Hermit crab

Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an asymmetric abdomen concealed by a snug-fitting shell. Hermit crabs' soft (non-calcified) abdominal exoskeleton means they must occupy shelter produced by other organisms or risk being defenseless.

The strong association between hermit crabs and their shelters has significantly influenced their biology. Almost 800 species carry mobile shelters (most often calcified snail shells); this protective mobility contributes to the diversity and multitude of these crustaceans which are found in almost all marine environments. 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.

Classification
The hermit crabs of Paguroidea are more closely related to squat lobsters and porcelain crabs than they are to true crabs (Brachyura). Together with the squat lobsters and porcelain crabs, they all belong to the infraorder Anomura, the sister taxon to Brachyura.

However, the relationship of king crabs to the rest of Paguroidea has been a highly contentious topic. 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. 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. As such, in 2023, the family Lithodidae was placed back into Paguroidea after having been moved out of it in 2007.

Nine families are formally recognized in the superfamily Paguroidea, containing around 1200 species in total in 135 genera.
 * Calcinidae Fraaije, Van Bakel & Jagt, 2017 – seven genera
 * Coenobitidae Dana, 1851 – two genera: terrestrial hermit crabs and the coconut crab
 * Diogenidae Ortmann, 1892 – 20 genera of "left-handed hermit crabs"
 * Lithodidae Samouelle, 1819 – 15 genera of "king crabs"
 * Paguridae Latreille, 1802 – 76 genera
 * Parapaguridae Smith, 1882 – 10 genera
 * Parapylochelidae Fraaije et al., 2012 – two genera
 * Pylochelidae Bate, 1888 – 9 genera of "symmetrical hermit crabs"
 * Pylojacquesidae McLaughlin & Lemaitre, 2001 – two genera

Phylogeny
The placement of Paguroidea within Anomura can be shown in the cladogram below, which also shows the king crabs of Lithodidae as sister taxon to the hermit crabs of Paguridae:

Fossil record
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, as well as a specimen of a diogenid hermit crab from the Upper Jurassic of Russia. The earliest record of the superfamily extends back to the earliest part of the Jurassic, with the oldest known species being Schobertella hoelderi from the late Hettangian of Germany.

Aquatic and terrestrial hermit crabs
Hermit crabs can be informally divided into two groups: aquatic hermits crabs and terrestrial hermit crabs.

The first group is the aquatic hermit crabs (almost all marine, with a single species, Clibanarius fonticola, in freshwater). These crabs spend most of their lives underwater as aquatic animals, living in depths of saltwater that range from shallow reefs and shorelines to deep sea bottoms, rarely leaving the water for land. They breathe through gills but do not have to carry around their water to do so, and most can survive briefly out of water as long as their gills are damp. However, this ability is not as developed as it is in land hermit crabs. A few species do not use a "mobile home" and inhabit immobile structures left by polychaete worms, vermetid gastropods, corals, and sponges.

The second group, the land hermit crabs, spend most of their life on land as terrestrial species in tropical areas, though even they require access to both freshwater and saltwater to keep their gills damp or wet to survive and to reproduce. They belong to the family Coenobitidae.

Description
Most species have long, spirally curved abdomens, which are soft, unlike the hard, calcified abdomens seen in related crustaceans. The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract. Most frequently, hermit crabs use the shells of sea snails (although the shells of bivalves and scaphopods and even hollow pieces of wood and stone are used by some species). The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell. Most hermit crabs are nocturnal.

Development and reproduction
Hermit crab species range in size and shape, from species with a carapace only a few millimetres long to Coenobita brevimanus, which can live 12–70 years and can approach the size of a coconut. The shell-less hermit crab Birgus latro (coconut crab) is the world's largest terrestrial invertebrate.

The young develop in stages, with the first two (the nauplius and protozoea) occurring inside the egg. Most hermit crab larvae hatch at the third stage, the zoea. In this larval stage, the crab has several long spines, a long, narrow abdomen, and large fringed antennae. Several zoeal moults are followed by the final larval stage, the megalopa.

