One doesn’t really expect a purple animal.
But the most common sea urchin on Oregon’s ocean edge is very purple. Purple sea urchins (Strongylocentrotus purpuratus) have both purple spines and thin purple flesh between the spines. The brittle “test” that protects the animal’s innards is usually faintly greenish, however.
You may have seen tests - or bits of them - in a tidepool or on a nearby beach. “Bits” because the tests are more fragile than you might think. The test, and therefore the body, of a purple sea urchin is shaped like a slightly-flattened, hollow sphere with a small, almost round hole on the top and a larger almost-five-sided hole on the bottom.
Between top and bottom stretch rows of small round bumps, tiny points where the spines pivot. The rows of the largest bumps are in pairs, two rows of pairs close together form five double-rays of bumps from top to bottom.
Rows of smaller bumps stretch top to bottom, too, loosely filling out the surface of the test for places the spines attach. In life, the purple spines articulate on the bumps, moved around by the thin layer of tissue on the outside of the test. There are also rows of tiny holes in the test where the long-stalked tube feet emerge from inside the body to wave around the spines and stretch to hang on to things, such as the rock.
An ocean animal with a five-parted body and tube feet that reach out and suck on to a surface… Remind you of a sea star?
Sea urchins and sea stars, as well as brittle and basket stars, sea cucumbers and crinoids, are related: they’re all echinoderms, the phylum of invertebrates with (mostly) spiny (“echino”) skins (“derm”). Purple sea urchins can survive high enough in the intertidal that we can easily see them. In deeper waters off Oregon’s shore—rarely in tidepools, there are also red sea urchins; red sea urchins have noticeably longer spines than the purples. Sand dollars are flat relatives of sea urchins that live on the surface of the sand, under the surf; living sand dollars have very short, fine spines that make them look velvet-covered.
In addition to the color in this species, it is the spines that impress us about the live animals: most sea urchins are pincushiony spheres, bristling with spines that move slowly, helping them walk about. You might notice the five-parted pattern in the rows of spines on a relaxed purple sea urchin, resting with its spines sticking straight out.
While exploring the rocky shore, you may have seen purple sea urchins settled in pits that seem to fit just right. They usually fit just right because the urchin slowly grinds down the rock by moving the spines around - perhaps also by scraping with its teeth - while hanging on to the rock with the tube feet. It makes sense that the pit would protect the urchin from predators and wave action, and possibly help keep them from drying out if they’re exposed to the air at low tide.
Purple sea urchins usually walk around to feed and may perhaps return to their home pit, though not all purple sea urchins spend much time in a pit. A given pit might be ground out by one or few long-living sea urchins or by generations of sea urchins. In the meantime, the space between the urchin and the rock can be favored habitat for smaller, more delicate animals.
No, the spines on our purple sea urchins aren’t poisonous, but I can personally attest that jabbing one in your hand by falling on it is not a pleasant experience and a deeply embedded spine tip may leave a purple spot for a very long time, even if it doesn’t introduce an infection to the hole.
Purple sea urchins eat kelp and other seaweeds using an “Aristotle’s lantern,” a complex mechanism of five sharp teeth with points that meet in the center. In addition to helping the urchin walk or stay put, the tube feet move food to the teeth. In life, the Aristotle’s lantern is just inside the larger hole on the bottom of a sea urchin’s test. (The smaller top hole is - you guessed it - the anus.)
In turn, our sea urchins are eaten by sea otters and seastars, particularly the large and speedy many-rayed seastar, Pycnopodia. The skeins of eggs inside female sea urchins are also sometimes eaten by people.
Pacific Coast sea urchins have been in the news the last couple of years because of their huge and sudden overpopulation. First, it was the loss of sea otters through hunting. Now an additional, bigger hit is the near-extinction of the many-rayed seastars due to seastar wasting disease that took hold of the region in 2013/14.
Without predators, today in many places the ravenous, rampaging urchins have mowed down huge areas of kelp beds, creating “urchin barrens” where rich and diverse kelp forests used to thrive. Increasing sea temperatures will further weaken the kelp and its forests, making the urchin demolition even more damaging and widespread.
A couple environmental turns and these fascinating balls of purple spines are now wreaking havoc.
For information on how you can arrange your own exploration of our fascinating natural history, contact Marty at firstname.lastname@example.org, www.facebook.com/wavecrestdiscoveries, or by calling 541/267-4027. Questions and comments about local natural history are welcome. www.wavecrestdiscoveries.com
On your way to the rocky shore to take advantage of this month’s very low tides?
Remember that tidepool life is adapted to withstand the rigors of limited stranding from their marine environment, but it’s not well adapted to withstand the rigors of people. Human visitors should step carefully and touch gently.
Never pry or pull animals off rocks or seaweeds; immediately return any rock or seaweed you move if you explore underneath. Note that most easily accessible, rich rocky shores on the Oregon Coast are protected by law to preserve these remarkable places. We visitors should take the memories of our delightful discoveries with us, but leave the animals, seaweeds and shells in their homes.
While we have a good number of minus tides coming up this month, know that tidepooling is fun and rewarding on low tides or +1 or lower. (Here’s one of many online resources with local tide tables: https://www.tides.net/oregon/ )
Watch your footing on slippery rocks and do pay attention to the tides and keep track of your path back to shore so you aren’t stranded or knocked over by incoming water.