how to measure hermit crab shells

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

Last Updated on October 14, 2023 by Joanne Harper

Hermit crab shells can be bought from pet stores, craft stores, and online retailers. Alternatively, you can forage for shells on the beach, but they must be cleaned first.

No matter where you get shells, they must be free from cracks, holes, and jagged edges. Hermit crabs can modify their shells if necessary, but try to give them healthy and robust selections.

As hermit crabs grow, they need shells ¼ an inch wider at the opening than their existing shell. To measure the current shell, lay a ruler across the widest point of the opening.

Hermit crabs are selective about new shells, so offer 3 to 5 each. The shells should never be painted. The best option is natural sea snails, which are roomy and afford protection from falls.

How To Measure Hermit Crab Shells

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

You must measure the shells to ensure they’re big enough to accommodate the hermit crab.

A shell is measured from the widest point of its opening. Using a ruler, measure the hermit crab’s existing shell. Note the diameter of the shell at its widest point.

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 several options.

It’s also sensible to match the general shape of the existing shell, especially at the opening. Hermit crabs favor “O” and “D” shape openings.

Hermit crabs make improvements to their shells to make them more suitable.

Selecting Good-Quality Shells

No matter where you source shells, check for imperfections, holes, sharp edges, cracks, and fractures.

Hermit crab shells can be found in pet stores or online in bulk. Sometimes, you can find suitable shells at craft stores or gift shops.

hermit crab shell size chart

Hermit Crab Shell Size Chart

Here’s a chart to reference when sizing up hermit crab shells.

Compare the opening of a hermit crab’s existing shell to determine its size category:

Small Hermit Crab⅜ – ½ inch opening
Medium Hermit Crab½ – 1-inch opening
Large Hermit Crab1 – 1 ⅜ inch opening
Jumbo Hermit Crab3+ inches opening

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

After molting the exoskeleton, they rely on having extra space, declining sub-optimal shells.  

How To Prepare Shells for Hermit Crabs

Pet stores sell sanitized shells in readiness for hermit crabs. Wash the shell to remove dirt and debris in primed water. Then, they can be put into the tank.

If you’ve brought back shells from the beach, they could be affected by parasites and harmful bacteria. Avoid shells with barnacle growths, algae, or defects like cracks or holes.

Check if a hermit crab is inside the shell before taking it from the beach. Hermit crabs or snails may tuck themselves away from sight. It’s common to bring home a hermit crab accidentally.

Once you have a selection of shells, they must 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.  

Each hermit crab should have 3-5 different shells to prevent unnecessary competition.

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. Hermit crabs won’t choose colorful shells unless they match the surrounding environment.

Can I Paint My Hermit Crab’s Shell?

Painted shells may be cute, but they should always be avoided.

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

Hermit crabs can become irreversibly stuck in a shell if the paint isn’t fully cured. 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 home, removing it unwillingly from a shell is a bad idea.

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

Hermit crabs can be fussy when selecting a new shell, even if their current one is a tight fit. They can become 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 try another shell type if 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 require 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 ready for a hermit crab needing to change shells.

It’s hard 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 easily it can retract into the shell. It’s getting too big for the shell if it’s struggling to hide.

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 change shells in preparation.

Best Shells for Hermit Crabs

As discovered by Animal Behaviour, hermit crabs show a preference for the shells of certain species. The shells left behind by sea snails are recommended.

Avoid shells used by land snails because they’re fragile and offer minimal protection.

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 tiny hermit crabs.

Hermit crabs may have different preferences depending on their size, body shape, and species.