Snail tales, part I

My Brooklyn backyard is a wall- and fence-enclosed concrete rectangle some 14 by 25 feet in size. A metal balcony and stair overshadowed about one third of the space. Very little sun reaches it during the winter, but come spring it is much less like the bottom of a well. In summer, it’s hot and bright on clear days. During the warm months, I grow flowers and vegetables in pots. The soil, enriched with homemade compost, anchors blooms and greens, which in turn are the foundation of an ecosystem I’ve been discovering bit by bit.

One day last July, I noticed several dozen tiny somethings attached to the brick wall outside my bedroom window. At first I thought they were spider prey, all wrapped up and ready for eating. Numerous spiders build sheet-webs in the corners between the metal poles that support the balcony and the house wall. These are horizontal triangles of cobweb, much messier looking than classic orb webs a la Charlotte. Tucked into the far corner of these triangles is a little sack, in which the spider crouches. The webs looked abandoned and existed for weeks (orb web weavers may make a new web every day). The spiders are quite shy, but I occasionally see one out on its sticky killing field.

I’d been wondering what the spiders eat and now I thought I’d found out. But then I looked closer. The little objects were actually small conical shells. They were about a quarter inch long, glossy and lightly amber in color and partially translucent. The snails within were visible as black bodies filling the farther reaches to the shell’s whorl. I consulted my found-on-a-stoop paperback copy of Shells from Cape Cod to Cape May by Jacobson and Emerson. The book, originally published in 1961 as Shells of the New York City Area, notes “The northeastern United States is the only region in the world where practically all the species of larger land snails have shells that are depressed or flat. Cionella lubrica, […] small as it is, still enjoys the distinction of being the largest elongated land snail in our area.” From there I moved to the Internet, to confirm the identification and find more up-to-date information. C. lubrica is commonly called the glossy pillar snail. They are slippery and best looked at with a magnifying glass. Like many of our backyard lifeforms, they are an invasive species. Jacobson and Emerson are right: they are beautiful little things.

And it turns out that these snails have a respectable Internet presence, I mean, for a tiny, unobtrusive creature that might otherwise warm the heart of only a few experts and collectors. But it also happens to be one of the intermediate hosts for a parasite, the Lancet or lanceolate, liver fluke, Dicrocoelium dendriticum. This fluke infects domestic animals (cattle, sheep), and rarely, humans, so its lifecycle is well-documented.

This lifecycle is rather extraordinary. There is a diagram of it on the Center for Disease Control: the snails ingest fluke eggs that get on their vegetable diet via mammal feces; ants pick up the parasite via snail slime, which is one of the ants’ sources of moisture; mammals get it because they ingest infected ants, and around it goes again. But why would a cow or a sheep eat ants? Can the fluke depend on the utter randomness of a ruminant accidentally slurping up an infected ant?
Not quite. Fluke-infected ants are taken over by the parasite, which attacks their nervous system and makes them suicidal. Or zombies, as one entomologist has described the state. At night, an infected ant will defect from the colony and climb up a blade of grass. Attached to the grass, there it will stay until morning if not consumed along with the grass by a passing ruminant. Come daylight, the ant will retire back to the colony as if nothing were amiss, then return to it grassy suicide post night after night until it is scarfed up or dies a natural death.

In the crowded complexity of the web of life, it is awfully easy to digress. One path is quickly crossed over by another, weaving together. The famed biologist E.O. Wilson, has written that it is possible to “spend a lifetime in a magellanic voyage around the trunk of a single tree.” I believe he was thinking of a tropical tree, but it still holds in temperate climates. And the same can be said of a backyard. Even one that is mostly concrete. Even in Brooklyn, where the sight of an animal, say a squirrel in the park, has been known to frighten children.