The banded dye-murex (Hexaplex trunculus) is a carnivorous snail from the Muricidae family of mollusks. The snail has historical and halakhic importance as it was used to produce the tekhelet. In addition, the snail is considered food in many countries. This snail is indigenous to the coasts of the Mediterranean and Black Seas, occupying the seabed at depths of 1-20 metres.
The shell has a wide cone shape and is 4–10 centimeters long. The adult snail has a tall spire with seven bends. Dark stripes on the width of the shell vary in colour and shape, hence the name “banded”. The sides of the shell have dark protrusions, giving it a coarse texture.
The banded dye-murex feeds on mollusca, snails, shells, bony and cartilaginous fish, and a variety of meat waste from sea or land sources.
The banded dye-murex was identified as the snail from which the tzitzit cords, parts of the Tabernacle, the Temple and the High Priest’s clothing were dyed in ancient times. The characteristics of the snail are described in Tractate Menachot 44a: “The hilazon resembles the sea in its colour, and in shape it resembles a fish; it appears once in seventy years, and with its blood one dyes the tekhelet; and therefore it is so expensive.”
The dye as such is not formed in the banded dye-murex, but is produced only after crushing its the shellfish’s hypobranchial gland. A chemical reaction occurs spontaneously, when the substance is exposed to oxygen from the air. Hues ranging from hyacinth-blue to violet are obtained depending on the chemical composition of the dye obtained. The main ingredient in the tekhelet dye is 6-monobromo-indigotin (MBI), with small quantities of two other components, – 6,6′-dibromo-indigotin (DBI) and indigotin. MBI is produced as a violet substance that changes to blue when slightly heated: it is unique to tekhelet. DBI is also the dyestuff of Biblical argaman (Tyrian purple). Indigotin is also the dyestuff of the plant-derived dyes isatis, kela ilan, indigo and woad.
Thanks to this characteristic composition, one can identify ancient remnants of textiles dyed with tekhelet using chemical analysis. Shells of banded dye-murex were found in many archaeological sites around Israel, together with shards of jugs with tekhelet-dye marks, dyeing installations, bits of tekhelet-dyed fabric, etc.
The question is asked how is it possible that an impure animal was used for religious purposes in the Temple [“that the law of the Lord may be in thy mouth” (Exodus 13) – that which is permitted in your mouth” – from here one learns that religious artifacts must be made from pure animals (Shabbat 28b, 108a )]. Amongst the many reasons given, we can state that the substance undergoes a significant change from the moment it is taken from the snail; we have a completely new material (“נשתנה מראיתו ופנים חדשות באו הנה”). Another option is that we are not using the snail’s substance itself, but merely imparting a colour by dyeing (“חזותא הוא”).