The sexual behavior exhibited by hermit crabs varies from species to species, but a general description is as follows. If the female possesses any larvae from a previous mating, she moults and lets them go. Female hermit crabs are ready to mate shortly before moulting, and she may come in contact with a male. In certain species the male grabs the pre-moult female for sometimes hours. Prior to the female moulting, and usually continuing after she has moulted, the male performs precopulatory behaviors. These vary widely but the most common are rotating/shaking the female, and jerking the female towards the male.

After some time, the female moves the chelipeds in her mouth region, signaling the male. Then they both move their bodies mostly out of their shells, and mate. Both crabs then go back inside their shells, and they may mate again. In some species the male performs post-copulatory behavior until the female has the eggs on her pleopods.

Shells and shell remodeling
As hermit crabs grow, they require larger shells. Since suitable intact gastropod shells are sometimes a limited resource, competition often occurs between hermit crabs for shells. 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. Hermit crabs kept together may fight or kill a competitor to gain access to the shell they favour. However, if the crabs vary significantly in size, fights over empty shells are rare. Hermit crabs with undersized shells cannot grow as fast as those with well-fitting shells, and are more likely to be eaten if they cannot retract completely into the shell.

Shells used by hermit crabs have usually been remodeled by previous hermit crab owners. This involves a hermit crab hollowing out the shell, making it lighter. Only small hermit crabs are able to live without remodelled shells. Most big hermit crabs that have been transferred to a normal shell die. Even if they were able to survive, hollowing out a shell takes precious energy, making it undesirable to any hermit crab. They achieve this remodeling by both chemically and physically carving out the interiors of their shell. These shells can last for generations, explaining why some hermit crabs are able to live in areas where snails have become locally extinct.

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). This can even create a chain reaction of fatality, because a dead hermit crab will release a signal to tell others that a shell is available, luring more hermit crabs to their deaths. More specifically, they are attracted to the scent of dead hermit crab flesh.

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. Other very close symbiotic relationships are known from encrusting bryozoans and hermit crabs forming bryoliths.

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.

Shell fighting
Shell fighting is a behavior observed in all hermit crabs. It is a process in which the attacker hermit crab attempts to steal the shell of the victim, using a fairly intricate process. It usually only occurs if there is no empty shell suitable for the growing hermit crab. These fights are usually between the same species, though they can also occur between two separate species.

If the defending crab does not retreat to the inside of its shell, an aggressive interaction will usually take place, until the defending crab retreats, or the attacker flees. After the defender has retreated, the attacker will usually turn the shell over multiple times, holding it with its legs. It then places its chelipeds into the shell's opening.

Then the crabs start the "positioning" behavior, this consists of the attacker moving side to side, over the opening of the defender's shell. This movement usually forms a figure 8. The attacker then goes into the aptly named "rapping" behavior. The attacker holds its legs and chepelothorax stationary, while it moves its shell down on the defender's shell. It is done quite rapidly, and is usually enough to produce an audible sound. It seems like little to no contact happens directly between the two crabs.

After a number of "raps", the defender may come out of its shell completely, usually positioning itself of one of the shells. The attacker then checks the now free shell, and then changes shell rapidly. As the crab tries its new shell, it usually holds its old shell, as it may decide to come back to the old one. The defeated crab then runs to the empty shell. If the defeated crab does not stay close to the shells, it is usually eaten.

Several hermit crab species, both terrestrial and marine, have been observed forming a vacancy chain to exchange shells. When an individual crab finds a new empty shell, or steals one from another, it will leave its own shell and inspect the vacant shell for size. If the shell is found to be too large, the crab goes back to its own shell and then waits by the vacant shell for up to 8 hours. As new crabs arrive they also inspect the shell and, if it is too big, wait with the others, forming a group of up to 20 individuals, holding onto each other in a line from the largest to the smallest crab. As soon as a crab that is the right size for the vacant shell arrives and claims it—leaving its old shell vacant—all the crabs in the queue swiftly exchange shells in sequence, each one moving up to the next size. If the original shell was taken from another hermit crab, the victim is usually left without a shell, and gets eaten. Hermit crabs often "gang up" on one of their species with what they perceive to be a better shell, and pry its shell away from it before competing for it until one takes it over.

