26 July 2005

Sleeping the summer away 2: converging in on an epiphragm

The land snail on the left is a subadult specimen of Helminthoglypta dupetithouarsi. I found it half buried in soil under a log near Monterey, California last month. The snail had sealed its aperture with a hard calcareous epiphragm before it became dormant (there was also soil stuck to it). Epiphragms reduce water loss by evaporation through the aperture1.

Contrast the epiphragm of H. dupetithouarsi with that of Neohelix albolabris from Maryland in the right-hand picture. There is no dry season in Maryland; some form of precipitation is expected throughout the year. Thus, there is no serious danger of water loss and the snails seal their apertures, even during the winter, with slime that dries to form a thin, membrane-like epiphragm.

As I discussed in the first post in this series, along the coasts of the Mediterranean Sea the summers are very dry and the land snails, like their Californian counterparts, build calcareous epiphragms. The picture on the right shows a subadult Helix lucorum that I found under the rubble of an old building in August 2000 on Burgazada, an island off Istanbul, Turkey.

Some years ago I had in captivity some live specimens of the land snail Rumina decollata, originally from the Mediterranean coasts of Turkey. Even when the inside of their container was wet, they would occasionally close the apertures of their shells with hard calcareous epiphragms. This example suggests that the type of epiphragm a snail makes is genetically determined.

The emergence of the same type of calcareous epiphragm in distantly related families of snails (Helminthoglyptidae and Helicidae) under the influence of the Mediterranean climate in different parts of the world is an example of convergent evolution (as opposed to parallel evolution). In other words, similar environmental conditions induce the evolution of similar adaptations in species with different ancestors. There is a detailed discussion of convergent, divergent and parallel evolution in chapter 7 of Niklas2. The diagram below, modified from Fig. 7.1 in Niklas, may help explain the difference between convergent (red yellow purple and blue purple) and parallel (red yellow green) evolution.

Colors denote different states of a phenotypic trait.

1. Machin, J. 1967. Journal of Zoology 152:55-65.
2. Niklas, K. J. 1997. The Evolutionary Biology of Plants. University of Chicago Press.

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