22 June 2005

Sleeping the summer away

Land snails are well known for their abilities to survive adverse environmental conditions, especially the lack of water and food. In one oft cited case, first reported in the 19th century1, one specimen of the Middle Eastern species Eremina desertorum survived almost 4 years attached to a display case in the British Museum. Sphincterochila boissieri, another tough species, lives in the Negev Desert in Israel. The body temperature of one snail aestivating in the sun was recorded at 50.3 °C, about 5 °C below lethal temperature2.

The Mediterranean summers are notoriously hot and dry. Rains usually end in May and don't start again until October. For example, along western and southern coastal areas of Turkey the average monthly total rain is barely 1 mm in each of the months July and August.

The land snails that inhabit such places spend about five months of the year in aestivation. Many species aestivate under the rocks or buried in soil, where they obtain protection from the direct heat of the sun. But, some species aestivate on exposed surfaces of rocks, sometimes fully in the blazing sun, and survive.

Recently, I reported the survival of some specimens of the clausiliid land snail Albinaria without getting liquid water from the outside for up to about 13 months3. This is possible apparently because the aestivating snails metabolize at a very low rate2, and consequently, their metabolic reserves can last a long time. In the long run, this over-adaptation must be an indispensable safety mechanism, because, without it, one unusually long dry period lasting, for example, 7 months rather than the ususal 5, would kill all the snails.

Albinaria caerulea aestivating attached to limestone rocks near Istanbul, Turkey (June 2004).

1. Stearns, R.E.C. 1877. On the vitality of certain land mollusks. American Naturalist, 11:100-102.
2. Schmidt-Nielsen, K., C.R. Taylor & A. Shkolnik, 1971. Desert snails: problems of heat, water and food. Journal of Experimental Biology, 55:385-398.
3. Örstan, A. 2005. Aestivation survival in some Turkish Albinaria species. Spirula, No. 342, pp. 3-4.

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