A protective shell

A protective shell

Charles Darwin seems to have had a boundless interest in the many forms life takes on earth. He was particularly intrigued by what he referred to as “contrivances,” the various biological devices through which creatures make their livings or disperse their young.
Even the most pedestrian species seized his imagination. Take the Roman land snail Helix pomatia, for instance. If one is not a lover of escargot, this common European snail would inspire little attention. But not so for Darwin. He was gripped, and troubled, by the mere existence of land snails.

In 1857, Darwin wrote his first letter to the naturalist Alfred Russel Wallace, who was then making his way across the Malay Archipelago. Wallace’s own voyages and observations would independently lead him to the concept of natural selection.
To support his revolutionary theory that all species arose naturally from ancestors, Darwin was eager to find evidence of how land snails, which he knew were easily killed by salt, contrived to reach and populate oceanic islands. At home, he was conducting all sorts of experiments with submerging snails and their eggs in seawater for weeks at a time. He suggested in On the Origin of Species that they might be transported to islands by adhering to the feet of birds or to driftwood.
When the world celebrated the 150th anniversary of the publication of Darwin’s magnum opus recently, I thought that it would be most fitting to highlight some recent discoveries about some nifty contrivances that I am very sure would have delighted the great naturalist. And they all revolve around land snails.

The shell, asymmetric, coiled...
If we look closely at the greatest contrivance of snails, the protective shell, we can see that it is not only coiled, but also asymmetric. If we hold a shell with the opening facing us and the pointed tip facing up, the opening is usually on the right. These shells are referred to as dextral or right-handed. In rare individual snails from right-handed species, however, the shells will have the opening on the left and are referred to as sinistral, or left-handed. Some entire species are sinistral. Biologists have known for almost a century that a simple genetic basis is behind shell handedness, at least in some species, such that the mutation of a single gene can cause right-left reversal. The mystery then is why, if both forms can occur and reversal is genetically easy, are most species either entirely right- or left-handed? An answer may be found in one of the logistical challenges that living with most of the body inside a shell imposes on snails, mating. Here is where handedness matters a lot. Snails with opposite handedness are misaligned, making it physically difficult for them to mate.
Any rare reversed individual snail would obviously have difficulty mating. As Darwin knew so well, if one cannot mate, it is the end of the line. Once of a given handedness, a species and its descendants would tend to stay that way.

A new, reversed population
But what if there were some other reversed individuals around? They could mate with one another and might form a new, reversed population. If members of that population continued to fail to mate successfully with snails of opposite handedness, that population might evolve into a new species. And that seems to be exactly what has happened several times in a group of Japanese snails, and perhaps very many times in land snails in general.
The genus Euhadra is unusual in that it contains multiple left-handed species, as well as right-handed species. By tracing species pedigrees through DNA, biologists Rei Ueshima from the University of Tokyo and Takahiro Asami from Shinshu University found that several right-handed species of Euhadra snails appeared to have evolved from a left-handed ancestor and that closely related left- and right-handed species could not successfully mate.
Both observations make perfect sense, but there may be something more to the explanation of why some species are left-handed than mating difficulties. There appears to be a greater diversity of left-handed land snails in Southeast Asia than elsewhere. Asami, Masaki Hoso and Michio Hori of Kyoto University have uncovered an astounding reason why this may be so, right-handed-snail-eating snakes.
About a dozen species of small Southeast Asian snakes in the Pareatinae subfamily specialise in eating land snails. But these escargot connoisseurs cannot crush the shell, so they extract the soft snail body from the shell by alternately retracting their left and right jawbones, or mandibles. Because the snails have asymmetric bodies, the researchers thought that the efficiency of extraction would be affected by different actions of the left and right mandibles. They examined the snakes’ jaws in detail and found that they were right-handed or, better, right-mandibled. In one species, the left mandible bears 17 teeth and the right mandible 25 teeth. This asymmetry was present in all 12 snail-eating specialists, whereas the jaws of two nonsnail-eating species of the same subfamily were symmetrical, like those of all other snakes and vertebrates.

Tests on snakes
To test whether this contrivance was in fact useful to the snakes, the researchers conducted experiments in which they compared the efficiency of feeding on left- and right-handed forms of the same snail species. The snakes failed more often at consuming left-handed snails, apparently because their upper jaws barely reached the outer shell on which they needed to anchor. Even when the snakes managed to eat left-handed snails, the process of extraction required more time and effort.
The work shows that snail asymmetry has driven the evolution of a unique asymmetry in snakes, but, as Darwin would have predicted, the snakes also appear to be driving snail evolution. Hoso and Hori identified a snail subspecies that had evolved a narrower aperture that impeded a snake’s ability to work its jaws over the body. These snails escaped capture and predation more frequently than did their close relatives with larger apertures.
Darwin’s genius was in finding great general truths among the details of humble, often obscure creatures.
NYT News Service

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