Start Navigation Menu!

Banded Snails (Cepaea Held 1838)


Heartfelt thanks to Mica Brugsch for her many colourful specimens of banded snails, I was allowed to photograph!

Introduction


Quintuple banded specimen of the grove snail. [RN]
 

The small and medium sized snails of the Helicidae family come in many different colours. Because of their sometimes present characteristic bands those snails are also referred to as banded snails. Their richness in colours and patterns is astounding even for gastropods, taking into account that there are many colourful species of marine gastropods or sea shells.

 
Left: Garden banded snail (Cepaea hortensis, front) and grove snail (Cepaea nemoralis).
Right: Garden banded snail and Viennese banded snail (Cepaea vindobonensis). [RN]

In Central Europe there are four species of the genus Cepaea (Held 1838). Of those two are generally very well known: The larger grove snail or black mouthed banded snail, Cepaea nemoralis, and the smaller garden banded snail or white mouthed banded snail, Cepaea hortensis. Both are different in size (though a small nemoralis may be as big as a large hortensis) and the colour of their apertural lip.

The other two species of banded snails in Central Europe are the Alpine banded snail (Cepaea sylvatica), occurring from the higher altitudes in Switzerland down to the Upper Rhine Valley, and the Viennese banded snail (Cepaea vindobonensis) appearing in an area neighbouring to the East, from Vienna (as the name says) and the rest of Eastern Austria to the Balkan Peninsula and the Black Sea.

Helicidae II (Helicinae): Banded snails, Roman snails and their nearest relatives.

While the Alpine banded snail and the Viennese banded snail are rather constant in colour and pattern, the former two species of banded snails are different in being very various in colour and in pattern.

There are different ground colours between yellow, rose, red and brown, also there are different types of banding, from unbanded forms, single band forms up to multiple band forms, such as five bands. Contrary to all this variability, an anatomical examination, especially of the genital apparatus and the love dart, will prove, that it is always the same species, so an unbanded yellow grove snail (Cepaea nemoralis) is more closely related to a rose coloured, quintuple band other grove snail than to an equally unbanded and yellow coloured garden banded snail (Cepaea hortensis).

Hainbänderschnecke (Cepaea nemoralis)   Hainbänderschnecke (Cepaea nemoralis)
Grove snail (Cepaea nemoralis). Left: Two banded varieties (yellow and rose). Centre left: Three different colour varieties. Centre right: Three different shades of red. Right: A specimen between brown and rose colour. [RN]

But how could shell colour evolve to such a unique variability among the European terrestrial pulmonate snail species (Stylommatophora)?

Genetics of Banded Snails

Gene Attribute Alleles
C Ground colour of shell CB Brown
CDP Dark pink
CPP Pale pink
CFP Faint pink
CDY Dark yellow
CPY Pale yellow
B Presence of absence of bands B0 Unbanded
BB Banded
I Punctuate bands II Punctuate
I- Unmodified
S Spreading of band pigment SS Spread bands
S- Unmodified
P Pigmentation of bands and (apertural) lip PN Normal (Dark brown)
PL Light brown
PA White (albolabiate)
PT White lip and translucent bands (hyalozonate)
T1: Attributes of shell colour in grove snails (Cepea nemoralis) inherited in
combination. Dominance decreasing from top to bottom.
 

The colour and pattern of a banded snail's shell is based on genetics and the rules of biological heredity. The bases of banded snails' polymorphism (diversity of forms) in Mendelian rules of dominance and recessivity has been determined since the beginning of the 19th century (Lang, 1904, 1908). Since then the shell colour of banded snails is a basic example of Mendelian genetics in the animal kingdom. Alas, Mendelian rules cannot explain sufficiently the heredity of shell colour among banded snails. Extensive cross-breeding experiments since the 50s (see description in Murray, 1975) have only in younger time led to the picture we have of banded snails' genetics today.

Several genes of grove snail's (Cepaea nemoralis) shell colour are located on the same chromosome and therefore usually are inherited in combination. There is the gene C for the shell's ground colour, described with several alleles, such as brown, rose coloured (pink) in different stages and yellow. Another gene on the same chromosome is B for the presence or absence of bands, with two alleles: B0 for an unbanded shell and BB for one with bands. Further genes comprise the gene I for punctuate (discontinuous) bands, S for a spreading of band pigment and P for the pigmentation of bands and apertural lip, which can be dark brown, as is typical for the species, light brown or even white or translucent.

 
Gene Attribute Alleles
U Suppression of bands 1, 2, 4 and 5 U3 One band
U- Unmodified
T Suppression of bands 1 and 2 T345 Three bands
T- Unmodified
D Skin pigmentation DR Reddish pigmentation
DG Grey pigmentation
Q Quantity of pigmentation QM Medium grey
QP Very pale (yellowish)
R Darkening bands R- Unmodified
RD Bands gradually darkening towards the aperture
O Orange bands O- Unmodified
OO Orange bands
T2: Attributes of shell colour in grove snails (Cepea nemoralis) inherited indepen-
dently. Dominance decreasing from top to bottom.

Further genes are located on other chromosomes, as they are inherited independently from the formerly mentioned genes, as well as from each other, as far as the available information allows the conclusion. Among those are the genes U for the suppression of the bands 1, 2, 4 and 5 (which leads to the existence of a single banded variety), T for the suppression of the 1. and 2. bands, D for the pigmentation of the soft body's skin, R for a gradual darkening of the bands from apex to aperture and finally O for orange coloured bands.

