Taxonomic revision of Hopliancistrus Isbrücker & Nijssen, 1989 (Siluriformes, Loricariidae) with redescription of Hopliancistrus tricornis and description of four new species

Hopliancistrus is an Ancistrini genus diagnosed by having few and very strong cheek odontodes on interopercular area, and a patch of strong and stiff odontodes on the antero-lateral border of the snout. The type species is herein redescribed based on types and recently collected specimens. In addition, four new congeneric species are described based on specimens collected in other parts of the Rio Xingu and Rio Tapajós basins. Hopliancistrus tricornis is distributed in the lower Rio Tapajós and is diagnosed by the possession of four branched anal-fin rays and relatively large white to yellow spots on trunk and pectoral and pelvic fins, and dark brown spots on dorsal, caudal and anal fins. Hopliancistrus munduruku is described based on specimens from Rio Jamanxim (Rio Tapajós basin) and Rio Curuá (Rio Xingu basin) and is diagnosed by the possession of five branched anal-fin rays and large yellow blotches on trunk and dark brown to black spots over the fins. Hopliancistrus wolverine is distributed in the rapids of the lower and middle Rio Xingu and is diagnosed by the possession of five branched anal-fin rays and conspicuous small yellow dots on head, trunk and fins. Hopliancistrus xikrin is distributed in medium- to small-sized tributaries of the lower portion of Rio Xingu basin, and is diagnosed by absence of contact between the transverse process of the first dorsal-fin pterygiophore and the transverse process of the second pterygiophore. Hopliancistrus xavante is distributed in the tributaries of upper Rio Xingu basin, and is diagnosed by having a thick skin covering the nuchal plate; by having large white spots on trunk and fins; and by the possession of five branched anal-fin rays. An osteological description and a key for species identification are also provided.


Introduction
Loricariidae is the fifth largest family of vertebrates in the world and the second largest in the Neotropics [1,2], with more than 1,000 valid species, of which approximately 22% were described only in the last ten years [3]. Among loricariids, Ancistrini is the most diverse tribe of Hypostominae, with approximately 260 valid species. These numbers indicate that freshwater fish diversity is still underestimated, especially in highly species-rich areas such as the Brazilian Amazon.
Hopliancistrus tricornis was described as a new species and new genus [4] belonging to a new tribe (Hopliancistrini) within the former Ancistrinae (sensu [4]). Despite being wellknown in the aquarium trade (code L212; [5]), the only known preserved specimens of Hopliancistrus available in collections at the time of its original description were from the lower Rio Tapajós (collected by the Expedição Permanente da Amazônia in 1970), and from Cachoeira von Martius in upper Rio Xingu basin (collected by Jean-Pierre Gosse in 1964).
During recent expeditions in the main channel and tributaries of the Rio Xingu and Rio Tapajós, several specimens of Hopliancistrus were collected and are now available in distinct fish collections. The aim of this paper is to review the diversity and geographic distribution of Hopliancistrus, which resulted in the description of four new species. In addition, new diagnoses, a key to species identification and osteological descriptions are provided.

Material & methods
Most specimens analysed in this study were obtained from museum collections. Recent collected fish were immediately anesthetized using water containing a lethal dose of eugenol (clove oil), and then fixed in 10% formalin, following the guidelines of the Brazilian Society of Ichthyology Lucena et al. [6]. No experimentation was conducted on live specimens. All material was collected in accordance with Brazilian law, under scientific collection licence (SISBIO 31089-2). This study is part of the project number 033/2012 approved by Brazilian ethics committee Comissão de Ética no Uso de Animais (CEUA) of Instituto Nacional de Pesquisas da Amazônia (INPA). Morphological measurements were taken as point-to-point linear distances with a digital caliper to the nearest 0.1 mm following Boeseman [7] and Fisch-Muller et al. [8]. Standard length (SL) is expressed in millimeters and all other measurements are expressed as percents of standard length or head length (HL). Measurements were recorded exclusively in specimens above 60.0 mm SL. Specimens were preserved in alcohol (alc); prepared as cleared and counterstained (cs) for bone and cartilage according to methods of Taylor & van Dyke [9]. Additional osteological observations were made from skeletons prepared from formalin-fixed specimens according to the methods of Bemis et al. [10] and is quoted as "skel" in the list of examined material. Vertebral counts include Weberian Apparatus (five) and ural complexes (one), following Lundberg & Baskin [11]. The nomenclature of the dermal plates on the lateral series follows Schaefer [12]. Osteological nomenclature follows Schaefer [13]. In the color pattern description, we standardized the use of the term "dot" for minute and well-defined round marks, smaller than pupil diameter; and "spot" for marks that are larger than pupil diameter, sometimes irregular in shape, and with blurred contour. Some specimens were excluded from the type series and listed as non-types, for their poor state of preservation, or due to an excessive number of specimens. Museum abbreviations include: ANSP, Academy of Natural In dorsal view, head roughly quadrangular with snout outline; body gradually narrowing from cleithrum to caudal-fin origin. Greatest body width at cleithrum. Head and trunk moderately depressed, greatest depth at dorsal-fin origin. In lateral view, dorsal profile straight or slightly convex from snout tip to supraoccipital process; straight from tip of supraoccipital process to dorsal-fin origin, then straight or gently declining to first procurrent caudal-fin ray. Ventral profile straight from snout tip to caudal-fin origin.
Body anterior region roughly trapezoidal, depressed dorsoventrally, and slightly oval at caudal peduncle in cross section. Weak ridge from snout tip to anterior nare, and from posterior nare to eye. Snout completely covered by plates except for small naked area on tip. Lateral border of snout covered by short stiff odontodes. First four plates of mid-ventral series gently keeled. Ventral surface of caudal peduncle flat; small and delicate keel at three or four last ventral plates series of caudal peduncle. Remaining of body plates without keels.
Head large and wide. Eye moderate in size, dorsolaterally positioned at midpoint of head length; iris operculum present. Orbit not elevated; interorbital area nearly straight. Parietosupraoccipital almost indistinct from other skull bones. Supraoccipital process not elevated, slightly rounded posteriorly. Parieto-supraoccipital limited posteriorly by four roughly triangular to trapezoidal plates. Predorsal area with three pairs of predorsal plates, plus one very small nuchal plate anterior to dorsal-fin spinelet.
Mouth and lips of moderate size; oral disk elliptical; lips almost completely covered with small round papillae densely packed, slightly larger on central portion of lower lip; smooth area surrounding premaxillary and dentary, without papillae. Lower lip large, but not reaching pectoral girdle. Maxillary barbel short and with small portion free from lower lip, except in some individuals with maxillary barbel completely fused into lower lip. Teeth short, thin, delicate, and bicuspid. Mesial cusp slightly larger and wider than lateral cusp; lateral cusp reaching two-thirds of mesial cusp length. Premaxilla and dentary of similar size and disposed parallel to anterior border of snout. Single and reduced buccal papilla between premaxilla present in most specimens.
