A new species of Hisonotus is described from the headwaters of the rio Xingu. The new species is distinguished from its congeners by having a functional V-shaped spinelet, odontodes not forming longitudinal aligned rows on the head and trunk, lower counts of the lateral and median series of abdominal figs, presence of a single rostral fig at the tip of the snout, absence of the unpaired figlets at typical adipose fin position, yellowish-tipped teeth, absence of conspicuous dark saddles and stripe on the body and higher number of teeth on the premaxillary and dentary. The new species, Hisonotus acuen, is restricted to headwaters of the rio Xingu basin, and is the first species of the genus Hisonotus described from the rio Xingu basin. Hisonotus acuen is highly variable in aspects of external body proportions, including body depth, snout length, and abdomen length. This variation is partly distributed within and among populations, and is not strongly correlated with body size. PCA of 83 adult specimens from six allopatric populations indicates the presence of continuous variation. Therefore, the available morphological data suggest that the individuals inhabiting the six localities of rio Xingu represent different populations of a single species. Low intraspecific variation in mitochondrial Cytochrome oxidase subunit I (COI) provides corroborative evidence.

Biodiversity, Cascudinhos, Fresh-water, Neotropical fish, Taxonomy

The subfamily Hypoptopomatinae is a monophyletic group of Loricariids (Schaefer, 2003) composed of 19 genera and 139 species (Eschmeyer and Fong 2014). Within this subfamily, Hisonotus Eigenmann & Eigenmann, 1889 comprises 33 valid species (Eschmeyer 2014). The genus Hisonotus was resurrected from the synonymy of Otocinclus by Schaefer (1998) based on the reduced or absent snout figs anterior to the nostril, rostrum with enlarged odontodes, and thickened figs forming the lateral rostral margin. However, the phylogenetic relationships in this genus are not well resolved (Britski and Garavello 2007) and, according to molecular (Chiachio et al. 2008; Cramer et al. 2011) and morphological (Martins et al. 2014) data, Hisonotus is a polyphyletic genus.

Although there is no definition of Hisonotus that supports its monophyly, many authors have considered this genus as valid. In the past decade, 18 species of Hisonotus have been described (Britski and Garavello 2007; Carvalho et al. 2008; Carvalho and Reis 2009; Carvalho and Reis 2011; Martins and Langeani 2012; Carvalho and Datovo 2012; Roxo et al. 2013; Roxo et al. 2014). Recently, during a collecting trip in tributaries of the rio Xingu basin, we found fish specimens that have the generally accepted characteristics of Hisonotus listed above but do not match any known species. Herein we describe the rio Xingu specimens as a new species.

All measurements and counts were taken on the left side of specimens. Measurements were taken point to point to the nearest 0.1 mm with a digital caliper. Body fig and osteology nomenclature followed Schaefer (1997) and measurements followed Carvalho and Reis (2009), as shown in Table 1. Abbreviations used in the text followed Carvalho and Reis (2009). Morphometrics are given as percentages of standard length (SL), except for subunits of the head region, which are expressed as percentages of head length (HL). Specimens were cleared and stained (c&s) according to the method of Taylor and Van Dyke (1985). Vertebral counts also include the five vertebrae that comprise the Weberian apparatus. Dorsal-fin ray counts include the spinelet as the first unbranched ray. All examined specimens were collected according to the Brazilian laws, and are deposited under permanent scientific collection licenses. After collection, specimens were euthanized using 1% benzocaine in water, fixed in 10% formaldehyde for morphological studies and preserved in 70% alcohol. For molecular studies specimens were fixed directly in 95% alcohol. Sequencing and molecular analysis followed Roxo et al. (2012). Institutional acronyms follow Fricke and Eschmeyer (2014). All samples are deposited at the LBP, Laboratório de Biologia e Genética de Peixes, Universidade Estadual Paulista, Botucatu; MZUSP, Museu de Zoologia, Universidade de São Paulo, São Paulo; NUP, Coleção Ictiológica do Nupelia, Universidade Estadual de Maringá, Maringá. Zoological nomenclature follows the International Code of Zoological Nomenclature (4th Ed.). The GenBank accession numbers for Cytochrome oxidase subunit I (COI) sequences are: H. acuen – KM365043, KM365044, KM365045, KM365046, KM365047, KM365048, KM365049, KM365050, KM104473; H. chromodontus – KM104474, KM104475, JN998567, JN998566, JN998567, JN998565, JN998564, KM365054; H. insperatus – KM104485, GU701888, GU701749, GU701748, GU701747, GU701746, KM365055, KM365056, KM365057, KM365058, KM365059, KM365060, KM365061; H. notatus – JN998579, JN998581, JN998580; H. oliveirai – KM104486, KM365062, KM365063; H. paresi – KM104490, KM365042; H. piracanjuba – KM104487, KM104488, KM365051, KM365052, KM365053.

