I would guess it's just the audio out of a Dot feeding the audio signal to the fish?
In the early days of Skype, a friend gave me a little creature with a speaker whose mouth would flap and arms would twist as the person on the other end of the skype call spoke. Very irritating. Had a driver on disk for PC and USB connection to PC. It was made in England. Leave it to the Brits to come up with the goofy gadget.