Dorsal body surface completely covered by large plates, except immediately around dorsalfin base. Ventral surface entirely devoid of plates from snout to anal-fin origin. Base of first anal-fin pterygiophore covered by skin, preanal plate absent. Caudal peduncle completely covered by plates. Median series plates [23][24][25]. Five series of lateral plates at caudal peduncle. Five

PLOS ONE
Taxonomic revision of Hopliancistrus Isbrücker & Nijssen, 1989 to seven oblong plates on caudal-fin base. Opercle exposed, sickle-shaped and covered by odontodes; plates immediately posterior to opercle small, sometimes reaching one-third of length of exposed opercle. Plates of supraopercular area small and few in number, leaving large naked area around opercle. Three large, strong, and curved hook-like odontodes on cheek plates, reaching posterior margin of branchial opening; fleshy and thick odontode sheath, sometimes covering two-thirds of cheek odontodes. All plates covered with small and aligned odontodes, slightly larger on opercle, postopercular plate and pectoral-fin spine.
Dorsal-fin origin on anterior portion of body, slightly anterior to vertical through pelvic-fin origin. Dorsal-fin II,7; spinelet present and very reduced, dorsal-fin locking mechanism not functional. Dorsal fin short and low, not reaching adipose fin when adpressed, nor even reaching preadipose plate in some specimens. Four to five plates separating dorsal from adipose fin. Adipose fin short and low. Usually two unpaired dorsal plates between adipose fin and first procurrent caudal-fin ray. Caudal-fin i,14,i, slightly emarginated, with oblique distal margin, ventral lobe slightly longer. Pectoral fin I,6, large, surpassing pelvic-fin base when adpressed. Pectoral-fin spine thick and strong, not pungent, covered by odontodes. Pelvic fin i,5, reaching end of anal-fin base when adpressed. Anal fin short, i,4, (except in two specimens of MNRJ 35602 and MNRJ 35604 with i,3 rays in anal fin). All rays covered by numerous short odontodes on their free surface. Anteriormost branched rays of pectoral, pelvic and anal fins longer than unbranched rays. Four to five dorsal and ventral procurrent caudal-fin rays. Vertebrae 28 (6). Seven (4) or eight (2) rib pairs.  Color in alcohol. Body ground color light brown at dorsum and sides. Head covered by yellowish-white dots, smaller than pupil diameter; dorsum and sides covered by larger yellowish-white blotches, larger than pupil diameter. All fins with dark brown or black marks; dorsal and caudal fins covered by peculiar conspicuous black rings that encircling the lepidotrichia, giving overall aspect of black wings; marks roughly aligned on bands. Pectoral and pelvic fins with light brown membranes, covered by dark spots on dorsal surface also aligned on bands. Ventral surface cream colored (Fig 3).
Color in life. Body ground color dark brown at head, dorsum, and sides. Head covered by small yellow dots; dorsum and sides covered by yellow spots larger than pupil. Dorsal, caudal and anal fins yellowish-white, covered by dark brown to black spots over rays. Pectoral and pelvic fins with dark brown membranes, covered by yellowish-white spots on dorsal surface. Ventral surface white, with gray blotches. In specimens from Rio Tapajós and main tributaries as Rio Jamanxim, background color and spots on body and fins less conspicuous than specimens from smaller tributaries, as Leitoso stream (Figs 4 and 5A).
Distribution. Hopliancistrus tricornis is apparently restricted to the middle portion of the Rio Tapajós basin in the São Luiz rapids, and its main tributaries, such as the lower portions of Rio Jamanxim and Rio Itapacurá. This species is also found in small tributaries, such as Igarapé Leitoso, tributary of the Rio Cupari, Rurópolis municipality, Pará State (Fig 8).
Remarks. Isbrücker & Nijssen [4] included only the measurements and counts of the holotype in the description of Hopliancistrus tricornis, and possibly did not notice the difference in the number of anal-fin rays in the paratypes from the Xingu basin. However, they reported that the specimens from the Xingu basin were distinct from those from the Tapajós basin by having more conspicuous light-colored blotches on head and body. This difference proved to be consistent in our study and useful in recognizing the two species as distinct. One lot of paratypes (MZUSP 24304) of H. tricornis included a small specimen (  Diagnosis. Hopliancistrus munduruku is distinguished from its congeners except H. tricornis by having large yellowish-white spots along the body, and dark brown spots on fins (vs. body covered by conspicuous small greenish-yellow dots of similar size on head, trunk and fins in H. wolverine; yellowish-white spots on posterior portion of the body moderate in size, usually smaller than pupil in H. xikrin; all fins covered by large yellowish-white spots in H. xavante). Hopliancistrus munduruku can be distinguished from H. tricornis and H. wolverine by the connection strut between the anterior process of the compound pterotic and main body shaped as a continuous sheet (vs. connection strut narrow and bar-shaped, leaving a large posterior gap, see Fig 6). Hopliancistrus munduruku also differs from H. tricornis by the possession of five branched anal-fin rays (vs. four), and from H. wolverine and H. xikrin by pectoral-fin spine length 24.5-30.9% of SL (vs. 32.1-38.4% of SL and 32.1-35.7% of SL, respectively). It differs from H. xikrin by the transverse processes of first and second dorsal-fin pterygiophores sutured to each other (vs. absence of contact between the transverse processes of first and second dorsal-fin pterygiophores). It differs from H. xavante by caudal peduncle depth 10.1-11.3% of SL (vs. 11.5-12.9% of SL); by a narrow nasal bone plate (vs. broad nasal, sometimes slightly triangular, see Fig 2); and by having nuchal plate exposed and covered by odontodes (vs. nuchal plate covered by thick skin and usually lacking odontodes, see Fig 7).
Description. Body shape and pigmentation in Figs 1A, 5B and 9. Morphometric data and counts in Table 1. Small-sized loricariid, with largest examined specimen reaching 155.3 mm SL. Head roughly square anteriorly in dorsal view; gradually narrowing from cleithrum to caudal-fin origin. Greatest width of body at cleithrum. Head and trunk wide and depressed, greatest depth at dorsal-fin origin. In lateral view, dorsal profile slightly convex from snout tip to dorsal-fin origin, gently declining from dorsal-fin origin to first procurrent caudal-fin ray. Ventral profile straight from snout tip to caudal-fin origin.
Body anterior region roughly trapezoidal, depressed dorsoventrally, and slightly oval at caudal peduncle in cross section. Weak ridge from snout tip to anterior nare and from posterior nares to eye. Snout completely covered by plates except for small naked area on its tip. Lateral border of snout covered by short stiff odontodes. First four plates of mid-ventral series gently keeled. Ventral surface of caudal peduncle flat; small and delicate keel at three or four last ventral plates series of caudal peduncle. Remaining of body plates not keeled.
Head large and wide. Eye rounded and moderate in size, dorsolaterally positioned at midpoint of head length; iris operculum present. Orbit not elevated; interorbital area nearly straight. Parieto-supraoccipital almost indistinct from rest the other bones of the skull. Supraoccipital process not elevated, slightly rounded posteriorly. Parieto-supraoccipital limited posteriorly by four roughly triangular plates. Predorsal area with three pairs of predorsal plates, plus one very small nuchal plate anterior to dorsal-fin spinelet.
