Purple Sea Urchins – Urchin Facts

By D.C. Demetre •  Updated: 08/02/22 •  6 min read

Sea urchins are marine animals that can be found in all of the world’s oceans. Most species have a spherical shape and spines on the upper surface of their shell. They also have many other unique features, including a mouth with teeth, a thin inner skeleton structure that supports its organs with calcium carbonate plates, and tube feet made up of tiny, beaded plates attached to the ends.

Unsurprisingly, one defining characteristic of the purple sea urchin is its colour. It has a purplish-black exterior with a red interior.

There are two main types of purple sea urchins. The purple sea urchin ( scientific name Strongylocentrotus purpuratus) lives in the Pacific Ocean waters off the coastline from Mexico to British Columbia. The Atlantic purple sea urchin (Arbacia punctulata) lives in the north Atlantic Ocean coastal waters from Massachusetts to Cuba and the Gulf of Mexico from Florida to Texas.

Both species are widely used in research as experimental model organisms, particularly in developmental biology. Reproduction of sea urchins features transparent eggs that can be handled easily in the laboratory. The Atlantic purple sea urchin is also used to model response to marine sediment toxicity.

Purple Sea Urchins Habitat

purple sea urchins habitat

Strongylocentrotus purpuratus in a temperate rocky reef. Credit: Ed Bierman CC-BY


Purple sea urchins are found in the Pacific, Atlantic and Indian Oceans, in shallow waters, usually on rocky, sandy, or shelly bottoms. One place these urchins call home is temperate reefs.

A temperate reef is different than the coral reefs of tropical waters. They are located in cooler waters between the tropics and the poles and feature ecosystems with seaweeds, invertebrates, fishes, mollusks, and worms.

During their grazing activity, temperate reef sea urchins excavate rock and form pits and crevices they nestle into. This may lead to the substantial bioerosion of temperate reefs, according to a 2018 study.

The research found that each purple sea urchin excavated around 32g of sandstone over a year – an average-sized urchin pit could be carved out in under five years. But granite excavation was 37 times slower, so much so that it would take over a century to form a pit.

“What shocked us was the rate of bioerosion, particularly on sandstone. In the course of feeding, sea urchins scrape the rock surface using their self-sharpening, regenerating teeth, which act as ‘rock picks,’ and this process results in the excavation of pits,”

said biologist Michael P. Russell, lead author.

What Do Purple Sea Urchins Eat?

Sea urchins can remain dormant for a long time without food. These are called “zombie urchins” and contain little nutrition for predators.

But when sea urchins do feed, they go full bore. Purple sea urchins are omnivorous, but their diet consists mainly of algae such as red and brown kelp. They will also eat plankton, dead fish, mussels, sea sponges and barnacles.

The mouth of a sea urchin has a complex jaw structure, known as Aristotle’s lantern, consisting of five calcium carbonate plates, each having teeth. They can use their spines and tube feet to grab hold of food.

What Eats Purple Sea Urchins

Purple sea urchins are eaten by sea otters, California sheephead, sunflower stars, crabs, lobsters, and sharks. Some sea otters eat so many purple sea urchins that they develop purple bones and teeth stains after many years of consuming them. Scientists even have a name for this: echinochrome staining.

Sea otter consumption of purple sea urchins aids in guarding kelp forests from destruction by overgrazing. Sunflower sea stars also play an essential role in kelp forests, which help remove CO2 from the atmosphere.

Additionally, a team of Canadian scientists found that sea otters feed on large sea urchins, and sunflower sea stars eat the small and medium-sized urchins. The density of kelp growth was highest in areas with sea otters and sunflower stars.

Purple sea urchins that do not get eaten will live an average of 20 years. Some can live to be up to 70 years old.

Facts about Purple Sea Urchins

Purple sea urchins are a type of echinoderm, a phylum of animals with spines. The word echinoderm comes from the Ancient Greek words ekhînos, meaning hedgehog, plus dérma, meaning skin.


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