The Madanei Yom Tov*1 (Chulin 66b Rosh # 67 sk 5 ) writes, when he was Rov in Vienna,
R.Ahron Rofeh (Doctor) showed him a poisonous fish, caught off the Spanish Sea, called
“ Shtinkus Marinus”*2. The fish has scales but no fins, instead of fins it has legs.
Chazal say (Mishna Niddah 51b) all fish with scales have fins. R.Ahron R. Asked “ Do
we consider the legs as fins and therefore it is Kosher? If it is kosher, then the question is why would Hashem create a Kosher poisonous fish? If on the other hand the legs are not considered fins, then how do you reconcile Chazal that all fish with scales must have fins.
*1 Also known as the Tosfos Yom Tov (R. Yom Tov Lipman Heller Wallerstein)
*2 In the Torah it says כל אשר במים seems to indicate all sea animals with fins & scales are kosher.