Coenobita brevimanus is a common terrestrial hermit crab on Marshall Islands atolls. Larger ones tend to occupy the common Turbo setosus and T. argyrostomus shells that measure up to about 80mm long.
The large claws of these hermits are strong and they tend to not let go when they grab onto something. I once made the mistake of holding one in the palm of my hand. It grabbed the flesh below my middle finger and would not let go for some time. Not wanting to rip off the arm of the crab, I endured the painful pinch until he finally released his grip.
The pincher!
The factor limiting the population of these and most other hermit crabs is the availabillity of suitable empty shells to occupy. Sometimes they will take substitutes. The one immediately below found a broken seed pod that will do for a while.
One of the sadder shell substitutes was this rusty nut.
Seeing what some of these poor crabs had to accept as living quarters prompted us to set up a shell exchange program in our backyard. We would often find large empty Turbo shells while diving, particularly around dens of octopus who had pulled out and eaten the original snail inhabitants. The shells were dried and set in shallow tubs, where they would be quickly taken by neighborhood crabs. Sometimes we marked the different batches with spray paint to give us an idea of how much the crabs move around and how often they swap. Swapping would start almost immediately, with larger crabs quickly taking the larger shells and leaving behind their smaller ones, like those in the shallow dish on the left. Smaller crabs would than upgrade their shells with the castoffs from the previous group. Within days, we would be left with only very small shells or ones that did not pass muster, perhaps because of holes in the shell that made them unacceptable to the crabs.
Shell swapping in progress. Smaller shells have been left behind by those who took some of the new offerings.
Created 10 August 2018
Updated 8 August 2021