Hermit crab

A hermit crab has a soft body.

It needs a shell to stay safe. The crab finds an empty snail shell. It crawls inside to hide.

As the crab grows, it needs a bigger home. It may fight other crabs for a shell.

Some crabs live on land. They still need water to stay wet. This helps them breathe.

They are very smart. They can even swap shells with friends!

Hermit crabs are special animals. Most of them have soft bodies.

Because their bodies are soft, they need a hard home. They find empty snail shells to live in. The crab crawls inside the shell to hide.

This shell protects their weak bellies from danger.

As a hermit crab grows, it needs a bigger shell. Sometimes, crabs will fight over a good shell.

They can even swap shells with each other! One crab will tap its shell against another. Then, they try to switch homes.

Some hermit crabs live on land. These are called terrestrial hermit crabs.

They live in warm, tropical places. They must stay near water. They need to keep their gills wet to breathe.

To grow, they must molt. Molting is a way they shed their old skin. The crab crawls out of its old shell. It then grows a new, hard skin. This can take up to 60 days. After it is done, the crab eats its old shell.

Hermit crabs are fascinating animals that have found a clever way to stay safe. Most of them belong to a group called Paguroidea. Unlike many other crabs, their abdomens are soft and do not have a hard shell.

Because their bellies are so vulnerable, they must find a home to hide in. They scavenge for empty snail shells to use as mobile shelters.

This habit of carrying a house has helped them thrive in almost every marine environment on Earth. There are more than 800 different species of these shell-seeking creatures.

Finding a good home is a very important part of their lives. As a hermit crab grows, it eventually outgrows its current shell. This means they must constantly look for larger ones.

Sometimes, two crabs might fight over the same perfect shell. They even perform a special behavior called shell exchanging.

One crab will approach another and tap its shell against the other's. They may even pull each other out of their homes to see if a new shell fits better. This helps them find the best protection for their growing bodies.

Scientists have studied how these animals are related to other sea life. Hermit crabs are actually more closely related to squat lobsters and porcelain crabs. They are part of a group called Anomura.

For a long time, experts argued about king crabs. They wondered if king crabs were actually just a special kind of hermit crab. In 2023, new data helped settle this. Researchers placed the king crab family, Lithodidae, back into the Paguroidea group. This shows how much our understanding of nature changes with new discoveries.

We can even see how hermit crabs lived a very long time ago. Fossils show that some hermit crabs used ammonite shells in the past.

The oldest known species is called Schobertella hoelderi from Germany. This species lived during the Jurassic period. Some hermit crabs can live for a long time. For example, the Indos Crab can live for 12 to 70 years. They can grow to be as large as a coconut!

Hermit crabs live in many different places. Some live in the ocean, but others live on land. Land hermit crabs belong to the family Coenobitidae.

They live in warm, tropical areas. Even though they live on land, they must stay near water. They need to keep their gills wet so they can breathe. Some land crabs even use man-made things like glass bottles or light bulbs for homes. This is a hard job because these items are not natural like snail shells.

Hermit crabs are specialized crustaceans belonging to the superfamily Paguroidea. Unlike many other crabs, they possess a soft, non-calcified abdomen. This lack of a hard exoskeleton makes their undersides extremely vulnerable to predators.

To compensate, they have adapted to live inside empty shells scavenged from gastropods, such as sea snails. This unique lifestyle of carrying a mobile shelter is a defining trait of the group. There are over 800 species of hermit crabs found in almost all marine environments. This protective mobility has allowed them to thrive across the globe.

The biological mechanism of their survival relies on the physical relationship between the crab and its shell. Most species have a long, spirally curved abdomen that is specifically shaped to fit inside a shell. The tip of the abdomen is adapted to clasp strongly onto the columella, which is the central pillar of a snail shell.

This tight fit allows the crab to retract its entire body inside for safety. While most use gastropod shells, some species use bivalves, scaphopods, or even pieces of wood and stone. This reliance on external structures is so strong that it has significantly influenced their entire biology.

Hermit crabs undergo a complex developmental process involving metamorphosis. They begin as symmetric, free-swimming larvae. The first two stages, known as the nauplius and protozoea, occur inside the egg.

Most larvae hatch at the third stage, called the zoea, which features long spines and large, fringed antennae. After several molts, they reach the megalopa stage. This final larval stage leads to their adult form, which is morphologically asymmetric and lives on the seafloor. This transition from a swimming life to a shell-seeking life shows the extensive evolutionary changes within the superfamily.

Growth in hermit crabs requires a regular process called molting. To grow, they must shed their hard exoskeleton. This process can take up to 60 days and involves four distinct stages. First is intermolt, where the crab stores energy. This is followed by proecdysis, where the old exoskeleton begins to shed and a new one forms underneath. The third stage is ecdysis, the main phase where the crab crawls out of the old shell. Finally, in postedysis, the new exoskeleton hardens. During this time, the crab may even eat its old exoskeleton to recover nutrients.

Classification of these animals has been a subject of intense scientific study. Hermit crabs belong to the infraorder Anomura. This makes them more closely related to squat lobsters and porcelain crabs than to true crabs, which belong to Brachyura.

For many years, the classification of king crabs, or the family Lithodidae, was highly contentious. Some argued they were a separate group, but molecular data eventually proved they are actually derived hermit crabs. Consequently, in 2023, the family Lithodidae was officially placed back into the Paguroidea superfamily.

Hermit crabs exhibit fascinating social and survival behaviors, such as shell exchanging. This is an elaborate interaction where one crab, the initiator, attempts to trade shells with a non-initiator. The initiator may approach the other, use its antennae for contact, or even perform "shell rapping." This involves striking its own shell against the other's in rhythmic movements.

The goal is to obtain a better-fitting shell. Because suitable shells are a limited resource, competition can become intense. Crabs may fight or even kill competitors to gain access to a preferred shell.

Species vary greatly in their habitats and sizes. Terrestrial hermit crabs, found in the family Coenobitidae, live mostly on land in tropical areas.

While they live on land, they must stay near water to keep their gills damp for breathing. Sizes range from a few millimeters to the Indos Crab, which can reach the size of a coconut and live up to 70 years. The coconut crab is the world's largest terrestrial invertebrate. Interestingly, recent research in 2024 found that some land species are now using human debris, like glass bottles and plastic, as makeshift homes.