Purple sea urchin

Purple sea urchins live by rocky shores on the Pacific coast of North America.

Some live in tide pools. Others live underwater in forests of tall seaweed called kelp.

Purple sea urchins live along the Pacific coast of North America, from Alaska to Baja California in Mexico. They are found from the lowest part of the shore, uncovered only at the lowest tides, down to about 100 meters deep, but they are most common in shallow water.

They like places with strong waves and churning water, and many live near the holdfasts that anchor giant kelp to the sea floor. Some even scrape out a hollow in the rock, which helps protect them from waves and predators.

ADW gives the range as Alaska to Cedros Island, Mexico, and the Aquarium of the Pacific as Alaska to Baja California, in the low intertidal and shallow subtidal to about 100 meters, most abundant in shallow water. ADW describes it as thriving amid strong wave action and aerated water, often near giant kelp holdfasts.

Using its Aristotle's lantern and spines, it can wear depressions into intertidal rock, giving it a firm hold and some protection from predators and environmental stress. The map shades the United States, Canada and Mexico; it lives only along their Pacific coasts.

Purple sea urchins eat seaweed, especially kelp.

They scrape it with five little teeth. They also catch bits of seaweed floating by.

Purple sea urchins are grazers that eat kelp and other algae, and their favorite is giant kelp. They catch pieces of drifting seaweed with their spines and tube feet, and they scrape algae off rocks with a remarkable mouth called Aristotle's lantern, which has five strong teeth.

When urchins are in balance with their predators, they eat drift kelp, pieces that break off and float down, and help control algae. They can also survive for a long time without food.

The Aquarium of the Pacific describes purple urchins as grazers feeding primarily on kelp and other algae; when populations are balanced they consume drift kelp and help control algal growth, and they can survive prolonged starvation. ADW notes a preference for giant kelp, Macrocystis pyrifera.

Food is captured with tube feet, spines and pedicellariae and passed to the mouth, and the Aristotle's lantern, a jaw apparatus with five teeth, scrapes algae from rock. In urchin barrens, where kelp has been eaten away, the Aquarium of the Pacific notes that the urchins' gonads can be almost empty because there is so little food.

Sea otters and big sea stars eat purple sea urchins.

When there are not enough of them, there can be too many urchins. Then the urchins eat lots of kelp.

Purple sea urchins have predators, including sea otters and several kinds of sea stars, such as the sunflower sea star. These predators help keep urchin numbers in balance.

When the predators disappear, urchins can multiply. Large numbers of hungry urchins can overgraze kelp forests, and places where urchins have eaten away the kelp are called urchin barrens.

ADW lists predatory sea stars (Solaster stimpsoni, Pycnopodia helianthoides and Astrometis sertulifera) and sea otters as primary predators. The Aquarium of the Pacific attributes the recent urchin explosion partly to the decline of sea otters, reduced historically by fur trapping, and of sunflower sea stars, devastated by sea star wasting disease.

It describes severe overgrazing of kelp forests along the Pacific coast as the result, and notes that marine heatwaves have also contributed to kelp declines, making purple urchins both a threat to and a victim of changing ocean conditions; whether heatwaves affect the urchins directly is not yet clear.

Purple sea urchins are not rare. In some places there are too many!

Too many urchins can eat up a kelp forest. Sea otters and sea stars help keep urchins in balance.

The purple sea urchin has not been assessed for the IUCN Red List, and it is not rare. In fact, the problem is the opposite: in some parts of California, purple urchin numbers have risen by up to 1,000 percent. The jump began in 2014, right after sunflower sea stars collapsed from disease in 2013 and 2014.

People have eaten purple urchins for hundreds of years, and today some are harvested for food. The Aquarium of the Pacific says that balancing conservation with careful harvesting will be important for keeping kelp forests, and the animals that live in them, healthy.

Strongylocentrotus purpuratus has not been evaluated for the IUCN Red List (GBIF's Red List copy: Not Evaluated). The Aquarium of the Pacific's report card (January 2025) describes a strong increasing trend, with some areas seeing increases of up to 1,000 percent; California monitoring shows the rise beginning in 2014, coinciding with the 2013 to 2014 sunflower sea star collapse, greatest in the Central Coast region and essentially stable on the North Coast.

Indigenous peoples of the Northwest Coast and California have eaten purple urchins for hundreds of years, and a limited commercial harvest, by permit in California, may help reduce their impact. Recent studies cited by the Aquarium of the Pacific suggest purple urchins have ample genetic variation to cope with ocean acidification.