Post Purchase Syndrome (also known as PPS) is a catch-all term used to describe the many reasons why newly purchased hermit crabs will often suffer from health problems and even death. All hermit crabs are wild caught, meaning that they undertook a huge journey from the beach or forest where they lived at the equator to finally end up in your tank. During this journey they would have passed through many hands and many locations, which means there were many opportunities for something to happen to them. Even if the store you purchased them at had ideal conditions, the damage may already have been done before the store ever received them.
Symptoms include leg loss, lethargy, not eating, not responding, slowing down, sitting in one location for days at a time. Sadly, even perfectly normal crabs will sometimes be found dead with no warning. Hermit crabs have a way of hanging onto life for weeks after a life-threatening occurrence. The damage that caused the death could have happened weeks or months ago.
The threat of PPS is over once the hermit crab molts successfully for the first time.
What causes it?
We don't know much about hermit crab diseases and parasites, but it's likely that they do suffer from both. Because of the stress of their collection and journey, and the fact that they were exposed to hundreds (if not thousands) of other crabs from different locations and who were of different species, they could have been exposed to something they had no natural immunity to. Both diseases and parasites can take over a stressed immune system, leaving the crab unable to fight off the infection.
They could also have been seriously injured during their journey, with no way for us to know if they are hurt until it's too late. (Aside from broken limbs, which most hermit crabs recover from completely.) Some hermit crabs are able to molt and repair any major damage they sustained, while others end up too badly hurt or just do not have the strength to heal.
Another issue could be dehydration or starvation from not being able to obtain enough food or water during their time in captivity. With so many crabs being packed so tightly together, the competition for resources is high, and many pet stores don't offer proper food or water for them to replenish themselves. These types of deprivations can cause permanent damage, and a weak crab may not be able to eat or drink once it gets home.
Temperature and humidity are important for them to stay healthy. If they had been kept too cold or too dry for too long it could have caused permanent damage. Low humidity especially can severely injure their gills. Often times there's no way to find out what the conditions were or how long they had been kept that way before they got to the store where you purchased them. If crabs were shipped during winter, they may have frozen during transport.
Hermit crabs need to be able to molt. If they put it off for too long a certain hormone builds up in their system, and if it builds up over time for too long the hermit crab can die from it. Since we have no way of knowing how long it had been since their last molt, this could also be a cause of PPS.
And finally, captivity is probably very hard to adjust to. Some hermit crabs may not be able to make the transition from the open world they came from to the tiny glass tank that we keep them in. There have been cases of hermit crabs who seemed to sink into a depression and simply gave up trying.
There are no veterinarians for hermit crabs. There also isn't much research into their health. We can only guess for now what the actual causes are, but all of the above are very likely scenarios and would explain why so many new hermit crabs pass away even when given perfect care. The HCA is always on the look out for new research that could explain why PPS occurs and how we can treat it successfully, and we will update as we learn more.
So what can you do to possibly prevent PPS, or treat the crab if it is showing signs?
Keep the new crab as stress free as possible. Do not handle him or her until after they molt for the first time. Try to keep the room where they are kept quiet, and stick to a normal day/night lighting cycle.
Make sure to offer as much healthy food as possible so they can gain the energy they need to heal. Great foods for this include:
Fresh Apples and Pears
Mineral Supplements or Cuttlebone
Using dechlorinated water is essential. Offering saltwater along with fresh is important as well. (Saltwater should be made using a marine water mix - please see the Water FAQ for more information.)
The new crab will also need to be able to bury in order to destress or even molt right away. Make sure that the substrate is moist and deep enough for the crab to be comfortable. If not using the main tank for this, then a smaller ISO should be set up with substrate that is at least 2 to 3 times the height of the crab. (Please see the Isolation Tank FAQ and the Molting FAQ for more information.)
The debate about Environmental Stress
There are two differing opinions on how to maintain their environment while a hermit crab is new. One method is to adjust their conditions slowly from what they were in at the store to the conditions in your normal tank so they can adjust gradually. The other method is to get them into ideal conditions immediately so that they don't have to stay in sub-par conditions which could cause further damage to their gills.
Neither approach has been proven better than the other, the choice of which to use has to be left up to each crab owner to decide which they are more comfortable with. The adjustment methods is provided below for those that wish to use it.
Please note - both sides agree that once a hermit crab has been put into good conditions, the adjustment method should not then be used. It's too much stress and shock for the crab to bounce around that much with heat and humidity. Never go backwards (lower) with conditions - only move the humidity and heat higher.
In order to prevent environmental stress, we need to start them off in an ISO tank near the conditions of the pet store tank. Temperature isn’t as important as humidity for this, so keep them in an ISO at around 72-75oF (assuming the pet store was within +/- 5oF of that), and start the humidity at around 55% relative. Each week for four weeks, increase the humidity by 5%. The four-week schedule is 55%, 60%, 65%, and 70%. Your main tank should be around 75-80% and 75-85oF, so that will be the last step-up.