Crabs, Lobsters, and Relatives

This red rock crab is one of 25,000 living species of malacostracans.
Dream Pictures / Getty Images.

Crabs, lobsters, and their relatives (Malacostraca), also known as malacostracans, are a group of crustaceans that includes crabs, lobsters, shrimp, mantis shrimp, prawns, krill, spider crabs, woodlice and many others. There are about 25,000 species of malacostracans alive today.

The body structure of malacostracans is highly diverse. In general, it consists of three tagmata (groups of segments) including a head, thorax and abdomen.

The head consists of five segments, the thorax has eight segments and the abdomen has six segments.

The head of a malacostracan has two pairs of antennae and two pairs of maxillae. In some species, there is also a pair of compound eyes that are located at the end of stalks.

Pairs of appendages are also found on the thorax (the number varies from species to species) and some of the segments of the thorax tagma may be fused with the head tagma to form a structure known as the cephalothorax. All but the last segment of the abdomen bears a pair of appendages called pleopods. The last segment bears a pair of appendages called uropods.

Many malacostracans are brightly colored. They have a thick exoskeleton that is further strengthened with calcium carbonate.

The world's largest crustacean is a malacostracan—the Japanese spider crab (Macrocheira kaempferi) has a leg span of up to 13 feet.

Malacostrocans inhabit marine and freshwater habitats.

A few groups also live in terrestrial habitats, though many still return to water to breed. Malacostrocans are most diverse in marine environments.


Malacostracans are classified within the following taxonomic hierarchy

Animals > Invertebrates > Arthropods > Crustaceans > Malacostracans

Malacostracans are classified into the following taxonomic groups

  • Crabs, lobsters, and shrimp (Eumalacostraca) - There are about 40,000 species of lobsters, crabs, shrimp, and their relatives alive today. Members of this group include krill, lobsters, crabs, shrimp, prawns, mantis shrimp and many others. Within this group, the most familiar subgroups include the crabs (a group more than 6,700 species of 10-legged crustaceans that have a short tail and small abdomen that lies beneath the thorax) and the lobsters (of which there are several groups—the clawed lobsters, spiny lobsters and slipper lobsters).
  • Mantis shrimp (Hoplocarida) - There are about 400 species of mantis shrimp alive today. Members of this group bear a superficial resemblance of the praying mantis (which is an insect and thus is not closely related to mantis shrimp).
  • Phyllocaridans (Phyllocarida) - There are about 40 species of Phyllocaridians alive today. Members of this group are filter-feeding crustaceans. The most well-studied member of this group is Nebalia.
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Klappenbach, Laura. "Crabs, Lobsters, and Relatives." ThoughtCo, Feb. 18, 2018, Klappenbach, Laura. (2018, February 18). Crabs, Lobsters, and Relatives. Retrieved from Klappenbach, Laura. "Crabs, Lobsters, and Relatives." ThoughtCo. (accessed March 24, 2018).