Featured picture: Thinned snail shells

Wing snail
(Image: Lisette Mekkes / Naturalis Biodiversity Center)

When the pH of the seawater drops, it also affects the shell development of pteropods, as this image shows. The left shell comes from a snail from an acidic sea area near the coast, the right from the open sea.

The oceans store more than a quarter of the carbon dioxide that humans release into the atmosphere annually through the burning of fossil fuels. The gas dissolves in the water and becomes carbonic acid, which causes the pH of the oceans to drop – with consequences for many marine life. Living organisms with a calcareous skeleton such as corals, mussels and snails can gain less calcium carbonate from the acidic water for the construction of their shells.

An important reason for the increasing acidification of many marine areas is the rising CO2 levels in the atmosphere, but there are also ocean areas that naturally have a lower pH value. These include parts of the Pacific off the US west coast. There, cool, carbon dioxide-rich deep water rises to the surface, making these coastal areas more acidic than the open sea.

Researchers led by Lisette Mekkes from the University of Amsterdam have now investigated the effects of this additional acidification. To do this, they collected tiny planktonic sea snails (Limacina helicinaaus) on the coast of Washington and Oregon and further out in the open sea. These snails from the order of the pteropods produce delicate shells and are very sensitive to changes in the pH value of the water. The research team therefore checked the shell thickness of 80 animals using 3D X-ray scans and a scanning electron microscope.

It showed that the shells from coastal waters were 37 percent thinner – as our photo illustrates. The bluish coloration of the left shell indicates a shell thickness of less than ten micrometers. In contrast, the snail living in the open, less acidic sea area on the right produced shell walls a good ten to 18 micrometers thick.

These differences are not due to different snail populations, but to the immediate environmental conditions: “Our research shows that pteropods, which are transported by the currents from the open ocean into the acidic, coastal waters, have difficulties within two to three months, to build their bowls, ”explains Mekke’s colleague Nina Bednarsek.

It is still uncertain what consequences the thinner shells could have for the pteropods. “Bowls protect against predators and infections, but the production of thinner bowls could also be an adaptation or acclimatization strategy,” suspects Mekke’s colleague Katja Peijnenburg. “An important question, however, is how long can pteropods continue to produce thinner shells in rapidly acidic waters?”

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