For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
It’s one of those things that feels really obvious if you cook a lot of east/south Asian dishes - shrimp sauce and mushroom soy sauce have a pretty similar aftersmell to them because they’re so concentrated
I’ll be honest, I’m not much into cooking Asian. I’m also not a frequent crustacean eater, but I eat mushrooms regularlish. I’ll pay more attention from now on, but I would have never otherwise thought of making a link between the two