We used to love crabs: crabs with butter, crabcakes, Krabby Patties, Sad Crab… but now that we've read Nausea, crabs just seem existentially confusing.
At several points in this book, Antoine compares either himself or part of himself to a crab. Like a crab, Antoine tries to protect himself from the outside world with a "shell" of distance, never really letting anyone get close to him. But at the same time, he realizes that being a crab also makes him unwanted by the rest of society, not quite human, as he suggests in the passage that says:
If I grabbed one of them by the back of the coat, if I told him: "Come, help me," he'd think, "What's this crab doing here?" and would run off, leaving his coat in my hands. (24.292)
Apart from being a social outcast, the crab represents for Antoine something that spends its life crawling around on the ocean floor. Like the crab, Antoine feels like he lives a bug-like existence, crawling around and scavenging for whatever mental comforts he can find.
One day, while staring at this open hand on a table, he thinks:
I see my hand spread out on the table […] It is lying on its back […] I amuse myself by moving them very rapidly, like the claws of a crab which has fallen on its back. (22.32)
In this case, the image of the crab on its back shows that even thought Antoine tries to put a shell around himself, he also feels very vulnerable at times.
And we don't even really have to mention that ol' Antoine has a tendency to be very crabby.