Hermit crabs are fascinating crustaceans that have become popular pets. Part of their unique appeal is because they wear empty mollusk shells to protect the soft sections of their body.
Although hermit crabs use shells to cover and protect the soft parts of their bodies, the relationship is not truly symbiotic. The snail neither gains nor loses anything from its shell being used. The crab needs a steady supply of shells to survive. This phenomenon is known as commensalism.
If you keep pet hermit crabs, it is essential to provide a good supply of empty shells that your growing pet can slip into as they grow. Let’s explore the close relationship between these fascinating crustaceans and the shells they wear on their bodies.
Hermit Crabs and Shell Symbiotic Relationship
Unlike their name suggests, hermit crabs are highly sociable creatures that often move about in large groups in the wild. They get their name because, unlike other crabs that have hard exoskeletons all over their bodies, they use empty snail shells to protect the soft parts of their body and quickly retreat to safety if threatened.
Symbiosis refers to a situation where two different creatures mutually benefit from their interaction; however, the snails neither gain nor lose anything due to their shells being reused when they die. Hermit crabs depend on rigid structures for their survival. This kind of symbiotic relationship is called commensalism.
In the case of a mollusk shell, it is the exoskeleton of an animal that has died. The soft inner snail parts have either decomposed or been eaten by another predator, so the shell, mainly composed of calcium carbonate or chitin, is the durable remaining outer part of the animal that created it.
Commensalism is an unusual form of symbiosis. Although in the case of a hermit crab and a snail, they have a long-term association, the snail would not be affected by the disappearance of the hermit crabs, while the hermit crab depends on a steady supply of empty shells to support its curved, soft body.
Hermit crabs rarely prey on snails unless forced to due to food deprivation or stress, so snails live their lives, and when they die, their shells become available to hermit crabs. Whether or not a hermit crab chooses a particular shell to inhabit is of no benefit to the snail or the remnants of the hard exoskeleton shell.
The Relationship Between Hermit Crabs and Sea Anemones
One of the most fascinating symbiotic relationships of the sea is the partnership between hermit crabs and sea anemones. This unlikely duo often teams up while they are small and stay together long-term, with the hermit crab moving its chosen sea anemone on to new larger shells when it does a swap.
The relationship between the hermit crab and the sea anemone is a truly symbolic liaison. The crab gains the protection of the sea anemone’s formidable sting, while the latter hitches a ride around on the hermit crabs shell and gets to feast on whatever morsels come drifting its way at each meal.
There are several sorts of symbiotic relationships that can occur between different kinds of creatures. The relationship between a hermit crab and sea anemone is mutualistic because both animals benefit and neither is harmed.
Hermit crabs also share a form of symbiosis with snails since they are dependent on a constant supply of larger shells for their survival. This form of symbiosis is called commensalism, as the snails, neither benefit nor suffer because of this.
Why Do Hermit Crabs Need Extra Shells?
Unlike regular crabs with hard exoskeletons, hermit crabs have soft, vulnerable abdomens. They have adapted to protect themselves by slipping into empty mollusk shells.
Although this is smart in terms of protection, it does leave a growing hermit crab with the ever-present challenge of finding perfectly sized new shells. Baby hermit crabs are tiny and need to seek the protection of a hard covering as soon as possible to avoid becoming easy pickings for a predator.
Depending on the species, hermit crabs can range in size from a few inches to almost as big as a coconut. So when keeping them as pets, one needs to keep a constant and varied supply of empty shell options available.
Hermit crabs need a curved or spiral shape inside the shell to securely twist their abdominal section around. They attach very securely to the hard shell. If your hermit crab seems loose inside its shell, it is quite possibly sick.
While the hard outside may look perfect to a human, hermit crabs have their own specific choice requirements. The shell needs to be the correct size, shape, and not have any cracks or holes in its structure to fit comfortably.
Although you may not observe any visible structural damage in a shell, even the slightest crack can affect the hermit crabs’ ability to regulate conditions around their gills. That is why it is vital to provide a selection of options for your pets – what looks perfect to a human may not be ideal for the hermit crab.
The most vital aspect for a hermit crab when it chooses a shell is to ensure that the opening, known as the aperture, is the correct size and shape. Even if the rest of the shell is a little too big, so long as the hermit crab can defend the opening with the hard front section of its body, the shell can be used.
To keep your pet hermit crabs comfortable, supply them with various shells. However, do not be tempted to use painted shells and always stick with natural ones. The paint can quickly peel or flake and often poisons the animal.
Final Thoughts
Hermit crabs and snails share a symbiotic relationship known as commensalism. This is a slightly different type of symbiosis because the snail neither benefits nor is harmed by the hermit crab using their shells.
Shells are the hard exoskeleton formed by mollusks, and when they die, their soft internal parts decompose or are eaten by predators. Hermit crabs use these empty shells as protection for their soft abdominal sections.
I have a bachelor’s degree in Film/Video/Media Studies, as well as an associates degree in Communications. I began producing videos and musical recordings nearly 15 years ago. I am a guitarist and bassist in Southwest MI and have been in a few different bands since 2009, and in 2012 I began building custom guitars and basses in my home workshop as well. When I’m home, I love spending time with my three pets (a dog, cat, and snake) and gardening in my backyard. I also like photographing wild birds, especially birds of prey.