A new, narrowly distributed, and critically endangered species of Characidium (Characiformes: Crenuchidae) from the Distrito Federal, Central Brazil

A new species of Characidium is described from the Cerrado biome, in Brasília, Distrito Federal, Central Brazil. The new species can be readily diagnosed by the presence of two or three longitudinal rows of dots along the body sides, absence of bars, by the short pectoral fin, which does not reach the vertical through the dorsal fin origin. Additional useful diagnostic characters are the scaled isthmus, absence of the adipose fin, and the terminal mouth. The new species is only known from the córrego Taquara and its tributaries, a tributary of ribeirão do Gama, upstream from lago Paranoá, in the upper rio Paraná basin. In accordance to the IUCN Red List Categories and Criteria, the new species is categorized as Critically Endangered.

The Brazilian Cerrado has high endemism of freshwater fishes because this biome contains the watersheds of some of the major South-American hydrological basins (Nogueira et al., 2010;Reis et al., 2016). In the past 20 years, the taxonomic knowledge regarding the diversity of Characidium in the Brazilian Cerrado has received increased attention, resulting in descriptions of eight species: C. barbosai Flausino Junior, Lima, 3/18 ni.bio.br | scielo.br/ni collected at the Reserva Ecológica do Instituto Brasileiro de Geografia e Estatística (RECOR). In addition, the conservation status of the species is also discussed.

MATERIAL AND METHODS
Morphometric and meristic data were obtained from the left side of specimens, under a binocular stereomicroscope, according to Buckup (1993b), with modifications proposed by Melo, Oyakawa (2015). Measurements were taken using a digital caliper to 0.1 mm. Meristic data are given along the description, followed by the frequencies of each count in parentheses, and an asterisk (*) indicating the values for the holotype. Terminology for pigmentation of body follows Leitão, Buckup (2014) andFlausino Junior et al. (2020). Osteological features were observed in cleared-and-stained specimens (cs), prepared according to Taylor, Van Dyke (1985). Osteological nomenclature follows Weitzman (1962) and Vari, Harold (2001), with the addition of posterior cleithral process. The vertebral counts included the four anterior-most elements modified into the Weberian Apparatus counted individually and the compound element of caudal fin was counted as a single element. Supernumerary elements in dorsal and anal fins were counted only from cleared and stained individuals. Osteological illustrations were made on a Zeiss Discovery V12 stereomicroscope connected to a camera with auto-montage mechanism. The gonads examined macroscopically through a small incision made in the flank, and sex and stage of gonadal development were evaluated according to Vazzoler (1996).
General abbreviations are as follows: SL, Standard Length; HL, Head Length; cs, cleared and stained. Acronyms for fish collections are as follows: MNRJ, Museu Nacional, Rio de Janeiro; MZUSP, Museu de Zoologia da Universidade de São Paulo; ZUEC, Museu de Zoologia da Universidade de Campinas; IBGE, Reserva Ecológica do Instituto Brasileiro de Geografia e Estatística fish collection. Abbreviations of main areas of conservation are as follows: EEJBB -Estação Ecológica Jardim Botânico de Brasília; FAL-UNB, Estação Experimental Fazenda Águas Limpas da Universidade de Brasília; EEC-UNB, Estação Ecológica da Universidade de Brasília; RECOR -Reserva Ecológica do Instituto Brasileiro de Geografia e Estatística. Only specimens in good conditions were selected as types. Maps presented in figure 4 were produced using Quantum GIS (QGIS.org, 2020), with the QuickMapServices plugin. Maps presented in Figure 5 were produced using Google Earth Pro (Google, 2020), with the aid of the timeline function. The extent of occurrence was calculated with the aid of Google Earth Pro (Google, 2020), drawing the smallest polygon in which no internal angle exceeds 180 degrees and which contains all the sites of occurrence (IUCN Standards and Petitions Subcommittee, 2019). A regression analysis was performed using the computer program Microsoft Excel (MICROSOFT, 2020), comparing the standard length with the ratio between standard length and body depth at dorsal-fin origin. Diagnosis. Characidium