“Chilazon” – The aquatic or semi-aquatic creature

In the Talmud, in Menachos 44a, we find the following description: “The ‘Chilazon’s’ body has the color of the sea, and its form is that of a fish. It appears only once in seventy years (In Masechet Tzitzit, the probable source of this information, ‘seventy’ is replaced by ‘seven’), and ‘Techeilet’ is made from its blood; therefore it is very expensive.” Bava Metzia 61b says that the color of the dye made from the “Chilazon” was identical to the color “indigo.” And Menachot 43b says that “Techeilet” was a permanent dye.

https://www.ou.org/judaism-101/glossary/chilazon/