Mouth and lips of moderate size; oral disk elliptical; lips almost completely covered with small round papillae densely packed, slightly larger on central portion of lower lip; lips area surrounding premaxillary and dentary smooth, without papillae. Lower lip large, but not reaching pectoral girdle. Maxillary barbel short and with small portion free from lower lip. Teeth short, thin, delicate, and bicuspid. Mesial cusp slightly larger and wider than lateral cusp; lateral cusp reaching three-quarters of mesial cusp length. Premaxilla and dentary of similar size and disposed parallel to anterior border of snout. Single very short buccal papilla between premaxilla.
Dorsal body surface completely covered by large plates, except immediately around dorsalfin base. Ventral surface entirely devoid of plates from snout to anal-fin origin. Base of first anal-fin pterygiophore covered by skin, preanal plate absent. Caudal peduncle completely covered by plates. Median series plates 23-25. Five series of lateral plates at caudal peduncle. Six to seven oblong plates on caudal-fin base. Opercle exposed and covered by odontodes, exposed part sickle-shaped; plates immediately posterior to opercle small, sometimes reaching onethird of length of exposed opercle. Plates of supraopercular area small and few in number, leaving large naked area around opercle. Three large, strong, and curved odontodes on cheek plates, reaching posterior margin of branchial opening; fleshy and thick odontodes sheath, sometimes covering two-thirds of cheek odontodes. All plates covered with small and aligned odontodes; odontodes slightly larger on opercle, postopercular plate and pectoral-fin spine.
Dorsal-fin origin on anterior portion of body, slightly anterior to vertical through pelvic-fin origin. Dorsal-fin II,7; spinelet present and very reduced, dorsal-fin locking mechanism not functional. Dorsal fin short and low, not reaching adipose-fin when adpressed; reaching preadipose plate in some individuals. Three (2), four (24) and five (12) plates separating dorsal from adipose fin. Adipose fin short and low. Two unpaired dorsal plates between adipose fin and first procurrent caudal-fin ray. Caudal-fin i,14,i, slightly emarginated, with oblique distal margin, ventral lobe slightly longer. Pectoral fin I,6, large, reaching urogenital opening when adpressed. Pectoral-fin spine thick and strong, not pungent, covered by large odontodes on mature males. Pelvic fin i,5 reaching end of anal-fin base when adpressed. Anal fin i,5, moderate in size, reaching insertion line of adipose-fin spine. All rays covered by numerous short odontodes on their free surface. Anteriormost branched rays of pelvic and anal fins slightly larger than unbranched rays. Four to five dorsal and ventral procurrent caudal-fin rays. Vertebrae 28 (3). Eight (3) rib pairs. Color in alcohol. Very similar to Holiancistrus tricornis in preserved specimens. Background color dark brown at head, dorsum and sides. Head covered by whitish-yellow small spots and trunk with irregular whitish-yellow blotches, sometimes present in trunk of few large specimens. All fins light brown covered with brown to black wing-like marks (as H. tricornis). Ventral surface of pectoral and pelvic fins cream colored without spots. Ventral surface of body uniformly cream to whitish-yellow (Fig 9).
Color in life. Background color gray or olive green at head, dorsum, and sides. Head covered by small yellow spots and trunk with irregular yellow blotches. All fins whitish-yellow, covered by small dark gray to black spots over rays of dorsal, caudal, and anal, and on dorsal surface of pectoral and pelvic fins. Distal portion of caudal-fin lobes yellow, more conspicuous in juvenile specimens. Ventral surface yellowish-white without dots (Fig 5B).
Distribution. Hopliancistrus munduruku is currently known from upper Rio Jamanxim, Rio Tapajós basin, and upper Rio Curuá, tributary of Rio Iriri, Rio Xingu basin, in the Novo Progresso municipality, Pará State (Fig 8). This disjunct distribution between upper tributaries to Tapajós and Xingu basin suggests, like in other taxa, drainage capture by geologic events.      Diagnosis. Hopliancistrus wolverine is distinguished from its congeners by having body color black, dark olive, or dark gray covered by conspicuous greenish-yellow small dots of similar size on head, trunk, and fins (vs. head with small spots, and large whitish-yellow spots or blotches posteriorly towards caudal fin, and dorsal, caudal, and anal fins covered by dark brown spots over rays). Hopliancistrus wolverine can be further distinguished from its congeners by a combination of additional characters: from its congeners except H. tricornis by a narrow, barshaped connection strut between anterior process of compound pterotic and main body, leaving a large posterior gap (vs. connection strut shaped as a continuous sheet, see Fig 6). Additionally, Hopliancistrus wolverine is distinguished from its congeners except H. xikrin by pectoral-fin spine length 32.  2); and by having the nuchal plate exposed and covered by odontodes (vs. nuchal plate covered by thick skin and usually lacking odontodes, see Fig 7). Hopliancistrus wolverine also differs from H. tricornis by the possession of five branched rays on the anal-fin (vs. four).
Description. Body shape and pigmentation in Figs 1B, 5C and 10-12. Morphometric data and counts in Table 2. Small-sized loricariid, with largest examined specimen 181.8 mm SL. Head roughly square anteriorly in dorsal view; body gradually narrowing from cleithrum to caudal-fin origin. Greatest width of body at cleithrum. Head and trunk wide and very depressed, greatest depth at dorsal fin origin. In lateral view, dorsal profile slightly convex from snout tip to dorsal-fin origin, then straight or flat to caudal-fin origin. Ventral profile straight from snout tip to caudal-fin origin.
Body anterior region roughly trapezoidal, depressed, and slightly oval at caudal peduncle in cross section. Weak ridge from snout tip to anterior nare and from posterior nare to eye. Snout completely covered by plates except for small area on its tip. Lateral border of snout covered by strong, short and stiff odontodes. First four plates of mid-ventral series gently keeled. Ventral surface of caudal peduncle flat; small and delicate keel at three or four last ventral plates series of caudal peduncle. Remaining of body plates without keels.
Head large and wide. Eye rounded and moderate in size, dorsolaterally positioned at midpoint of head length; iris operculum present. Orbit not elevated; interorbital area flat or nearly straight. Parieto-supraoccipital almost indistinct from rest of the skull bones. Supraoccipital process not elevated, slightly rounded posteriorly. Parieto-supraoccipital limited posteriorly by four roughly triangular plates. Predorsal area with three pairs of predorsal plates; plates of third pair connected anteriorly but separated posteriorly by small nuchal plate.
Mouth and lips of moderate size; oral disk elliptical; lips almost completely covered with small round papillae densely packed, slightly larger on central portion of lower lip; lips area surrounding premaxillary and dentary smooth, without papillae. Lower lip large, but not reaching pectoral girdle. Maxillary barbel short and with small portion free from lower lip. Teeth short, thin, delicate, and bicuspid. Mesial cusp slightly larger and wider than lateral cusp; lateral cusp reaching two-thirds of mesial cusp length. Premaxilla and dentary of similar size and disposed parallel to anterior border of snout. Single and reduced buccal papilla between premaxilla.
