how to measure hermit crab shells

Finding Shells for Hermit Crabs (with Size Chart + Measurements)

Hermit crab shells can be bought from pet stores, craft stores, and online retailers. You can also forage for shells at the beach. No matter where you get your shells from, they need to be whole and without cracks, fractures, holes, or jagged edges. Hermit crabs will modify shells if desired, but it is still best to give them healthy shells from the get-go.

As hermit crabs grow, they should be given shells 3/8 – 1/2 an inch wider at the opening than their existing shell. To measure your hermit crab’s current shell, lay a ruler across the widest point of the shell’s opening. This diameter is how you gauge the size of the next shell. New shells should be cleaned before being placed in the tank. Likewise, wild-foraged shells need to be sterilized in boiling primed water for 10 minutes.

Hermit crabs are fussy about new shells, so offer each one 3-5 shells to choose from. These shells should never be painted or decorated in any manner. Such shells are unhealthy and even toxic for hermit crabs. The best options come from natural sea snails, as they are strong, roomy, and afford ample protection from falls.

How To Measure Hermit Crab Shells

Hermit crabs don’t grow their own shells. Instead, they recycle empty gastropod shells and turn them into homes. This means that hermit crabs need to trade in their shells for bigger ones as they grow. To provide an appropriate selection, you will need to measure the shells to ensure they are big enough.

A shell is measured across the widest point of its opening. Using a ruler, carefully measure your hermit crab’s existing shell. The diameter of the shell at its widest point is what you need to note down. Once you have your measurements, you can do the same for potential new shells.

New shells should be roughly a ¼ inch wider than the hermit crab’s existing shell. Pick 3-5 new shells that vary in size to give the hermie a few options. It is also wise to match the general shape of the existing shell, especially at the opening. “O” shape and “D” shape openings are the most popular with hermit crabs.

Hermit crabs are known to conduct “home improvements” on shells to make them more suitable for each hermit. It is normal for these critters to modify their homes, even if it’s a perfect size.

Selecting Good Quality Shells

No matter where you are sourcing your shells, be it from stores or wild foraging, you should closely inspect the options. Look for imperfections, holes, and sharp edges. Avoid shells with cracks or fractures as well. These can easily hurt your pets or do serious damage if your hermit crab takes a tumble.

Hermit crab shells can be found in pet stores or online, where they are sold in bulk. Aside from that, you can often find appropriate shells at craft stores or gift shops. Be careful of those sold outside of pet stores, however. Sometimes they are cleaned with harsh chemicals.

hermit crab shell size chart

Hermit Crab Shell Size Chart

Here’s a quick chart to reference when sizing up your hermit crab shells. Compare the opening of your hermit crab’s existing shell to determine which size category it fits into:

Small Hermit Crab3/8 – ½ inch opening
Medium Hermit Crab½ – 1 inch opening
Large Hermit Crab1 – 1 3/8 inch opening
Jumbo Hermit Crab3 inch or larger opening

Remember, the new shells need to be up to ¼ inch wider than the existing shell. This gives your hermies room to grow and maneuver. After a molt, they’ll rely on having extra space and will turn down shells that don’t fit the bill.  

How To Prepare Shells For Hermit Crabs

Pet stores will offer shells that have been sanitized and treated in preparation for being used by hermits. As such, all you should need to do is gently wash the shell of any dust or dirt in primed water. They can be placed straight into the tank afterward.

If you have foraged the shells from your local beach, you need to undertake safety measures. There is a broad range of parasites and bacteria that could befall your sheltered hermies. Likewise, cracks or imperfections may cut up your hermit crabs by accident.

Avoid any shells with barnacle growths, algae, or defects like cracks or holes. Most importantly, you should check that there isn’t already an animal using the shell as a home. Hermit crabs or snails may tuck themselves away from sight. It’s not uncommon for owners to accidentally bring home a new hermit crab (or a danger to their existing hermies).

Once you have a selection of shells, they will need to be cleaned and sterilized. Do this by:

  1. Boiling the shells for 5-10 minutes.
  2. Leave them in the water and allow them to cool to room temperature.
  3. Remove the shells from the water and gently shake any trapped water out.
  4. Pop the clean, cooled shells into the tank.  

