Debate Responses
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Tosafos (ד”ה הצד ) says that upon removing the chilazon from water, “שמא חלזון דרכו לפרכס ולקרב מיתתו ” – perhaps the chilazon tends to quiver and hasten its death. In other words, perhaps it does, but perhaps it doesn’t.
[The following Tosafos (ד”ה ולחייב ) says: והכא נמי במה שממיתו יש תקון קצת שאינו מפרכס לכאן ולכאן ונוח ליטול ממנו הדם , — that killing the chilazon [by piercing it to remove the dye] is a תיקון קצת , in that it will not quiver and thus inconvenience the (further) removal of dye. While we don’t know to what extent murex would be מפרכס if the פציעה wasn’t fatal, it certainly moves while alive (as evident in the techeiles documentary) and hence the תיקון קצת . Importantly, תוספות הרא”ש here switches the word מפרכ ס to מתנדנד (a weaker term relating to movement) even though the Rosh does retain the term מפרכ ס in context of the previous Tosafos regarding the possibility that the chilazon hastens its own death (where the Rosh indeed maintains that this possibility is merely “אפשר ”).]
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Warning: the following answer is non-Yeshivish.
The murex trunculus sea snail wasn’t available in the regions that the Rishonim writing about its’ characteristics lived. Furthermore, the first significant public zoo in the modern sense was the Menagerie du Jardin des Plantes in Paris, France, which opened as a public zoo in 1793. Before that, without access to exotic animals from distant countries which we take for granted, the only visual access the public had to these creatures were in special manuscripts called bestiaries. Often, the exotic creatures weren’t accurately drawn and the artists drew them with a religious Christian agenda. The above said, that would have been the only visual access to these creatures everyone, including our Rishonim, would have had. See images.
I was tipped off on this from the fact that elsewhere, Tosafos writes that elephants have the ability to jump. Factually speaking, that’s impossible, since the bones are so brittle due to its large size that they would crush on impact. We tend to forget that elephants were never indigenous to Europe: only in Africa and parts of Asia. The only visual reference to it would have been in a bestiary where a distorted drawing was to be found. And from what’s been found, the drawings depict an elephant with hind legs capable of jumping.