Dorsal body surface completely covered by large plates, except immediately around dorsalfin base. Ventral surface entirely devoid of plates from snout to anal-fin origin. Base of first anal-fin pterygiophore covered by skin, preanal plate absent. Caudal peduncle completely covered by plates. Median series plates 24. Five series of lateral plates at caudal peduncle. Six to seven oblong plates on caudal-fin base. Opercle exposed sickle-shaped and covered by odontodes; plates immediately posterior to opercle of moderate size, sometimes reaching half of opercle length. Plates of supraopercular area of moderate size and few in number, leaving large naked area around opercle. Three large, strong, and curved odontodes on cheek plates, reaching the posterior margin of branchial opening; fleshy and thick sheath sometimes covering two-thirds of cheek odontodes. All plates covered with small and aligned odontodes lines; odontodes slightly larger on opercle, postopercular plate and pectoral-fin spine. Dorsal-fin origin on anterior portion of body, slightly anterior to vertical through pelvic-fin origin. Dorsal fin II,7; spinelet present and very reduced, dorsal-fin locking mechanism not functional. Dorsal fin short and low, reaching adipose-fin when adpressed. Four to five plates separating dorsal from adipose fin. Adipose fin short and low. Two unpaired dorsal plate between adipose fin and first procurrent caudal-fin ray. Caudal fin i,14,i, slightly emarginated, with oblique distal margin, and ventral lobe slightly longer than dorsal. Pectoral fin I,6, very large, reaching urogenital opening when adpressed. Pectoral-fin spine thick and strong, not pungent, covered by large odontodes. Pelvic fin i,5 reaching end of anal-fin base when adpressed. Anal fin i,5, moderated. All rays covered by numerous short odontodes on their free surface. Anteriormost branched rays of pelvic and anal fins larger than unbranched rays. Four to five dorsal and ventral procurrent caudal-fin rays. Vertebrae counts 28 (4). Seven (3) or eight (1) rib pairs.
Color in alcohol. Background color light to dark gray on head, trunk and fins. Head, trunk, and dorsal, caudal, and anal fins, and dorsal surface of pectoral and pelvic fins covered by small whitish-yellow dots, smaller than pupil. Caudal fin distal band yellowish-white when visible. Ventral surface cream-colored or light brown (Fig 10).
Color in life. Background color dark olive to black on head, dorsum and sides, fins slightly clearer than body; ventral surface of pectoral and pelvic fins light gray or yellowishwhite. Small greenish-yellow dots covering body and fins. Caudal fin with yellow distal band, wider in juveniles and narrowing along ontogenetic development. In adults, caudal fin band, when present, becoming reduced to blotch on distal portion of both lobes. Ventral surface light gray to yellowish-white without dots; some individuals with dots on lateral portion of abdomen and around anus and anal fin. Considerable variation occurs in density, size and color intensity of dots (Figs 5C, 11 and 12).
Distribution. Hopliancistrus wolverine is currently known from the middle Rio Xingu, from immediately downriver the Belo Monte Village, throughout the Volta Grande do Xingu to the vicinity of São Félix do Xingu, and in the Rio Iriri, immediately upstream its confluence to Xingu until the mouth of the Rio Curuá (Fig 8).
Etymology. The specific name is an allusion to the Mustelidae Gulo gulo, also known as wolverine, glutton, carcajou, skunk bear, or quickhatch because of it's blunt stature, strong claws, and ferocity. Name in apposition.
Remarks. This species is referred to in some aquarium fish publications as L017 and LDA15 [16], but the code L017 was coined to depict a Pseudancistrus (Stawikowski [20]: 174 [Fig 9]) today known as P. asurini. One individual of the lot INPA 40877 shows a hypertrophied membrane posterior to last ray of dorsal-fin, covering four plates immediately behind it, but not reaching the preadipose plate, thus a similar but distinct condition than what is observed in species of Baryancistrus Rapp Py-Daniel, 1989.     xavante). Hopliancistrus xikrin also differs from H. tricornis and H. wolverine by the connection strut between anteroventral process of compound pterotic and the main body shaped as a continuous sheet (vs. connection strut narrow and bar-shaped, leaving a large posterior gap, see Fig 6). Hopliancistrus xikrin differs from H. xavante by having dorsal, caudal, and anal fins covered by dark brown to black spots (vs. all fins covered by large yellowish-white spots); by narrow nasal bone plate (vs. nasal broad, sometimes slightly triangular, see Fig 2); by having nuchal plate exposed, and covered by odontodes (vs. nuchal plate covered by thick skin and usually lacking odontodes, see Fig 7); and by caudal peduncle depth 10.0-11.0% of SL (vs. 11.5-12.9% of SL). Hopliancistrus xikrin also differs from H. tricornis by the possession of five branched rays on anal-fin (vs. four).

PLOS ONE
Description. Body shape and pigmentation in Figs 5D and 13. Morphometric data and counts in Table 2. Small-sized loricariid, with largest examined specimen 141.6 mm SL. Head roughly square anteriorly in dorsal view; body gradually narrowing from cleithrum to caudalfin origin. Greatest width of body at cleithrum. Head and trunk depressed, greatest depth at dorsal-fin origin. In lateral view, head dorsal profile from snout tip to eyes sloped, flat from eyes to dorsal-fin origin and nearly straight or slightly declining to first procurrent ray of caudal-fin. Ventral profile straight from snout tip to caudal-fin origin.
Anteriorly, body roughly trapezoidal, depressed, and slightly oval at caudal peduncle in cross section. Weak ridge from snout tip to anterior nare, and from posterior nare to eye. Snout completely covered by small plates except for small naked area on tip. Lateral border of snout covered by short and stiff odontodes. First four plates of mid-ventral series gently keeled. Ventral surface of caudal peduncle flat; small and delicate keel at three or four last ventral plates series of caudal peduncle. Remaining of body plates without keels.
Head large and wide. Eye rounded and moderate in size, dorsolaterally positioned at midpoint of head length; iris operculum present. Orbit not elevated; interorbital area nearly straight. Parieto-supraoccipital almost indistinct from other skull bones. Supraoccipital process not elevated, slightly rounded posteriorly. Parieto-supraoccipital limited posteriorly by four roughly triangular plates. Predorsal area reduced, with three pairs of predorsal plates, plus one small plate anterior to dorsal-fin spinelet. Nuchal plate situated between plates of third predorsal pair.
Mouth and lips of moderate size; oral disk elliptical; lips almost completely covered with small round papillae densely packed, slightly larger on central portion of lower lip; area around premaxillary and dentary smooth, without papillae. Lower lip large, but not reaching pectoral girdle. Maxillary barbel short with small portion free from lower lip. Teeth short, thin, delicate, and bicuspid. Mesial cusp slightly larger and wider than lateral cusp; lateral cusp reaching twothirds of mesial cusp length. Premaxilla and dentary of similar size and disposed parallel to anterior border of snout. Single and reduced buccal papilla between premaxilla.
