The Torah teaches that in order for a fish to be kosher it must have fins (senappir) and scales (kaskeset – see Vayikra 11:9-12). This rule is stated both as a positive commandment (verse 9) and a negative commandment (verse 12). Senapirim – fins – are bony protrusions that extend from the stomach of a fish on its side that serve as paddles for swimming. The other fins – on the top of the fish and on its tail – do not move. Kaskasim – scales – are flat protrusions on the fish’s skin that overlap one another like shingles. There are different types of scales that are unique to a given type of fish based on their shape, how they are connected to the body of the fish and so forth. Some fish – including certain types of tuna – lose their scales as they age, but remain kosher fish.
The Gemara on today’s daf quotes a Mishna from Massekhet Nidda (daf 51b) that teaches that all fish that have scales also have fins, but a fish might have fins, yet not have scales. Thus, if it has scales it will also have fins and it is a kosher fish; if it has fins without scales the fish is not kosher. According to Tosafot, this rule is a tradition handed down from the first man, Adam, who examined each and every living creature when he named them (see Bereishit 2:20) – or, perhaps, this is a tradition handed down from Moshe on Mount Sinai.
At various times in history, creatures with scales but no fins were brought before the Rabbinic leadership to determine their status, given that the animal appeared to negate the principle taught in this Mishna. In his Ma’adanei Yom Tov, Rav Yom Tov Lippman Heller argued that this rule applied only to fish and not to other sea creatures. Rav Yonatan Eibeshutz suggests that when the Mishna says that there are no animals with scales but no fins it simply means that the vast majority of fish with scales have fins, as well. In the whole of nature we are bound to find exceptions to every rule and the principle that was taught is referring to the majority of cases.