When adding newly purchased or acquired crabs to an existing colony, for the long-term health of all, we at CSJ recommend the use of an isolation tank. Placing newly purchased or adopted hermit crabs into an existing healthy colony without a quarantine period risks the unnecessary exposure of your healthy hermits to shell disease or parasites.
If you are just starting out with hermit crabs and do not have an existing colony or crabitat in place, use of the PPS Reduction Method will allow you to monitor your new crabs for 30 days and bring them slowly up to ideal environmental conditions.
The PPS Reduction Method should be used with all newly purchased crabs unless you have visually confirmed that they are from a setup with sufficient heat and humidity. This is a 30-day process, allowing sufficient time to identify signs of illness or parasites, and reduces the number of hermit crab deaths due to PPS, most of which occur within the first month but can occur up to eighteen months post-purchase. The first molt for a hermit crab in your care is the biggest hurdle in overcoming Post-Purchase Syndrome.
For adoptive hermits coming from an ideal environment, quarantine conditions should be set at recommended heat and humidity levels for an established crabitat. However we still recommend the use of shallow substrate to allow for easy visual inspection. If you prefer to allow them to dig down, plan to quarantine for 60 days instead.
If the hermit crabs are coming from an unknown environment, the PPS Reduction Method provides the needed quarantine time not only for combating PPS but also for shell disease or parasites.
Shell Disease Syndrome includes a number of exoskeleton-related bacterial infections that are easily transmitted to healthy animals. Most infections are external-only and do not affect underlying tissue unless there is cross-exposure and re-contamination while molting. (There is, however, one variant that causes the exoskeleton to fuse with underlying tissue, preventing a successful shed and resulting in death or deformity.) Crustacean shell disease is highly contagious and spreads rapidly in an artificial environment. Wild-harvested hermit crabs are warehoused in dirty, overcrowded conditions and then shipped to pet stores. One crab could expose many other crabs, particularly those made vulnerable from physical stress or injury. New crabs should always be inspected for damaged exoskeletons both at the time of purchase and prior to adding to an existing colony. When purchasing a hermit crab with visible damage, said crab should be kept in quarantine until successive molts have repaired all signs of damage or disease. At the time of writing this article, it is unclear if the bacteria that causes shell disease can survive in the substrate, so it may prove beneficial to give the hermit crab a gentle bath in ocean water before placing in the main crabitat. As a rule we do not recommend forced bathing but in this instance you do not want to risk carrying contaminated substrate into the main crabitat, thereby placing your entire colony at risk.
Mites or other parasites can hide in the shell of newly purchased hermit crabs. This is especially true of hermit crabs purchased from filthy conditions where dead hermits are present. Before selecting new hermit crabs, check them over carefully for mites or other bugs, both on the body and the shell—exterior and interior. Red mites attached to the leg joints or gills are parasitic and harmful. In rare cases there may be fly larvae inside the shell where you are unable to see it. If the store habitat is especially dirty there is a higher likelihood of phorid flies and other fly larvae being present. You may not be able to easily see mites or parasites so visual inspection does not mean it is safe to place them in the main crabitat with your healthy colony. Quarantine for 30 days and check daily for signs of mites.
If shell disease appears in an existing colony, the scenario is more complicated due to the possible contamination of substrate coupled with molters still underground. Infected crabs should be moved immediately to a quarantine tank with minimal substrate until they have molted and show no signs of infection. This may precipitate a surface molt, but that is preferable to re-infection from digging into contaminated substrate. For the main crabitat, consider replacing the substrate and sanitizing items in the tank once all molters are above ground. This may be exercising an abundance of caution, but I do believe that an episode of contaminated substrate (ten years ago) is what killed my colony of Coenobita compressus (as well as some of the other species), halting their molts and trapping them in their exoskeletons.
We do have a recipe for a hermit crab medicinal bath that may be beneficial in treating shell disease.
Note: If you suspect shell disease, attempt to wipe the spot off. Certain foods (such as Spirulina powder) may cling to or stain the exo skeleton and appear to be shell rot. A damp q-tip should be enough to tell if it is staining or infection/disease.