Purple Sea Urchin (Strongylocentrotus purpuratus): Facts

What does a purple sea urchin look like?

The body is a rigid, slightly flattened sphere called a test, built from interlocking plates of calcium carbonate under a thin layer of skin. It bristles with short, stout spines that are bright to deep purple in adults. Young urchins look different: their spines are mostly pale green, tinged with purple.

Each spine sits on a tiny ball-and-socket joint. Muscles at the base swivel it in any direction, and a special connective tissue can stiffen to lock the spine in place, wedging the urchin firmly into a crack.

Between the spines are hundreds of tube feet: thin, stretchy, water-filled tubes tipped with suction discs, arranged in five double rows from top to bottom. Scattered among them are pedicellariae, tiny pincers on short stalks that snap at small animals and keep the surface clean. The mouth is in the middle of the underside, and the anus is on top.

Like its relatives the sea stars, sand dollars and sea cucumbers, the purple urchin has a body built in five parts and no brain. A ring of nerves around the mouth sends five nerves along the body. It also has no eyes, although it can sense light; see Do sea urchins have eyes?

Two relatives share its coast. The red sea urchin (Mesocentrotus franciscanus) is much larger, with longer spines in shades from red to dark burgundy, and dark individuals can look purple. The green sea urchin (Strongylocentrotus droebachiensis), common in the north, has finer greenish spines.

How big do purple sea urchins get?

The test of an adult purple sea urchin is usually 5–8 cm (2–3 in) across and occasionally reaches about 10 cm (4 in). The spines add a little to the overall width. Typical adults weigh roughly 30–160 g (1–5.5 oz), depending on size and on how full their gonads are.

Growth depends heavily on food. Urchins in healthy kelp forests grow faster and carry much larger gonads than urchins in barren areas. In a classic 1967 study, the ecologist Thomas Ebert even showed that starving purple urchins can shrink.

Habitat and range

The purple sea urchin lives along the Pacific coast of North America from Alaska to Isla Cedros in Baja California, Mexico, and is most common from British Columbia southward. It is found from the low intertidal zone, the part of the shore uncovered only at the lowest tides, down to about 160 m (525 ft), but most live in shallow water.

It thrives on wave-beaten rocky shores and in kelp forests of giant kelp (Macrocystis pyrifera) and bull kelp (Nereocystis luetkeana). Where waves are strong, urchins cluster in crevices, under ledges and around kelp holdfasts, the root-like bases that anchor kelp to rock.

On many shores, purple urchins live in cup-shaped pits that they grind into the rock with their teeth and spines. A pit gives a firm grip against breaking waves and holds a little water at low tide. Spines worn down by the scraping simply keep growing. An urchin that starts its pit when young may enlarge the inside until it is too big to climb out, and then spends the rest of its life there, feeding on whatever drifts in.

In tide pools, purple urchins often hold pebbles, bits of shell and seaweed over their tops with their tube feet. Scientists think this covering behavior shades them from strong sunlight, slows drying and may help hide them from gulls.

What do purple sea urchins eat?

The purple sea urchin is a herbivore. Its favorite food is giant kelp, but it also eats bull kelp, many red, brown and green seaweeds, and the thin films of microscopic algae that coat rock. Like many urchins, it may take small amounts of animal material, but plants and algae make up almost all of its diet.

Where kelp is healthy, purple urchins are mostly patient drift feeders. They stay in their crevices and catch pieces of kelp that break off and roll past, grabbing them with tube feet and spines and passing them down to the mouth. The jaw that chews this food is a remarkable structure of five teeth and dozens of muscles, described in What is Aristotle’s lantern?

When drifting kelp runs short, behavior changes. Hungry urchins leave their shelters and move across the reef in dense groups, chewing through the stems and holdfasts of living kelp. A single mass of grazing urchins can turn a forest into bare rock.

Purple sea urchins and kelp forests

In a balanced kelp forest, predators keep purple urchins scarce and hidden. The sea otter is the best-known urchin hunter, but sunflower sea stars, California sheephead and spiny lobsters also eat large numbers.

When those predators disappear, urchins multiply and can graze kelp forests down to urchin barrens: rocky seabeds covered with urchins and pink crusts of coralline algae, with few fish or seaweeds. Barrens can persist for decades, as explained in What are urchin barrens?

