HOW SNAILS BUILD A SHELL

All they need is a specialised organ, some calcium-rich food and time

WORDS SCOTT DUTFIELD

For more than 500 million years, marine snails have been growing their own homes, along with many other mollusc relatives. Around 150 million years ago, a clade of snails called Stylommatophora moved out of the ocean onto land, bringing their shells with them to form the world’s terrestrial snails, also known as gastropods. And like their water-dwelling cousins, snails start forming their back-bound homes before they hatch.

While developing inside their eggs, snails start to grow the precursor to a shell, called a protoconch. After around a month inside their eggs, the juvenile snails hatch with their protoconch in tow and begin foraging for food rich in calcium to build shells, which are almost entirely made from calcium carbonate. The hunt for calcium starts with devouring their egg. In marine snails, calcium can be drawn from seawater, while land snails have to rely on food – like decaying leaves, fruits and vegetables – and even soil to get their calcium fix. A snail’s quest for calcium is never over, and these slimy invertebrates will continue to grow their shells until they die. To do so, they use a specialised organ called a mantle. This biological builder uses an electric current to move calcium and bicarbonate ions from the snail’s digestive system and bind them to proteins to secrete a matrix, which eventually solidifies into a hard shell.