Shell protects from heat: desert snail
The shell of some desert snails helps them survive extreme heat using light reflectance and architecturally-derived, insulating layers of air.
"It will be a surprise to many biologists that snails are found in large numbers on the dry, barren surfaces of certain hot deserts. The present study is concerned with one such snail, Sphincterochila boissieri, which occurs in the deserts of the Near East. Live specimens of this snail, withdrawn in the shell and dormant, can be found on the desert surface in mid-summer, fully exposed to sun and heat. The surface temperature of these deserts may reach 70 °C and more than a year may pass between rains…
"The maximum air temperature, reached at noon, was 42.6 °C, and the maximum soil surface temperature in the sun, reached at 13.00, was 65.3 °C. Under the snail, in the space between the soil surface and the smooth shell, the maximum temperature was 60.1 °C, or 5.2 °C below the adjacent soil surface in the open sun. The lower temperature under the shell is expected, for the shell provides shade for that particular spot of the soil surface on which it sits. Inside the shell in the largest whorl, located in contact with the ground, the maximum temperature was 56.2 °C. In the second and third whorls the temperature was lower, reaching a maximum of 50.3 °C.
"It is important that the animal, when withdrawn, does not fill the shell and leaves most of the largest whorl filled with air…The snail, withdrawn to the upper parts of the shell, is significantly cooler…
"Why does the snail not heat up to the same temperature as the soil surface? The answer lies in its high reflectivity in combination with the slow conduction of heat from the substrate. Within the visible part of the solar spectrum (which contains about one-half of the total incident solar radiant energy) the reflectance of these snails is about 90%. In the near infrared, up to 1350 nm, the reflectance is similar to that of magnesium oxide and is estimated to be 95%. In the total range of the solar spectrum, therefore, we can say that the snails reflect well over 90% of the incident radiant energy.
"…heat flow, however, is impeded by two important circumstances. Firstly, the snail shell is in direct contact with the rough soil surface only in a few spots, and a layer of still air separates much of its bottom surface from the ground, forming an insulatng [sic] air cushion. Next, and perhaps more important, the snail is withdrawn into the upper parts of the shell and the largest whorl is filled with air; this constitutes a further impediment to heat flow into the snail." (Schmidt-Nielsen et al. 1971:385, 388-9)
Learn more about this functional adaptation.