Striking Open-coiled Snails
The Exquisite False Dial, Spirolaxis centrifuga (Monterosato, 1890) is a delicate and truly attractive species of the sundial snail family Architectonicidae. The species lives in the eastern and western Atlantic Ocean, reaching about 5 mm* in diameter.
As the photo above shows, the Exquisite False Dial shell is normally open-coiled, with successive whorls** not touching each other, yet maintaining regular, proportional distances from the adjacent whorls. Other gastropods with open-coiling are, for instance, some members of the nutmeg family Cancellariidae, such as Extractrix milleri (Burch, 1949), from the eastern Pacific Ocean (below).
Some wentletraps (family Epitoniidae) also present open-coiling. This is the case, for instance, of Cycloscala revoluta (Hedley, 1899), from the Indo-West Pacific (below) And even some rare species of land snails, such as Blaesospira echinus infernalis (Torre and Bartsch, 1941) from Cuba, (below) are "open-coilers." The possible advantages for these species to "wear" open-coiled shells have not yet been fully explored by malacologists (mollusk researchers.)
*One millimeter (1 mm) = 0.04 inch.
**A whorl is one complete shell revolution, or “turn.”