Next to a tree, you find a crab playing Japanese chess (aka shogi). Hey, why not? In New York people play chess in the park, and in Japan crabs play shogi next to a tree.

Sometimes this also means go, which is a different strategic board game from China. Kind of confusing, but just think "strategic East Asian board game" and you're in the right territory for this kanji.

If you can beat the tree crab at a game of Japanese chess, it'll give you key (き). What kind of key? The key to whatever treasure you desire. Pretend you're in a crab- and shogi-based JRPG, and this crab is the final boss. A magic key is a fitting reward, right?