Translated from Rabbi Avraham Rom’s sefer (and also hinted at here):
In Halacha, the identification of an object can be made through “simanim”. Simanim can be used to identify lost objects or a deceased husband. An object that has a סימן מובהק ביותר is considered a valid siman “min hatorah” (שלחן ערוך אבן העזר סימן י”ז סעיף כ”ד).
A סימן מובהק ביותר is defined as a feature which is found in less than one in a thousand of the general objects באר היטיב אבן העזר סימן י”ז ס”ק ע”ד.
To date, of all the thousands of fish and mollusks that were studied, no other fish has been found that can produce the techeiles color. Seeing that the ability to procure a techeiles dye from a given fish is an occurrence with a likelihood of one in many thousands, we can consider this property to be a סימן מובהק ביותר that identifies the Murex snail as the true chilazon.