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 from the Upper Jurassic of Russia. The fossil record of in situ hermit crabs using gastropod shells stretches back to the Late Cretaceous. 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. A few species do not use a "mobile home" and inhabit immobile structures left by polychaete worms, vermetid gastropods, corals, and sponges. However, this ability is not as developed as it is in land hermit crabs. 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. 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. The first group is the aquatic hermit crabs (almost all marine, with a single species, Clibanarius fonticola, in freshwater). Eight families are formally recognized in the superfamily Paguroidea, containing around 1100 species in total in 120 genera. 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.
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. However, the relationship of king crabs to the rest of Paguroidea has been a highly contentious topic. Hermit crabs are more closely related to squat lobsters and porcelain crabs than they are to true crabs ( Brachyura). Such physiological and behavioral extremes facilitate a transition to a sheltered lifestyle, revealing the extensive evolutionary lengths that led to their superfamily success. In most species, development involves metamorphosis from symmetric, free-swimming larvae to morphologically asymmetric, benthic-dwelling, shell-seeking crabs. 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. The strong association between hermit crabs and their shelters has significantly influenced their biology. Hermit crabs' soft (non- calcified) abdominal exoskeleton means they must occupy shelter produced by other organisms or risk being defenseless. There are over 800 species of hermit crab, most of which possess an asymmetric abdomen concealed by a snug-fitting shell.
Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons.
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