It’s just as well for some of our common marine molluscs that we don’t have emperors and do have synthetic dyes. Sea snails of the Murex group in the Mediterranean Sea were once very intensively harvested because they were the source of the Tyrian purple dye used by royalty. Other shells from the family are still used by Mexicans today.

The Cartrut shell (Dicathais orbita) is one of the largest and most common shells of this family found on our shores. It grows to 75mm and its heavy shell has deep spiral ridges with grooves between (hence its common name). Since it lives just below mean low tide level, the ridges help to strengthen the shell to withstand the pounding from waves.

Cartrut shells are carnivores. They become active at high tide seeking out and devouring their prey: other molluscs, barnacles and Galeolaria tube worms. Like their relatives the Mulberry shells (Morula marginalba – featured in a previous column), they bore into their prey, softening the shell with acid they dribble from their mouths, then rasping away the weakened calcium carbonate with their rough tongues (radulas).

The Cartrut shells use the dye’s ingredients to help protect their egg capsules from microbes. Sometimes these capsules can be found washed up on the beach and some may still have a tinge of purple. It was unfortunate for murex snails that their dye proved so attractive to humans. The dye starts off as a pale yellowish mucus, secreted by a gland above the mollusc’s gills. If left exposed to the sun the mucus will change from yellow through green and blue then eventually to a deep red-purple colour. In strong sunlight this happens very quickly.

Although the Minoans of ancient Crete are believed to have first used the dye around twenty centuries before Christ, it was the Phoenecians of Tyre that mainly produced and sold it during the Roman empire. Tens of thousands of snails have to be processed to get a few grams of dye, so naturally it was very expensive. In Tyre the shells were left to decompose in huge vats before the dye was extracted; the smell must have been horrendous.

In some parts of coastal Mexico today, another relative of the Cartrut shell is still used to dye yarn but the shells live to secrete another day. They are taken from the water and poked until they exude a few drops of the mucus. The drops are then rubbed into a thread of yarn and the yarn left to colour in the sun. Needless to say the process is very labour intensive and slow but at least it is sustainable.