A Helix pomatia snail king (on the right) together with a normal
dextral Helix lucorum. Only the sinistral specimens of Roman
snails are called snail kings. Picture: [RN].
There is another very well documented phenomenon among the Roman snail (Helix pomatia). Among thousands of specifically dextral shells there may be one or the other rare sinistral specimen. Those then are colloquially referred to as snail kings.
The coiling direction of a snail shell's whorls are determined genetically. Because of the rarity of sinistral Helix shells one would certainly assume, that shell genetics is based on a normal dominant-recessive pattern of heredity. The character "sinistral" shell then would be the recessive and would disappear when characters are mixed, which in genetics is called heterozygote. The recessive character is suppressed by the dominant wild-type (those types of characters in genetics are also called alleles).
It may, though, happen, that after the mating of two dextral Helix snails the offspring is exclusively sinistral. That is impossible, if one assumes the pattern of heredity to be conventionally dominant-recessive.
In 1923 the American geneticist Alfred Henry Sturtevant published a discovery in this subject in the magazine Science. Which is why we know today, that shell coiling in gastropods is indeed inherited in a dominant-recessive pattern.
Snail kings hibernate, too. What is the genetic background of
this fascinating phenomenon?
Picture: Monika Samland (German Institute for Heliciculture).
The direction, in which a snail's shell whorls finally coil, however, depends on the direction of cell cleavages in the fertilised egg cell. Snail's cells divide in a spiral pattern (other than mammal cells, for example). And the direction, in which this spiral turn, determines, among other things, the direction of the coiling direction of the shell.
So the genetic information that determines a snail shell's coiling direction can not be found in the snail's own cells, but in those of the mother animal - in the case of the hermaphroditical land snails the snail the egg cell was from.
That means, the 'mother's' genome is important to a snail's coiling pattern, not its own. Such a pattern of heredity is called a maternal effect of matrocline heredity. Snails that are externally dextral (that is called the phenotype) may have exclusively sinistral offspring, because they are genetically sinistral (that is called the genotype). And besides their genotype must, of course, be homozygote, because the character "sinistral" is the recessive allele.
The genetics of snail shell coiling: Heredity pattern with crossing diagrams.
Besides the direction, in which a snail's shell turns, also its body is different to a commonly dextral snail: The sexual opening on the wrong side of the body may make mating difficult for a sinistral snail. But, as we can see in the picture on the right, it does not stop a determined sinistral snail.
A picture with the value of rarity: A snail king (right) mating
with a normal dextral Helix pomatia.
Picture: Peter Leonhardt (Felix Helix Project).
Besides the fundamental difference in the coiling direction it is clearly visible externally, that both snails are Helix pomatia.
Living snail king, though, are very rare (one in several thousand) and almost never encountered in nature. Only an incredible fortunate coincidence may be the reason, that like in the case of Karin and Peter Leonhardt, somebody gets hands on a living snail king twice in succession! The circumstances may be seen in their homepage mentioned below.
A Japanese study has found out that sinistral snails of the Camaenidae family have an advantage in regions where there are snail eating snakes (Pareas iwasakii). Those snakes are adapted to hunting snails to such an extent that they have more teeth on the right side, than on the left. That makes it easier to grab snails. It is however, decidedly more difficult when the snail is sinistral. That is why there are noticeably more sinistral Camaenidae snails in regions with snail eating snakes.
In Central Europe, there are snail eating snakes as well, such as the grass snake (Natrix natrix). But they are usually too small to eat a Roman snail and they make no difference between coiling directions. That would explain, why sinistral specimens of snails are so rare here: There are no selective advantages for them to compensate for their disadvantages in mating.
Literatur used for genetic basics