Dorsal body surface completely covered by large plates, except immediately around dorsalfin base. Ventral surface entirely devoid of plates from snout tip to anal-fin origin. Base of first anal-fin pterygiophore covered by skin, preanal plate element absent. Caudal peduncle completely covered by plates. Median plates series 23-24. Five series of lateral plates at caudal peduncle. Five to six oblong plates on caudal-fin base. Opercle exposed sickle-shaped and covered by odontodes; plates immediately posterior to opercle small, sometimes reaching half of length of exposed opercle. Plates of supraopercular area moderate-sized and few in number, leaving large naked area around opercle. Three large, strong, and curved odontodes on cheek plates, reaching posterior margin of branchial opening; fleshy and thick odontodes sheath covering two-thirds of odontodes. All plates covered with aligned small-sized odontodes; odontodes slightly larger on opercle, postopercular plate and on pectoral-fin spine.
Dorsal-fin origin on anterior portion of body, slightly anterior to vertical through pelvic-fin origin. Dorsal fin II,7; spinelet present and very reduced, dorsal-fin locking mechanism not functional. Dorsal fin large and low, reaching preadipose plate when adpressed. Four to five plates separating dorsal from adipose fins. Adipose fin short and low. Two unpaired dorsal plates between adipose fin and first procurrent caudal-fin ray usually present. Caudal fin i,14,i, slightly emarginated, with oblique distal margin, and ventral lobe slightly longer. Pectoral fin I,6, large, reaching urogenital opening when adpressed. Pectoral-fin spine thick and strong, not pungent, covered by large odontodes. Pelvic fin i,5 reaching end of anal-fin base when adpressed. Anal fin i,5, moderate-sized. All rays covered by numerous short odontodes on their free surface. Anteriormost branched rays of pelvic and anal fins larger than unbranched rays. Four to six dorsal procurrent caudal-fin rays and four to five ventral procurrent caudalfin rays. Vertebral counts: 27 (1) to 28 (2). Seven (1), eight (1) or nine (1) rib pairs. Color in alcohol. Body uniformly dark brown with inconspicuous mottled clear blotches. All fins uniformly dark; some specimens with clear spots larger than pupil on fins; dorsal fin with irregular dark spots on rays. Ventral surface from snout tip to border of upper lip dark gray; lower lip cream, whitish-yellow from lower lip to pectoral girdle, then light gray to caudal fin origin (Fig 13).
Color in life. Body ground color light to dark brown at head, dorsum and flanks. Head covered by very small inconspicuous white dots; trunk and flanks covered with inconspicuous whitish spots, smaller than pupil size. Dorsal fin smudged with light brown to dark spots over rays. Pectoral, pelvic, anal, and caudal fins gray, covered by white spots on caudal and anal fins and dorsal surface of pectoral and pelvic fins. Caudal fin with little yellow blotch on distal portion of both caudal fin lobes in juveniles, absent in adults. Ventral surface from snout tip to border of upper lip dark gray, whitish-yellow from lower lip to pectoral girdle, then gray to caudal fin origin (Fig 5D).
Distribution. Hopliancistrus xikrin is known from small to medium-sized tributaries of the middle portion of Rio Xingu, like Rio Bacajá and Rio Bacajaí, at the Volta Grande area (Fig 8).
Etymology. The new species is named after the Xikrin, an indigenous ethnic group inhabiting the Rio Bacajá margins. The Xikrin people belong to the "Je" linguistic trunk, calling themselves "Mebengokre". They are related to the Kayapo people with whom they share the same self-denomination, cultural characteristics such as body painting patterns, language, residence and marital relations, rituals, chants, and naming system.  Diagnosis. Hopliancistrus xavante can be distinguished from its congeners by the nuchal plate covered by thick skin (Fig 7), and usually lacking odontodes (vs. nuchal plate exposed and covered by odontodes); by having a broad nasal bone plate (Fig 2), sometimes slightly triangular (vs. narrow nasal); and by the trunk and fins covered by large yellowish-white spots (vs. dorsal, caudal, and anal fins covered by dark brown to black spots in H. tricornis, H. munduruku, and H. xikrin; body covered by conspicuous small greenish-yellow dots of similar size on head, trunk, and fins in H. wolverine). Hopliancistrus xavante can be further distinguished from its congeners by a combination of additional characters: it differs from its congeners except H. wolverine by caudal peduncle depth 11.5-12.9% of SL (vs. 9.7-11.3% in H. tricornis, 10.1-11.3% in H. munduruku, 10.0-11.0% in H. xikrin). Hopliancistrus xavante differs from H. tricornis and H. wolverine by connection strut between anterior process of compound pterotic and main body shaped as a continuous sheet (vs. connection strut narrow and bar-shaped, leaving a large posterior gap, see Fig 6). It differs from Hopliancistrus xikrin by the transverse processes of first and second dorsal-fin pterygiophores sutured to each other (vs. absence of contact between the transverse processes of first and second dorsal-fin pterygiophores). Hopliancistrus xavante also differs from H. tricornis by the possession of five branched rays on anal-fin (vs. four).

Hopliancistrus xavante, new species
Description. Body shape and pigmentation in Figs 5E, 14 and 15. Morphometric data and counts in Table 3. Small size loricariid, with largest examined specimen reaching 116.5 mm of SL. Head roughly square anteriorly in dorsal view; from cleithrum to dorsal insertion line straight, then gradually narrowing to caudal-fin origin. Greatest width of body at head. Overall body corpulent, strong. Head and trunk wide and moderately depressed, greatest depth at dorsal-fin origin. In lateral view, dorsal profile sloped and slightly convex from snout tip to eye, then plane with low decline to first procurrent caudal-fin ray. Ventral profile straight from snout tip to caudal-fin origin.
Anteriorly, body roughly trapezoidal, depressed, and slightly oval at caudal peduncle in cross section. Weak ridge from snout tip to anterior nare and from posterior nare to eye. Snout completely covered by plates except for small naked area on tip. Lateral border of snout covered by strong, short and stiff odontodes. First four plates of mid-ventral series gently keeled. Ventral surface of caudal peduncle flat; small and delicate keel at three or four last ventral plates of caudal peduncle. Remaining of body plates without keels.
Head large and wide. Eye rounded and moderate in size, dorsolaterally positioned just posterior to midpoint of head length; iris operculum present. Orbit not elevated; interorbital area nearly straight. Parieto-supraoccipital almost indistinct from rest the other bones of the skull. Supraoccipital process not elevated, slightly rounded posteriorly. Parieto-supraoccipital limited posteriorly by four roughly triangular plates. Predorsal area with three pairs of predorsal plates, nuchal plate not exposed, covered by skin and usually lacking odontodes. Mouth and lips of moderate size; oral disk elliptical; lips almost completely covered with small round papillae densely packed, slightly larger on central portion of lower lip; lip area surrounding premaxillary and dentary smooth, without papillae. Lower lip moderate in size, not reaching pectoral girdle. Maxillary barbel short and with small portion free from lower lip. Teeth long, thin, delicate, and bicuspid. Mesial cusp slightly larger and wider than lateral cusp; lateral cusp reaching two-thirds of mesial cusp length. Premaxilla and dentary of similar size and disposed parallel to anterior border of snout. Single and small buccal papilla between premaxilla.
