Tide Pools: Holes and the Hidden

The wind pours into our faces. We walk the short path from the parking lot at Gerstle Cove in Salt Point State Park toward the shore. We're walking past low bushes and a few purple blooms of Douglas iris. From up on the path, it doesn't look like a particularly low tide. At a gradually sloping beach, low tides are obvious—the beach stretches way out— but at a steep shore like this one it's harder to tell. We clamber down the honeycombed sandstone toward the churning waves. Now the low tide we've stumbled upon (one of the lowest of the month) becomes obvious.

There are crimson bat stars and rock walls textured in gooseneck barnacles. Shag carpets of sunburst anemones wave teal and pink tentacles.

Peering and picking our way, we discover abalone and red sea urchins—dwellers of the lower section of the intertidal zone that are only revealed by quite low tides. We even come upon a gumboot chiton, a wrinkled red burl the size of a small loaf of bread. I've only seen one once before in years of tide pooling.

A gumboot chiton lives low on rocky shores, crawling very slowly, scraping off algae with its radula (an organ like a tongue). Its radula is covered in tiny teeth tipped with magnetite. Magnetic teeth!

We see huddles of pacific purple sea urchins, each tucked into its own perfectly-sized hole in the sandstone. Did they somehow carve these homes or did they just find these spots and move in? We had seen somewhat similar holes in the sandstone much higher on the shore where no urchin could live, but these holes seem so perfectly matched to their residents.

Later, I read about a study by Michael Russell at Villanova University in which he and his team documented pacific purple sea urchins eating holes into mudstone, sandstone, and even granite. Apparently, the five teeth that a sea urchin uses to eat kelp are also used to break off and eat bits of rock. In one year, the urchins in the study ate an average of 63 cubic centimeters of sandstone each. Eating a hole into rock provides an urchin with a safer home.

These wonders we saw are almost always covered by water. We only got to see them because we caught the cycle of the tide as it pulled back the water to nearly the farthest reach, revealing what is always there but almost always hidden. There are other moments in natural cycles like this. In the fall, the yearly cycle of temperature causes leaves of deciduous trees to lose their chlorophyl. When this green pigment is pulled away, it reveals the orange, red, and yellow colors that were there in the leaves all along, but hidden by the green. Each night the light of the Sun is pulled away, revealing the stars. The stars are there in the daytime too but sunlight hides them from our view. Two days ago, I walked in snowy winter woods in Michigan. I saw several bird nests in the bare branches of the trees, holding snow rather than eggs. These nests were built in spring, but would have been invisible then, hidden by leaves.

Salt Point State Park
Tide Tables
Information about Gumboot Chitons from the Monterey Aquarium
New Scientist Article About Rock-eating Urchins