Tag Archive for humidity

Coenobita respiration


A hermit crab’s gills are enclosed in the branchial chamber, which functions as a lung. The branchial chamber is on the sides of the thorax, above the crab’s legs. A hermit crab breathes through its gills and branchial chamber, which must be kept moist in order to function. If the branchial chamber and gills dry out, the crab will die. Compared to aquatic crabs, land hermit crab’s gills are reduced in size, and if the adults are kept underwater too long, they will drown. [2]

There are tufts of setae at various sites on the ventral surface that enable moisture from the substrate to be passed to the branchial chamber. [1]

Maintaining the correct relative humidity inside the crabitat is crucial to survival. Without sufficiently humid air your hermit crabs will slowly suffocate and die. Once the gills are damaged from too dry air they can not repair themselves.

Unless you live in a native climate and do not use any form of heat or air conditioning your crabitat will require a lid to maintain the correct humidity levels. Your house is likely 40-50% relative humidity.

A hermit crab requires 70-80% relative humidity. This range is for the purple pincher hermit crab (Coenobita clypeatus) native to Florida and the Caribbean. Other species of land hermit crabs enjoy a slightly higher relative humidity of 85%. You will need a solid lid for the crabitat (tank or aquarium) and a good quality hygrometer and thermometer to accurately measure the air inside of the crabitat.

All hygrometers should be calibrated prior to use.

Different ways to create humidity in your hermit crab habitat.

This tank is set up correctly with solid glass lids.

Humidity levels maintained above 85% are not harmful to the hermit crab directly but can lead to an unsafe environment in the crabitat. Floods, excessive surface mold and mildew, slime mold and bacterial blooms are just of the unsafe conditions that can develop from maintaining excess humidity levels.

Treat your hermit crab in the same manner you would treat a fish and refrain from removing them from their humid environment unless absolutely necessary. A very brief photo from time to time is acceptable. Taking your hermit crab from the tank on a regular basis (for any purpose) is strongly discouraged.

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1. Biology of the Land Crabs Warren W. Burggren and Brain R. McMahon
2. Hermit crabs : everything about anatomy, ecology, purchasing, feeding, housing, behavior, and illness Sue Fox

Central Heating and Air Conditioning

Is my hermit crab’s daytime behavior normal?

Heather submitted the following question:
My hermit crab like to explore during the day and is in a sleep-like position at night. Is that normal?

Check the substrate temperature too!

Check the substrate temperature too!

Well Heather while hermit crabs are mostly nocturnal, they are also seen being active during the day. It sounds like possibly your tank is too cold and your hermit crab is active during the day when it is warmer and lethargic at night because it’s too cool. Minimum temperature should be 75F and maximum should be 82F. Your tank should provide a range of temperatures between the minimum and maximum. It is also important to check the substrate temperature.

To eliminate the possibility that something is wrong in the environment causing this behavior you should check your temperature range in the tank and ensure the tank is in the ideal range. If your temperature and humidity are in acceptable ranges I would not get concerned about your non conformist crabbie.

This article discusses the reasons that a hermit crab is more active at night: Why is my hermit crab inactive?

This article discusses the optimal temperature for your tank: Air Temperature versus Substrate Temperature



Your friend in a pinch,


The Newbies Guide to Hermit Crabs

Coenobita compressus

Let’s Get Started!

First and foremost use the SEARCH BOX and SEARCH, SEARCH, SEARCH! Our site has a ton of information and nearly every question that a new crabber is trying to answer, has been asked here before. We work hard to maintain a current and extensive library of articles to help you. We do have a forum but it is not monitored and you may not receive an answer right away.



Have you found yourself an unprepared parent of a hermit crab or two? Are you prepared to give them the proper habitat in which they can survive long term? If so, here’s a very basic list of supplies to get you started.

A glass tank

  •  go as big as you can afford
  •  it doesn’t have to hold water
  •  it doesn’t have to be new (check the newspaper,craigslist, yard sales)

A secure lid for the tank

  • a mesh lid with a solid barrier on top to retain humidity is ideal
  • you can use saran wrap in a pinch but not ideal long term
  • a piece of plexiglass or coroplast is a good cover for the mesh lid
  • hermit crabs are escape artists! Place heavy weights or locks on your lid

Substrate for the tank

  • 6-12 inches deep or 3 times as deep as your largest crab is tall
  • Play sand meant for a sand box is perfect http://amzn.to/1PLQOMu
  • Coco fiber which is shredded coconuts http://amzn.to/1KhJaHb
  • Mix the play sand and coco fiber together, roughly 5 parts DRY sand to 1 part DRY coco

Heat source

Heat mat – We recommend Ultratherm http://amzn.to/2EVPmb2

  • Can be insulated with Reflectix Reflective Roll Insulation http://amzn.to/1PLR1zg
  • Place on the back wall of the tank to provide gradient heat. (Warm and cool zones)

Light source

  • If you have a well lit room that will illuminate the tank during the day you can get by without a lamp. No direct sunlight on the tank.
  • Be aware of how the room temperature rises during summer.
  • They need a normal light/dark 12/12 hour cycle.