Dorsal body surface completely covered by large plates, except immediately around dorsalfin base. Ventral surface entirely devoid of plates from snout to anal-fin origin. Base of first anal-fin pterygiophore covered by skin, preanal plate absent. Caudal peduncle completely covered by plates. Median series plates 24-25. Five series of lateral plates at caudal peduncle. Six to seven oblong plates on caudal-fin base. Exposed opercle sickle-shaped and covered by odontodes; plates immediately posterior to opercle of moderate size, sometimes reaching half of opercle length. Plates of supraopercular area small and few in number, leaving large naked area around opercle. Three large, strong, and curved odontodes on cheek plates, reaching the posterior margin of branchial opening; fleshy and thick odontodes sheath covering two-thirds of cheek odontodes. All plates covered with small and aligned odontodes; odontodes slightly larger on opercle, postopercular plate and pectoral-fin spine.
Dorsal-fin origin on anterior portion of body, slightly anterior to vertical through pelvic-fin origin. Dorsal fin II,7; spinelet present but very reduced, dorsal-fin locking mechanism not functional. Dorsal fin short and low, not reaching adipose-fin spine when adpressed. Four to five plates separating dorsal from adipose fins. Adipose fin short and low, slightly more developed than congeners. Two unpaired dorsal plates between adipose fin and first procurrent caudal-fin ray usually present. Caudal fin i,14,i, slightly emarginated, with oblique distal margin, and ventral lobe slightly longer. Pectoral fin I,6, moderate in size, reaching end of pelvicfin base when adpressed. Pectoral-fin spine thick and strong, not pungent, covered by large odontodes. Pelvic fin i,5 not reaching end of anal-fin base when adpressed. Anal fin i,5, moderated in size. All rays covered by numerous short odontodes on their free surface. Anteriormost branched rays of pelvic and anal fins slightly larger than unbranched rays. Four to six dorsal procurrent caudal-fin rays and four to five ventral procurrent caudal-fin rays. Vertebrae counts 28 (6). Eight (2) or nine (4) rib pairs. Color in alcohol. Body ground color light brown at dorsum and sides. Head, dorsum, and sides covered by yellowish-white spots, sometimes absent, smaller than pupil diameter on head and larger than pupil on trunk. Dorsal, caudal, and anal fins, and dorsal surface of pectoral and pelvic fins covered by large yellowish-white spots. Ventral surface cream to yellowish white (Figs 14 and 15).
Color in life. Body ground color brown at head, trunk, and fins. Head covered by small yellowish-white dots; dorsum, flank, and dorsal, caudal, and anal fins, and dorsal surface of pectoral and pelvic fins covered by large yellowish-white spots, larger than pupil diameter. Ventral surface light brown (Fig 5E).
Distribution. Hopliancistrus xavante is currently known from small tributaries of the middle portion of Rio Xingu (Fresco and Jauri rivers) and upper Rio Xingu basin at Cachoeira von Martius and its tributaries such as Rio Culuene, Rio Curisevo, and Rio Sucuri (Fig 8).
Etymology. The new species is named after the Xavante, an indigenous ethnic group nowadays inhabiting several Indigenous Lands that form part of its earlier territory of traditional occupation, in the region comprised by the Serra do Roncador and the valleys of the Rio das Mortes, Rio Culuene, Rio Couto de Magalhães, Rio Batovi, and Rio Garças, in the eastern Mato Grosso State.

Osteology of Hopliancistrus
Skull. Depressed; cranial roof-bones delicate and completely covered by odontodes (Fig  2). Dorsal profile of cranial-roof bones convex in lateral view; skull triangular, anteriorly narrow and posteriorly expanded in dorsal view; larger width at pterotic-supracleithrum (Fig 16).
Mesethmoid short and slender, cylindrical in cross section. Dorsal surface smooth and covered by small plates. Middle portion wider than rest of bone, narrowing anteriorly. Anterior tip of mesethmoid flattened and expanded laterally, narrower than middle portion; ventrally, mesethmoid disk well-developed and perforated. Hopliancistrus tricornis, H. munduruku, H. xavante and H. xikrin with anterior border of mesethmoid rounded, H. wolverine with small notch medially. Mesethmoid with dorsal expansion sutured to frontals posteriorly and laterally to lateral ethmoid (Fig 16); ventral surface with large rectangular notch sutured to vomer (Figs  6 and 16).
Lateral ethmoid well-developed and strongly ossified, triangle-shaped in ventral view (similar condition in Peckoltia, see Armbruster [25]), and posteriorly contacting frontal, and orbitosphenoid ventrally; anteriorly connected to vomer. Lateral ethmoid posterolateral processes well-developed and forming anterior margin of orbit, and contacting ventral margin of dermal plate between frontal and sixth infraorbital. Lateral ethmoid narrow anteriorly, with welldeveloped articular facet with palatine on ventral area. Condyle articulating with posterodorsal margin of metapterygoid, ovoid, large and positioned on posterior concavity of lateral ethmoid. Ridge on lateral ethmoid thin, extending from anterior point of contact with metapterygoid to ovoid condyle; in Hopliancistrus tricornis, H. wolverine and H. xikrin this ridge moderately deep, whereas in H. munduruku and H. xavante distinctly deep. Nasal capsule enclosed in lateral ethmoid near ridge (Figs 6 and 16).
Vomer elongated and relatively narrow, slightly longer than mesethmoid, and expanded medially. Anterior half sutured to mesethmoid; posterior half sutured laterally to lateral ethmoid, and posteriorly to parasphenoid and posteriorly reaching vertical through anterior margin of orbitosphenoid (Figs 6 and 16).

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Parasphenoid elongated and narrow, anterior portion between contralateral lateral ethmoid, at midline sutured to orbitosphenoid and posteriorly to prootics. Parasphenoid posteriorly expanded and sutured to basioccipital; pair of small lateral wings sutured to anterior internal facet of prootic. Anteriorly narrow and deeply divided by vomer, fork-shaped, posterior to wings deeper than orbitosphenoid and lateral ethmoid (Figs 6 and 16).
Orbitosphenoid ranging from trapezoidal to slightly square-shaped, of similar size of prootic, and with ventral profile convex. Posteriorly connected to prootic, anteriorly to lateral ethmoid, medial margin to parasphenoid and lateral margin free (Figs 6 and 16). Sutured to frontal dorsally, forming internal facet of orbit, along with pterosphenoid and lateral ethmoid.
Prootic large, forming floor of neurocranium. Ventral facet plane with two large foramina at anterolateral margin. Dorsally prolonged sutured to compound pterotic and to sphenotic and pterosphenoid forming posterior wall of trigeminofacial and optic foramina (FTF+FOP [13]). Lateroposteriorly sutured to basioccipital and posteriorly with synchrondral articulation to exoccipital and compound pterotic. Anteriorly connected to orbitosphenoid, medial facet connected to parasphenoid, and lateral margin with cartilaginous condyle that articulates hyomandibula (Figs 6 and 16).
