They are similar to the Snails of Spelunky HD in terms of behavior, but with a few differences.
Hermit Crabs make their "home" hiding under Chests, Crates, Push Blocks, and even Powder Boxes. When not close enough to them while hiding in an object, they are indistinguishable from the normal version. Getting too close to a Hermit Crab in this state will cause him to pop out and blow poisonous bubbles which do no damage but will poison the player if they are not already. These bubbles float upwards infinitely until destroyed from touching any tile or receiving damage of any kind.
If hurt, they will be kicked out of their shell, and if not in a Push Block or a Powder Box, they may be stomped out of it as well. When out of their shell, they will move back and forth, regularly blowing bubbles regardless of proximity to the player.
They are able to climb on the climbable dragon poles in the Tide Pool, occasionally stopping and blowing poison bubbles.
They shoot their bubbles to the side before they rise upwards, making them always vulnerable from above. They sometimes spawn in tight corridors, with no way to get above them. In this case, it is best to whip them when they come out to attack, but before they start spitting bubbles.
When a Hermit Crab is hiding in a Powder Box, players should dispatch of them using thrown items or other ranged options. Note that the Hermit Crab is likely to be flung towards the player after the explosion. If their corpse hits the player, it will ignite and detonate any flammable pack items that the player may be wearing, such as the Jetpack.