The most dramatic recent example came from northern California. From 2013, sea star wasting disease killed most of the region’s sunflower sea stars, and a marine heat wave from 2014 weakened the kelp. Purple urchin numbers exploded, and the canopy of bull kelp fell by more than 90 percent along over 350 km (220 mi) of coast. Urchins living in barrens can survive for years on almost nothing, with nearly empty gonads, which is why divers nicknamed them “zombie urchins”.

Behavior and senses

Purple sea urchins move slowly on their tube feet, helped by their spines, and often stay in one spot for long periods. If turned upside down, an urchin grips the rock with tube feet on one side and slowly rolls itself upright.

They defend themselves with spines and pedicellariae. When an ordinary sea star reaches toward a purple urchin, the urchin can bend its spines aside and let its pincers clamp onto the sea star’s tube feet, often making it retreat. Against the large, fast sunflower sea star this does not work, and the urchin’s best response is to wave its spines and move away. A sunflower star that catches an urchin swallows it whole.

Although it has no eyes, the purple urchin senses light through cells in its tube feet and can detect dark shapes. It also responds to touch and to chemicals in the water, such as the scent of predators. Sea urchins breathe largely through their tube feet, helped by five pairs of small gills around the mouth.

Reproduction and life cycle

Purple sea urchins have separate sexes, but males and females look identical. Inside the test are five gonads, the orange or yellow tissue eaten as uni. In California, spawning happens mainly from about January to March, although some ripe urchins are found into summer.

Spawning is a mass release. Females push orange eggs and males white sperm out of small pores on top of the test, and fertilization happens in open water. A large female can release millions of eggs in a season.

The fertilized egg soon develops into a swimming embryo and then into a larva called an echinopluteus, a tiny see-through creature with long arms supported by skeletal rods. It drifts in the plankton for several weeks to a few months, feeding on microscopic algae, while a miniature urchin forms inside it. The larva then settles on rock and transforms into a juvenile less than a millimeter across. Purple urchins reach maturity at about two years old, when they are roughly 2.5 cm (1 in) wide.

Lifespan, predators and threats

Purple sea urchins are long-lived. Many probably reach 20 years or more, and studies of growth and survival by Thomas Ebert suggest the oldest exceed 50 years. That is modest next to the red sea urchin, which can live more than 100 years and appears among the longest-living animals.

Predators include sea otters, sunflower sea stars, California sheephead, spiny lobsters and wolf eels, plus some smaller sea stars and crabs. Sea otters that eat many purple urchins can even have their bones and teeth stained purple by the urchins’ pigments.

Natural threats include storms, disease and unusually warm water. Sea urchins are also sensitive to pollution and poor water quality; stressed urchins stop moving and their spines droop. Laboratory studies suggest that seawater with more dissolved carbon dioxide, which makes the ocean more acidic, produces smaller larvae.

Conservation status

The purple sea urchin has not been evaluated by the IUCN Red List and has no special protection. It is abundant across most of its range, and in parts of northern California and Oregon it has become more numerous than at any time in recent decades.

It is a native species, not an invasive one. The problem is not the urchin itself but the loss of the predators and healthy kelp that once kept its numbers in balance.

Purple sea urchins in science

Sea urchins have been laboratory favorites since the 19th century, because their eggs are easy to collect and fertilize in a dish and their embryos are transparent. The purple sea urchin became one of the most important species in developmental biology, used to study fertilization, cell division and how genes build an embryo.

In 2006, an international team from more than 70 institutions published its genome, the first ever sequenced for an echinoderm. It contains about 23,000 genes, many shared with humans. It surprised researchers with a huge set of immune genes, including more than 200 Toll-like receptor genes, compared with about 10 in humans.

Relationship with people

Indigenous peoples of California ate the gonads of purple sea urchins long before urchins became a commercial seafood. Today the red sea urchin dominates the uni fishery, because purple urchins are smaller and those from barrens hold little roe. Some companies now collect barren urchins and feed them in tanks until their gonads fill, turning a kelp problem into a product.

For most people, the purple sea urchin is a tide-pool favorite, easy to spot as a purple pincushion tucked into a rock pit. Look closely at its moving spines and tube feet, but leave it in place.