In the episode "TKO" of the science-fiction show Babylon 5, Lt. Commander Susan Ivanova, a Jew, has dinner with her rabbi and they have a discussion about the food that they are eating.
Rabbi Koslov: What did you say this was again?
Ivanova: It's Treel. It's a sort of fish. The Centauri raise them.
Rabbi Koslov: Treel? It's kosher?
Ivanova: That, I can't say.
Rabbi Koslov: I don't recall Treel being mentioned in the Torah, so… [Takes a bite] It's wonderful.
The creator of the show, J. Michael Straczynski, stated that one version of the script delved further into whether or not Treel would be kosher, but found that it just didn't make for very good television:
At one point, there was a discussion in the scene about the whole gills/scales/fins issue, to define kosher...but it really brought the scene to a screaming standstill, and we needed to concentrate on the relationships at that moment. In addition, as we looked at it, you would have to get into the question of how alien gills/scales/fins compare to earthly gills/scales/fins, because they're going to be very different in many ways. In short order it became a massive Talmudic discussion, and we only have an hour for the show....
While we don't ever see a live Treel on the show and the shortened dialogue leaves few clues as to their nature, it seems clear from his statement that he thought there were significant enough differences that might lead to debate. Unfortunately, Straczynski didn't state what the outcome of that discussion might have been.
So would an alien fish be kosher? What might be the outcome of this discussion if there were time to consider the sort of differences in gills/scales/fins between an alien and earthly fish?