onca can be distinguished from its congeners except C. chancoense Agudelo-Zamora, Ortega-Lara & Taphorn, 2020, C. japuhybense Travassos, 1949, C. lauroi Travassos, 1949, C. oiticicai Travassos, 1967, C. pellucidum Eigenmann, 1909, C. phoxocephalum Eigenmann, 1912, C. pteroides Eigenmann, 1909, C. schubarti Travassos, 1955, C. stigmosum, and C. travassosi Melo, Buckup & Oyakawa, 2016 by small dots on the side of body present (vs. spots absent), and from these exceptions by the dots arranged in two or three well-marked, longitudinal lines (vs. dots irregularly distributed); and by the pectoral fin short, not reaching the vertical that passes through the origin of the dorsal fin (vs. tip of pectoral fin reaching or extending posterior to the vertical that passes through the origin of dorsal fin). It further differs from C. chancoense, C. japuhybense, C. lauroi, C. helmeri, C. pellucidum, C. phoxocephalum, C. pteroides, and C. schubarti by the adipose fin absent (vs. adipose fin present); from C. stigmosum, C. japuhybense, C. longum, and C. pteroides by the bars absent (vs. bars present); from C. 6/18 ni.bio.br | scielo.br/ni japuhybense, C. lauroi, C. helmeri, C. oiticicai, C. schubarti, and C. travassosi by the isthmus scaled (vs. isthmus scaleless); and from C. pellucidum and C. pteroides by the blotches on dorsum absent (vs. sickle-shaped, reddish-brown blotches present, formed by the fragmentation of vertical bars on sides of body present), and longitudinal stripe present (vs. absent).
Description. Morphometric data summarized in Tab. 1. Largest examined specimen 40.1 mm SL. Body elongate. Dorsal profile moderately convex between snout tip and dorsal-fin base, gently convex between dorsal and caudal-fin. Ventral profile gently convex between dentary tip and anal-fin origin, slightly convex at anal-fin base; almost straight between anal and caudal-fin bases. Belly strongly accentuated arched in females with well-developed ovarium. Greatest depth of body at dorsal-fin origin.
Pseudotympanum immediately posterior to supracleithrum, underneath anterior part of longitudinal stripe, antero-dorsally elongate, limited dorsally by lateralis superficialis, anteriorly and posteriorly, by obliquus inferioris, and ventrally by obliquus superioris; lined dorsally by lateral-line nerve. Humeral hiatus divided into anterior and posterior chambers by pleural rib of fifth. Fatty tissue filling humeral hiatus absent (Fig. 4).

Coloration in alcohol.
Ground color of head and trunk light yellow, darker on dorsum, lighter on belly (Figs. 1-2). Cheek, distal portion of upper jaw, ventral part of head and opercle pale with widely spaced melanophores. Dorsal portion of snout and head light brown. Oblique preorbital stripe extending from snout tip to anterior portion of eye present and conspicuous. Postorbital stripe present, conspicuous. Bars on body absent. Three thin stripes present on dorsal part of body; dorsal stripe extending from head to dorsal-fin base; middle bar extending from head to caudal peduncle; and ventral bar extending from head to base do dorsa-caudal fin ray. Midlateral stripe, stout, extending from supracleithrum to caudal peduncle, not reaching base of middle caudalfin rays. Humeral blotch absent. Basicaudal spot inconspicuous, vertically elongated. Dots arranged in two or three longitudinal rows; dorsal row of dots extending from level of cleithrum to caudal peduncle, slightly dorsal to, or overlapped by, midlateral stripe; middle and ventral rows of dots extending from posterior angle of opercle to caudal peduncle, ventrally to lateral line. Pectoral, pelvic, dorsal, anal and caudal fins mostly hyaline, or caudal and pectoral fins dusky; melanophores present only on lepidotrichia.  The body depth at origin of dorsal fin has a statistically significant, positive growth in both females and males, varying from 22.2 to 27.0% in SL (mean 24.3%) in immature females; from 26.5 to 29.6% in SL (mean 27.3%) in mature females; and from 21.8 to 25.5 % in SL (mean 23.4%) in mature males (Fig. 5). The mature females with fully developed eggs have a largely expanded belly, resulting on a considerable deeper body and high standard deviation for the body depth at origin of dorsal (Tab. 1). Further secondary sexual dimorphic characters, such as differences in the color pattern or the lepidotrichia bony process, are absent.

