Published online by Cambridge University Press: 05 December 2011
The place of crayfish among decapod crustaceans
Arthropods are undeniably the most successful animal group on Earth. When only terrestrial arthropods are considered, insects, spiders, mites and myriapods are almost ubiquitous, but in fact crustaceans are the most widely distributed group among the arthropods. They are at home in the sea, in fresh waters and in moist areas on land (notably crabs, isopod woodlice and some amphipods), limited only by their requirements for gas exchange, excretion and osmoregulation. No other group of animals or plants on the planet exhibits the range of morphological diversity seen among the extant Crustacea (Martin & Davis, 2001), of which there are an estimated 50000–67000 species. The group also includes the largest of the mobile invertebrates – some crabs, clawed and spiny lobsters, and freshwater crayfish may reach several kilograms in weight, and some stone crabs have leg spans of more than two metres. At the other end of the spectrum are parasitic barnacles and copepods and tiny parthenogenic cladocerans less than one millimetre in total length. Such diversity also has its parallel in the range of habitats exploited, making the Crustacea ideal for any study of biodiversity.
Class Malacostraca, the largest and most advanced group (containing more than 23000 species), includes almost all large and edible crustaceans, also the smaller but very abundant Peracarida (Isopoda, Amphipoda) and mysids as well as stomatopods (mantis shrimps) and euphausiids (krill).