snails

By: Kaylin Martin, M.Sc. and Timothy A. Pearce, PhD

Asymmetries in nature are noteworthy because they usually mean something interesting is going on. Most snail-eating snakes in the family Pareidae are remarkable for having more teeth on the right lower jaw than on the left. The vast majority of snails worldwide coil clockwise (dextral) while, in contrast, the counter-clockwise coiling (sinistral) snails tend to be scarce, usually on the order of 1/10,000 to 1/100,000.

Recent experiments demonstrate that pareid snakes are more successful at eating dextrally coiling snails, evidently because having more teeth in the right jaw helps the snake to extract the snail’s body from the shell. Upon striking a dextral snail, with the aperture on the right, the snake advances and retracts its mandibles along the snail’s forebody. The sequential movements of this mandibular walk extract the snail’s soft body from its shell. Conversely, when a pareid snake strikes a sinistrally coiled snail, it finds the snail’s aperture on the left, and consequently the snake’s stereotypical right-handed behavior is less successful at grasping the snail’s body. The asymmetry in the snake’s mandibles means that sinistrally coiled snails escape predation by these snakes more often than do dextrally coiled snails.

Could the pareid snakes be an evolutionary force that favors sinistrally coiled snails? The ranges of Pareidae and Amphidromus almost entirely overlap, both groups occurring in Southeast Asia from China to Indonesia. Quite a few other land snail species in that part of the world are known to coil sinistrally, although in most of these other genera, the whole species is sinistral, rather than showing polymorphism (showing both forms) for coiling direction. The two facts, that sinistrally coiled snails escape predation more often, and that the ranges of the predator and the prey largely overlap, both support the idea that the asymmetry in the snake’s jaw provides an evolutionary force resulting in a greater proportion of sinistral snails in Southeast Asia. This conclusion was also reached in a study by Hoso et al. (2010).

The snake Pareas carinatus and the snail Amphidromus inversus are both tree-dwelling. In controlled lab experiments, the snake is known to eat Amphidromus, as well as other genera of snails. However, we are not sure whether the snake actually eats Amphidromus inthe wild because data are scarce on Pareas diets in their natural environment. So, whether the snake could have influenced the unusual predominance of left handedness in Amphidromus species makes logical sense, but remains unresolved.

Dozens of other snail eating snakes exist, for example many species in the genus Sibon throughout the tropical Americas, but their jaws do not show asymmetry, so they would not influence snail coiling direction.

We know of no other predator that is known to specialize in prey that have a particular “handedness.” Further studies on diets of pareid snakes would advance scientific understanding of specialized predator-prey interactions, ecological adaptation, and coevolution between the arboreal snakes and snails of southeast Asia.

And given that we are talking about snakes and snails, we must also mention puppy dog tails. The tails of many dogs do coil, and of those that coil, many of them coil off to the side. As judged by a survey of coiling dog tails in a Google Image search, dog tails that coil to the left or to the right appear to be about equally represented. So, puppy dog tail coiling direction also appears to be polymorphic…

Timothy A. Pearce, PhD, is the head and curator of collections of the Section of Mollusks at Carnegie Museum of Natural History. Kaylin Martin, M.Sc, is the curatorial assistant in the Section of Amphibians and Reptiles. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.

Literature Cited

Hoso, M., Kameda, Y., Wu, S.P., Asami, T., Kato, M. & Hori, M. 2010. A speciation gene for left–right reversal in snails results in anti-predator adaptation. Nature Communications, 1:133; DOI: 10.1038/ncomms1133.