Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons.

Hermit crabs' soft (non-calcified) abdominal exoskeleton means they must occupy shelter produced by other organisms or risk being defenseless.

In most species, development involves metamorphosis from symmetric, free-swimming larvae to morphologically asymmetric, benthic-dwelling, shell-seeking crabs.

Such physiological and behavioral extremes facilitate a transition to a sheltered lifestyle, revealing the extensive evolutionary lengths that led to their superfamily success.

Together with the squat lobsters and porcelain crabs, they all belong to the infraorder Anomura, the sister taxon to Brachyura.

Many studies based on their physical characteristics, genetic information, and combined data demonstrate the longstanding hypothesis that the king crabs in the family Lithodidae are derived hermit crabs descended from pagurids and should be classified as a family within Paguroidea.

[4][5][6][7] The molecular data has disproven an alternate view based on morphological arguments that the Lithodidae (king crabs) nest with the Hapalogastridae in a separate superfamily, Lithodoidea.

The fossil record of in situ hermit crabs using gastropod shells stretches back to the Late Cretaceous.

Before that time, at least some hermit crabs used ammonite shells instead, as shown by a specimen of Palaeopagurus vandenengeli from the Speeton Clay Formation, Yorkshire, UK, from the Lower Cretaceous,[14] as well as a specimen of a diogenid hermit crab from the Upper Jurassic of Russia.

They spend most of their life on land in tropical areas, though they require access to water to keep their gills damp or wet to survive and to reproduce.

The abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract.

[19] The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell.

[20] Hermit Crab young develop in stages, with the first two (the nauplius and protozoea) occurring inside the egg.

The availability of empty shells at any given place depends on the relative abundance of gastropods and hermit crabs, matched for size.

An equally important issue is the population of organisms that prey upon gastropods and leave the shells intact.

Even if they were able to survive, hollowing out a shell takes precious energy, making it undesirable to any hermit crab.

These shells can last for generations, explaining why some hermit crabs are able to live in areas where snails have become locally extinct.

[31] There are cases when seashells are not available and hermit crabs will use alternatives such as tin cans, custom-made shells, or any other types of debris, which often proves fatal to the hermit crabs (as they can climb into, but not out of, slippery plastic debris).

[33] For some larger marine species, supporting one or more sea anemones on the shell can scare away predators.

The sea anemone also benefits, because it is in a prime position to consume fragments of the hermit crab's meals.

[34] In February 2024, Polish researchers reported that 10 of 16 terrestrial hermit crab species were observed using artificial shells, including discarded plastic waste, broken glass bottles and light bulbs, in lieu of natural shells.

[22] Several hermit crab species, both terrestrial and marine, have been observed forming a vacancy chain to exchange shells.

The first one consists of the crab lifting its whole body (shell included), and spreading its legs, then moving its cheliped forward until the dactylus (top part of the claw) is perpendicular with the ground.

The second variation called the "cheliped extension", is usually a purely visual movement, though it may sometimes be used to strike a crab.

[22] Some species such as Clibanarius tricolor, Calcinus tibicen and Pagurus miamensis are semi gregarious, showing unique behaviors in groups.

Specimens of P. hemphilli tolerated the presence of their guest, while other hermit crab species attempted eating them.

[40][41] Some of the exterior associates are the epifauna, such as barnacles and Crepidula, which may be a hindrance to the crabs, as they may ruin the stability or just add weight to the shell.

[42] Despite their reputation as 'throwaway' and 'low maintenance' pets, hermit crabs can actually live for up to 15 or more years with proper care.

[48] Hermit crabs should be kept in glass tanks of an appropriate size in order to maintain the humidity and temperature needed.

All water should be treated to remove chemicals, and saltwater should be prepared using a marine grade salt mix.