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True Crabs  Hermit Crabs
Crabs typically walk sideways  (a behaviour which gives us the word crabwise). This is because of the articulation of the legs which makes a sidelong gait more efficient . However, some crabs prefer to walk forwards or backwards, including raninids , Libinia emarginata  and Mictyris platycheles]. Some crabs, notably the Portunidae and Matutidae, are also capable of swimming].

Crabs are mostly active animals with complex behaviour patterns. They can communicate by drumming or waving their pincers. Crabs tend to be aggressive towards one another and males often fight to gain access to females . On rocky seashores, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes .

Crabs are omnivores, feeding primarily on algae , and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness .

Crabs are known to work together to provide food and protection for their family, and during mating season to find a comfortable spot for the female to release her eggs .

Hermit crabs are decapod crustaceans of the superfamily Paguroidea. They are not closely related to true crabs. Hermit crabs are quite commonly seen in the intertidal zone: for example, in tide pools.

Most species have long, soft abdomens which are protected from predators by a salvaged empty seashell carried on the crab's back, into which the crab's whole body can retract. Most frequently hermit crabs use the shells of sea snails; the tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell . As the hermit crab grows in size, it has to find a larger shell and abandon the previous one. This habit of living in a second hand shell gives rise to the popular name "hermit crab", by analogy to a hermit who lives alone[citation needed].

Of about five hundred known species, most are aquatic and live in varying depths of saltwater, from shallow reefs and shorelines to deep sea bottoms. However, tropical areas host some terrestrials.

The king crabs in the family Lithodidae were formerly considered to be derived hermit crabs, but are now placed in a separate superfamily.

As hermit crabs grow they require larger shells. Since suitable intact gastropod shells are sometimes a limited resource, there is often vigorous competition among 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 .

A hermit crab with a too-small shell cannot grow as fast as those with well-fitting shells, and is more likely to be eaten if it cannot retract completely into the shell .

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.

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