HCA: Hermit Crab Association

How often do crabs switch shells with each other in this manner? Crab A goes to crab B. B leaves the Shell. A goes into B's former shell. B goes in A's shell.
I'm asking because this morning i saw either a shell fight or something like that happening. I got several straws yesterday, and one is especially timid. He was hiding in the back of the tank when an older crab came up to him. When i saw them, the new one had left his shell and the old one was checking it out. I separated them easily enough, and within a minute or two the straw was back in his old shell. I put him in a personal iso with food, both kinds of water, and several shells while i was at work, but he opted not to change shells. They're getting along for now, but should i keep a close eye on the two of them? I did dip the new ones in salt water

Edit: Just to be a little safer overnight, i took all the shells from the second floor shell shop and spread them out in case a straw gets bullied. If they get bullied like that, do crabs just go into the nearest shell, then work on finding the right one?

Bigger crabs might demand shells of less dominant crabs... in the wild shell rapping & some shoving might lead to an exchange between crabs, as part of a non-lethal dominace (and shell) transaction.

I know salt water hermits (pagurids) have well studied shell swapping & dominance behaviour. Interestingly, there are intertidal hermit species where larger shells can be often found on smaller crabs... these small crabs have been hypothesized to rely in part on bigger crabs trading down shells with them to get the most appropriate fit.

But... I worry more about this because it's Straws & I know they can be more delicate. I'll defer to anyone more experienced with straws... I feel for you, 'cause I'd be nervous too if I thought a new straw might end up naked!

P.s. how many straws per shell... an how big is the tank?

I have essentially a 40 gallon tank as it's a regular 20 with a 20 terrarium topper. There's 1 small pp, 3 medium pps, 3 medium straws, and a medium-large straw. There are at least 15 shells the size of the larger ones, which make it that, even though the tank is based on two 20 longs, little sub shows through the shell layer. The crab he's tormenting is in a shell that's smaller than several of the larger shells, and there are several about the same size

IMO, having 9 crabs in what essentially a glorified 20g tank is going to have issues in the future, either when mass moulting event or when they grow up. Particularly with exotics, they really do appreciate larger tanks and space in comparison to PPs. Also, each cran species usually go after a certain kind of shells, eg. Straws and PPs prefer o shaped shells like turbos... and rarely go after D shaped shells like Babylonians. So increasing the amount of turbo shells for example, would be ideal in reducing the chance of 'shell less crabs'

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That's what perplexes me: all the available shells are various kinds of turbo, other than the magpies two are wearing and the unknown kind the small came in

Edit: it appears Courage (new guy) went shell shopping and found something. Bro moved into his shell, but may be moving back. At least now i can rest more easily tonight! I hope i never go through anything like a shell fight again!

Edit 2: Bro went back to his old shell when i wasn't looking, so all this stress for nothing! Hopefully Courage chose the new shell because he wanted one rather than because he was being bullied, but at least he was able to find something and the stress seems like it's over for now