Basioccipital, small and strongly sutured to parasphenoid and small portion of prootic anteriorly, and laterally connected to exoccipital. Posterior portion of bone connected to Weberian complex, and with pair of thin lateral wings connected to transcapular process (Figs 6 and 16). Base of skull with large groove to accommodate facial and cranial musculature. Exoccipital slightly triangular and small; ventral facet plane with posterior border extended ventrally and connected to anterior facet of transcapular process; medial facet connected to basioccipital. Exoccipital anterolaterally shares synchrondral joint with prootic and compound pterotic.
Pterosphenoid large, rectangular; ventral surface contributing to dorsal wall of trigeminofacial and optic foramina along with prootic. Anteriorly connected to orbitosphenoid, dorsally to frontal and posteriorly to sphenotic, forming internal wall of orbit (Fig 16).
Frontals large, slightly triangular in dorsal view, expanded medially forming dorsal border of orbit (Figs 2 and 16). Anteriorly, frontals narrow and connected to nasal plate, and forming small portion of border of nasal capsule. Frontals medially sutured together (cranial fontanel absent); posteriorly connected to sphenotic and parieto-supraoccipital. Nasal elongated in Hopliancistrus tricornis, H. munduruku, H. wolverine and H. xikrin, and broad in H. xavante. Nasal capsule completely encased ventrally by lateral ethmoid.
Sphenotic roughly rectangular with rounded edges in dorsal view. Ventral process thin and elongated, ventrally surpassing half height of orbit, and forming posteromedial margin of orbit, with external contact with eighth infraorbital (Figs 2 and 6).
Parieto-supraoccipital large (only smaller than compound pterotic), flat, octogonal in shape and forming posterior portion of skull dorsally covered by odontodes on dorsal surface. Posterior process broad, rounded and not raised, posteriorly limited by four large plates (Figs 2 and 16).
Compound pterotic rectangular and greatly expanded laterally, with small perforations uniformly distributed on dorsolateral surface. Anteroventral process of compound pterotic separated medially from main body of bone (Figs 2, 6 and 16).
Infraorbital canal running through eight platelets, eighth and seventh platelets forming posteroventral border of orbit (Fig 2).
Splanchnocranium. Metapterygoid large, triangular, positioned on anterolateral portion of suspensorium, sutured to quadrate anteroventrally and hyomandibula posteriorly. Metaptertygoid anterior border sinuose, gently deflected ventrally. Metapterygoid channel large, deep, long-walled, and with thin anterior process straight, and elongated (with tip slightly expanded). Ventral wall of metapterygoid channel more developed than dorsal, and with edentate suture to lateral ethmoid anteriorly, with condyle posteriorly. Dorsal wall elongated, with border triangular, resting on large facet of lateral ethmoid. Ventral surface of metapterygoid with groove more developed in Hopliancistrus wolverine and H. xikrin (Fig 17).
Hyomandibula large (largest bone in suspensorium), synchondrally sutured to metapterygoid anteriorly. Hyomandibula separated from quadrate by symplectic cartilage; anterolateral facet jointed to preopercle. Hyomandibula has two posterior connections to skull: facet with cartilaginous condyle connected to prootic and digitized suture to compound pterotic. Posteromedial portion of hymandibula forming ventral wall of orbit. Dorsal surface with well-developed levator arcus palatini crest, with profound conical fossa on levator arcus palatini crest base (Fig 17). Posterior region of hyomandibula greatly deflected ventrally. Portion of hyomandibula posterior to opercle developed into shelf, with thin and pointed process extending posteriorly from opercular condyle, to which opercle has secondary attachment. Opercular condyle of hyomandibula well-developed.
Quadrate narrow, elongated and anteriorly expanded (L-shaped in dorsal view); dorsal surface flat, longitudinal ridge absent. Quadrate sutured to metapterygoid and laterally to preopercle, bearing small edentate suture to hyomandibula posteriorly. Quadrate anterior tip
Preopercle roughly rectangular, sutured to quadrate anteriorly and hyomandibula posteriorly. Preopercle-hyomandibular ridge high, continuous and covered by small platelets. Symplectic foramen cartilage present on anterior portion of preopercle. Ventral facet of preopercle with small condyle articulating with posterior ceratohyal (Fig 17).
Opercle with pointed anteriorly directed process, anterior to opercular condyle, which fits within depression on lateral face of preopercle. Posteriorly, opercle connected to preopercle by extensive synchondral joint (Fig 17).
Autopalatine short and robust, anterior tip slightly curved laterally and enlarged (Fig 6). Anterior tip of autopalatine connected to maxilla by large block of cartilage. Posteriorly with well developed rounded condyle connected to lateral ethmoid. Autopalatine posterior condyle with three processes, two lateral and one medial, latter two serving as insertion site for extensor muscles [13]. Maxilla relatively broad and flattened, with size similar to that of autopalatine. Maxilla articulated to autopalatine via two condyles.
Premaxilla roughly rectangular; dorsal surface with well developed groove and condyle for articulation with palatine cartilage. Dorsal ridge well-developed to insertion of adductor muscle. Premaxillary bones joint mesially, and articulated to mesethmoid disk through cartilage block. Ventral facet with rectangular opening, housing teeth. Teeth numerous and small, bicuspid with medial cusp diminute. Dentary somewhat rectangular, with well-developed rounded coronoid process and concave posteriorly (dentary deeper in H. tricornis). Dentary with bony laminar expansion strongly sutured to anguloarticular posteriorly, and connected via thick cartilage anteriorly. Anguloarticular L-shaped in cross section. Anterior area of anguloarticular with slit above condyle of articulation with quadrate. In H. tricornis slit very narrow, and dorsal facet wider than anguloarticular condyle. In H. munduruku slit more developed than in H. tricornis, but condyle smaller than in other species, and dorsal facet much wider than anguloarticular condyle. In H. wolverine and H. xavante slit wider and deeper than in H. tricornis, H. munduruku and H. xikrin, with dorsal facet narrower than condyle. In H. xikrin slit narrow but with dorsal facet narrower than condyle (Fig 18).
Posterohyal roughly triangular, with lateral border largest, and L-shaped in cross section. Lateral wall of the posterohyal pouch laterally curved externally extending lateral border of posterohyal (Fig 19). Condyle for articulation with preopercle small and positioned on ventrolateral margin. Posterohyal synchondrally connected to anterohyal except small suture on anterior portion (Fig 20). Four branchiostegal rays; first distinctly wider, and with small condyle of contact with ceratohyal. Last branchiostegal contacting adjacent cartilage between posterohyal to anterohyal (Fig 20). Anterohyal wide and flat dorsoventrally, with anterior margin (in dorsal and ventral views) straight from medial tip to approximately half bone, then distinctly convex towards suture with posterohyal. Anterohyal bearing small rounded foramen for afferent mandibular artery on posterior half of bone. Anterohyal connected to hypohyal by cartilage. Dorsal and ventral hypohyal not distinguishable, probably fused. Hypohyal short and rectangular, connected to counterpart, as well as to dentary via thick cartilage. Dorsal surface with large fossa for insertion of anterior process of urohyal. Urohyal well-developed, wider than longer, bearing strong anterior processes that articulate to hypohyals.
