Cause of Hermit Crab Deaths?

[Crabbie19]

Since you've come to the emergency forum, we know you want a fast answer to your question. In order for us to figure out the problem as quickly as possible, we ask that you answer the following questions as best you can. Some of them may seem odd, but they're all designed to give us the information we need to give you a good solution for your problem. The things in the [ brackets ] are there to make this post easier to read once submitted. Thanks!


1. What kind of substrate is used in your tank and how deep is it?
Mix of play sand and eco earth, in a roughly 3:1 ratio

2. Do you have gauges in the tank to measure temperature and humidity? If so, where are they located and what temperature and humidity do they usually read?
Yes. I have four temperature gauges. Three are located in the right, center, and left of my 50-gallon tank, and one is located in the center of my 29-gallon front-opening aquarium (reptile aquarium). Those two tanks are connected via a 10-gallon topper. I have two humidity gauges: one in the center of the 50-gallon aquarium and one in the center of the 29-gallon aquarium.

3. Is a heat source used in the tank? If so, what?
I have two heat mats adhered to the back of my 50-gallon tank, which take up the entire length of the tank.

4. What types of water are available (fresh or salt) and how is the water treated (what brands of dechlorinator or salt mix and what ratio is used to mix it)?
I have two fresh water bowls and two salt water bowls, of varying depth. I use distilled water, without dechlorinator, and instant ocean powder.

5. What kinds of food do you feed and how often is it replaced?
I feed a wide variety of foods. Many food sources are kept in the tank at all times: brown leaves, hay, corn on the cob, and calcium blocks. Other foods are provided in dishes and rotated on a roughly weekly basis. These include oyster shell powder, dried shrimp, and various mixtures of flower pedals, dried fruits, vegetables, seaweed, seeds. I also feed fresh food on a random basis.

6. How long have you had the crab and what species is it, if known?
I have 18 purple pincher hermit crabs. I've had 3 since July 2020, 3 since March 2021, 3 since July 2022, 1 since July 2023, 3 since April 2024, 5 since June 2024. Additionally, I've had 4 crabs die of post purchase stress over the past 5 years (all within the first several weeks after getting them), and I've also had 2 crabs very unexpectedly die in the past week (which this post is about).

7. Has your crab molted, and how long ago did it happen?
All except 3 of the new crabs that I got in June 2024 have molted, some numerous times (depending on how long I've had them).

8. What type of housing are the crabs kept in, what size is it and what kind of lid is on the housing?
I have a 50-gallon glass aquarium (36x18x19) connected to a 29-gallon glass front opening reptile terrarium (30x12x18) connected via a 10-gallon glass topper aquarium (20x10x12). Part of the 50-gallon tank is covered with a glass-lid, part is covered with the 10-gallon topper, and part is covered with a secured wooden board. The 29-gallon tank is covered with a different wooden board. The gaps are sealed with tinfoil to allow for humidity and temperature control.

9. How many crabs are in the tank and about how large are they?
At the beginning of last week (before the deaths), I had 20 crabs: 3 tiny (dime-sized), 2 small, 4 small-medium (golf ball sized), 5 medium, 3 medium-large (tangerine sized), and 2 large crabs. The two who died were small-medium and medium sized.

10. How many extra shells are usually kept in the tank, if any?
I probably have 100 extra shells in the tank. There are 6 different shell bowls, all with shells of varying sizes.

11. Have there been any fumes or chemicals near the crabitat recently?
No.

12. How often do you clean the tank and how?
I do spot checks daily when I turn the lights on/off, change food items roughly weekly and water bowls monthly (they have filters). I watch carefully for mold growth as I had problems with that several years ago.

13. Are sponges used in the water dish? If so, how are they cleaned?
I do not use sponges.

14. Has anything new been added to your crabitat recently?
I altered the layout of the tank after I found my first crab dead last week. Originally, I had the 50-gallon tank with two toppers, the 10-gallon and the 29-gallon. However, I did not see as much usage of the toppers as I had hoped and I had the first ever surface molt in my tank a few weeks ago (I isolated him), so I decided to move the 29-gallon tank to the ground to allow for more molting space.

