Archolaemus, , Korringa, 1970

Vari, Richard P., Santana, Carlos David De & Wosiacki, Wolmar B., 2012, South American electric knifefishes of the genus Archolaemus (Ostariophysi, Gymnotiformes): undetected diversity in a clade of rheophiles, Zoological Journal of the Linnean Society 165 (3), pp. 670-699 : 675-676

RHEOPHILY OF ARCHOLAEMUS

In their multifaceted ecological and taxonomic analysis of what they considered to be populations of A. blax, Schwassmann & Carvalho (1985: 236) reported that individuals of the species took refuge during the day ‘inside crevices and between rocks at places of high current velocities’ at localities in the Rio Xingu (Altamira and Belo Monte) and the Rio Tocantins (Rio Itacaiunas) basins. Our results demonstrate that Schwassmann & Carvalho’s life-history observations were actually of two species, A. janeae (Rio Xingu localities) and A. blax (Rio Tocantins), both of which are clearly rheophiles (those authors did not make life observation in the Rio Araguari system, the home of the third species included in their concept of A. blax ). Archolaemus blax also inhabits high water flow subterranean settings within karstic formations in the São Domingos region of the upper Rio Tocantins system ( Bichuette & Trajano, 2003). Ferreira, dos Santos & Jégu (1988: 345) reported swift water habitat preferences for A. ferreirai (identified as A. blax in that study) in the rapids of the Rio Mucajaí of the Rio Branco system. Ferreira (1995: 51), in turn, indicated that A. luciae (identified as A. blax in that study) is resident in swiftly moving waters within the Rio Trombetas. Observations of A. luciae in the Rio Tapajós system, A. orientalis at its type locality in the Rio São Francisco, and information on the type locality of A. santosi in the Rio Madeira basin (see Habitat in the accounts for these species) indicate that these three species similarly dwell in high-energy settings. Habitat preference information is unavailable for many of the lots examined of Archolaemus in this study, but it is noteworthy that many of these samples originated at, or proximate to, localities identified as falls (cachoeiras), attesting to the rapid water conditions in those localities. A preference for swiftly flowing waters is clearly an attribute that is general across the species of Archolaemus .

Recent advances in the knowledge on the alphataxonomy, habitat preferences, and phylogenetic relationships among Neotropical electric fishes demonstrate that life in shallow waters, including swift water conditions, evolved several times independently across the Gymnotiformes ( de Santana & Vari, 2009, 2010a, b). Crampton (2011) noted that there have been multiple transitions within the Gymnotiformes from deep river habitats to life in high-energy systems, such as rapids, with no apparent reversals in habitat preferences. It is interesting that only two genera in the Gymnotiformes , Archolaemus and Megadontognathus , are composed entirely of rheophiles: species specializing in life in a high-energy aquatic environment. Megadontognathus includes only two species (Camposda-Paz, 1999), which makes Archolaemus , with six species, by far the most species-rich clade of exclusively rheophlic species in the Gymnotiformes .

Rheophily, the behaviour of inhabiting high-energy aquatic environments, such as occurs in all species of Archolaemus , is a phenomenon that is widespread among freshwater fishes, albeit limited to a small subset of the total number of species in the geographically extensive ichthyofaunas encompassing such swift water specialist species ( Kullander, 1988; Lima & Zuanon, 2004; de Santana & Vari, 2010a). Adaptations for rheophily are myriad, but in some instances are apparently limited to behavioural strategies, or at least would seem to be so restricted based on externally obvious morphological adaptations. The observations by Schwassmann & Carvalho (1985: 237) document that behaviour undoubtedly contributes to the utilization of rapid flowing waters by A. blax in the Rio Itacaiunas and A. janeae in the Rio Xingu. Behavioural adaptations are supplemented in many rheophilic species by, sometimes manifold, morphological specializations for life in rapids and proximate to waterfalls. One of these specializations, the possession of a subconical downwardly oriented snout ( Kullander, 1988), characterizes the species of Archolaemus , but many of the other specializations common in other rheophiles (e.g. dorsoventrally flattened head and body) are not feasible within the body plan of the Sternopygidae .