Macrobrachium

Macrobrachium is a genus of freshwater prawns or shrimps characterised by the extreme enlargement of the second pair of pereiopods, at least in the male. The genus is cosmopolitan, found throughout the tropical, freshwater and estuarine ecosystems of both the Old and New Worlds. The genus has a difficult taxonomic history due to morphological conservation between its species (plesiomorphy), and conversely, variations within a single species (polymorphism), leading to the discovery of cryptic species which were only revealed through genetic analysis and morphometry. This polymorphism may be especially apparent among males, where it is termed heterogeneous individual growth (HIG); in males, this determines the size and morphology that they may grow into, as well as their physiology and behavior. Many species of Macrobrachium are of high commercial value, especially the larger species such as those from the M. rosenbergii species group; these are both shrimped and cultivated throughout the world. Escapees of aquaculture operations have been found to have established breeding populations, though their ecological impact is little studied. In addition to their commercial value, these crustaceans are thought to provide various ecosystem services to humans: a study in the Senegal River basin found that large dams have been implicated in the spread of schistosomiasis, a disease caused by Schistosoma flatworms carried by snails. In general, dams inhibit the reproduction and migration of various riverine species, including Macrobrachium spp.; as these prawns are "voracious" predators of snails in all life stages, damming has been linked to marked increases in human schistosomiasis around dammed areas. It is estimated that restoration of Macrobrachium populations could benefit around 300 million people, one third to one half of the at-risk population.