The Medusa and the Snail: More Notes of a Biology Watcher (public library

How a Jellyfish and a Sea Slug Illuminate the Mystery of the Self
A humbling evolutionary antidote to the hubris of exceptionalism, with a side of etymology.
By Maria Popova



Around the same time, a poet laureate of the life process — the great physician, etymologist, poet, and essayist Lewis Thomas (November 25, 1913–December 3, 1993) — explored the confounding nature of the self with uncommon insight and originality in the title essay of his altogether magnificent 1979 collection The Medusa and the Snail: More Notes of a Biology Watcher (public library).Lewis Thomas (Photograph: NYU archives)

Thomas writes:


We’ve never been so self-conscious about our selves as we seem to be these days. The popular magazines are filled with advice on things to do with a self: how to find it, identify it, nurture it, protect it, even, for special occasions, weekends, how to lose it transiently. There are instructive books, best sellers on self-realization, self-help, self-development. Groups of self-respecting people pay large fees for three-day sessions together, learning self-awareness. Self-enlightenment can be taught in college electives.




The original root was se or seu, simply the pronoun of the third person, and most of the descendant words, except “self” itself, were constructed to allude to other, somehow connected people; “sibs” and “gossips,” relatives and close acquaintances, came from seu. Se was also used to indicate something outside or apart, hence words like “separate,” “secret,” and “segregate.” From an extended root swedh it moved into Greek as ethnos, meaning people of one’s own sort, and ethos, meaning the customs of such people. “Ethics” means the behavior of people like one’s self, one’s own ethnics.

Embedded in this evolutionary history of our language is something wholly uncorroborated by the evolutionary history of our biology — the misplaced hubris of exceptionalism. Thomas writes:


We tend to think of our selves as the only wholly unique creations in nature, but it is not so. Uniqueness is so commonplace a property of living things that there is really nothing at all unique about it. A phenomenon can’t be unique and universal at the same time. Even individual, free-swimming bacteria can be viewed as unique entities, distinguishable from each other even when they are the progeny of a single clone.Art by William Saville-Kent from the world’s first encyclopedia of the Great Barrier Reef ecosystem. (Available as a print and as a face mask.)

Thomas points out that creatures large and small exhibit properties that, in their human manifestation, we call individuality — they are, in other words, distinct selves. Single-cell microorganisms swimming in the same water, when examined closely enough, can be distinguished from one another by the way they twirl around their flagellae. Beans carry glycoproteins that serve as self-labels. Coral polyps are endowed with a biological self-consciousness that allows them to recognize other polyps of the same genetic line to fuse with, rejecting polyps of different lines. Fish and mice can tell individuals of their species by their smell. (Decades after Thomas composed this essay, we know that trees also differentiate between and communicate with individual others.) He considers the biological function of the self:


The markers of self, and the sensing mechanisms responsible for detecting such markers, are conventionally regarded as mechanisms for maintaining individuality for its own sake, enabling one kind of creature to defend and protect itself against all the rest. Selfness, seen thus, is for self-preservation.

In real life, though, it doesn’t seem to work this way. The self-marking of invertebrate animals in the sea, who must have perfected the business long before evolution got around to us, was set up in order to permit creatures of one kind to locate others, not for predation but to set up symbiotic households. The anemones who live on the shells of crabs are precisely finicky; so are the crabs. Only a single species of anemone will find its way to only a single species of crab. They sense each other exquisitely, and live together as though made for each other.



Thomas locates the most compelling and sobering illustration of this in two obscure species inhabiting the Bay of Naples, melded into one — a common nudibranch sea slug and the medusa of a tiny jellyfish, permanently affixed to the shell-less snail’s mouth as a vestigial parasite. 


The attached parasite, although apparently so specialized as to have given up living for itself, can still produce offspring, for they are found in abundance at certain seasons of the year. They drift through the upper waters, grow up nicely and astonishingly, and finally become full-grown, handsome, normal jellyfish. Meanwhile, the snail produces snail larvae, and these too begin to grow normally, but not for long. While still extremely small, they become entrapped in the tentacles of the medusa and then engulfed within the umbrella-shaped body. At first glance, you’d believe the medusae are now the predators, paying back for earlier humiliations, and the snails the prey. But no. Soon the snails, undigested and insatiable, begin to eat, browsing away first at the radial canals, then the borders of the rim, finally the tentacles, until the jellyfish becomes reduced in substance by being eaten while the snail grows correspondingly in size. At the end, the arrangement is back to the first scene, with the full-grown nudibranch basking, and nothing left of the jellyfish except the round, successfully edited parasite, safely affixed to the skin near the mouth.




It is a confusing tale to sort out, and even more confusing to think about. Both creatures are designed for this encounter, marked as selves so that they can find each other in the waters of the Bay of Naples. The collaboration, if you want to call it that, is entirely specific; it is only this species of medusa and only this kind of nudibranch that can come together and live this way. And, more surprising, they cannot live in any other way; they depend for their survival on each other. They are not really selves, they are specific others.

The thought of these creatures gives me an odd feeling. They do not remind me of anything, really. I’ve never heard of such a cycle before. They are bizarre, that’s it, unique. And at the same time, like a vaguely remembered dream, they remind me of the whole earth at once. I cannot get my mind to stay still and think it through.One of Ernst Haeckel’s stunning drawings of jellyfish. (Available as a print and as a face mask.)

The essays in The Medusa and the Snail, which include Thomas’s beautiful meditation on altruism and the scientific poetics of friendship, Thomas on our human potential and our cosmic responsibility.