C. F. Keil and Franz Delitzsch on Leviticus 11:9-12

Lev 11:9-12 (cf. Deu 14:9 and Deu 14:10). Of water animals, everything in the water, in seas and brooks, that had fins and scales was edible. Everything else that swarmed in the water was to be an abomination, its flesh was not to be eaten, and its carrion was to be avoided with abhorrence. Consequently, not only were all water animals other than fishes, such as crabs, salamanders, etc., forbidden as unclean; but also fishes without scales, such as eels for example. Numa laid down this law for the Romans: ut pisces qui sqamosi non essent ni pollicerent (sacrificed): Plin. h. n. 32, c. 2, s. 10. In Egypt fishes without scales are still regarded as unwholesome (Lane, Manners and Customs).