The chirality of proteins in biology is something I either forgot or never learned. And that's an consideration of interstellar colonization that I never knew about. Trying to colonize another world with right-handed protein biology would be a bigger challenge, but possibly one solvable with chemistry if a sleeper ship was prepared for it. The upside of being surrounded by hostile alien microbes but being bioincompatible with them is a funny thought.

Something very conspicuously fish-shaped should be present in alien oceans.

Do you think it's likely that these would be vertebrate? I can't think of something that's evolved to that shape without a backbone.