SilverEar writes "Imagine a creature that swims and preys on others. But once it eats a certain kind of plant, that plant grows inside it, the predator loses its ability to prey and starts using sunlight to make its food. Its preying mouth is replaced by an eye that is needed to find sunlight. This is the Hatena "enigma" in Japanese. The kicker: when Hatena reproduces, one offspring is a peaceful photosynthesizer with the sun-seeking eye, while the other is yet again a predator with the voracious mouth. Well, at least until it eats the "magic algae". Read about it in Byte Size Biology."