The new species has a functional V-shaped spinelet (Fig. 2a–f). Carvalho and Datovo (2012) first reported this structure in H. bockmanni, H. chromodontus, H. insperatus and H. luteofrenatus. Subsequently, Roxo et al. (2014) reported this character in H. oliveirai and H. paresi. The functional V-shaped spinelet (Fig. 2a–f) is a putative apomorphic character within Hisonotus, and may distinguish a monophyletic group within the genus (Carvalho and Datovo 2012). However, Martins et al. (2014) have a different interpretation, in which the spinelet in H. chromodontus, H. luteofrenatus and H. piracanjuba is reduced, and the locking mechanism is not functional (Martins et al. 2014, Fig. 11A), the same character state found in H. armatus, H. depressicauda, H. francirochai and H. notatus. Martins et al. (2014) also suggested that in H. insperatus the spinelet is absent (Martins et al. 2014, Fig. 11b). However, in our interpretation, H. acuen, H. chromodontus, H. bockmanni, H. insperatus, H. luteofrenatus, H. oliveirai, H. paresi, and H. piracanjuba exhibit a functional V-shaped spinelet, which is not present in H. depressicauda and H. notatus (Fig. 2). Therefore, despite the fact that the genus Hisonotus may not represent a monophyletic unit, we include H. acuen within Hisonotus pending a formal phylogenetic analysis of Hypoptopomatinae, and the species-level composition is established.

Hisonotus acuen exhibits a large amount of variation in external body proportions across its range (Fig. 5), especially in body depth at dorsal-fin origin (13.5–22.8% of SL), snout length (34.2–57.2% of HL), and abdomen length 10.2–24.4% of SL). This variation is partly distributed within populations, and partly between populations, and is not strongly correlated with body size. We performed a PCA to evaluate the morphometric variation within this new species. We compared the morphometric data of six populations found in different tributaries of the rio Xingu, and our results suggest that the range in morphology has a continuous distribution. The lack of phenotypic discontinuities among populations suggests they are not different species (Fig. 8). Additionally, we found that the genetic variation of the Cytochrome Oxidase I (COI) gene within the populations of H. acuen is 1%, and that variation among closely related congeners (i.e. H. chromodontus, H. insperatus, H. oliveirai, H. paresi and H. piracanjuba) is more than 17% (see Table 3 and Fig. 9; sequences can be downloaded from GenBank using the accession numbers provided in Methods). Therefore, the available morphological and molecular data support the recognition of the individuals inhabiting the six localities of rio Xingu and representing different populations as a single species.

Figure 8. 

Scatter plot of Principal Component Analysis (PCA) of six allopatric populations of H. acuen (n = 83) indicating the presence of continuous external morphology variation. Purple circle = affluent of rio Toguro; Red cross = affluent of rio Culuene; Dark blue star = affluent of rio Suiá-Missu; Blue square = affluent of rio Feio; Yellow triangle = córrego Xavante; Green diamonds = rio Coronel Vanick. All are tributaries of rio Xingu, Mato Grosso State in Brazil.

Figure 9. 

Phenogram constructed using Neighbor-Joining (NJ) method, based on the COI gene (581 pb). Numbers above branches are bootstrap values obtained from 1,000 pseudoreplicates. Values below 90% are not shown.

The new species H. acuen is the first described species of Hisonotus from the rio Xingu basin, and is externally very similar to H. chromodontus, a species from the rio Tapajos basin. The coloration of the caudal fin and the tip of the teeth distinguish these species that also are very different genetically (i.e. 19.3% of genetic divergence; Table 3 and Fig. 9). Britski and Garavello (2003) discussed the coloration of the teeth of H. chromodontus, reporting that in more than one hundred specimens examined, varying from 12.0 to 32.2 mm SL, all tooth-tips have a reddish-brown color. We analyzed more than one hundred specimens of H. chromodontus from the museum collections of LBP and NUP, and found the same reddish-brown tooth tips. This tooth features appears to be unique within the genus Hisonotus. A very similar external morphology, as well as the presence of the functional V-shaped spinelet among H. acuen and H. chromodontus, could suggest a close relationship between these species.

