crab nomenclature
I think the reason you can get live king crabs or kegani in Tokyo pretty easily is because it's feasible to truck them in from
Hokkaido. (Whereas Alaska is much further away from the rest of the U.S.)
I did see a king crab in a tank in Chinatown (New York) recently but otherwise I don't think I've seen live king crab (i.e. raw unfrozen king crab) in the U.S.
I think "matsuba gani" is one of several fancy "brand names" applied to snow crab (standard name: "zuwai gani").
"Echizen gani" is also snow crab, and the very small female snow crabs are sometimes sold as "seko/seiko gani". These females have something called "uchiko" inside (undevelped eggs or something?? <NSS>) with a very rich taste, fairly close to either sea urchin.
Unless you're an expert I don't think you can visually distinguish a permium snow crab from a mediocre-tasting one. I've had good snow crab that someone brought us from Quebec.
Though I have never had it, I've seen TV shows where freshwater crabs ("mokuzu gani" or "mozuku gani") are being caught and consumed locally. "mokuzu" refers to the tufts of algae on their claws.
The "Shanghai crab" is also a similar freshwater species that comes in from China August ~ September ish.
There are also the "sawagani" or creek crabs. You can catch these out in the suburbs, etc. I've not eaten these but I've seen them in restaurants and supermarkets deep-fried in oil ("kara-age" not tempura).
I have also wondered whether the "soft-shelled" concept could be extended to other crustaceans (i.e. soft-shell lobster, king crab, tiger shrimp, shako[mantis shrimp], etc.)
I saw Iron Chef Morimoto make "drunken shrimp" using spiny lobsters and throwing them in a whole keg of sake. That might be neat to try on a king crab as well.