K. Description of the Tekhelet-Hillazon in Maimonides’ Code
Maimonides’ description of the hillazon is noteworthy in itself and also because of its association with a movement for the restoration of Tekhelet-shel–sisit after centuries of desuetude.
“It is a species of fish the colour of which resembles that of Tekhelet and its blood is black like ink and it is found in the salt-sea” (i.e., not in sweet water). By the “blood which is like ink,” Maimonides doubtless understands the dye-secretion of the species. Maimonides, it will have been observed, makes no mention of the septuagenarian or septennial “appearance” or “coming up” of the hillazon. His severe critic R. Abraham b. David whose wont it is to challenge the slightest deviation of Maimonides from the Talmud here maintains complete silence. Again the indication concerning the inklike blackness of the “blood” is not traceable, so far as I know, to any traditional source. Here too R. Abraham b. David’s silence is significant. It would seem that in scientific matters Maimonides’ critic fully recognised his (Maimonides’) superiority.