All from Brazil, except when stated otherwise: Hisonotus aky (Azpelicueta, Casciotta, Almirón & Koerber, 2004): MHNG 2643.039, 2, 33.1−34.2 mm SL, paratypes, arroio Fortaleza, Argentina; Hisonotus bocaiuva Roxo, Silva, Oliveira & Zawadzki, 2013: MZUSP 112204, male, 24.2 mm SL, holotype, córrego Cachoeira, Bocaiúva, Minas Gerais; LBP 9817, 9, 3 c&s, 18.3−23.2 mm SL, paratypes, córrego Cachoeira, Bocaiúva, Minas Gerais; Hisonotus carreiro Carvalho & Reis, 2011: MCP 40943, 3, 33.6−35.8 mm SL, arroio Guabiju, Guabiju, Rio Grande do Sul; Hisonotus charrua Almirón, Azpelicueta, Casciotta & Litz, 2006: LBP 4861, 1, 35.9 mm SL, arroio Guaviyú, Artigas, Uruguai; MHNG 2650.051, 1, 34.2 mm SL, paratype, arroio Aspinillar, Uruguay; Hisonotus chromodontus Britski & Garavello, 2007: LBP 7964, 25, 24.0−28.3 mm SL, 3 females c&s, 26.5−28.9 mm SL, 1 male c&s 24.9 mm SL, rio dos Patos, Nova Mutum, Mato Grosso; LBP 7974, 26, 17.7–24.8 mm SL, rio dos Patos, Nova Mutum, Mato Grosso; LBP 12278, 2, 26.7−28.7 mm SL, 1 unsexed c&s, 26.7 mm SL, rio Sumidouro, Tangará da Serra, Mato Grosso; MZUSP 45355, 25.9 mm SL, holotype, affluent of rio Preto, Diamantino, Mato Grosso; MZUSP 70758, 7, 19.4−23.9 mm SL, paratype, riacho Loanda, Sinop, Mato Grosso; NUP 10924, 24, 19.5−31.5 mm SL, rio Preto, Diamantino, Mato Grosso; Hisonotus depressicauda (Miranda Ribeiro, 1918): MZUSP 5383, 24.4 mm SL, paralectotype, Sorocaba; LBP 17474, 5 c&s, 18.1−24.0 mm SL, rio Araquá, Botucatu, São Paulo; Hisonotus francirochai (Ihering, 1928): LBP 13923, 22, 25.7−35.7 SL, córrego sem nome, Capitinga, Minas Gerais; MZUSP 3258, 29.4 mm SL, lectotype, rio Grande, São Paulo; Hisonotus heterogaster Carvalho & Reis, 2011: LBP 3335, 39, 20.8−30.1 mm SL, arroio sem nome, rio Grande, Rio Grande do Sul; Hisonotus insperatus Britski & Garavello, 2003: LBP 1299, 3, 23.5−29.6 mm SL, 1 female c&s, 24.8 mm SL, rio Araquá, Botucatu, São Paulo; LBP 1316, 2, 24.1−27.4 mm SL, 1 female c&s, 24.7 mm SL, 1 male c&s, 23.9 mm SL, rio Araquá, Botucatu, São Paulo; LBP 1344, 2, 22.9−24.9 mm SL, rio Araquá, Botucatu, São Paulo; LBP 1373, 1, 25.8 mm SL, rio Araquá, Botucatu, São Paulo; LBP 1405, 2, 22.2−27.3 mm SL, rio Araquá, Botucatu, São Paulo; LBP 4699, 17, 19.6−26.9 mm SL, 4 females c&s, 20.3−26.8 mm SL, 3 males c&s, 24.3−26.1 mm SL, ribeirão Cubatão, Marapoama, São Paulo; LBP 4945, 5, 27.3−28.5 mm SL, 2 females c&s, 28.2−29.9 mm SL, Botucatu, São Paulo; LBP 6770, 5, 25.1−28.2 mm SL, 3 females c&s, 20.0−27.0 mm SL, ribeirão Cubatão, Marapoama, São Paulo; LBP 13336, 1 female c&s, 26.0 mm SL, rio Capivara, Botucatu, São Paulo; LBP 13337, 2 females c&s, 27.4−28.6 mm SL, rio Araquá, Botucatu, São Paulo; MZUSP 22826, 1, 25.4 mm SL, paratype, córrego Água Tirada, Três Lagoas, Mato Grosso; MZUSP 24832, 1, 23.8 mm SL, paratype, rio Corumbataí, Corumbataí, São Paulo; MZUSP 78957, 29.6 mm