Ah excellent, I read about the fish and donkey conundrum on an email list years ago and have hunted and asked well versed folks if they knew the reference but no luck. And hey, good luck doing a search for a specific talmud reference ;-0
It's a really fun thought experiment at the level of Schrödinger's poor cat, with interesting nontrivial moral and ethical paradoxes.
> Deut. 22:10: "You shall not plow with an ox and an ass together."
The question in the Talmud:
> Bava Kamma 55a: "The Sage Raḥava raises a dilemma: With regard to one who drives a wagon on the seashore with a goat and a shibbuta, a certain species of fish, together, pulled by the goat on land and the fish at sea, what is the halakha? Has he violated the prohibition against performing labor with diverse kinds, in the same way that one does when plowing with an ox and a donkey together, or not? The two sides of the question are as follows: Do we say that since the goat does not descend into the sea and the shibbuta does not ascend onto the land, they are not working together at all, and so he has not done anything forbidden? Or perhaps, since in any event, he is now driving the wagon with both of them, he thereby transgresses the prohibition?"
So it seems like the issue is not specifically related to the types of animals being used together. Rather, the question is: what constitutes "together"?
Does together mean "working on the same task at the same time"? If so, then the fish and the goat would apparently be working together and this would be prohibited.
Or does together imply something about the areas in which they are working? If so, perhaps the fish and the goat are not together and thus are allowed.
It's a really fun thought experiment at the level of Schrödinger's poor cat, with interesting nontrivial moral and ethical paradoxes.