Extreme environments are characterised by the presence of physicochemical stressors and provide unique study systems to address problems in evolutionary ecology research. Sulphide springs provide an example of extreme freshwater environments; because hydrogen sulphide's adverse physiological effects induce mortality in metazoans even at micromolar concentrations. Sulphide springs occur worldwide, but while microbial communities in sulphide springs have received broad attention, little is known

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... bout macroinvertebrates and fish inhabiting these toxic environments. We reviewed qualitative occurrence records of sulphide spring faunas on a global scale and present a quantitative case study comparing diversity patterns in sulphidic and adjacent non-sulphidic habitats across replicated river drainages in Southern Mexico. While detailed studies in most regions of the world remain scarce, available data suggests that sulphide spring faunas are characterised by low species richness. Dipterans (among macroinvertebrates) and cyprinodontiforms (among fishes) appear to dominate the communities in these habitats. At least in fish, there is evidence for the presence of highly endemic species and populations exclusively inhabiting sulphide springs. We provide a detailed discussion of traits that might predispose certain taxonomic groups to colonize sulphide springs, how colonizers subsequently adapt to cope with sulphide toxicity, and how adaptation may be linked to speciation processes. OPEN ACCESS Diversity 2014, 6 598 Species Locations References Synbranchidae Ophisternon aenigmaticum El Azufre II (Rio Tacotalpa drainage) and La Gloria (Rio Pichucalco drainage) springs, Mexico This study Cyprinodontidae Cyprinodon bobmilleri † Baños de San Ignacio (Rio San Fernando drainage), Mexico [60]; Schlupp (personal communication) Aphanius ginaonis † Ginao spring (Hormuzgan drainage), Iran [61,62] Aphanius dispar * Khurgu, Faryab, and Howba springs (Hormuzgan drainage), Dalaki and Mirahmad springs (Helleh drainage), Iran [62,63] Aphanius furcatus * Khurgu and Faryab springs (Hormuzgan drainage), Iran [62,63] Poeciliidae Acanthophacelus reticulata Poza Azufre (Rio San Juan drainage), Venezuela [64,65] Brachyrhaphis roseni Spring near David (Rio David drainage), Panama S. Ingley (personal communication) Gambusia affinis Vendome Well/Black Sulphur Springs (Red River drainage), Oklahoma [66,67] Gambusia eurystoma † Baños del Azufre (Rio Pichucalco drainage), Mexico [68,69] Gambusia holbrooki * Green (St. John's River drainage), Newport (Wakulla River drainage), and Panacea Mineral Springs (Dickerson Bay area), Florida [70] Gambusia sexradiata Mogote del Puyacatengo (Puyacatengo drainage), Mexico [71] (Lake Enriquillo basin), Dominican Republic [72,73] Poecilia formosa Baños de San Ignacio (Río San Fernando drainage), México Schlupp (personal communication) Poecilia latipinna Panacea Mineral Springs (Dickerson Bay area), Florida [74] Poecilia mexicana limantouri Baños de San Ignacio (Río San Fernando drainage), México Schlupp (personal communication) Poecilia mexicana mexicana El Azufre springs and Cueva del Azufre (Tacotalpa drainage), Mexico [75,76] Poecilia mexicana mexicana La Lluvia and Puyacatengo springs (Puyacatengo drainage), Mexico [76,77] Poecilia sulphuraria †, * Baños del Azufre and La Gloria springs (Pichucalco drainage), Mexico [76,77] Poecilia thermalis † La Esperanza springs (Ixtapangajoya drainage), Mexico [77] Poeciliopsis elongata Spring near David (Rio David drainage), Panama S. Ingley (personal communication) Priapichthys annectens Spring in Rincón de la Vieja National Park (Rio Colorado drainage), Costa Rica J. Johnson (personal communication) Priapichthys panamensis Spring near David (Rio David drainage), Panama S. Ingley (personal communication) Pseudoxiphophorus bimaculatus La Gloria springs (Pichucalco drainage), Mexico This study Xiphophorus hellerii * La Lluvia (Puyacatengo drainage) and La Gloria springs (Pichucalco drainage), Mexico This study † Species that were described from and are endemic to sulphide springs. * Species that potentially include populations that have independently colonised sulphide spring environments.