prickly aristotelians


The eastern shoreline of Salt Creek State Park on the Olympic Peninsula is a jumbled mass of black basalt, sometimes carved into sheer, rugged cliffs or broken up at irregular intervals to form sea stacks, archways, and boulders. As a child I remember walking along the fog-shrouded beach at low tide, picking my way across the treacherous rocks slick with kelp and Fucus algae. There was much to see, of course, but my most intriguing find came at the far end of a cobbled spit jutting toward the strait, deep in the lower intertidal zone. Wedged there in the basalt were purple and green sea urchins, dozens of them, each occupying a hole scarcely large enough to accommodate its spiny circumference. “How did they get in there?” I wondered, trying (and failing) to pry one out. “And what could’ve drilled these immaculate holes?” I was stumped. I soon gave up on the urchins, moving on to other, more pliant curiosities, but their mystery has stuck with me ever since.

Puget Sound and its adjoining straits and inlets are home to several urchin species, notable of which are the green (Strongylocentrotus droebachiensis) and the purple (Strongylocentrotus pupuratus) sea urchins, both common denizens of rocky intertidal areas.  They are echinoderms (from the Greek ekhinos, “prickly, hedgehog-like” and dermatos, “skin”), a phylum of marine invertebrates known for their radial symmetry, tube feet, regenerative prowess, and often spiny skin. Theirs is an ancient, sea-bound lineage: the first known echinoderms appeared in oceans more than 500 million years ago, at the start of the Cambrian Period, and thenceforth nary a one seems to have bothered to leave its salty embrace.


Purple urchin—note the tube feet

Echinodermata is the second-largest phylum of deuterostomes—animals that, in their embryonic development, sprout an anus before a mouth, instead of the other way around as in protostomes—comprising 7,000 extant species, trailing only the chordates in number. Because of their developmental dispositions, these prickly invertebrates are more closely allied to humans than to insects, crabs, or clams, protostomes all. It can thus be said that the Dickensian street urchin and the sea urchin are not all that dissimilar, zygotically speaking, but you’d have to look real close to see the resemblance. Echinodermata includes such unlikely confreres as the sea star, sand dollar and sea cucumber, but it is inarguably the urchins that best exemplify the ekhinos of their phylum.

Green and purple urchins are round, radially symmetrical creatures, slightly flattened dorsoventrally, each residing within a calcium carbonate exoskeleton called a test. Generally they approach the size and shape of Satsuma oranges, but S. purpuratus grows larger, to around four inches in diameter, not including the spines. Both species are positively bristling with these hard, tapered excrescences—also of calcium carbonate make—which are attached to teensy muscles and protrude from the test via ball-and-socket joints. They assist the urchin in moving across uneven terrain and also serve the obvious role of defense: if threatened, the urchin will raise its spines and even point them in the direction of the harasser. Broken spines are quickly regenerated, as are any non-terminal punctures in the test.

Despite their spines, green and purple sea urchins are preyed upon by a great many creatures, including crabs, sunstars, spiny lobsters, gulls, otters, and numerous species of fish. In the towering kelp forests off California’s coast, sea otters feast so extensively on S. pupuratus that their bones and teeth are stained purple from the urchin’s spines. Humans of course harvest urchins for their roe (actually the five gonads of the adult), its richness and flavor considered a delicacy in many coastal cuisines. Urchins are dioecious, with male and female members that are almost indistinguishable for much of the year. Come spawning season, though—usually lasting from winter to early spring—the sexes tend to assort themselves based on their needs. Males prefer outcroppings elevated off the seafloor, the better to have their cloudy milt broadcast by currents. Females stick to the bottom, where their fertilized eggs are less likely to be disturbed amid the rocks and sandy substrate. Like many marine creatures, urchins favor the “spray-and-pray” style of reproduction, widely distributing huge quantities of eggs and sperm in the hopes that a fortunate few will survive into adulthood.

All echinoderms use tiny sucker-tipped appendages called tube feet as their primary means of locomotion. Controlled by the so-called water vascular system, tube feet are muscular structures composed of two parts—the hollow ampulla and the suckered podium—that swell and elongate with seawater and then extend, suction, and contract by dint of hydraulic pressure. In this manner the urchin appears to crawl across the substrate, pulled along by its thousands of little feet. When fully engorged they stretch beyond the urchin’s spines, flailing about its test in a tubular halo, feeling around for something on which to attach. Possibly these feet function as taste and light receptors, acting as sensory inputters for an animal lacking a head. They are torn off regularly and grow back apace. Tube feet are important also in respiration and food and waste transport, ferrying vital materials back and forth between the urchin and its environment.

Green urchin—note the lantern

Green urchin—note the lantern

Perhaps the most fascinating aspect of the urchin’s anatomy lies dead-center on the test, situated on the creature’s underside directly opposite its anus. Known as the Aristotle’s Lantern—after the fourth-century Greek polymath who studied urchins, among much else, and likened their mouths to five-sided lanterns—this fiendish-looking aperture is where urchins take their food. Composed of five calcite-crystal plates that—and this should come as no surprise—continuously grow and shed to keep their honed edges, the Aristotle’s Lantern works like the rotating blades of a lawnmower, shearing and masticating algae and other foodstuffs into easily digestible bits. The urchin simply hoovers along, grazing algae beds and sometimes using its feet to capture water-borne morsels, which are then passed foot to foot all the way down to the lantern.

At last we return to the puzzle of those holes drilled in the basalt. (Perhaps, at this point, the solution is a tad predictable.) Using its perma-sharp lantern plates, the urchin literally bores through rock to carve a niche for itself, scraping out the burrow’s sides with its renewable spines. It is a formidable redoubt, years in the making and nigh unassailable. There are times, however, when the ensconced urchin becomes trapped by its own rock-solid defense. Young urchins will drill holes and occasionally hunker down for months on end—or sometimes longer; S. purpuratus can live for seventy years or more—subsisting on bits of food washed in by the waves. When at last they make to move, they find their tests and spines have grown too large to pass the entrance, and thus they are stuck, self-immured in stone.

For a chance to see these spiny enigmas in King County, pick a rocky beach like Alki Point in West Seattle or Seahurst in Burien. Check crevices and cracks in boulders for urchins waiting out the low tide—they’re often crammed in to an almost impossible extent, just a glimpse of purple or green poking out from the rock. Say hello to (or, you know, acknowledge) a fellow deuterostome—one that, in the case of S. pupuratus, boasts a genome almost as complex as our own, with a surprising number of analogous genes—and hopefully, after all that, you’ll get an inkling of what struck me so long ago on that basaltic strait-side beach.

Test shot(painting by tracy effinger)

Test shot
(painting by tracy effinger)


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