For many years a cooler was in operation when keeping Pagurus bernhardus. This certainly helps, and the temperature was kept to a maximum of 22° C but because the small ones can be kept, two reasons for the failure could be guessed at:
1) Inadequate nutrition
2) Old age.
The same problems occurred with both Diogenes pugilator and Galathea squamifera. I suspect that all these species eat live plankton as well as organic material.
If the cooled aquaria only were included, the average life span may be increased to 7 months, perhaps. Predation upon has not always been excluded. Specimens with the anemone Calliactis parasitica were interesting, with anemone not living more than 3 months after the Hermit Crab had died.
The other species of Hermit Crab I have kept is Pagurus prideauxi, which again lived for between 2 and 6 months, but I think with these I concentrated on the water quality at the expense of food.
I think they may survive best in dirty tanks with changes of seawater say 50% a week, which used to be practical, but it is no longer (I have moved).
These forums are very slow to start up, and there are more little things to think about then I imagined. The EForum was not really planned. Smart Groups approached me.