I think there's only one documented instance of horizontal gene transfer in fish and it increased fitness dramatically - https://www.quantamagazine.org/dna-jump

I think there's only one documented instance of horizontal gene transfer in fish and it increased fitness dramatically - https://www.quantamagazine.org/dna-jumps-between-animal-species-no-one-knows-how-often-20210609/ https://www.quantamagazine.org/dna-jumps-between-animal-spec...