As discussed, each hermit crab in your colony should be provided with 3-5 shells to choose from. Ample choices allow for two things: no fights between colony members, which can be brutal, and finding the right fit.

Do Hermit Crabs Like Colorful Shells?

Hermit crabs are small animals with a limited ability to defend themselves. Their shells are their primary defense against predators for the protection they afford. An additional trait that hermit crabs rely on is the ability to camouflage.

According to the Journal of Experimental Biology, hermit crabs can distinguish the level of contrast between shells and the surrounding environment. The researchers found that hermit crabs would choose shells with minimal contrast over those with strong contrast. This allowed them to better hide from predators.

As such, hermit crabs will not choose colorful shells unless they match the surrounding environment. If pushed to desperation, though, a hermit may compromise and pick a colored shell anyhow. In this case, however, it will be highly stressed and believe that it is overly exposed to predators. This can shorten its overall lifespan.

Can I Paint My Hermit Crab’s Shell

Painted shells may be cute, and decorating them can be a fun crafting activity. But will your hermit crabs appreciate the effort? Of course not.

Not only will hermit crabs avoid painted shells where possible, but they are actually dangerous for these little critters. The paints used to decorate such shells are often toxic, even if it says non-toxic on the label. As such, all manner of harmful chemicals are leached into the water and absorbed through contact.

Hermit crabs can also become irreversibly stuck in a shell if the paint is not fully cured before they settle in. The paint effectively glues the hermit crab into the shell. There is no way to remove the hermie safely. It will eventually die when it grows too big for the shell if the toxins don’t kill it first.

Avoid painting your hermit crabs’ shells. There are plenty of beautiful shells naturally available, without the danger.

How To Help Hermit Crabs Switch Shells

Never force a hermit crab to leave its shell. Even if you know it needs to move house, removing it unwillingly from a shell is a terrible idea. Aside from traumatizing it, you risk wounding it, perhaps fatally. To encourage your hermit crab to move, there are a few things you can do.

Hermit crabs can be quite fussy about selecting a new shell, even if their current one is a tight fit. They can become very attached to a shell and will be reluctant to leave sometimes. When a hermit crab rejects new shells, you’ll need to offer it some new ones.

Look for ones with different colors, shapes, openings, and so forth. You may even try a different type of shell, as long as it is suitable for hermit crabs.

Ensure that the substrate in the tank is deep enough for the hermit crab to burrow and submerge itself in. Depending on your hermit crabs’ size, this may mean anywhere between 3-8 inches of substrate. Hermit crabs will often molt before changing shells, and they need to burrow to do so.

my hermit crab is too big for its shell

My Hermit Crab Is Too Big For Its Shell

Keep spare shells in the tank in preparation for a hermit crab needing to change shell. However, there are times when hermit crabs reject all the shells on offer, and you are none the wiser.

It is difficult to track a hermit crab’s growth accurately. Instead, to monitor when your hermit crab needs a new shell, you need to perform a quick visual test. Pick the hermit crab up and angle it so that the shell’s opening faces you. Watch for how easily it can retract into the shell. If it struggles to hide, then it’s getting too big for the shell.

With that said, this isn’t the most reliable way to judge when your hermit crab needs a shell change. Hermits change shells roughly twice every 12-18 months. Just keep a good variety in the tank.

Hermit crabs may also change shells on a whim. If your pet finds one that it likes more than its current home, it may swap on the spot. If it is preparing to molt, it may also swap shells in preparation.

Large hermit crab shells can be slightly harder to come by. You may need to resort to a specialty aquarium or pet store, or buying online.

Best Shells For Hermit Crabs

There are certain types of shells that are more suited to hermit crabs than others. It is important to distinguish the good from the bad. As found in Animal Behaviour, hermit crabs show a preference for the shells of certain species.

Avoid shells used by land snails at all costs. These are too fragile and delicate, even for a hermit crab. Choosing those left behind by sea snails are far better. Look for options from the following species:

  • Turbo Snails – Thick shells with a round opening.
  • Murex Snails – Larger shells with spines or lumps.
  • Babylonia Snails – Solid shells with beautiful patterns.
  • Nerita Polita Snails – Good shells for very small hermit crabs.

Hermit crabs may have differing preferences, depending on their size, body shape, and species. Providing a selection of different shells from different species can be good if your hermit crab is fussy.