Hexaplex trunculus (also known as Murex trunculus, Phyllonotus trunculus, or the banded dye-murex) is a medium-sized species of sea snail, a marine gastropod mollusk in the family Muricidae, the murex shells or rock snails.
This species is known in the fossil record from the Pliocene to the Quaternary period (age range: from 3.6 to 0.012 million years ago). Fossil shells within this genus have been found in Morocco, Italy, and Spain.
This species of sea snail is important historically because its hypobranchial gland secretes a mucus that most ancient people's of the mediterranean from the Minoans to the ancient Canaanites/Phoenicians and classical Greeks used as a distinctive purple-blue indigo dye. One of the dye's main chemical ingredients is dibromo-indigotin, and if left in the sun for a few minutes before becoming fast, its color turns to a blue indigo (like the dye used in blue jeans).
This species lives in the Mediterranean Sea and the Atlantic coasts of Europe and Africa, specifically Spain, Portugal, Morocco, the Canary Islands, Azores.
This murex occurs in shallow, sublittoral waters.
Hexaplex trunculus has a broadly conical shell about 4 to 10 cm long. It has a rather high spire with seven angulated whorls. The shell is variable in sculpture and coloring with dark banding, in four varieties. The ribs sometimes develop thickenings or spines and give the shell a rough appearance.
Secretions from this species of snail were used as dye in ancient times. The snail itself is still used as a valuable food source in Portugal.
The ancient method for mass-producing purple-blue dye from Hexaplex trunculus has not yet been successfully reproduced (because the purplish hue degrades too quickly, resulting in blue only). Nonetheless the use of this species in dyeing "purple-blue" has been confirmed in the archeology of Phoenicia, where large quantities of the shells have been recovered from inside ancient live storage chambers that were used for harvesting. Allegedly, 10-12,000 murex were needed to produce one gram of dye. The dye was highly prized in ancient times. Sometimes known as royal blue, it was prohibitively expensive and was only used by the highest ranking aristocracy.
A similar dye, Tyrian purple, which is purple-red in color, was made from a related species of marine snail, Murex brandaris. This dye (alternatively known as imperial purple, see purple) was also prohibitively expensive.
The Hebrew Bible mentions a specific blue dye, called Tekhelet (Hebrew: תְּכֵלֶת, pronounced [ˈtχelet]) for use in the Priestly garments as well in the layman's tzitzit, the formal tassels or fringes of clothing, which refers to the indigo dye from the Hexaplex trunculus when kept in the sun. The Hexaplex trunculus is the tekhelet mentioned in the Bible and Talmud.
That is, research by Otto Elsner (Shenker College of Fibers, Ramat Gan, Israel) and Ehud Spaneir (University of Haifa, Haifa, Israel) showed that by performing what is commonly referred to as "vat dyeing" based on the dyestuff from the trunculus, they could achieve colors varying from blue to purple depending on exposure to the sun when the dye solution was in its leuco (reduced) state. This phenomenon was attributed to the dyestuff being composed of indigo, mono-bromo-indigo and di-bromo-indigo. Dibromo-indigo presents itself as purple whereas indigo is blue. It was demonstrated that when the reduced solution of trunculus dye is exposed to sunlight, the UV-rays from the sun act to break the bromine bonds such that when oxidation occurs following the removal of the dyed fabric from the solution, pure indigo bonds to the wool, while the bromine atoms are left in the vat.
This feature of final color varying according to exposure to the sun was indeed known by the ancients, as noted in the writings of Vitruvius (1 c. BCE), “Purple exceeds all colors in costliness and superiority of its delightful effect. It is obtained from a marine shellfish. ...It has not the same shade in all the places where it is found, but is naturally qualified by the course of the sun”.