Invertebrates
Purple Sea Urchin
Strongylocentrotus purpuratus · also called purple sea urchins
Where sea otters once patrolled a healthy kelp forest, purple sea urchins now cover bare rock in staggering numbers, each one scraping away the last traces of algae with a feeding structure so effective it can convert thriving habitat into an underwater desert.
Overview
The purple sea urchin, Strongylocentrotus purpuratus, is a spiny, algae-grazing invertebrate native to rocky coastal and kelp forest habitat along the eastern Pacific coast of North America. Its capacity to devastate kelp forests when predator populations decline makes it one of the more ecologically consequential small animals in the entire coastal ecosystem.
A grazer that needs to be kept in check
On its own, the purple sea urchin is a modest algae-eater, but remove its natural predators and its collective grazing pressure alone can reshape an entire coastline, turning a lush kelp forest into barren rock within a surprisingly short span of time.
Status
Least Concern — widespread and often locally abundant across suitable rocky coastal habitat along the eastern Pacific.
Purple Sea Urchin: common questions
How can a small sea urchin destroy an entire kelp forest?
When natural predator populations such as sea otters or sunflower sea stars decline, purple sea urchin numbers can explode dramatically, and the resulting overgrazing can strip a kelp forest down to bare rock entirely, a transformation marine biologists call an urchin barren, since without enough predators to keep urchin numbers in check, their collective grazing pressure becomes overwhelming.
What is Aristotle's lantern?
Aristotle's lantern is the specialised internal feeding structure a purple sea urchin uses to scrape algae directly off rock surfaces, made up of several hard plates and teeth working together in a coordinated grinding motion, a structure so distinctive that the ancient philosopher Aristotle himself first described its lantern-like shape.
Why are sea otters so important to purple sea urchin populations?
Sea otters are one of the purple sea urchin's most significant natural predators, and healthy otter populations help keep urchin numbers in check, preventing the overgrazing that would otherwise threaten kelp forests, making the otter-urchin-kelp relationship a classic example of a trophic cascade in marine ecology.
How long does a purple sea urchin live?
A purple sea urchin can live for several decades, with some individuals documented living for fifty years or more, a remarkably long lifespan for a relatively small marine invertebrate that most people would assume lives a much shorter life.