Rashi has an alternative reading in the Gemara: Dolphins are fruitful and multiply by coupling with human beings.According to this reading, the Gemara is talking about mermaids, or sirens – half human, half fish – that engage in sexual relations with people. In ancient literature and until modern times we find many references to such creatures.
Our Rabbis taught: An unclean fish breeds, but a clean fish lays eggs. All animals that give birth, also nurse. And all that lay eggs, supports its brood by picking up food for it, except the bat, for although it lays eggs, it nurses its young. Dolphins are fruitful and multiply by coupling like human beings. What are dolphins? — Said Rav Yehudah: Humans of the sea.
In many non-kosher fish, such as Cartilaginous fish, a fertilized egg develops in the body of the mother and offspring are born as live fish. Kosher fish, on the other hand, lay eggs that develop and hatch after time.
Generally speaking, the statement that animals that give birth nurse and animals that lay eggs gather food for their young is fairly accurate. There are very few exceptions – monotremes indigenous to Australia, like the platypus – that lay eggs but nurse their young. According to current scientific knowledge, bats give birth to live offspring and nurse them.
Some commentaries identify the dolphins mentioned in the Gemara with the sea creature that we know as a dolphin. Similarities between dolphins and humans include extensive shows of affection prior to mating, and that sex takes place with the male and female facing each other. Beyond these issues, it is well-known that dolphins are mammals that nurse their young and assist one-another, as do people; they have been known to help people who are in distress in the water.