וְכֹ֨ל אֲשֶׁ֧ר אֵֽין־ל֛וֹ סְנַפִּ֥יר וְקַשְׂקֶ֖שֶׂת לֹ֣א תֹאכֵ֑לוּ טָמֵ֥א ה֖וּא לָכֶֽם׃ (ס)
and whatsoever hath not fins and scales ye shall not eat; it is unclean unto you.
Read kabbalistic (mystical) interpretation of Deuteronomy 14:10: and classical mystical sources.