Why sea snails are pretty in pink

Identifying these pigments gives scientists an idea of which genes are involved in producing shell colours - a vital step towards understanding how the snails evolved their distinctive appearance.

The team found two compounds, known as uroporphyrin I and uroporphyrin III, in the snails' shells, with their distribution corresponding to the areas of intense pink-red and yellow-brown colouration.

They also discovered eumelanin in the darker spots on the snails' shells. Eumelanin is one of the pigments responsible for human skin and hair colour.

'It's a really exciting finding,' says Dr Williams. 'These snails, also known as the strawberry top shell and the beautiful clanculus, have been collected for their bright colours for hundreds of years. But until now, we didn't know where this colour came from, or how it evolved.

'This research is the first step towards understanding the evolution of colour in sea snails.'

Shining a light on pigments

The uroporphyrins belong to a group of pigments called porphyrins, which are often brightly coloured. For example, haem, which gives blood its red colour, is a porphyrin, and chlorophyll, which is responsible for the green colour of many plants, is derived from a porphyrin.

The uroporphyrin pigments have an unusual characteristic: they glow red when ultraviolet light shines on them. This helped the researchers identify their presence in the Clanculus shells, as both the yellow-brown and the pink-red regions of the shells look red when viewed under ultraviolet light.

Pigmentation process

Colour plays a vital role in many aspects of animal survival and behaviour, from camouflage to attraction. By helping species pass on their genes - through avoiding predators or by attracting mates - colour is honed by natural selection over many generations.

But how snail species developed these colours in the first place is unclear, according to Dr Williams.

'We actually know very little about how colour evolved in snails,' she explains.

'Identifying which molecules are behind the colours is an important first step, because it helps us identify the genes that could be responsible.'

In the case of the Clanculus snails, the discovery that uroporphyrin molecules are involved could point to a very specific biological process in the body, known as the haem pathway.

This process is used to produce haemoglobin, the protein that allows red blood cells to carry oxygen around the body. However, if the process is disrupted, it can lead to the production of uroporphyrins.

This suggests that the snails' colour may have evolved from an adaptation to this pathway - something Dr Williams hopes to investigate in future research.

Converging on colour

The research could also lead to a better understanding of the evolution of colour in other snails. The team found that a snail with similar yellow-brown and pink colouring, the European painted top shell (Calliostoma zizyphinum), has only trace amounts of uroporphyrins in its shell - not enough to account for its colouration.

It lives in European waters, from Norway to the Mediterranean Sea, whereas the Clanculus snails are found in the tropics of the Indian and Pacific Oceans.

'Despite their different habitats and their different evolutionary trajectories, these sea snails evolved a similar colouring,' says Dr Williams. 'This suggests that there may be some adaptive benefit to these colours and patterns.'

However, this also means that researchers will have to be careful when classifying snails based on their appearance. Two species with similar colours may be unrelated, having evolved the same colours independently - a process known as convergent evolution.

'This shows there is a real benefit to identifying the shell pigments before trying to establish an evolutionary history,' says Dr Williams. 'Not only does it reduce the risk of misclassification, but it also offers additional clues to the evolutionary history of the species.'