15. Is there any other information you would like to share that might be helpful (anything that is regularly part of your crab care, playtime, bathing, etc.)?
I try not to touch my crabs as much as possible, so as not to stress them out.

16. Please describe the emergency situation in detail.
A few weeks ago, I noticed that the crab I have had the longest (since March 2019) was acting lethargic and hanging around the water bowls more than usual. Additionally, I noticed drag marks in the sand from his shell and that his skin color was very vibrant. I kept close watch of him, but assumed he was preparing for a molt. However, approximately a week and a half later, I found him at the bottom of the freshwater bowl, dead. He had switched into a smaller shell that day, but he had fallen out of the shell after he died in the water bowl. I removed him from the water when I found him and placed him in an isolation tank on the off chance that I'd gotten to him in time, but after a few days it was clear he had passed. All I could think of as a possible cause of death was overcrowding. I had introduced more crabs than ever before (9) into the tank in a short period of time (3 months), so I moved the 29-gallon tank to provide additional molting space. I don't believe that he could have drowned because there were a sufficient amount of leaves and sticks to get out, but I bought two additional smaller water pools just in case (placed in the 29-gallon tank). Now, tonight, I have just found another one of my crabs (I've had since April 2024) in the bottom of the same water bowl. I had removed the leaves and changed the filter and water since the previous crab died, so the only constant was the piece of chola wood. It is dark brown on the bottom, and I was wondering if it is possible it has grown some form of infectious mold? Also, four days ago I found a third crab without a shell in the saltwater bowl for a prolonged period of time. The crab seemed otherwise healthy, returned to its shell following the incident, and then subsequently switched shells. So, I wasn't sure if this may simply be a case of an uncomfortable shell, or if I have a more malignant problem in my tank? I plan to buy completely new water bowls and entrance materials for the 50-gallon tank, as well as dechlorinator because I have read that there can still be contaminants in distilled water. I switched brands of distilled water within the past couple months, so maybe that's it? Does anyone have any advice or insight? I am worried about the rest of my crabs.

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[Altaccforhermitsssss]

Overcrowding may be a problem, it could have stressed him out but i was thinking he got attacked because of it? Mabye not if he was not showing signs of being hurt but it mat have just been old age or something as a natural death, nothing else sticks out to me, but hope the rest of your crabs are doing ok.

[I<3Crabo's]

I would say over crowding too but if you have your 29 not as a topper and connect the two with just the 10 and if you have enough substrate in the 29 and 50 (I'd say 8 plus inches cause you have big crabs) then you should be fine for now.

[wodesorel]

Ditching shells for the water bowl tends to be a death behaviour. I've always assumed it has something to do with comfort, or because of electrolyte or osmosis imbalance due to organs shutting down. It's not necessarily a sign of any one specific thing but is a sign of general severe illness.

The new crabs could have potentially brought something in, however studies on wild crabs don't really note contagions or harmful parasites. I'd also expect something to have happened sooner and the newest crabs to have shown major signs of problems as they'd be the weakest of the group. Your oldest should have been among the healthiest.

Distilled water is lacking minerals naturally found in surface water. It's not recommended for hermits, and treated tap water is honestly a better option as it'll supply more trace elements in their diet. (Distilled has been found to cause major health issues in other pets like reptiles and dogs because it can leach minerals from their bodies instead.)

Bacterial contamination is always my first worry with unexplained deaths. The funky cholla could be growing something unpleasant but it is used in shrimp culture so as long as the water quality is being measured (ammonia, nitrites and nitrates) and the water is being replaced or filtered then the risk should be super low. When I've had groupings of otherwise unexplained deaths in the past, I would change out my substrate and it would stop the deaths immediately. Could have been totally unrelated but if I suspect something wrong that's my go to (pain in the butt I know) first step now.


Commercial calcium blocks can have unwanted preservatives, something to check. Are they getting more sources of protein than just shrimp? I doubt it's an issue just from diet alone but never hurts to make sure that's well rounded.