Crabs – The Animal Kingdom

Crabs are short-tailed crustaceans belonging to the order Decapoda (phylum Arthropoda). They are familiar as shrimp, lobster, crayfish and hermit crabs.

The tail and reduced abdomen are folded under the thorax or midsection, and covered by a hard exoskeleton. Crabs live in all oceans and on land. The Portunidae, or swimming crabs, have a fifth leg modified into a paddle-like dactyl to allow them to swim.


Eucarida is a superorder of crustaceans, comprising the decapods and the marine shrimp-like Euphausiacea, commonly known as krill. It is characterised by a carapace fused to all thoracic segments and the possession of stalked eyes.

The Pacific sand crab has a egg-shaped carapace and three teeth in front. It lives lower in the swash zone of wave-swept sandy beaches. It migrates up and down the beach with the tide and appears on Washington beaches mainly after El Nino events.

The phylogenetic relationships of eucarids have been disputed. Based on molecular data, the eucarids can be classified in two monophyletic groups. One group is paraphyletic and includes the order Euphausiacea, the other is polyphyletic and contains the orders Amphionidacea and Reptantia. Recent fossil sequences have established the clade of Reptantia as sister to the clade of Caridea and Decapoda.


Crabs, crayfish, lobsters and shrimp belong to this order of crustaceans (phylum Malacostracana). They are mostly marine, but many species have added freshwater or terrestrial adaptations. Their slender bodies usually have a carapace covering their heads and thorax. They possess a number of pairs of appendages, and the front three pair are usually enlarged and claw-like (‘chelate’) to perform functions including prey capture, defence and mating signals.

Males of some crayfish and crab species have larger claws than females to allow them to compete in courtship and mating. They also possess clasping organs to hold the female during spawning. Their legs are joined to the thorax by five pairs of segments which are capable of carrying biramous pleopods (swimming legs). They are respirational, absorbing oxygen through the front chelate pair of appendages, which function as gills when they are in water. This order is the most abundant in shallow warm waters. Larvae have a nauplius, protozoea and zoea stage before settling into adulthood.


Malacostracans are crustaceans of all sizes that occupy every zone from the tidal pools to coral reefs, from polar regions to deep sea depths. They are largely marine, but some amphipods and isopods have formed symbiotic or commensal relationships with fishes, aquatic reptiles, and other animals, while hemicaridean and gnathiid isopods are fully parasitic of decapods.

In brachyuran and astacidean lobsters and shrimps the first pair of thoracic legs is modified as large claws, known as cheliform appendages. The other thoracic legs are used for locomotion. These are jointed and adapted for swimming or walking, with the second pair usually being able to cling, or climb in inchworm fashion.

These are a diverse group that includes the familiar crabs, lobsters, hermit crabs, and shrimp. Most are gonochoristic, but some hermaphrodites occur as well. The gonads are located in the sixth thoracic segment of females and in the eighth segment of males. Many species have free-swimming naupliar larvae.


A class of widespread arthropods with a hard shell and segmented body. Crabs, lobsters, shrimp and crayfish are examples of crustaceans. They are chiefly aquatic, but some such as pill bugs (order Isopoda, family Gnathiidae) and wood lice (order Isopoda, family Oniscoidea) live on land.

Most crustaceans are free-swimming, scavengers that prey on living or recently dead animals and plants. Others are detrivores that recycle plant nutrients into the water.

Crustaceans are typically characterized by two pairs of antennae, mandibles and compound eyes, which are usually on stalks. Their bodies are divided into distinct tagmata – regions that have particular functions. The head, thorax and abdomen are the main tagmata.

The earliest crustacean larva is called the nauplius, which has three pairs of appendages and a single eye. Some anamorphic crustaceans skip this stage and emerge from eggs at a more advanced level of development, the zoea. Zoea larvae have both a cephalon and a thorax, and they swim using biramous thoracic appendages.

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