Forty thousand crustaceans share one twenty-segment body plan
Malacostraca is the second-largest pancrustacean class after insects, with about 40,000 living species across sixteen orders—crabs, lobsters, shrimp, krill, isopods, amphipods, mantis shrimp, and more. Aristotle’s “soft shell” label stuck via Latreille in 1802, though the cuticle hardens after each moult.
A shared tagma plan ties the class together: a five-segmented head, eight-segmented thorax, and usually a six-segmented abdomen plus telson—seven abdominal segments only in Leptostraca. Nearly every segment bears jointed appendages. Two pairs of antennae, stalked compound eyes in many taxa, and abdominal pleopods set them apart from most other crustaceans. Thoracic legs often end in seven articles; decapod claws form when the dactylus pinches against a propodus outgrowth. A tail fan of uropods and telson powers the famous backward escape flick.
Inside, a gastric mill grinds food before midgut caeca absorb nutrients, and haemocyanin carries oxygen in an open circulation. Habitats span every ocean, fresh water, and land: woodlice, some amphipods, and several crab lineages live ashore. Most species have separate sexes; gonopores sit on thoracic segments six (female) and eight (male). Molecular work in 2023 split the class into Phyllocarida, Peracarida, and Stomatocarida and found classical Eucarida and Eumalacostraca paraphyletic. Leptostracans, often called the most primitive living forms, retain Cambrian-deep roots and seven abdominal segments.
Stomatopods—mantis shrimps—range about 5 to 36 centimetres as ambush predators with folded raptorial claws, roughly three hundred species strong. Eucaridan giants such as crabs, shrimp, and krill dominate species counts, while peracarids brood young in a marsupium. Mating studies on the freshwater shrimp Caridina ensifera link multiple paternity to lower relatedness between successful sires and mothers, hinting at sperm competition or female choice against inbreeding.
Source: Malacostraca
