Squat lobsters are dorsoventrally flattened crustaceans with long tails held curled beneath the cephalothorax. They are found in the two superfamilies Galatheoidea and Chirostyloidea, which form part of the decapod infraorder Anomura, alongside groups including the hermit crabs and mole crabs. They are distributed worldwide in the oceans, and occur from near the surface to deep sea hydrothermal vents, with one species occupying caves above sea level. More than 900 species have been described, in around 60 genera. If one considers the members of the family Aeglidae as squat lobsters, then those numbers rise to more than 1000 species in 61 genera, and they occur at an altitude up to 3600 meters above sea level. Some species form dense aggregations, either on the sea floor or in the water column, and a small number are commercially fished.
Description
The two main groups of squat lobsters share most features of their morphology. They resemble true lobsters in some ways, but are somewhat flattened dorsoventrally, and are typically smaller, ranging from in length. Squat lobsters vary in postorbital carapace length (measured from the eye socket to the rear edge), from in the case of Munidopsis aries, down to only a few millimetres in the case of Galathea intermedia and some species of Uroptychus.
thumb|left|upright=1.2|Dorsal (left) and ventral (right) views of the [[holotype of Kiwa puravida (Kiwaidae); two pereiopods have broken off on the animal's left side.]]
The cephalothorax is made of 19 body segments (somites), although the divisions are not obvious and are most easily inferred from the paired appendages. From front to back, these are the two pairs of antennae, six pairs of mouthparts (mandibles, maxillae, maxillulae and three pairs of maxillipeds), and five pairs of pereiopods. In Munida sarsi, the farther the mouthparts are located from the mouth, the more complex in movement and functional scheme they are. The trend of larger numbers of eggs and smaller sized eggs is mostly found in lower latitudes and cooler temperatures in order to accommodate for the longer incubation time.thumb|Squat lobsters can form dense aggregations.
Ecology and behavior
Aggregation and migration
Benthic aggregation
Squat lobster aggregation is theorized to be proportional to the amount of available organic particulate carbon reaching the seafloor. As such, many species of benthic squat lobsters aggregate into groups of very high population density around a number of different types of highly productive areas of the deep sea, like hydrothermal vents, cold seeps, sites of food falls like whale falls or wood falls, and shipwrecks. Squat lobsters seem to aggregate in these areas because they are associated with the fauna that thrives in them, like Bathymodiolus mussels and vestimentiferan tubeworms, but they are also attracted to the three-dimensional structures found in them. Burrows, crevices, or debris from shipwrecks are suggested to serve as shelter against predators and as food accumulation sites. A variety of squat lobster called Emmunida picta were found to almost exclusively reside on some type of structure, mostly Lophelia pertusa.
Squat lobster species found on seamounts typically have smaller bodies with shorter larval stages, as opposed to rise and ridge habitats. It has been suggested that this is due to the difference in substrates at these habitats.
Claw position behavior
In a resting posture, squat lobsters rest their claws on the substrate in front of them. E. picta were observed most frequently in all conditions with their claws extended into the water column, perpendicular to the substrate. In the case of this study, the behavior was thought to be an avoidance response to the surveillance submersible, hence why this behavior was so often observed. In general, it is thought that this behavior may be a mechanism to increase the perceived size of the squat lobster as an aggressive or perhaps illusory display to ward off predators, as well as an active "fishing" strategy to catch prey. When aggressive displays do occur, as a result of competition for mates or food, the aggressive behavior is ignored 70% of the time, met with submission 20% of the time, and met with reciprocal aggression 10% of the time. These species of squat lobster also displayed very short interbrood intervals, or time between mating, generally not longer than a few days. As well as being used for human consumption, there is demand for squat lobster meat to be used as feed in fish farms and shrimp or prawn farms. This is in part because they contain astaxanthin, a pigment that helps to colour the meat of farmed salmon and trout.
In Central America, the primary species of squat lobster targeted by fisheries is a species of Pleuroncodes. There is a great deal of confusion over both scientific names and common names, and the exact species is often unknown. In El Salvador, for instance, the commercial catch is generally referred to as "P. planipes", but is in fact P. monodon. Chirostyloidea contains the families Chirostylidae, Eumunididae, and Kiwaidae. Galatheoidea contains the families Galatheidae, Munididae, Munidopsidae, and Porcellanidae. The systematics of squat lobsters and the classification of deep-sea squat lobsters is an area of active research due to the limited fossil record. Molecular and morphological data indicate that Galatheoidea is not a monophyletic group; Galatheidae, Porcellanidae, Kiwaidae, and Chirostylidae have independent origins. divided into over 900 recognized species; more than 120 undescribed species likely exist. It is likely that squat lobsters underwent deep sea colonization multiple times in evolutionary history. Squat lobsters underwent rapid diversification in the late Oligocene through the Miocene likely due to a variety of selection pressures, beginning in the Southwest Pacific. Fossil galatheoid squat lobsters have been found in strata dating back to the Middle Jurassic of Europe. No fossils are currently assigned to the Chirostyloidea. A split in the galatheid fauna exists between the Eastern Pacific, containing much less species richness, and the Western Pacific.