Exterior decorating for snails

You may think of snails as boring, everyday creatures, but in the land snail Cepaea nemoralis, a lot more is going on than meets the eye. Nonetheless, much of the intrigue of this species sits on the surface, in plain view for all to see. Also known as the ‘grove snail’ or the ‘brown-lipped snail’, this is one of the most common snail species in Europe, occupying a wide range of habitats. It has long been studied because of the considerable amount of variation in the colour and banding of shells between individuals.

The shells of Cepaea nemoralis come in one of three colours: pink, yellow, or brown. They also have either zero, one, three, or five dark bands patterning their shell. Each individual has a specific colour and banding pattern- a huge number of combinations gives the possibility of many different morphs (Cepaea are highly ‘polymorphic’)Frequencies in which the different morphs occur varies depending on location, altitude, climate, predators present, as well as other factors, as long as it is genetically possible.

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Variety of morphs: different shell colours and bands within a population of Cepaea nemoralis in Poland. Photograph by Robert Cameron.

Shell colours that are similar to the colour of the snail’s habitat reduce its predation risk as they help the it blend in, and banded morphs may have an advantage over non-banded morphs in some habitats if they help the snail camouflage. The type of shell therefore directly affects Cepaea‘s chances of being eaten by predators who use visual cues, such as the song thrush. Darker shells also help snails warm up quicker in the sun by absorbing heat better, so that they can become more active.

Interestingly, much of the data we have about the geographical distributions of the morphs came from citizen science project “Evolution MegaLab“. Run by the Open University since 2009, people throughout Europe were asked to record the number of different morphs found in their local area. Gaining geographical data would allow scientists to study how the frequencies of morphs vary depending on habitat type and the number of song thrushes in the area, as well as how climate change and global temperature increases are changing the distributions of different shell colours.

Surely, if there are selection pressures acting on shell colour and banding pattern, such as predation or unfavourable climates, certain morphs will be at a disadvantage and will be removed from the population in that area? Obviously this is not happening, as can be seen from the beautiful range of shells in the photo above.  The polymorphism persists in almost all habitats, even though proportions vary.

So how is the variation maintained in the population?

This is an interesting yet complex question that has puzzled biologists for decades. If we can understand how these traits remain so varied under such precarious conditions, it could shed new light on Cepaea‘s genetics, ecology, and evolution.  One of the most famous and comprehensive papers on Cepaea by J. S. Jones, “Polymorphism in Cepaea: A Problem with Too Many Solutions?” published in 1977 brought together the genetic nature of this polymorphism and the ecological factors affecting it, ultimately suggesting that there are too many aspects involved to provide a single definitive reason. Often several selection pressures act on an individual at once, pulling its appearance in opposite directions. For example, in a cooler region it may be important that the shell is darker in colour, in order to get enough energy to survive. But what if darker shells are more easily picked out by predators? Then, the number of lighter shells will increase due to frequency-dependent selection. At the same time, there may be gene flow from neighbouring populations altering the frequency of each colour, as well as genetic restrictions on what is physically possible (such as a certain colour and number of bands being inherited together). These mechanisms will vary between populations.

One thing to consider is looking at the situation from a different perspective in terms of predation- what if birds view colour different to us? We may see three distinct colours, but structures in the eyes of avian predators may allow them to see a more precise scale of colour, meaning they can differentiate between two shells that look the same to humans.

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Bird’s eye view: the song thrush is a predator of Cepaea nemoralis. Frequency of morphs in an area may be related to the abundance of song thrushes in the area, and avian predators may view colour different to humans. Photo by LubosHouska on Pixabay.

This year I am going to be working on this wonderful creature, quantitatively measuring the exact colour of their shells to help determine whether what is thought to be three distinct colours (pink, yellow and brown) is actually a more varied array of colours, like a spectrum. I look forward to learning so much more about Cepaea nemoralis and hopefully contributing some new information to this subject!

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