How To Measure Shells For Your Hermit Crabs

Few individuals are aware of the critical importance of providing appropriately sized shells for their hermit crabs. Many owners tend to acquire an assortment of shells, place them in the tank, and rely on chance. Captive crabs often do not behave in that manner. Some can be highly selective about the shells they choose for habitation.

Providing appropriately sized shells for each of your crabs is crucial. An oversized shell can impede their mobility within the tank, while an undersized shell prevents them from fully retracting to safeguard themselves from predators, such as other crabs.

How can you determine if a shell fits perfectly? When you pick up your crab, observe as it retracts into its shell. The large claw should comfortably cover the shell's opening, functioning like a door. This provides protection for the crab's body from potential predators. If the crab is fully retracted yet still protrudes from its shell, it indicates that the shell is too small, necessitating an upgrade to a larger one. If the crab can fully retract into its shell and there remains a significant amount of space at the front of the opening that the crab does not occupy, the shell is too large for the crab.

How to Measure the Shell Opening of Your Hermit Crab

Utilize a digital caliper, ruler, or tape measure to accurately assess the shell opening. Carefully hold your crab and turn the shell opening towards you. Position your measuring instrument over the longest section of the shell opening. It is crucial to ONLY measure the interior of the shell, excluding the edges from your measurement. Note that the type of shell opening (D, round, or oval) can significantly influence the measurement outcome.

When measuring the shell of a crab, it is essential to handle only the shell itself. Avoid holding the crab by its legs, face, antennae, or body. Use your fingers to grasp the very back of the shell to prevent being pinched.

Record the measurement to ensure it is not forgotten. In Australia, centimetres are the most commonly used measurement unit for shells.

How Frequently Should I Measure My Crabs' Shell?

Measurements should be taken each time a new crab is introduced to your tank. This ensures that the new crab has access to appropriately sized shells.

We also recommend measuring the shell openings of your current hermit crabs at least three times a year. Each time a crab moults, it grows. If you observe that they appear slightly too large for their current shells, it is advisable to measure them.

While measuring their shell can be stressful for them, this procedure is brief and can be conducted safely within or on top of the tank, allowing for a prompt return to their habitat. Minimizing stress is a priority, yet it is essential to measure their shells as this is a critical process, irrespective of the crab's preference.

My Crab Won't Retract. How Can I Measure Its Shell Safely Without Getting Pinched?

Indeed, some crabs are quite stubborn and may emerge during shell measurement. This behavior is particularly common in crabs that cannot fully retract into their shells, as they have no other option but to come out while you attempt to measure their shell.

Due to the obstruction caused by their body when measuring the shell opening, it is advisable to use a digital caliper at this stage. We consistently recommend utilizing a digital caliper, as it provides the most precise measurement of a shell opening. Simply position the measuring device around the crab's body, coming down from the top, to assess the opening. This method will not harm the crab and ensures a more straightforward and accurate reading.

If you have a crab that tends to pinch (which is common in many tanks),

you can use gloves, oven mitts, or a towel to hold it and prevent being pinched.

When opting to measure your crabs outside of the tank, it is crucial to have a soft surface beneath them in case they are accidentally dropped. Suitable options include a couch, bed, blanket, or towel. Avoid measuring over tiles, concrete, marble benches, or tables, as a fall from such a height onto a hard surface could cause serious injury to your crab.

Occasionally, our crabs fit seamlessly within their shells, yet they may still be reluctant to retract, preventing us from obtaining an accurate shell measurement. A useful technique for encouraging a crab that is particularly sociable is to gently stroke or tap its claw or leg. This often prompts the crab to retreat into its shell. However, this method may not be effective with exceptionally outgoing crabs that are unaffected by interaction. Indeed, such personality-rich crabs do exist!

Presented below are photographs illustrating the proper methods for measuring shells, including various techniques, as well as guidance on identifying crabs that are either too large or too small for their shells.

For further information on hermit crab shells, please visit our shell blog here.

https://www.happyhermie.com.au/post/hermit-crab-shell-guide

This Hermit Crab is too big to fit securely inside their shell and will need to be offered a larger size.

This hermit crabs shell is excessively large, allowing it to retract completely out of sight. It should be provided with a smaller shell.

This crab comfortably fits into its shell, providing ample space to fully retract and ensure protection.

Utilizing a digital caliper to measure shell openings is the most precise method.

Tape measures are not as accurate and can be more difficult if there is a crab occupying the shell.

Rulers can also be used; however, they can be difficult to gain an accurate reading.