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how to measure hermit crab shells

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

(Last Updated On: August 20, 2022)

Hermit crabs’ shells can be bought from pet stores, craft stores, and online retailers. You can even forage for shells at the beach, but they’ll need to be cleaned first.

No matter where you get shells, they must be free from cracks, fractures, holes, and jagged edges. Hermit crabs can modify their shells if desired but give them healthy ones.

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 a 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.

Hermit crabs are selective about new shells, so offer 3-5 shells each. These shells should never be painted because it’s toxic for hermit crabs.

The best options are natural sea snails, which are strong, roomy, and afford ample protection from falls.

How To Measure Hermit Crab Shells

Hermit crabs don’t grow their shells. Instead, they recycle empty gastropod shells and turn them into homes. So, hermit crabs need to trade in their shells for bigger shells as they grow.

To provide a selection, you’ll need to measure the shells to ensure they are big enough.

A shell is measured from the widest point of its opening. Using a ruler, carefully measure the hermit crab’s existing shell. The diameter of the shell at its widest point is what you need to note down.

New shells should be ¼ inch wider than the hermit crab’s existing shell. Pick 3-5 new shells that vary in size to give the hermit crabs a few options.

It’s also wise to match the general shape of the existing shell, especially at the opening. “O” shape and “D” shape openings are most popular with hermit crabs.

Hermit crabs conduct “home improvements” on shells to make them more suitable.

Selecting Good-Quality Shells

No matter where you’re sourcing shells, look for imperfections, holes, sharp edges, cracks, and fractures.

Hermit crab shells can be found in pet stores or online, where they’re sold in bulk. Aside from that, you can often find appropriate shells at craft stores or gift shops.

hermit crab shell size chart

Hermit Crab Shell Size Chart

Here’s a quick chart to reference when sizing up hermit crab shells. Compare the opening of a 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 inches or larger opening

Remember, the new shells need to be up to ¼ inch wider than the existing shell. This gives your hermit crabs room to grow and maneuver.

After a molt, they’ll rely on having extra space and turn down sub-optimal shells.  

How To Prepare Shells for Hermit Crabs

Pet stores sell shells that have been sanitized in preparation for being used by hermits.

So, wash the shell to remove any dust or dirt in primed water. Once done, they can be placed straight into the tank afterward.

You must undertake safety measures if you have foraged the shells from your local beach. There’s a range of parasites and bacteria that could befall hermit crabs.

Avoid any shells with barnacle growths, algae, or defects like cracks or holes. Most importantly, check there isn’t already an animal using the shell as a home.

Hermit crabs or snails may tuck themselves away from sight. Unfortunately, it’s common for owners to accidentally bring home a new hermit crab.

Once you have a selection of shells, they’ll need to be sterilized:

  1. Boil the shells for 5-10 minutes.
  2. Leave the shells in water and allow them to cool to room temperature.
  3. Remove the shells from the water and remove any trapped water.
  4. Put the clean shells into the tank.  

As discussed, each hermit crab should be provided with 3-5 shells to prevent fighting.

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 their 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.

Hermit crabs won’t choose colorful shells unless they match the surrounding environment. If pushed to desperation, a hermit crab may compromise and pick a colored shell. In this case, it’ll be highly stressed, believing it’s overly exposed to predators, which can shorten its lifespan.

Can I Paint My Hermit Crab’s Shell?

Painted shells may be cute, and decorating them can be a fun crafting activity. However, painting hermit crab shells should be avoided.

The paints used to decorate shells are often toxic, even if it says non-toxic on the label. So, all harmful chemicals are leached into the water and absorbed through contact.

If the paint isn’t fully cured, hermit crabs can become irreversibly stuck in a shell. The paint glues the hermit crab into the shell, and there’s no way to remove the hermit crab safely.

It’ll eventually die when it grows too big for the shell if toxins don’t kill it first.

my hermit crab is too big for its shell

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 bad idea.

Aside from traumatizing the hermit crab, you risk wounding it, perhaps fatally. To encourage a hermit crab to move, there are things you can do.

Hermit crabs can be fussy when selecting a new shell, even if their current one is a tight fit. They can become very attached to a shell and reluctant to leave.

When a hermit crab rejects new shells, offer it some new ones. Look for shells with different colors, shapes, and openings. You can even try a different shell type, as long as it’s suitable for hermit crabs.

Ensure the substrate is deep enough for the hermit crab to burrow and submerge. Depending on the hermit crab’s size, this may mean 3-8 inches of substrate.

Hermit crabs often molt before changing shells, and they must burrow.

My Hermit Crab Is Too Big For Its Shell

Keep spare shells in the tank in readiness for a hermit crab needing to change shells. However, sometimes hermit crabs reject all the shells on offer, and you’re none the wiser.

It is difficult to track a hermit crab’s growth accurately. Instead, perform a quick visual test to monitor when a hermit crab needs a new shell.

Pick up the hermit crab and angle it so the shell’s opening faces you. Check how easy it can retract into the shell. If it struggles to hide, it’s getting too big for the shell.

This isn’t the most reliable way to judge when a hermit crab needs a shell change. Hermit crabs change shells roughly twice every 12-18 months, so keep a wide variety in the tank.

Hermit crabs may also change shells on a whim. If a hermit crab finds a shell it likes more than its current shell, it may swap immediately. If it’s preparing to molt, it may swap shells in preparation.

Large hermit crab shells can be harder to find, so you may need to go to a specialty aquarium.

Best Shells for Hermit Crabs

Certain shells are more suited to hermit crabs than others. As discovered by Animal Behaviour, hermit crabs show a preference for the shells of certain species.

Avoid shells used by land snails because they’re too fragile, even for a hermit crab. Choosing the shells left behind by sea snails is recommended. Look for shells 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 different preferences depending on their size, body shape, and species. Providing a selection of different shells from various species can be good if your hermit crab is selective.