As displayed in the tables T1 for attributes inherited in combination and T2 for attributes inherited independently, there are mostly clear dominance relationships in the genes C (brown is dominant over pink, which in turn is dominant over yellow) and B (unbanded is dominant over banded), as well as U (single banded is dominant over five bands). Clear relationships of dominance are also present in the remaining genes.

The dominance relationships between the different shades of rose or pink are insecure. Only those colours are difficult to determine in the field anyway. Sometimes a banded snail's shell colour can be determined clearly only by scratching away the shell skin (periostracum).

Following the findings of those genetic examinations, in nature mainly brown, unbanded grove snails should occur.


D1: The number of banded snails (Cepaea nemoralis) found on a wall in Bad Kreuznach.
 

The result of a snail counting in Bad Kreuznach in the German state of Rhineland-Palatinate (Nordsieck, R., 2009, Diagram D1) as part of the Evolution Megalab (see further down) however proves, that this is not at all the case. Of nearly 70 counted snails on a wall overgrown with ivy, the large majority was yellow and unbanded, followed by yellow, single banded snails and yellow snails with five bands. Brown snails were not found at all.

 
D2: The number of banded snails (Cepaea nemoralis) found on a wall in Berlin-Dahlem.

A more valuable statistics is given by the counting of a larger number of banded snails as part of the Evolution Megalab in Berlin (Brugsch, M. and Nordsieck, R., 2009, Diagram D2) on a comparable wall. Here as well yellow snails made by far the majority (186 of 274 snails, equalling about two thirds). But here the largest part (nearly half of all counted shells) was yellow and had five bands, which according to genetics makes a combination of not less than three recessive attributes - yellow, banded and five bands!

Selection by predators

That the genetic bases of shell colouring in banded snails are no expressed in nature as they should, is for ecological reasons.

Banded snails, as well as many other snail species, are among the favourite prey of numerous bird species, among them the song thrush (Turdus philomelos). Those birds hunt on sight and so they catch and eat those snails most they see first in the vegetation. The snails camouflaged better in front of a given background survive and are allowed to breed.


Juvenile song thrush (Turdus philomelos). Picture: Rudolf Dick.
 

Song thrushes have become rare in many places. They look out for a special stone (also a kerbstone or a piece of wall), which they use to crack shells of banded snails they caught to reach the nutritious contents. In nature we can recognize those stones by the numerous broken snail shells around them. They are also called thrush anvils.

 
D3: The relation between bright and dark, banded and unbanded shell va-
rieties of Cepaea nemoralis depending on different habitats:
: Wood; : Hedge; : Meadow. [RN]

Dark specimens of banded snails turn out to be least visible on dark underground such as earth without vegetation. Yellow specimens disappear in bright open grassy areas, whereas the banded and rose coloured specimens are camouflaged best on the branches of bushes and shrubs. Because vegetation in nature is so various, there are always the specimens camouflaged worst to be eaten and the others to survive and to reproduce. Those see to the survival of the species.

Additionally there is a seasonally different selection by predators: In the seasons, where the vegetation is lessening, the more brightly coloured snails are caught and eaten to a larger extent than the inconspicuous brown specimens. On the other hand those become conspicuous when the vegetation grows, thus relieving the brighter varieties.

The multitude of shell colours and patterns in banded snails is called polymorphism. As already mentioned, polymorphism has nothing to do with differences of species of the developent of species. In contrary, by the multitude of colours and patterns ensure the species' survival in an equal multitude of environments.


Grove snails (Cepaea nemoralis) of two different finding sites: Left: Gardens of the Applied Genetics Department (FU Berlin)
[RN], right: A wall overgrown with ivy in Berlin-Dahlem (see diagram D2), Picture Micaela Brugsch.
 

It is interesting, that freshly hatched banded snails are always unbanded. Only some weeks after hatching, the bands appear.

The already mentioned international project "Evolution Megalab" in the Darwin year 2009 has got the task of collecting the results of field countings of the banded snail species Cepaea nemoralis and Cepaea hortensis. Those results, as shown in the diagrams D1 and D2 and divided between the three ground colours (yellow, pink and brown), as well as the band patterns (unbanded, single banded, multiple banded).

A further question posed is, if the increasing rarity of the song thrush (Turdus philomelos) descreases the rate of banded snails' selection by predators. According to the results displayed above it becomes clear: The natural relation between the different varieties of banded snails is not the expression of the genetic bases. If the reason for this is not (any more) the song thrush, then it is other bird species or a factor so far unknown.

These results are supposed to be compared to historical results, hoping for the possibility to show an evolutionary trend.

Finally, banded snails' success in nature speaks for itself. And there seems to be no end to the multitude of colours possible.

Further Information

Helicidae II (Helicinae): Banded snails, Roman snails and their nearest relatives.
Cook, L.: Cepaea genetics  - what we think we know and what we don't know. Evolution Megalab, as of 20.10.2009. (Also the source of tables T1 and T2).


The Europe-wide banded snail project in the Darwin year 2009.


A burgundy coloured specimen of the grove snail (Cepaea nemoralis). [RN]