedging into crevices, and defence.
What is an urchin barren and why does it matter?
An urchin barren is a stretch of shallow rocky sea floor that has been grazed clear of kelp and other seaweed by a dense population of sea urchins, leaving mostly bare rock crusted with pink coralline algae. It forms when the predators that normally keep urchins in check, such as sea otters, wolf eels, sheephead, and lobsters, are removed by hunting or fishing. Freed from control, the urchins mow down the kelp forest and then persist at low food levels, cropping any new kelp before it can establish. Because kelp forests are among the most productive habitats in the sea, supporting fish, invertebrates, and marine mammals, the switch to a barren is a large loss of biodiversity and fisheries value, and barrens can last for decades.
Is a sea urchin sting dangerous?
For most people a sea urchin injury is painful but not serious. Standing on or brushing against one drives brittle spine tips into the skin, where they break off, and the punctures burn, swell, and can become infected, with the fragments often needing to be soaked out in hot water or removed by a doctor. A few tropical species raise the stakes. The long-spined Diadema urchins give a deep, intensely painful jab, and the flower urchin of the Indo-Pacific carries venomous pincer-like organs called pedicellariae among its spines that can deliver a sting causing severe pain and, rarely, more serious symptoms. The general rule on a rocky or reef shore is to wear footwear and not put a hand down blind.
What part of a sea urchin do people eat?
The edible portion, sold in Japanese cuisine as uni, is the reproductive tissue, the five gonads that hang inside the shell like segments of an orange. They are scooped out fresh and eaten raw, with a rich, briny, custard-like taste, and are among the most valuable seafood by weight. Demand, especially from Japan, supports urchin fisheries around the world and has led to overharvesting in some areas. It has also created an unusual conservation opportunity, since urchins taken from barrens are nearly empty and worthless but can be held in tanks and fattened on feed until their gonads fill out, which both produces seafood and helps clear the barren so kelp can return.