Water conditioner

  • We recommend Prime Aquarium Water Conditioner http://amzn.to/1PLRcu9

Ocean Salt Mix

  • Not freshwater aquarium salt!
  • Instant Ocean is the most common brand and is easy to find http://amzn.to/1SrKxnM
  • These other brands are also good and may be available in your particular area:  Fluval Sea Salt, Red Sea Salt, Coralife, Oceanic Sea Salt, Tetra Marine Sea Salt and Marinium.


  • Cheap Tupperware works great!
  • Water dishes should be deep enough for your largest crab to submerge.
  • Provide exit ramps for small crabs


  • If you can afford it, go wireless and digital. WalMart carries digital indoor/outdoor Thermometer Hygrometer You can also buy online. http://amzn.to/1KhJDZM
  • Hygrometers should be calibrated prior to use

Check out this video on how to set up a temporary crabitat in a storage bin: https://youtu.be/x29JaAr9w9U?list=LLCYR6QkuCDm4MJ8hEDeBqvQ.

Bins are not a permanent home, but may be needed in emergencies.

Setting up a Proper Crabitat

Why dry substrate?


How do I create and maintain humidity in my crabitat?
Calibrating your Humidity Gauge

Methods for heating your crabitat
Air Temperature versus Substrate Temperature

Can I Use A Light To Keep Them Warm?
Do I need a light?

Regulation of Crustacean Molting: A Multi-Hormonal System
CrabloverDon on Molting
All about molting
On molting
Is my hermit crab dead or molting?
What is molting?
Pre Molt Symptoms

Dropped or Missing Limbs-
Handicapped/limbless/sick Hermit Crab Care
Why do land hermit crabs drop limbs?
PPS (Post Purchase Stress) Minimizing the Impact

Shell-less Hermit Crab-
My Hermit Crab has left its shell! What do I do?

Growing your own hermit crab food
Going Natural Beginner’s List
Hermit Crab Food Recipes
Hermit Crab Nutrition Table
People food for hermit crabs
Should I feed my crabs meat?
The Power of Protein
What foods are good and bad for hermit crabs?

The importance of the right kind of salt
How do I mix ocean water?
Why can’t I use tap water?

Substrates for Hermit Crabs

How do I choose suitable shells for my hermit crab?
When do hermit crabs change shells?
Painted Shells

Escaped Hermit Crab-
Locating an Escaped Hermit Crab

Cost Cutting-
Cost Cutting Tips
A guide for setting up a large crabitat on a budget

Calibrating your Humidity Gauge

Analog or dial gauges need to be calibrated to be accurate

Analog or dial gauges need to be calibrated to be accurate

written by Rai Ahmes

Relative humidity (RH) is measured as a percentage–absolute and specific humidity are NOT. If it reads in % it’s a relative gauge. RH gauges are fairly inexpensive and commonly available. Gauges measuring absolute or specific humidity are hard to find and expensive; they also read in either g/kg or g/cubic meter (or the non-metric equivalent) not %. “Actual humidity” and “exact humidity” are not measurements.

Dial RH gauges are rarely calibrated properly by the time they reach your home. This is NOT the fault of the manufacturer. It is simply the way it is with this type of hygrometer whether it’s for your tank or for your house. This probably accounts for the wide variation of humidity levels that have been recommended. You can do something about this though 🙂

Calibrating your hygrometer:
First check to see if your hygrometer can be adjusted. Look at the back to see if the small center post has a slot for a small screwdriver. If it does you’ll need to have a screwdriver ready to make any adjustments as soon as you open the jar. If it doesn’t, don’t worry, this procedure will still allow you to use your hygrometer more effectively.

Half-fill a shot-glass or other small dish with table salt and add enough water to wet the salt completely but not dissolve it–it should be like wet sand. Put the shot-glass and hygrometer in a wide-mouth jar, and seal it airtight. After six hours (or more), the humidity in the jar is 75%. The hygrometer should read somewhere around 75%. If it does not, take the screwdriver and turn the screw on the back of the hygrometer until the meter reads 75%.

If your hygrometer is non-adjustable and reads other than 75% in the jar, you must correct all subsequent readings by this difference. Example: if the hygrometer reads 67% in the jar, it is reading 8% low. Thus, it will always read 8% less than the actual RH. Using this example if you wanted the humidity to be 60-70% then the reading on your hygrometer would have to be 52-62%.

