How to Choose Hermit Crab Shells by Shape and Size

It is important to keep the appropriate shells with your hermit crabs to allow them the opportunity to try a new home. This important advice will show you how to select the correct shells for your hermit crabs based on their size, environment, and species.

Every shell is different, so before reaching into the $2.99 shell box at the pet store, get the information you need to measure and select the best shells to avoid wasting money on inappropriate shells.

In this article...

How to size hermit crab shells • Choosing a comfortable shell • Shell cleaning • About hermit crab shell changes • Buying hermit crab shells online • Reader comments

Shell size is one of the most important considerations for choosing hermit crab shells. While most seashells sold for decoration are described by their total width (i.e. 3 inches from end to end), the only measurement that matters for your crabs is the diameter of the opening.

Shell size is measured across the longest diameter of the opening. Shell calipers are sold to accurately measure shell openings, but you can get a fairly accurate measurement by eyeballing the opening and a ruler. Small hermit crabs usually inhabit shells 3/8" to 1/2", medium crabs from 1/2" to 1", large crabs from 1" to 1 3/8", and jumbo crabs live in shells with up to 3" openings or larger.

Once you have determined the size of your crab, select a few shells that are 1/8 to 1/4 inch wider than the hermit crab's current shell. Also try to find the same shape opening, especially if the crab has consistently chosen shells with round or D-shaped openings.

Ordering online can be tricky, but until you place and receive an order for a specific size and actually see the shells next to your hermit crab, it can be difficult to know what size she needs. If you are unsure, get 2 shells that are the size you think you want, one that is 1/8" smaller, and one or two that are 1/8" larger. Once you've ordered, simply remember her size and select shells that are a size larger.

You can also buy convenient variety packs with shells of appropriate sizes for small and medium hermit crabs. However, when you are ready to buy a new shell you still haven't learned the correct size of your crab since the individual shells in a pack are not labeled with measurements.

Most common hermit crabs, especially Purple Pincher (C. clypeatus) prefer shells with round openings. Other species of hermit crabs, such as Ecuadorians (C. compressus), have been known to prefer shells with D-shaped openings because their bodies are more flattened and they are better able to protect themselves in shells with D-shaped openings.

In addition to considering the size and opening of the shell you should only buy natural, polished, or etched shells. Painted shells are hazardous to the health of your hermit crabs. If your child wants a decorated shell, there are many beautiful natural shells safely etched with dolphins, palm trees, and other designs.

Turbo, Murex, and other shells with round openings

Turbo shells are probably the best choice for hermit crabs of any size. With a broad, round opening and a heavy, thick construction these shells are roomy and offer good protection to their inhabitants. There are many types of turbo shells available in all-natural, polished, and etched designs (exposing the white pearlescent layer beneath the dark exterior of the shell).

Magpie, Green Turbo, Turbo Cinerius, Turbo Stripe, and Turbo Petholatus shells are readily available online and are quite affordable. For more expensive taste or for jumbo crabs, try the Jade Turbo or the very beautiful Turbo Sarmaticus shells.

Murex shells are more structurally lavish than turbo shells with spines and lumps. These may be more attractive for their shape, but know that a hermit crab is not likely to choose a shell with large branches or spikes because it will just make it more difficult to navigate the crabitat.

Pink Murex shells are very commonly found in craft stores at all sizes and are very inexpensive. Apple Murex shells are available as well in a nice mix of browns.

Babylonia, Whale Eye, and other shells with D-shaped openings

Babylonia shells are a nice mix of brown and white and are readily available for both small and large crabs. These shells are common in pet stores, craft stores, and online shell stores. Other shells with D-shaped openings include Nerita Polita shells (for the smallest of crabs), Whale Eye shells, Shark Eye shells, Crown Conch, and Nutmeg shells.

Land Snail Shells

The shells made by land snails are usually much thinner and more lightweight than those made by sea snails (turbo, babylonia, etc). Generally, sea snail shells are preferred over land snail shells so these are not recommended.

Shells must be prepared before you place them in the crabitat to ensure that they are safe for your crabs. Shells sold at craft stores are usually bleached beforehand, leaving chemicals which could harm the crab.

Shell cleaning instructions

1. Discard shells with unnatural cracks, holes, or jagged edges that may harm the crabs.
2. Dechlorinate enough warm water to cover the shells three times over.
3. Add 1/3 of the water to a bowl and stir in baking soda (about 1 tsp per cup of water).
   
4. Soak the shells in the baking soda solution for 10 minutes, be sure that the air bubbles out of each shell.
5. Shake each hermit crab shell over the sink to drain the water.
6. Add 1/3 of the dechlorinated water to a clean bowl and stir in aquarium salt mix, according to the directions on the package.
7. Soak the shells for 5 minutes.
8. Shake each hermit crab shell over the sink to drain the water.
9. Repeat steps 6-8 with fresh dechlorinated water.
   

Now the shells are ready for your hermit crabs! Place the shells with the opening facing up to entice the crabs to inspect them, but be patient - only the crab will know when it needs to switch shells and which candidate will be most comfortable. Inevitably, unused shells will fill up with sand over time, so be sure to clean them occasionally with dechlorinated fresh water.

When a hermit crab is ready to change shells, it may need to try several shells before settling on a new home. Purple Pincher crabs (C. Clypeatus) are particularly likely to switch shells several times in the span of an hour or two, while other crabs may spend more time looking for the perfect shell.

After inspecting the inside of the shell with their legs and antennae, the crab will carefully position the shell. Rising up out of its current shell, the crab will swing its soft white abdomen over and into the new shell, curling down into the new home. After up to an hour of rolling around and inspecting the new shell, the shell switch happens in less than a second!

Fighting Crabs

If you have several crabs that are the same size, they may have loud vocal and physical contests over shells. While this is rarely meant to harm each other, the battles can become dangerous and you may need to separate the crabs. However, if you have provided enough sand, the crabs should be able to bury to get out of the situation.

This is very common in the following scenario, and there is nothing you can do about it besides separating the crabs and buying more shells:

Crab A switches to a new shell. Crab B switches into Crab A's old shell. Crab A doesn't like the new shell and wants the old shell back. Crab B loves the shell and won't give it back.

If you notice the crabs making noises back and forth or trying to steal shells, buy a few more shells for them to choose from. I had a crab who held a grudge over this scenario for more than a month. "Crab B" didn't stay above ground for more than an hour at a time until "Crab A" switched into a new shell that I had bought and was no longer harassing the other crab for stealing her shell.

Don't make these mistakes...

Be careful not to make the following mistakes with your hermit crabs.

• Do not remove the old shell, they may need to switch back into it.
• Provide at least 2 shells that are the proper size for your crab.
• If you notice crabs fighting over shells, give them more shells to choose from.
  
• Never try to force a crab to switch shells.
  

If you do not live near the ocean, you probably do not have any seashell shops nearby. Depending on the size for your crabs, you may be able to get shells at the pet store or even at a craft store, but otherwise it is best to shop for top-quality shells available online at standardized sizes.

Be careful... most seashell shops will sell shells based on the outside measurements of the shell, only shells sold specifically for hermit crabs will include the opening diameter measurement. It is very difficult to order shells based on exterior measurements.

Remember, never buy painted shells because they are unsafe for your hermit crabs. Polished and etched natural shells are a beautiful alternative if you don't like the rugged all-natural look.

Hermit crab and shell photos used on this page were taken by the author, fritteritter. Other photos taken by photographers at sxc.hu, used with permission.