[bisonbison]

Can hermit crabs feel pain?

2 things define pain - the intensity of it and the memory of it.

Hermit craps experience intensity, but have little structural capacity for suffering. Their tools for learning and remembering are so crude that a crab that's been kept alive and treated terribly will only strangely resemble a mammal that's been traumatized.

While do they like crawling all over each other?

Because the presence of others of their species indicates that it is not a predator-prey situation, and with that fear drive or food drive suppressed, the sensing creature's drive for sensation and information takes over.