"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 Live specimens of this snail, withdrawn in the shell and dormant, can be found on the desert surface in mid-summer, fully exposed to sun and heat.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"It is important that the animal, when withdrawn, does not fill the shell and leaves most of the largest whorl filled with air…The snail, withdrawn to the upper parts of the shell, is significantly cooler… "Why does the snail not heat up to the same temperature as the soil surface?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 Next, and perhaps more important, the snail is withdrawn into the upper parts of the shell and the largest whorl is filled with air; this constitutes a further impediment to heat flow into the snail."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)
"During the second ever expedition to hydrothermal vents in the Indian Ocean, biologists spotted a snail with a strange-looking foot. Many snails can close the opening to their shell with a flat, round bit of shell called an operculum. But this snail instead protects itself with scales, a feature seen before only in long extinct species, although the vent snail itself evolved recently. Even more unusually, the scales are reinforced with the iron sulphide minerals fool's gold and greigite, giving them a golden colour. No other multicellular animal is known to use these materials." (Schrope 2005:38)
"…[T]he snail has evolved a tri-layered shell structure consisting of an outer layer embedded with iron sulfide granules, a thick organic middle layer, and a calcified inner layer. This creates a configuration in which the inner compliant layer is sandwiched between two rigid layers.
"Ortiz and her colleagues, including MIT Dean of Engineering Subra Suresh, used nanoscale experiments and computer modeling to determine the shell's structure and mechanical properties. They found that the unique three-layer structure dissipates mechanical energy, which helps the snails fend off attacks from crabs that squeeze the shell with their claws in an attempt to fracture it. The shell of the scaly-foot snail possesses a number of additional energy dissipation mechanisms compared to typical mollusk shells that are primarily composed of calcium carbonate." (Trafton 2010)
The Class Gastropoda includes snails and slugs. Most gastropods have a single, usually spirally coiled shell, but the shell is lost or reduced in some groups. Many snails have an operculum, a plate that closes the gastropod's opening. Shelled gastropods have mantles, while those without shells have reduced to absent mantles.
Gastropods have a muscular foot used for creeping in most species. In some, the foot is modified for swimming or burrowing. Most gastropods have a well-developed head that includes eyes at the end of one to two pairs of tentacles.
"A UC San Diego engineer has revealed a new mode of propulsion based on how water snails create ripples of slime to crawl upside down beneath the surface.
"Eric Lauga, an assistant professor of mechanical and aerospace engineering at the Jacobs School of Engineering, recently published a paper…that explains how and why water snails can drag themselves across a fluid surface that they can't even grip.
"Based on Lauga's research, the secret is in the slime. The main finding of Lauga's research is that soft surfaces, such as the free surface of a pond or a lake, can be distorted by applying forces; these distortions can be exploited (by an animal, or in the lab) to generate propulsive forces and move. Some freshwater and marine snails crawl by 'hanging' from the water surface while secreting a trail of mucus. The snail's foot wrinkles into little rippling waves, which produces corresponding waves in the mucus layer that it secretes between the foot and the air. Parts of the mucus film get squeezed while other parts are stretched, creating a pressure that pushes the foot forward." (Jacobs School of Engineering News 2008)
"And certain land snails, particularly desert dwellers, seal themselves inside their shells to avoid desiccation in dry conditions, secreting a special membrane across their shells' opening that reduces evaporation; they can remain encased for years if need be until rain returns." (Shuker 2001:105) Learn more about this functional adaptation.
Gastropods are distributed throughout the ocean, and on land, essentially everywhere except the most extreme polar regions. They occur as far north as Point Barrow, Alaska (USA) at 71°23′20″N (J. Nekola, personal communication, January 17, 2011) and as far south as the sub-Antarctic islands (Solem & van Bruggen, 1984). They do not occur on the Antarctic continent.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:74749 Specimens with Sequences:65859 Specimens with Barcodes:58246 Species:9734 Species With Barcodes:8557 Public Records:57430 Public Species:6993 Public BINs:10548