If a fish without scales, forbidden for consumption, swallowed a fish with scales and fins, permitted for consumption, the sages ruled that the swallowed fish is still permitted for consumption. The scholars asked: Because the early sages specifically stated that rule using the term ‘swallowed’ and did not merely say that a permitted fish which is found in the belly of a prohibited fish is permitted, it implies that if one did not see the swallowing the fish with scales and fins is forbidden, for we must assume that this fish is the spawn of the forbidden, scale-less fish and not its prey. The scholars lent support to this opinion from the words of the early sages, who taught a rule about the reproductive habits of fish: “A fish lacking scales will reproduce by live birth and a fish with scales and fins will expel eggs and from these eggs fish will hatch.” This implies that within the belly of a scale-less fish there are other fish, her offspring. The scholars asked: If so, if there is suspicion that the fish, even if it has fins and scales, is actually the offspring of the scale-less fish, even if we saw the forbidden fish swallow a permitted fish we should suspect that the fish which was swallowed was digested and the fish which we have found, complete with fins and scales, is actually the offspring of the forbidden fish. Answer: We permit a fish with fins and scales only when it is not found in the other fish’s womb or reproductive organs, as when it is found in the digestive system or the oral cavity. Another scholar, Rav Nachman, added that if the fish is whole it is reasonable to assume that it was the swallowed fish, for otherwise it would have already been birthed. One scholar, Rav Ashi, gave the opinion that even if we did not see the fish with fins and scales swallowed, yet we found it in the belly of a fish without scales, we should permit it for consumption. In his opinion most fish spawn fish like themselves, so we would not find a fish of one species in the belly of a different species of fish. This implies the fish was swallowed and it should be permitted for consumption. The sages also stated a rule about animal breeding and said that forbidden fish have live births while permitted fish expel eggs. Animals and birds which have live births are mammals and those which lay eggs are not mammals aside from the bat, which lays eggs yet is a mammal. A dolphin reproduces as does a human — according to Rashi that means that if a man mated with a dolphin she could become impregnated. Their child would be half man, half fish, what the sages called bnei yama [and Greek mythology called mermaids].
(Babylonian Talmud, Tractate Bechorot 7b-8a)