Sea urchins, the spiny, spherical relatives of sea stars and sea cucumbers, inhabit every ocean on Earth and can occupy territory from shallow tide pools to cavernous trenches miles below the surface. Years ago, before I started diving for them or cooking with their creamy reproductive organs (colloquially referred to as roe or by the Japanese as uni), I’d always assumed from sushi menu prices that urchins would be scarce, like truffles of the sea. This is true in some regions, such as the waters around Nova Scotia, where dwindling populations in recent years have made green urchins a precious, hard-to-come-by treasure. But today, on the West Coast, purple urchins are not. Like their larger, more commonly served red urchin cousins (which often look more purple-black than red), the purple Strongylocentrotus purpuratus flourish everywhere from Vancouver to Baja. Though their lovely colors may suggest otherwise, urchins are hardcore creatures: vaultlike orbs of reinforced calcium carbonate covered in hundreds of ball-and-socket-jointed spines that form a rippling conveyor system to drag food toward their rugged, five-toothed beaks. These marine wood chippers buzz around the ocean floor on legions of flexible tube feet that line their undersides. They feed mostly on algae but are opportunistic eaters, and their beaks can excavate everything from coral to rock to steel beams. Yet the coveted, edible sacks of pale orange roe within these living chambers—which taste like butter cultured from fresh, deep seawater—are as delicate and fragile as egg yolks.
Members of a healthy, balanced community of urchins live fairly solitary lives. They burrow into crevices and dine on a varied diet of whatever scraps of decaying algae, animals, and plants the tide impales on their spines. When an urchin population explodes, however, as is likely to happen whenever climate change or other human-catalyzed chaos disrupts the local food chain, a distorted hive mind takes over. They congregate in subtidal hordes, actively consuming everything in their path, including the holdfast tissue that anchors kelp to rocks. Even after obliterating an entire kelp forest, urchins won’t eat themselves to extinction—they just switch back to munching on the tidal drift. A single one can also live for 70 years.