In Tanach and the Gemara, we find various clues about the identity of the chilazon. First, we learn that it was famously produced outside of the Land of Israel, in the “Islands of Elisha” (Yechezekel 27:7), which are identified as Italy or Cyprus.Then, we have a list in the Gemara (Menachot 44a) of several aspects of the chilazon. There is debate about how well these clues match the trunculus – arguments can be made in both directions. The Gemara presents the following descriptions:
- Its body “resembles” the sea. This can be easily argued to match the trunculus, which grows algae over its shell and is virtually indistinguishable from the ocean bed where it resides.
- Its “formation” (briyato) is “similar” to that of a fish. This can be easily argued to match the trunculus, whose development is only similar to a fish, in that it is an aquatic living creature that reproduces via laying eggs, but is not actually a fish.
- The techelet dye is obtained from its “blood.” This can be easily argued to match the trunculus, since the Hebrew word dam can easily refer to the fluid contained in the gland of the trunculus.
- It emerges from the sea only once every seventy years. This can easily be understood to be a figure of speech, meaning that it rarely emerges. Additionally, it may mean that it only comes close to shore rarely, and is usually in deeper waters. All this matches the trunculus perfectly.
- Due to the previous factors, it is very expensive. Trunculus dye was indeed extremely expensive, due to the tiny amount of dye produced by each snail and the difficulty of obtaining them.
Particularly interestingly, we are told that a regular chilazon is not subject to the laws of trapping creatures on Shabbat (Talmud Yerushalmi cited by Tosafot to Shabbat 75a). Why would it have such an exception? The answer is that there is no prohibition of trapping a creature when it does not require any effort or skill. Such is the case with snails.
At the same time, although a terrestrial snail requires no effort or skill to catch, the Talmud says that the techelet-producing chilazon was caught with a net (Shabbat 74b). This is perfectly consistent with the trunculus – due to it generally inhabiting deeper waters, and its camouflage, the way they were caught was by baiting nets.
Finally, we are told that techelet made from the plant-based indigo is indistinguishable from that made from the chilazon (Bava Metzia 61b). Indeed, the dye produced from the trunculus is chemically identical to indigo and thus impossible to tell apart.
Now, as discussed, all these clues match the Hexaplex trunculus. Some match it very obviously so, whereas with others, it involves arguments that to my mind are straightforward but which some others dispute. But could it be claimed that perhaps there is some other, unknown creature which matches the clues even better and is the real chilazon?
Of course, such arguments strain credulity. Similar to the arguments against those who claim that the arnevet and shafan are unknown species of animal, it’s just not reasonable to reject known creatures that match the clues pretty well in favor of unknown and zoologically unlikely creatures for which, if they did exist, there would surely be some kind of evidence.
But there’s an even stronger argument.
The most important, basic halacha of techelet is that it is only kosher if it is made from the chilazon; if it is made from something else, it is invalid (Tosefta, Menachot 9:6). Although – and precisely because – indigo is identical to techelet, Chazal stressed that one does not fulfill the mitzvah by wearing indigo (Bava Metzia 61b).
Now, it is an indisputable fact that in the Biblical and Talmudic periods, there was a famous trade of producing an expensive dye, chemically identical to indigo, and with colors ranging from purple to blue, from the Hexaplex trunculus. There is endless evidence for this, and nobody at all disputes it.
And so the ultimate argument for techelet being the trunculus is as follows: Chazal clearly wanted to make sure that people were using real techelet and not indistinguishable indigo. Given that, and given that every culture was using dye from the trunculus which looked the same as indigo, how on earth would Chazal have spoken about it being a marine creature and not warned against using the trunculus?!
From Rationalist Judaism, here.