Geographical distribution.
Characidium onca is only known from córrego Taquara and its tributaries, itself being a tributary of the ribeirão do Gama which nowadays drains into the lago Paranoá, a tributary of the right margin of the rio São Bartolomeu, tributary of the left margin of rio the Corumbá, rio Paranaíba drainage, in the upper rio Paraná basin, Distrito Federal, Brazil (Fig. 6).
Etymology. The specific name onca refers to the Portuguese name onça, used for the jaguar Panthera onca (Linnaeus) (Mammalia: Felidae), in an allusion to the black spots in a bright gold-yellow body. It is derived from the Latin lyncea, meaning lynx (Italian "lonza", old French "l'once"), and should be pronounced as ˈõ.sɐ. A noun in apposition.

Conservation status.
Characidium onca is known only from córrego Taquara and its tributaries, which have about 6.5 km of extension. Since the 1980's, the second author (MCLBR) has been sampling the drainages in the Distrito Federal and surroundings building a collection of over 1,000 lots of Characidium hosted mainly at the IBGE-RECOR, MZUSP and ZUEC fish collections, but C. onca was never found in any other drainages. Additional ichthyofaunistic inventories conducted in the Distrito Federal also did not report the presence of C. onca elsewhere, supporting the evidence that it truly has a narrow distribution (Ribeiro et al., 2001(Ribeiro et al., , 2008Aquino et al., 2009;Ribeiro, 2012). The extent of occurrence of C. onca is calculated in 15 km 2 and considered a single location. Additionally, the species is naturally rare and have extremely low abundance in nature (MRSM and MCLBR, pers. obs.).  Since the construction of Brasilia in 1960's, the impacts on the ribeirão do Gama drainage are gradually increasing (Figs. 7A-D). The lago Paranoá is an artificial lake formed by damming the rio Paranoá in 1959, to provide water supply, electric power and to increase the air humidity in Brasília. The Lago Paranoá extends to the lower part of the ribeirão do Gama, slightly downstream (>8 km) from the collection sites of C. onca, resulting on habitat loss by changing the original characteristics of a lotic into a lentic system, and by the destruction of the original riparian vegetation. The formation of the lake was followed by the introduction of several exotic species, such as the Congo tilapia Coptodon rendalli (Boulenger, 1897), the largemouth bass (known in Brazil as black bass) Micropterus salmoides (Lacepède, 1802), the bluegill Lepomis macrochirus Rafinesque, 1819 and the carp Cyprinus carpio Linnaeus, 1758 (Ribeiro et al., 2001).
The construction of Brasilia also resulted in dynamic land use and vegetation coverage changes, with the replacement of natural areas of open fields (campo nativo) and Cerrado vegetation for agriculture and urban uses (Figs. 7A-D). Over the last two decades, the population in the Distrito Federal increased in more than 70%, from 1,737,813 in 199570%, from 1,737,813 in to 2,977,216, in 201670%, from 1,737,813 in (UNESCO, 2002. The rapid urbanization has replaced large areas of the natural Cerrado ecosystem with urban areas and agriculture land for cultivating soybean, corn, and bean. The intense use of surface and underground hydrological resources for such purposes caused water depletion and local climate changes of longer dry seasons and a predicted increase of temperature by up to 5 o C (Campos, 2004;Cadamuro, Campos, 2005;Lorz et al., 2012;IBGE, 2017). The most severe consequences of those impacts are loss of ecological integrity of both terrestrial and aquatic ecosystems, such as the reduction of water level in lakes, rivers and floodplains, the deterioration of water quality, changes in the fire regimes, loss of gallery forests, and the increase of river channel sedimentation that reduces habitats conditions for the species of Characidium (Alley et al., 1999;Ribeiro et al., 2018).