Dial RH gauges can be off by 20% or more! They are also usually only accurate to +/- 5% and often take up to 2 hours to finish stabilizing to a new reading if the humidity changes. Please remember that it is simply the nature of the beast and NOT necessarily poor quality or manufacturing. The spring mechanism is delicate and goes through a lot (shipping, etc) before reaching your home.

Note: Digital gauges rarely allow for adjustment. Simply record the adjustment to be made when checking your gauges. (IE: Humidity reads 2% low so when the gauge says 82% it is really 80%)

FAQ How do I create and maintain humidity in my crabitat?

FAQ How do I maintain humidity in my crabitat? Photo credit Amber Miner

The first step to creating and maintaining humidity in your tank, is a good lid. A screen lid alone is not sufficient. A glass/plexiglass/lexan lid, cut to fit is the best option.

Plexiglass/lexan can be purchased at most hardware stores and be cut to size for free. This can be placed on top of your existing screen lid for stability.

Coroplast is another good option for a lid. This can be placed on top of your existing screen lid for stability.

In the short term, you can use plastic wrap over the screen lid. Some people use cardboard and wrap it in plastic or tape.




Water Dishes:

Your tank should have two types of water bowls, salt and fresh. Use bowls that are wider than they are deep but still accommodate your largest crab and allow it to submerge. This increased surface area will create more humidity. Water dishes may be placed next to your heat pad or beneath your overhead light to produce additional humidity. Adding bubblers/air stones to the water pools will boost humidity.

Natural moss:

Crabs love moss! Placing moss anywhere in your tank and keeping it damp will raise your humidity, especially next to the heat pad. Rewet/replace as needed. You can buy moss in bags or compressed bricks. Sphagnum moss is a favorite and it holds up really well in the crabitat long term. If you can’t find sphagnum, look for Hiawatha or sheet moss, terrarium or frog moss, natural and un-dyed. Moss does not need to be cleaned or replaced for cleanliness.

What Moss is Safe?
Is Beaked moss safe?
Is Sphagnum moss safe
Is Cypress safe?
Don’t Forget the Moss

Wood: Choya, cork bark and other safe woods can suck up the humidity in the air but once saturated can help maintain it.

Things to avoid

Soaking your substrate is not the way to raise humidity.

SLIGHTLY moistening the substrate next to the heat pad is ok. Water logged substrate is not safe for molting hermit crabs and will encourage the growth of lethal bacteria.

Foggers: Foggers and misters can easily flood your tank and lead to lethal conditions for molting crab or trigger a bacteria bloom. Use with caution.

Sponges: Sponges were once considered a staple in the hermit crab tank but there are better solutions to the humidity issue. Sponges placed in the fresh water will raise humidity. Sponges should be changed out daily, sterilized and replaced with already sterilized sponges. Sponges placed in the salt water will become nasty fast. Due to the upkeep of sponges it’s easier to go with moss.

Emergency fix:

If you have just discovered that your crabs need humidity and you are not able to fix the problem immediately you can place a wet towel over the tank and some plastic wrap on top of the towel. The towel must be kept wet and it must have plastic wrap over it or as the towel dries out it will absorb the moisture from the tank. This is not a long term solution.

You can also mist the tank with dechlorinated water near the heat source. This is not a long term solution either as the humidity will fluctuate too much between misting to be suitable. You also risk flooding your tank by over misting.

Originally posted at All Things Crabby The Hermit Crab Care Blog

Understanding Humidity in the Crabitat

Originally written by Vanessa Pike-Russell-Updated by Stacy Griffith

What is Humidity?

Humidity is a measure of the amount of water vapor in the air. It can be expressed in several ways.

“Specific humidity” is the mass of water vapor per unit mass of combined dry air and water vapor, generally expressed in grams per kilogram. The specific humidity of an air parcel does not change with temperature.

“Absolute humidity” is the density of water vapor, expressed as grams per cubic meter of air.

“Relative humidity” is discussed below. Related terms are “saturation,” which describes the condition where water vapor is at a maximum concentration for the air temperature (warm air can hold more moisture than cold);

“dew point temperature” the temperature at which saturation occurs if air is cooled at constant pressure without addition or removal of water vapor; and

“vapor pressure,” which in meteorology is that part of the total atmospheric pressure due to water vapor content.

Reference: Arctic Climatography and Meteorology

What is the Ideal Humidity Range?

Humidity should be between 50-60% for Actual and around 70-80% relative humidity.

How can I measure humidity within my Crabitat?

You will need to buy a humidity gauge, also known as a hygrometer.
There are many kinds, some of the more popular are shown below.

Misting your Hermit Crab

Misting Your Hermit Crab

Misting is commonly considered to be unnecessary and potentially stressful by most crabbers. If your tank humidity is at a recommended level and you offer fresh and salt water pools deep enough for your crabs to wade in, there is no benefit in regular misting. If you choose to take your hermit crab out of its tank for playtime or exercise, it is a good idea to mist the gills.