Mussels are a prime source of B12 and manganese (stress, bones and metabolism). Crab is wayyy higher in b12, but it’s heavy in selenium and copper, both toxic in large amounts (neurologic/psychologic). In a starvation situation, I’d definitely be hunting mussels over crab, well, if I knew then what I know now. Maybe they knew, too?
To be clear: mollusks (clams, oysters, mussels) were considered food ONLY when faced with starvation by native peoples. They had no idea of the nutritional value we place on them. They had their own ideas what was fit to eat or not.
“By large amounts” means pounds of selenium and copper - not eating a crab or two. You’d have to eat selenium and copper by the handful - the rest is just new age hype designed to create vegetarians.
Don’t fall for the FUD.