Avel comes from a tribe who determines their gender at their first sexual experience. He'd always expected to become the chief, but after being saved by a member of the shark-descended rival tribe, Tukiri, the two are unexpectedly swept away by heat-like symptoms and mate. Now Avel has no choice but to be an offering to the rival tribe, but when he arrives, it's nothing like he thought. I liked this story more than I thought I would, but the mystery they build up around why the tribes do things the way they do has a disappointing conclusion, imo. Trigger warnings: dubcon(heat), voyuerism.
Samezoku e no sasagemono