The purple shore crab (Hemigraspus nudus) is perhaps the first marine invertebrate I learned to identify as a child, growing up near Alki Beach in Seattle. Purplish-black carapace (though sometimes olive green or white-flecked burgundy) usually an inch or two across, white-tipped chelapeds with purple spots, dainty hairless legs, googly little eyestalks: unmistakable, pretty much ubiquitous. Abundant to the point of banality, purple shore crabs are found under just about every intertidal rock from Alaska to the Baja Peninsula. They thrive in Puget Sound, tolerant of pollutants and variable salinity, feeding on algae and detritus with their frosted pincers.
There are rock-strewn beaches along the Pacific Coast where, at low tide, every footfall of the beachcomber is accompanied by a mad clattering of legs, claws, shells, as the shore crabs dive for cover. It is a strange, almost mechanical sound, like floppy disks settling in a heap. Lift a stone and watch them scatter at the compromising presence of light, crawling over one another, fearful for their lives. A threatened, cornered crab will raise its chelae in defiance—a puny, futile effort from the human’s perspective, yet oddly ennobling, I find, and admirable. The crab senses inimical forces at work and stands to fight, caring neither the size of its adversary nor the nature of its plight.
Can a crab actually care for such things, or for that matter, anything at all? Not likely—the animal’s brain is too small, too rudimentary, to form what we’d describe as thoughts. It relies instead on instinct, the genetically-programmed responses to stimuli within and without: reactions to temperature, hormonal changes, tidal flux, the movements of a perceived threat. Darkness is sheltering, safe for the crab; light means exposure to the vicissitudes of its environment, and thus is treated with circumspection. Instances in which injury is sustained are avoided to a fault—this harm-aversion is called nociception, the neural calculus of encoding and processing noxious stimuli. Damage to tissues is bad, potentially life-threatening, so the tissue-bound nociceptors (sometimes called pain receptors), when activated by a stimulus like heat or pressure, send an electric message to the brain via the spinal cord saying, in effect, “Move affected areas away from harmful stimulus, stat.” But is this nociception, documented in animals as diverse as sea slugs, fruit flies, and fishes, equivalent to feeling pain? As a crab is dropped into a pot of boiling water—or crushed under a mislaid beach rock—is it subject to pain as excruciating as we, the empathetic and “higher-level” organisms, imagine it?
A recent U.K. study found that the European shore crab (Carcinus maenas) exhibits pain-sensing behavior in laboratory tests when engaged in a kind of mild electric-shock therapy. (To differentiate from reflex-like nociception, the study authors describe pain as a learned response, helping the animal avoid future damaging experiences for longer-term protection.) In the tests, single crabs were given a choice between two darkened shelters in a tank. (Remember, crabs like the dark.) One of the shelters was wired to shock its occupant, the other was not. After choosing one and receiving a shock (or not), the crab was removed from the tank. Placed back in a little while later, the crabs would sometimes go for the wired shelter a second time, but by the third round, they had learned to avoid it. The crabs seemed to have remembered the shocks, altering their behavior accordingly. It certainly appears to be a response to pain. The authors acknowledge that more definitive analyses of pain recognition—involving, say, testing stress hormone levels—are needed before conclusions can be made.
I think perhaps that I give animals and plants the benefit of the doubt more often than is necessarily prudent. Of course I believe crabs feel pain—I’ve never harbored any illusions as to the brutal, painful, dog-eat-crab-eat-seaweed ethos that characterizes the natural world. Life is hard, and dying often hurts. The struggles of the living and breathing among us are, I believe, a constant source of wonder, perturbation, and hope.