Aggressive behaviors
Aggressive behaviors for hermit crabs are quite similar to one another, with some variations present between species. It usually consists of moving or positioning the legs and the chelipeds, also known as the claw or pincer. Usually these displays are enough to avoid confrontation. Sometimes two opposing crabs will do multiple actions, with no apparent pattern. These confrontations usually last a few seconds, though some may last a few minutes, for those especially stubborn crabs.

They can also raise a leg which is sometimes referred to as an "ambulatory raise". This can happen with multiple legs such as with the first two walking legs, or both the first and second pair. This is referred to as "double ambulatory raise", and "quadruple ambulatory raise", respectively. The exact form of this movement is variable between species. In some other species there is another distinct movement, where they move their leg away and upwards from the body, while it moves forwards, this same movement continues as the limb is brought down. This movement is sometimes called an "ambulatory poke".

They also use their chelipeds as a warning display, usually used in two distinct variations. 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. This movement is usually called an "cheliped presentation" This position may be more distinct in some species, such as those in the genus Pagurus. The second variation called the "cheliped extension", is usually a purely visual movement, though it may sometimes be used to strike a crab. The chelipeds move forward and upwards, until the limb is parallel with the ground, usually used to push another crab out of the way. If a larger crab pushes a smaller one, the smaller one may be moved multiple centimeters.

The crabs of the family Paguridae, have another distinct type of movement. Individuals may crawl upon another's crab shell. If the size is just right the crab climbed upon may move rapidly up and down or sideways, usually causing the other crab to fall off.

Grouping behavior
Some species such as Clibanarius tricolor, Calcinus tibicen and Pagurus miamensis are semi gregarious, showing unique behaviors in groups. While these three species all show gregarious behavior, C. tricolor, forms the densest and bigger groups. The crabs of Clibanarius tricolor congregate during the day, and usually stay with their same respective group, day after day. At 4:00 p.m. the crabs would start moving in their groups, and by 5:00 p.m. they had left their congregation. The congregations usually move in one general direction, and may be close to other crabs. This behavior seems to be lost under controlled conditions, however.

Associations with other animals
The shells of hermit crabs have multiple "associates" whose exact roles have not been well described. These associates are usually categorized into two groups, those who live in the interior of the shell and those who live on the exterior. Some of the interior associates include nereid worms which have a commensal relation, the worms help the hermit crabs keep their shell clean along with the crabs of the family Porcellanidae. It is not rare to see both the worms and crabs in the same shell.



There are also associations with Amphipods, such as the relationship between the hermit crab species Pagurus hemphilli and the amphipod genus Liljeborgia. The coloration of this amphipod matches the coloration of the hermit crab and the Crustose rhodophycean algae which commonly grows in their shells. Specimens of P. hemphilli tolerated the presence of their guest, while other hermit crab species attempted eating them.

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. Some species of hermit crabs have live colonies of Hydractina, while others rejected them. Some species just keep the colony in their shells, while others are actively detaching and re-attaching the sea anemone. Most hermit crabs attempt to place the most anemones as possible, while some others steal the anemone another hermit crab is carrying. There is a mutually beneficial relationship between the two, as they help defend against predators.

Hermit crabs as pets
Several marine species of hermit crabs are common in the marine aquarium trade. They are commonly kept in reef fish tanks.

Of the approximately 15 terrestrial species of genus Coenobita in the world, the following are commonly kept as pets: Caribbean hermit crab (Coenobita clypeatus), Australian land hermit crab (Coenobita variabilis), and the Ecuadorian hermit crab (Coenobita compressus). Other species, such as Coenobita brevimanus, Coenobita rugosus, Coenobita perlatus or Coenobita cavipes, are less common but growing in availability and popularity as pets.

Hermit crabs are often considered to be 'throwaway pets' that only live for a few months, but species such as Coenobita clypeatus have a 20-year lifespan if properly cared for, and some have lived longer than 32 years.