Because of its proximity to downtown Brasília, a considerable extension of the APA das Bacias do Gama e Cabeça de Veado is already occupied by urban areas, especially in the ribeirão do Gama left margin. The mosaic composed of the FAL-UNB, RECOR and EEJBB is also under strong agricultural and real state speculations and the plans to expand the city limits includes the construction of the Juscelino Kubitschek bridge in 2002 and several changes in delimitation and management categories of the protected areas occasioned by the revision of the Distrito Federal Master Plan (Plano Diretor de Ordenamento Territorial -PDOT, Lei Complementar Nº 17/1997, of January 23 rd , 13/18 ni.bio.br | scielo.br/ni 1997, revised by the Lei Complementar Nº 803/2009, of April 25 th , 2009, and instituted by the Lei Distrital N o 6.269, of January 29 th , 2019). The FAL-UNB, was replaced with the Estação Experimenal da UNB and, within that area, the Estação Ecológica da UNB (EEc-UNB) was circumscribed as a "Macrozona de Proteção Integral", the RECOR and the EEJBB were classified as a "Macrozona de Proteção Integral", but the latter had a part of its territory changed to "Urban Zone". The expansion of human occupation in the vicinity of the mosaic resulted on the habitat fragmentation and the loss of ecological integrity of both terrestrial and aquatic ecosystems, compromising the creation of buffer zones and ecological corridors (UNESCO, 2003;Ribeiro et al., 2018).
In summary, C. onca has a very narrow distribution in a single location that has observed impacts of human occupation and introduction of exotic species in the basin and nearby areas, and predicted impacts of reduction of the river flow and loss of water quality caused by the longer periods of the dry season and higher temperatures due to local climate changes allied to the intense use of surface and underground hydrological resources. Following the IUCN Red List Categories and Criteria (IUCN Standards and 14/18 ni.bio.br | scielo.br/ni Petitions Subcommittee, 2019), C. onca should be categorized as Critically Endangered (CR) because of its restricted geographic distribution (B1: Extent of occurrence <100 km 2 ) and the following two conditions: number of location equals one (a); and continuing decline observed, estimated, inferred or projected of area extent and/or quality of habitat (biii).
The Clade C4 is composed of the species with a single row of dentary teeth, the absence of the parietal branch of the laterosensory canal of head, and 12 or more vertical bars on body, such as C. cacah, C. bahiense Almeida, 1971, C. interruptum Pellegrin, 1909, and C. stigmosum (Buckup, 1993aNetto-Ferreira et al., 2013;Mendonça, Netto-Ferreira, 2015;Melo, Espíndola, 2016;Zanata et al., 2020a). In addition, the pigmentation pattern of C. stigmosum is very distinctive and unique among its congeners, as the spots are enlarged and vertically elongate (Melo, Buckup, 2002). Characidium onca does not share any synapomorphies with the species of Clade C4 and, therefore, we refute a hypothesis 15/18 ni.bio.br | scielo.br/ni of closer relationships between C. onca and C. stigmosum. Likewise, C. onca lacks the synapomorphies defining the Clade C5, which includes the species with narrow and fragmented bars, the reddish-brown blotches on dorsum, and a slender body, such as C. longum, C. pellucidum and C. pteroides (Buckup, 1993a;Taphorn et al., 2006).
Characidium chancoense and C. phoxocephalum are trans-Andean species with scaled isthmus and dots on the sides not assigned to any clade of Characidium (Agudelo-Zamora et al., 2020a). Characidium chancoense has considerable fewer dots, restricted to an irregular line below the longitudinal stripe, apparently associated to the transversal bars, being remarkably similar to C. japuhybense. Characidium phoxocephalum is the species that most closely resembles C. onca because of the numerous dots on body and the short pectoral and pelvic fins, and but can be externally distinguished by having an adipose fin. Nevertheless, osteological features suggest such external similarity would be the result of evolutionary convergence, with C. phoxocephalum having a well-developed postcleithrum 1 and an elongated and a finger-like posterior cleithral process (Agudelo-Zamora et al., 2020a) while, in C. onca, the postcleithrum 1 is reduced or absent, and the posterior cleithral process being short and triangular (Fig. 4)