Hypogean (cave, artesian) fishes have fascinated researchers even before they were described in the scientific literature in 1842. Since then, a number of scientists have used them to justify their own evolutionary ideas, from neo-Lamarckism to neo-Darwinism, from neutral evolution to selectionist approaches. Research in recent years has shown that these fishes are much more complex in their adaptations to the subterranean environment than previously believed: there are those with features expected from living in total darkness (complete blindness and depigmentation) and poor in nutrients (extremely low metabolic rates); others differ very little, if any, from their epigean (surface) ancestors in their morphology and physiology (but not so in their behavior). Some of them even live in nutrient-rich environments. Actually, one of the most overlooked facets of these animals is that there are more species of hypogean fishes without troglomorphisms (blindness, depigmentation) than with troglomorphic ones. The study of these apparently unadapted' fishes is providing new insights into our understanding of the evolution of phenotypic characters, founding effect, behavioral, and physiological adaptations. The 86 species of troglomorphic fishes described so far belong to 18 different families, many of which would hardly fit the notion that they were 'preadapted' to conquer the underground environment. Further, many troglomorphic species' show very little genotypic differentiation when compared with their putative ancestors, indicating that massive phenotype changes can be achieved via little genetic reorganization, a reorganization that mostly affects regulatory genes. These and many other topics are discussed in this volume containing 29 papers, written by 41 authors from 9 countries. Hopefully, this volume will convince many other researchers that hypogean fishes represent a unique opportunity to study a concept in evolutionary biology that is only superficially understood: convergent evolution.

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An introduction to the special volume on the biology of hypogean fishes; A. Romero. It's a wonderful hypogean life: a guide to the troglomorphic fishes of the world; A. Romero, K.M. Paulson. Scientists prefer them blind: the history of hypogean fish research; A. Romero. Nontroglobitic fishes in Bruffey-Hills Creek Cave, West Virginia, and other caves worldwide; W.J. Poly. Part 1: Anatomy, physiology and behavior. The mechanosensory lateral line system of the hypogean form of Astyanax fasciatus; J.C. Montgomery, et al. Metabolic rate and natural history of Ozark cavefish, Amblyopsis rosae, in Logan Cave, Arkansas; G.L. Adams, J.E. Johnson. Locomotory responses of the dave cyprinid Phreatichthys andruzzii to Chemical signals from conspecifics and related species: new findings; R. Berti, L. Zorn. Responses to light in epigean and hypogean populations of Gambusia affinis (Cyprinodontiformes: Poeciliidae); M.M. Camassa. Temporal organization in locomotor activity of the hypogean loach, Nemacheilus evezardi, and its epigean ancestor; A.K. Pati. Part 2: Ecology and conservation. Ecology of subterranean fishes: an overview; E. Trajano. Population biology and growth of Ozark cavefish in Logan Cave National Wildlife Refuge, Arkansas; J.Z. Brown, J.E. Johnson. Threatened fishes of the world: Ophisternon infernale (Hubbs, 1938) (Synbranchidae); R. Medina-González, et al. How hydrogeology has shaped the ecology of Missouri's Ozark cavefish, Amblyopsis rosae, and southern cavefish, Typhlichthys subterraneus: insights on the sightless from understanding the underground; D.B. Noltie, C.M. Wicks. Habitat and population data of troglobitic armored cave catfish, Ancistrus cryptophthalmus Reis, 1987, fromcentral Brazil (Siluriformes: Loricariidae); E. Trajano. The conservation status of hypogean fishes; G.S. Proudlove. Threatened fishes of the world: Ogilbia pearsei (Hubbs, 1938) (Bythitidae); G. Proudlove, et al. Status of the federally endangered Alabama cavefish, Speoplatyrhinus poulsoni (Amblyopsidae), in Key Cave and surrounding caves, Alabama; B.R. Kuhajda, R.L. Mayden. Part 3: Genetics and evolution. Genetic differentiation among populations of the cave fish Schistura oedipus (Cypriniformes: Balitoridae); R.B. Borowsky, L. Mertz. Origins and relationship of cave populations of the blind Mexican tetra, Astyanax fasciatus, in the Sierra de El Abra; L. Espinasa, R.B. Borowsky. Threatened fishes of the world: Caecobarbus geertsii Boulenger, 1921 (Cyprinidae); G.S. Proudlove, A. Romero. Morphological and physiological correlates of evolutionary reduction of metabolic rate among amblyopsid cave fishes; T.L. Poulson. Convergent adaptations to cave life in the Rhamdia laticauda catfish group (Pimelodidae, Teleostei); H. Wilkens. A review of morphological and behavioural changes in the cave molly, Poecilia mexicana, from Tabasco, Mexico; J. Parzefall. Part 4: Special cases. Troglomorphic sculpins of the Cottus carolinae species group in Perry County, Missouri: distribution, external morphology, and conservation status; B.M. Burr, et al. Milyeringa veritas (Eleotridae), a remarkably versatile cave fish from the arid tropics of northwestern Australia; W.F. Humphreys. Mexican blindcat genus Prietella (Siluriformes: Ictaluridae): an overview of recent explorations; D.A. Hendrickson, et al. A new blind cave fish population of genus Astyanax: geography, morphology and behavior;