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shellfish · The Trophy Room

Purple Sea Urchin

scientificStrongylocentrotus purpuratus
foundrocky intertidal and shallow reef · Pacific coast
seasonyear-round

A living pincushion of the tide pools — and, split open, the source of uni, the golden roe prized on the sushi counter. Purple urchins graze the rocky coast in grinding numbers, and their story is one of the more dramatic in the kelp forest: unchecked, they can mow an entire reef down to bare rock.

Identifying

  • hemispherical test (shell) densely covered in short, blunt purple spines
  • a central mouth underneath with a five-toothed jaw — "Aristotle's lantern"
  • moves on hundreds of tiny tube feet; grinds itself pits into solid rock
  • an echinoderm, not a true shellfish — kin to sea stars, not clams

Facts

  • the five wedges of uni inside are actually the animal's gonads — sweet, briny, and delicate
  • when sea otters and sunflower stars decline, urchin numbers explode and create bare "urchin barrens" where kelp forest used to be
  • a keystone grazer — the health of the whole kelp forest tracks how many of these are kept in check

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Purple Sea Urchin