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Squat lobsters are dorsoventrally flattened with long tails held curled beneath the cephalothorax. They are found in the two superfamilies Galatheoidea and Chirostyloidea, which form part of the Decapoda infraorder Anomura, alongside groups including the and Hippoidea. 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 the sea level. Some species form dense aggregations, either on the sea floor or in the water column, and a small number are commercially fished.
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, and three pairs of ), and five pairs of . The cephalothorax is covered with a thick carapace, which may extend forwards in front of the eyes to form a rostrum; this is highly variable among squat lobsters, being vestigial in Chirostylus, wide and often serrated in some genera, and long, narrow, and flanked with "supraorbital spines" in others. The degree of ornamentation on the surface of the carapace also varies widely, and there are almost always at least a few (bristles), which can be iridescence in some members of the Galatheidae and Munididae. A pair of also project on stalks from the front of the carapace; these are made up of ommatidium with square facets, which is typical of the "reflecting superposition" form of eye. Many deep-sea species have reduced eyes, and reduced movement of the eyestalks. In the families Munididae and Galatheidae, there is often a row of setae close to the eyes, forming "eyelashes".
The mouthparts consist of six pairs of appendages— three posterior cephalic appendages and the first three pairs of thoracic appendages. While their function was traditionally believed to be limited to food handling, the mouthparts have a more complex movement pattern that allows them to perform a variety of functions such as prey- and sediment-gathering, sediment transfer, and sediment sorting/particle rejection. In Munida sarsi, the farther the mouthparts are located from the mouth, the more complex in movement and functional scheme they are.
The most conspicuous appendages are the pereiopods, and the largest of these is the first pair. These each end in a chela (claw), and are therefore known as the "chelipeds"; they can be more than six times the body length, although some groups show sexual dimorphism, with females having proportionally shorter chelipeds. The following three pairs of pereiopods are somewhat smaller than the chelipeds and are without claws, but are otherwise similar; they are used for walking. The fifth pair of pereiopods are much smaller than the preceding pairs, and are held inconspicuously under the carapace. They each end in a tiny chela, and are generally believed to be used for cleaning the body, especially the , which are in a cavity protected by the carapace.
The pleon is made up of six somites, each bearing a pair of , and terminating in a telson. The first somite is narrower than the succeeding somites, and the last pair of pleopods are modified into , which flank the telson. The pleon is usually curled under the thorax, such that only the first three somites are visible from above. The form of the pleopods varies between the sexes. In females, the first one or two pairs are missing, while the remaining pairs are uniramous, and have long setae, to which the eggs can be attached. In males, the first two pairs are formed into , and are used to transfer the spermatophore to the female during mating; the first pair is often missing. The remaining pleopods can be similar to those of the females, or reduced in size, or entirely absent. In both sexes, the uropods are biramous.
Carcinisation has previously been explored in regards to outer morphology; however, the external change in body shaped has influence on the internal anatomical features as well. The use of micro-computer tomography and 3D reconstruction have brought to light anatomical disparity within Galatheoidea. Differences have been found in the ventral vessel system between porcelain crabs and squat lobsters. Carcinisation is also responsible for the loss of the caridoid escape reaction which caused a shift in gonads and the pleonal neuromeres for squat lobsters.
The development period of the embryo consists of five distinct stages in which the lasts several months. Throughout the five stages, both the diameter and volume of the egg increases. In the first stage of embryo development, the spherical egg is a uniform dark color. In stage II, the optic lobe and appendages begin formation. Stage III is when the abdominal segments and terminal spines begin to develop. In stage IV, there is pigmentation of the ocular lobe, segmentation of the maxillipeds, and cardiac movement. In the fifth and final stage, the eyes are enlarged and the abdomen is extended.
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.
Munida gregaria form aggregations in warm summer waters of the Pacific Ocean associated with river plume fronts, headland fronts, and shallow internal waves. Density of these aggregations are, on average, 2700 individuals per cubic meter. M. gregaria are able to aggregate in the pelagic region due to a number of unique features as compared to benthic squat lobsters, including fast swimming speeds, reduced density, reduced sinking rates as a result of greater morphological surface area, and optimized aerobic metabolism. M. gregaria also exhibit ontogenetic migration through larvae accumulation in highly productive nearshore waters, which then move toward the mid-continental shelf as they mature, and move completely offshore around full maturation.
In 2020, a study of squat lobsters determined that these crustaceans are far more diverse than previously thought. Through this study, 16 new species within the Leiogalathea genera were described. It was also revealed that diversity of squat lobsters in the Atlantic Ocean is relatively poor in comparison with the Pacific Ocean.
Squat lobsters are generally unaggressive toward each other, but instances can occur in particular scenarios. Individuals among dense populations will make decisions about whether to hunt for food or engage in deposit feeding on the basis of minimizing aggressive interactions. In general, squat lobsters exhibit no lasting dominance hierarchies, nor do they engage in territorialist behavior. 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. A 2001 study examined the effects of Serotonin and octopaminergic systems in Munida quadrispina, and found that injected serotonin elicits aggressive postures and behaviors, including increased likelihood and intensity of aggressive reactions to real or artificial squat lobsters, while injected octopamine reduced instances of aggressive behavior, including increasing the likelihood of escape responses.
Despite their worldwide distribution and great abundance, there are few functioning fishery for squat lobsters. Experimental fisheries have occurred in several countries, including Argentina, Mexico, and New Zealand, but commercial exploitation is restricted to Latin America, and chiefly to Chile. The main target species are Pleuroncodes monodon, P. planipes, and Cervimunida johni.
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 and , 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. Commercial fishing for squat lobsters in El Salvador began in the early 1980s; production increased markedly in the 2001 season, and has continued to grow, now when? making up 98% of the demersal resources landed in El Salvador, with annual catches peaking at 13,708 t in 2005. In Costa Rica, aggregations of squat lobsters are avoided, as the fishermen fear the squat lobsters will clog their nets. In Nicaragua, squat lobsters are heavily exploited, especially following a large increase in fishing effort in the 2007 season. In Panama, production reached 492 t in 2008. squat lobster fisheries initially targeted Cervimunida johni, beginning in 1953. By the mid-1960s, effort had largely switched to P. monodon. In an effort to conserve stocks, the Chilean government instituted quotas for squat lobsters, and the fishery is closely monitored. In New Zealand, Munida gregaria has been considered as a potential fisheries resource, particularly to feed farmed Chinook salmon ( Oncorhynchus tshawytscha).
In March 2022 it was reported that a squat lobster, possibly from the genus Munidopsis, had been filmed on the wreck of the Endurance, which sank in 1915 in the Antarctic. This was the first record of a living squat lobster in the Weddell Sea.
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