Who always has a soft body covered with a mantle and often, but not always, a hard exterior? With a single flat foot and a mouth full of chainsaw teeth? Who has green blood full of copper? A net of nerve cells with a row of some knotted together into clumps? Optional extras are either lungs or gills or being of one sex or another or both. Size? From microscopic to thirteen metres.

Preposterous as it seems, for 570 million years, in the sea, in fresh water and on land, these are mollusks. Their many forms are all variations of the basic body plan. As limpets, they have a single shell like a chinaman’s hat. As clams, oysters, mussels and such, they have two. Being chitons, they live under a set of plates hooked together like chain mail. The squid and cuttlefish forms have a remnant of a shell shaped like a supporting rod which they carry it inside themselves. As octopuses, they have no shells or remnants at all.

Shells and us

Finally, with bodies twisted 180 degrees and tapered to fit into their single coiled shell, they are snails. Surely at some moment in your life, you paid special attention to a snail? Or perhaps it was a snail’s shell?

Snail shells always mean something to humans. Cowry snail shells, threaded together into belts or woven into baskets, speak of wealth. They were used as trade goods from as long ago as 3000 BC, sourced from the Maldives. The shells became the standard currency for Africa, India and China. They were legal tender throughout the Middle East and in Europe. In parts of the world, they were used right into the mid 20th century.

Sex changes

For centuries, science workers and teachers have dissected marine snails, counted them on shores throughout the world and observed their lifestyles.

Although all snails on land are hermaphrodites, many marine snails have separate sexes. So in 1970-71, when researchers on both sides of the Atlantic Ocean reported an alarming change in females, everyone looked again to their snails.

Suddenly, adult female snails grew penises. This blocked the passageway used for the release of eggs. But the females did not become functional males. With their females sterilised, these snail populations died out.

By 1975, the entire southern coast of the UK reported a rapid decline of its dog whelk snails. This was directly related to imposex, as the deformity was called.

In Arcachon Bay, France, imposex wiped out the snails that drilled into commercial oysters. But the shells of the oysters themselves became deformed, their flat shape changing into very thick balls. In 1976, the oyster larvae did not survive. By 1980, the oyster fishery collapsed.

Not till 1986 was the cause traced to the use of organtins in antifouling paint on boats and fish cages. The persistence of tributyltin (TBT) in the sediments was finally understood in the 1990s. France, Japan, the US and several other countries limited the use of the biocide, but this did not halt contamination. The pollutant recognized no boundaries.

Toxic input

By 1998, the International Maritime Organisation legislated a mandatory global ban on the use of TBT on any craft after 2008. In 2009, a Japanese research team found female flounder larvae were becoming masculinised. This was the first report of a proven effect of TBT on a vertebrate animal.

From first alarm to global ban, 38 long years passed. All the while, the seas and their sediments continued to receive this toxic pollutant. Like an ocean liner at full steam changing direction, our global response was slow. The full repercussions of our ignorance and delay are not yet upon us.

Though they make no sound, marine snails now raise a new alarm. The small snails known as pteropods have flaps on both sides of their single foot. They use these to swim through their home, the open seas of the cold Southern Ocean.

Under certain conditions, they replace krill as the major food for many other species from small fish to great whales. But their delicate living shells are falling apart at the edges.

This prompted Australian science workers to comb through Antarctic data assembled over the past ten years. In 2008, they announced the collective weight of shells of one pteropod species has indeed declined. Another species simply does not appear as often anymore.

CO2 impact

This time, the cause is already known. The increase of carbon dioxide in the atmosphere continues to be absorbed by oceans around the world but not without consequences. The chemistry of the sea is being changed. As it becomes more acidic, it also holds less dissolved carbonate ions in solution. It is these carbonate particles that snails use to build their shells.

The edge of its mantle, filled with nerves, matches the growing edge of its shell. The nerves ‘read’ the pattern of the shell created to date, both the white carbonate part and the colours which are proteins. They then instruct the cells in the mantle to secrete the next row of carbonate and proteins as required by the pattern unique to its species. If the carbonate is missing, what can be built? Living in acidic waters, how to keep whatever’s made from dissolving?

The last time we ‘heard’ snails, we took forty years to respond. Facing climate change, we humans have choices: energy savings, renewable energy, alternative lifestyles and more. Now, how quickly will we act?