Hi and welcome to the forum
Does the hole stick up above the crab's shell?
If yes, it might be a barnacle.
Maybe post a few more pictures from different angles. It's interesting and weird
If the crab has been in freshwater for a while it shouldn't be a barnacle. As to what the hole is, no idea. Octopus will drill a hole into a crab shell and inject venom to kill the crab, but again, only if the crab is in salt water.It is raised around the edges and very hard BUT the hole looks likes it's INTO the shell... If it is a barnacle how do it get it off? I searched it and it says they don't live in freshwater... so idk.
Colin, how do you know all this obscure stuff ? That bit about the octopus drilling a hole and injecting poison weirded me out. Fish are deadly. Jeffrey Dahmer and Pinhead would blush.If the crab has been in freshwater for a while it shouldn't be a barnacle. As to what the hole is, no idea. Octopus will drill a hole into a crab shell and inject venom to kill the crab, but again, only if the crab is in salt water.
Assuming the hole is not caused by a fungus or bacterial infection, it should go when the crab moults its shell. However, it could die of an infection before that happens.
I would contact the company you got it from and ask them to replace the crab with a healthy one. Email them the pictures showing the hole and ask if they know what it is.