Kind of like an omegaverse, but only because the uke is from a clan where they can become female depending on some conditions (like winning or losing a fight at their ceremony of coming of age). Whereas the seme is from another clan where they are beastmen descendants from a human and a shark's child. And the ones with the most (powerful?) shark blood can even transform their lower body into shark mode.
Reminds me of Umi no Soko of course.
Samezoku e no sasagemono