SL, holotype, rio Capivara, Botucatu, São Paulo; MZUSP 78960, 31, 12.6−26.0 mm SL, paratypes, 5 c&s, 22.7−24.7 mm SL, rio Pardo, Botucatu, São Paulo; MZUSP 78965, 10, 15.6−28.6 mm SL, paratypes, 3 c&s, not measured, rio Araquá, Botucatu, São Paulo; MZUSP 78968, 5, 24.1−27.3 mm SL, paratypes, córrego da Figueira, Lins, São Paulo; Hisonotus iota Carvalho & Reis, 2009: LBP 13072, 5, 32.3−33.0 mm SL, rio Chapecó, Coronel Freitas, Santa Catarina; Hisonotus laevior Cope, 1894: LBP 3377, 1, 25.2 mm SL, arroio dos Corrientes, Pelotas, Rio Grande do Sul; LBP 6037, 8, 33.4−47.0 mm SL, rio Maquiné, Osório, Rio Grande do Sul; LBP 13187, 7, 19.4−45.8 mm SL, córrego sem nome, Camaquá, Rio Grande do Sul; Hisonotus leucofrenatus (Miranda Ribeiro, 1908): LBP 2085, 7, 38.3−50.6 mm SL, rio Sagrado, Morretes, Paraná; LBP 6837, 36, 35.1−43.5 mm SL, rio Fau, Miracatu, São Paulo; Hisonotus leucophrys Carvalho & Reis, 2009: LBP 13065, 6, 17.2−33.6 mm SL, rio Ariranhas, Xavantina, Santa Catarina; LBP 13073, 1, 36.8 mm SL, rio Guarita, Palmitinho, Rio Grande do Sul; Hisonotus luteofrenatus Britski & Garavello, 2007: MZUSP 62593, 28.6 mm SL, holotype, córrego Loanda, Cláudia, Mato Grosso; MZUSP 62594, 8, 22.4−30.5 mm SL, paratype, riacho Selma, Sinop, Mato Grosso; MZUSP 95940, 3, 26.1−28.5 mm SL, affluent of rio Teles Pires, Itaúba, Mato Grosso; Hisonotus megaloplax Carvalho & Reis, 2009: LBP 13108, 6, 36.4−37.8 mm SL, córrego sem nome, Saldanha Marinho, Rio Grande do Sul; Hisonotus montanus Carvalho & Reis, 2009: LBP 13051, 3, 26.4−27.2 mm SL, rio Goiabeiras, Vargem, Santa Catarina; LBP 13055, 5, 24.8−31.9 mm SL, rio Canoas, Vargem, Santa Catarina; Hisonotus nigricauda (Boulenger, 1891): LBP579, 16, 34.1−40.1 mm SL, rio Guaíba, Eldorado do Sul, Rio Grande do Sul; Hisonotus notatus Eigenmann & Eigenmann, 1889: LBP 3472, 20, 21.0−34.3 mm SL, 2 males c&s 25.8−26.5 mm SL, 1 female c&s, 25.0 mm SL, rio Aduelas, Macaé, Rio de Janeiro; LBP 10742, 25, 24.4−43.3 mm SL, rio Macabu, Conceição de Macabu, Rio de Janeiro; Hisonotus oliveirai Roxo, Zawadzki & Troy, 2014: MZUSP 115061, female, 26.4 mm SL, holotype, ribeirão Cambira, affluent of rio Ivaí, Cambira, Paraná; LBP 13332, 1 male, 23.2 mm SL, 1 unsexed c&s, 23.7 mm SL, paratype, rio Mourão, rio Ivaí basin, Campo Mourão, LBP 17578, 3 females, 27.7−30.4 mm SL, 2 males, 25.4−26.1 mm SL, paratypes, rio Mourão, rio Ivaí basin, boundary between Engenheiro Beltrão and Quinta do Sol; NUP 3578, 7 females, 27.8−28.1 mm SL, 8 males, 24.7−26.8 mm SL, 1 female c&s, 27.6 mm SL, 1 male c&s, 25.5 mm SL, paratypes, ribeirão Salto Grande, rio Ivaí basin, Maria Helena; Hisonotus paresi Roxo, Zawadzki & Troy, 2014: MZUSP 115062, female, 26.2 mm SL, holotype, riacho Águas Claras, affluent of rio Sepotuba, Santo Afonso; LBP 13351, 9, 14.7−24.3 mm SL, paratype, riacho Águas Claras, Santo Afonso; LBP 13352, 1, 23.7 mm