Systematics of the Genus Hypoptopoma Guenther, 1868 (Siluriformes, Loricariidae)

The systematics of HypoptopomaGuenther (1868a) is revised based on comprehensive evaluation of specimen collections and a phylogenetic analysis of the species. The genus Hypoptopomacomprises a distinctive assemblage of loricariid catfishes distributed in the lowland drainages of tropical, subtropical, and temperate latitudes of South America to the east of the Andes. Hypoptopomais uniquely diagnosed among genera of the Loricariidae on the basis of the presence of a laterally expanded nuchal plate. Members of the genus can be further distinguished from all other loricariids, except the hypoptopomatin genus Oxyropsis, by the depressed head with eyes placed ventrolateral and visible from below. Hypoptopomais further distinguished from all other Hypoptopomatini, including Oxyropsis, by the caudal peduncle posterior to the base of the anal fin ovoid in cross section and deeper in the dorsoventral axis. All species of Hypoptopoma, except H. spectabile, can be further distinguished among loricariids by the presence in adult stages of a column of variably enlarged and flattened odontodes positioned along the posterior margin of the trunk plates. Individuals of several Hypoptopomaspecies attain the largest body size for the subfamily Hypoptopomatinae, with standard length reaching 105mm. Species of Hypoptopomatypically occur in streams of slow to moderate current and muddy to sandy bottom with marginal emergent vegetation. Based on verified specimen records, the species is distributed in the Rio Amazonas basin, including the Ucayali, Madeira, and Tapajos rivers, as well as in the rivers east to the Ilha Marajo drainage (Para, Brazil), in the Tocantins and smaller coastal river drainages in northeastern Brazil (Mearim), the upper Rio Orinoco basin, the Essequibo and Nickerie river basins of the Guiana Shield, and in the rios Paraguay and lower Parana. There are no records of Hypoptopomain the Rio Uruguay, the Atlantic coastal drainages of Uruguay and Brazil south of Rio Mearim (Maranao), the upper Parana, and Rio Sao Francisco systems. Fifteen species are recognized in Hypoptopoma, seven of which are newly described herein. Phylogenetic analysis of Hypoptopomaspecies, based on analysis of 26 characters drawn from aspects of external morphology and internal osteology, recovered a well-supported but incompletely resolved nested set of clade relationships that suggests a widespread ancestral distribution for the group in central Amazonia, plus at least four instances o