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‘ non-calcified abdominal exoskeleton makes their exogenous shelter system obligatory. Hermit crabs 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.
The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract.  The tip of the hermit crab’s abdomen is adapted to clasp strongly onto the columella of the snail shell. A few species do not use a “mobile home” and inhabit immobile structures left by polychaete worms, vermetid gastropods , corals , and sponges .
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, the occurrence of fights over empty shells will decrease or remain nonexistent. As the hermit crab grows in size, it must find a larger shell and abandon the previous one. Several hermit crab species, both terrestrial and marine , have been observed forming a vacancy chain to exchange shells.
For some larger marine species, supporting one or more sea anemones on the shell can scare away predators. The sea anemone benefits, because it is in position to consume fragments of the hermit crab’s meals. 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 . ^ a b Patsy A. McLaughlin; Tomoyuki Komai; Rafael Lemaitre; Dwi Listyo Rahayu (2010). “Annotated checklist of anomuran decapod crustaceans of the world (exclusive of the Kiwaoidea and families Chirostylidae and Galatheidae of the Galatheoidea) – Chapter: Part I Lithodoidea, Lomisoidea and Paguroidea” (PDF) .
“Mechanoreceptors innervating soft cuticle in the abdomen of the hermit crab, Pagurus pollicarus “. “Plastic pollution has killed half a million hermit crabs that confused trash for shells” . “Bryoliths constructed by bryozoans in symbiotic associations with hermit crabs in a tropical heterozoan carbonate system, Golfe d’Arguin, Mauritania”.
“New species, genera and a family of hermit crabs (Crustacea, Anomura, Paguroidea) from a mid-Cretaceous reef of Navarra, northern Spain”.
Whats the biggest a hermit crab can get?
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.
Can hermit crabs bite you?
Do Hermit Crabs Pinch or Bite? Hermit crabs do not have teeth in the conventional sense, so they do not bite. … Hermit crabs know how to use these claws to defend themselves. It is rare for a hermit crab to use their chelipeds out of aggression, but it does happen.
How long do pet hermit crabs live?
Hermit crabs can live for more than 30 years in their natural habitats on tropical seashores, but after being purchased, most do not live for more than a few months to a year. 4.
Hermit crabs make great pets! Our habitat setup guide and care tips cover enclosure, substrate, temperature, humidity, lighting, accessories, diet, nutrition, behavior, handling, and safety.
For two small crabs, we recommend at least a 10-gallon plastic or glass tank with a screen top to prevent escape, but more room may be required as they grow. Hermies are natural diggers, and its important that you provide enough deep moist sand to protect your crabs during the molting process.
Avoid calcium carbonate sand as it doesnt retain moisture and can stick to your crabs legs and abdomen. Place a sea sponge in each dish to allows crabs to get in and out safely and to help maintain humidity. Decorate their home with branches, half logs, driftwood, caves, and shells.
During this time, isolate them from their roommates and be careful not to drop them onto a hard surface; doing so can injure or kill them. If you notice any of these symptoms of illness or distress, contact a veterinarian: increased appetite or activity, staying outside of the shell, excessive molting, lost claws or limbs, strong odor from inside the shell.
Lots of people wonder how big can hermit crabs get, but the funny thing is that since there are over 1100 species of this animal, sizing can be a little difficult. So, first lets go over some hermit crab basics then lets talk about the size of various species out there. Keep in mind, the size of the hermit crab in question will depend on the particular species in question.
The abdomen of a hermit crab is specially designed to be able to clasp and hold onto the inside of a shell. The first kind is the marine hermit crab, which spends most of its time underwater, usually salt water.
While they do spend most of their time on land, they do need access to fresh and saltwater to keep their gills damp and able to take in oxygen. Scientifically named Coenobita clypeatus, the Caribbean crab can grow to be anywhere from 2 to 6 inches in length from front to back, with one claw being larger than the other. They tend to be deep purple and brown in color, a tan head, and a little red or orange on them.
Not only are they called strawberry hermit crabs because of their red coloring, but because their body shape also resembles it. Ruggie crabs, also known as Coenobita rugosus are fairly small and are usually known to grow to only around 6 cm in size. The average size of a full grown hermit crab is around 4 inches in length, but they can be bigger.
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 crustaceans 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.
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.
Development and reproduction
Hermit crab species range in size and shape, from species with a carapace only a few millimetres long toThe 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.Hermit crabs are often seen as a ‘throwaway pet’ that would live only a few months, but species such as
Hermit crabs are more closely related to squat lobsters and porcelain crabs than they are to true crabs (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.