SL, paratype, riacho Águas Claras, Santo Afonso; NUP 10928, 2 males, 23.2−24.2 mm SL, paratype, 2 c&s, 23.6−24.2 mm SL, riacho Águas Claras, afluente of rio Sepotuba, Santo Afonso; NUP 10976, 3 unsexed, 16.7−20.5 mm SL, paratype, riacho São Jorge, Santo Afonso; Hisonotus piracanjuba Martins & Langeani, 2012: LBP 17256, 9, 17.2−26.3 mm SL, 1, c&s 27.1 mm SL, córrego sem nome, Morrinhos, Goiás; NUP 5059, 1, 24.7 mm SL, córrego Posse, Anápolis, Goiás; NUP 10979, 3, 21.4−21.8 mm SL, ribeirão Bocaina, Piracanjuba, Goiás; Hisonotus prata Carvalho & Reis, 2011: MCP 40492, 18, 19.5−33.2 mm SL, rio da Prata, Nova Prata, Rio Grande do Sul; LBP 9918, 14, 21.7−32.6 mm SL, Laguna dos Patos system, Nova Prata, Rio Grande do Sul; Hisonotus ringueleti Aquino, Schaefer & Miquelarena, 2001: FMNH 108806, 2, 25.7−32.2 mm SL, rio Quaraí basin, Uruguay; LBP 13148, 1, 24.5 mm SL, arroio Putiá, Uruguaiana, Rio Grande do Sul; Hisonotus sp.: LBP 8276, 1 c&s, 25.6 mm SL, rio Verde Grande, Jaíba, Minas Gerais; Microlepidogaster arachas Martins, Calegari & Langeani, 2013: LBP 10882, 3, 22.8−35.3 mm SL, rio Paraná basin, Araxás, Minas Gerais; Microlepidogaster dimorpha Martins & Langeani, 2011: LBP 10683, 2, 28.8−35.6 mm SL; rio Paraná basin, Uberaba, Minas Gerais; Otothyris travassosi Garavello, Britski & Schaefer, 1998: LBP 1971, 13, 14.0−27.2 mm SL; coastal drainage, Canavieiras, Bahia; Otothyropsis marapoama Ribeiro, Carvalho & Melo, 2005: LBP 4698, 6, 23.9−36.3 mm SL; rio Tietê basin, Marapoama, São Paulo; Parotocinclus aripuanensis Garavello, 1988: LBP 10981, 33, 15.63−18.47 mm SL, rio Lageado, Guajará Mirim, Rondônia; Parotocinclus aff. spilurus (Fowler, 1941): LBP 5624, 21.5−22.0 mm SL, rio Maravilha, Balsas, Maranhão; Parotocinclus cf. bahiensis (Miranda Ribeiro, 1918): LBP 7182, 3, 27.9−35.6 mm SL; rio Paraguaçu basin, Lençóis, Bahia; Parotocinclus maculicauda (Steindachner, 1877): LBP 2869, 15, 20.2−44.7 mm SL, rio Ribeira do Iguape basin, Miracatu, São Paulo; Parotocinclus prata Ribeiro, Melo & Pereira, 2002: LIRP 1136, 38, 19.8−41.9 mm SL; rio São Francisco basin, Presidente Olegário, Minas Gerais; Parotocinclus sp.: LBP 1572, 3, 19.0−24.0 mm SL, ribeirão Ínsula, Barra do Garça, Mato Grosso; LBP 2414, 19, 16.7−20.8 mm SL, 1 c&s 23.6 mm SL, córrego Fundo, Barra do Garça, Mato Grosso.

We wish to thank Guilherme José da Costa Silva, Jefferson M. Henriques, and Martin Taylor for their help during the collection expeditions. Martin Taylor, for the live fish and type locality photos. James S. Albert and Ricardo Britzke, for reading the manuscript and giving valuable suggestions. Maria Thereza for English review. This research was supported by the Brazilian agencies FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo, proc. 2010/01610-9 to FFR and 2012/01622-2 to GSCS) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico).