How many hermit crabs per gallon?

by Vanessa

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I have pondered the question for a long time. Lately I have concluded that there is no easy way to answer that question. It depends on several factors, which will be different for each situation, unless a standard tank developed with the same items, the same dimensions. Because this is not the case for the majority of land hermit crab owners, perhaps we need to revisit the rule of thumb and bring it into line with the 21st century.

Firstly, let's look at the Gallon, (or 4 Litres in Aus-speak).

The Gallon is a volume taking into consideration length, width and height.
Since most crab tanks are one level, and hermit crabs mainly use the surface area of a tank, is a 'per Gallon" guideline appropriate?

Unless you use levels so you are doubling or adding to the surface area of your tank, the amount of Gallons is not truly relational to the surface area. You could have a 50 Gallon tank but the surface area is actually very small because it is a fish tank, with a tall vertical height for the benefit of fish, which need it. Perhaps what would be more fitting is to say 'per square foot' or 'per square metre' so that we have an idea of how much actual surface area you are talking about. Then if people have levels, they can measure the square foot or metre dimensions and get an estimate as to how many hermit crabs they can keep in that tank.

To be honest, the best way to work out how many hermit crabs you can fit within an area is to put all the things you need within a tank, such as food and water dishes, hiding spots (caves, coconut huts, etc), greenery, wood items, etc until you have created a comfortable habitat for your hermit crabs. Then calculate how many hermit crabs can comfortably exist in the environment you have created while answering the following questions:

1. Does every land hermit crab have an area to go into in order to escape the stresses of captivity? Many animals need hiding spots and areas within their habitat so that they can distress and feel at ease. If you have a tank that includes: substrate, bowls, dishes, and no barrier from the bright lights and shadows, there is the chance that they may not be able to settle into their true behaviour patterns, or ever truly relax. There should be several hiding spots available so that each crab has a chance to feel at ease. Imagine yourself within the tank, and what it would feel like to be out in the open

2. Is there enough room in the tank for a moulting-friendly area within the tank? It is important that there is enough room for each hermit crab to be able to moult in peace without crowded tank mates digging them up in order to find their own place to hunker down and sleep or moult.

3. If you have a large hermit crab, or even a jumbo, it is important that they are able to have an area that is Jumbo-friendly, that is, deep enough substrate for them to bury properly, and large enough hiding caves and spaces to enable their survival.

4. Is there enough within the tank to facilitate climbing, play and exercise? It is important that a section of your tank makes use of the vertical height available, without lending itself to 'the great escape'. Items like Driftwood, coral, mangrove root etc are all great to have within a tank, and will a hit with your hermit crabs. Make sure to place these items before calculating how many hermit crabs you can fit within the tank

After you have considered the above factors, then estimate how many hermit crabs could comfortably fit within your crabarium, keeping in mind

a. How many hermit crabs are moulting at the one time?
b. How many different sizes you have or would like
c. Whether your hermit crabs sleep huddled up together, or in their own spaces
d. Whether you have a mix of PP's (Carribean, C. clypeatus) and E's (Pacific, C. compressus)
    and have provided for their moulting set-up needs.

You might be wondering why I mentioned a mix of PP's and E's. To read my views on the mixing of the species, and some of the needs of Ecuadorians, visit here. Quite a few friends who have owned both have found difficulty in creating a moulting set-up that was successful for both species. One species would moult perfectly; the other would have complications and pass on. Some members choose to keep both species within the tank, providing two moulting areas. This can be done either with the use of iso units that hermit crabs can climb in and out of, or by creating two ends of the tank which are set up for moulting with a divider in the sand and keeping it at varying dampness and depth. It is one way to meet the needs of both species, and simply refresh the damp sand where needed.

A simple adaption of the 'per Gallon' rule would be 'per Gallon divided by the percentage of vertical height used'. That is, if you only use the bottom third of the tank, calculate how many Gallons your tank holds, then divide by 3 to deduce how many hermit crabs you might comfortably keep within that crabitat.

From Matty's (dig.to/hermies) he explains how to deduce the Gallon or Litre volume of your tank

How Do I Work Out How Many Litres or Gallons My Tank Can Hold?

Measure the height, length and width in the tank.
Convert the measurements to cm.
Multiply the measurements together.
(How many mL your tank can hold!
Divide that figure by 1000 (mL to L).
(How many litres your tank can hold!
Multiply that by .26417
(How many gallons your tank can hold!)

One of the reasons why I am supporting the creation of custom made tanks, is because land hermit crabs are not fish, and the surface area needs to be taken into consideration when purchasing a tank. We have all seen hermit crabs climbing the sides of their tank like arachnids as they try to make their way around tank items due to the narrowness of their crabarium.

When I moved to Tasmania in December 2000, I purchased a 2ft by 1.5ft crabarium to fit within a nook in my lounge room. Not only was it cheaper to purchase than a standard 2ft fish tank, but I was able to fit more items comfortably within the tank, and just the extra half a foot width helped to make the crabarium a much happier place for my hermit crabs.

When my fellow Tasmanian and fellow-crabber Craig saw what a great idea it was to create a custom-made tank and the cubing effect, he had a 3ft by 3ft tank built, which gives him 9 square feet of surface area - 3 times that of my 2ft by 1.5ft tank.

Previously he had purchased a standard 3ft long, the surface area around a third less for the same amount of money. It also had a height that makes it difficult to lower and lift items to or from the tank. It just seems like common sense to me.

If you cannot afford a custom made tank, why not ask your local aquarium if they know of anyone that would create a more 'cubish' tank or perhaps look into a hexagonal style crabarium which will have a similar amount of surface area. In the meantime, try not to buy more hermit crabs just because a rule of thumb says that you can fit more based on the Gallons in your tank, but how many you can comfortably keep with the surface area available.