First hypobranchial ossified with cartilaginous caps, elongate and slender medially, with lateral portion slightly expanded. Second, third, fourth and fifth hypobranchials cartilaginous and indistinguishable from cartilaginous cap of ceratobranchials. First basibranchial ossified with cartilaginous caps and with half length of first hypobranchial. Second and third basibranchial cartilaginous; second being tiny and third robust and elongate, occupying space between bases of third to fifth ceratobranchials. Fifth ossified ceratobranchials, rod-like and with cartilaginous caps. First ceratobranchial with large accessory flange, as long as ceratobranchial. Fifth ceratobranchial with middle portion expanded, triangular-shaped, bearing lower pharyngeal tooth plates. Lower pharyngeal plate with vestigial teeth on its mesial border. Four ossified epibranchials, rod-like and with cartilaginous caps. First epibranchial bearing small ventrally directed process. Fourth with short anteriorly directed process and with well developed posterior shelf (Fig 20). Two infrapharyngobranchials, first connected to third epibranchial short and slender, expanded posteriorly; second square and connected to anterior facet of fourth epibranchial and upper pharyngeal tooth plates. Upper pharyngeal tooth plates slightly triangular; tooth plates with mesial shelf with raised bulbous area (comma-shaped in ventral view), bearing small teeth restricted to bulbous area and posterior edge of shelf.
Weberian complex centrum (WCC) relatively short and composed of vertebral centrum 1 to 5. Swimbladder encapsulated on ventral expansion of transverse process of WCC. Transverse process narrowing distally with rounded tip contacting compound pterotic (Fig 6). Ventral process of WCC roughly circular, short, not reaching the sixth vertebrae. First completely

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free, sixth vertebra with rectangular neural arch sutured to parieto-supraoccipital; sixth and seventh vertebrae expanded laterally in area they connect (Fig 6). Ribs of sixth vertebra (first rib) large and split medially, articulating to both centrum and neural arch; rib enlarged ( Fig 6). Seventh centrum with pointed anteriorly directed process on dorsal portion, not reaching the tuberculum of rib of sixth centrum (Fig 21A). Ribs on vertebrae six, eventually on eighth, and always present from centra 9 to 16. Vertebra 8 bearing ribs only in some specimens of H. xavante and H. xikrin. Vertebral centra 8, 9 and sometimes 10 lacking neural spines; seven to eight bifid neural spines; first bifid neural spine on tenth, or sometimes on eleventh centrum. Hopliancistrus tricornis, H. munduruku, H. xavante and H. xikrin with 11 vertebrae after last neural bifid spine, whereas H. wolverine with 10. Vertebrae with 16 to 17 haemal spines; first haemal spine on eleventh or twelfth centrum. Haemal spines of 14, 15, 16, 17 and 18th vertebrae with small bifurcation on posterior portion to support anal-fin pterygiophores (Fig 21A).
Caudal skeleton with parhypural, hypural 1, and 2 fused forming a ventral plate, uroneural and epural fused to hypurals 3, 4 and 5 forming a dorsal plat; ventral plate larger than dorsal plate; ventral and dorsal plates fused at base with notch on posterior border (notch smaller in H. tricornis); hypurapophysis triangular and laterally directed (Fig 21A)  and 7). First and second dorsal-fin pterygiophores fused and forming nuchal plate, and with condyle on dorsal surface to support spinelet. First and second dorsal-fin pterygiophore connected along entire posterior and anterior margins, respectively, with small edentate suture on dorsal portion (Fig 21B). First pterygiophore loosely connected to short neural spine of seventh centrum; second pterygiophore loosely connected to neural spines of seventh and eighth centra. Second pterygiophore largest and with transverse process (TPD2) distally connected to connecting bone (CNB) (Fig 16). Connecting bone extended to rib of sixth vertebra. Transverse processes present on first five (of eight) pterygiophores; eighth supporting sixth and seventh branched rays, and with par of elongate Y-shaped posterior processes (Fig 21B). Dorsalfin spinelet very reduced and V-shaped; short ventral arms of spinelet slide under transverse

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process of nuchal plate, but locking mechanism not functional. Dorsal-fin spine unbranched, robust.
Adipose-fin composed of single pre-adipose plate, spine, and posterior membrane. In Hopliancistrus tricornis, H. munduruku, and H. xikrin pre-adipose plate positioned over neural spine of vertebrae 22, and adipose-fin spine over 23; in H. wolverine and H. xavante pre-adipose plate over neural spine 23 and adipose-fin spine over 24.
Pectoral girdle strong, broad; cleithral process short, covered by odontodes on lateral facet. Anterior margin of cleithrum (in dorsal and ventral views) straight medially, and concave laterally. Posterodorsal process of cleithrum deep, and articulating to notch of compound pterotic. Ventral surface of pectoral girdle with large fossa for abductor muscles and base of pectoral-fin spine. Posterior process of coracoid relatively elongated, longer than last pectoralfin ray (Fig 22A and 22B). Ventral surface or pectoral girdle covered by skin.
Basipterygium composed of anterolateral, anteromedial, and posterior processes. Anterolateral process of basipterygium not reaching counterpart at midline; anteromedial process with wide base, and sutured to counterpart. Lateropterygium short (smaller than anterolateral process). Anterior fenestra of basipterygium rounded and small. Basipterygium sutured to counterpart, except for a small portion in which it is connected via cartilage (Fig 22C and 22D).
https://doi.org/10.1371/journal.pone.0244894.g021 of 14th vertebral centrum. In H. munduruku and H. xikrin first pterygiophore longer than in congeners, and anterior tip almost reaching haemal spine of 13th vertebral centrum. Unbranched ray of anal-fin supported by first pterygiophore; first and second branched rays supported by second pterygiophore, last pterygiophores supporting only one ray (Fig 21C and 21D).

Discussion
Species of Hopliancistrus of the Rio Tapajós basin are currently found in its middle portion only, even though several expeditions were conducted to its main upstream tributaries, the Juruena and Teles Pires rivers. On the other hand, Hopliancistrus are widespread in stretches with rapids and rocky substrate of the Rio Xingu basin. As far as we know, no other genus of Ancistrini has its distribution restricted to these two basins; the closest situation is that of Parancistrus, which is known only from the Xingu and Araguaia-Tocantins basins. Generally, Ancistrini genera that include species adapted to rapids present wider distributions that include the Araguaia-Tocantins basin and/or rivers draining the Guyana Shield at the northern margin of the Amazonas basin in Pará state, such as Baryancistrus, Hypancistrus, Panaqolus, Panaque, Peckoltia, Pseudacanthicus, Pseudancistrus, Scobinancistrus and Spectracanthicus.

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The current distribution of Hopliancistrus indicates that the most recent ancestral species of the genus emerged after the tectonic events that shaped the Amazon basin, such as the reversal of the direction of the Amazonas River [26,27]. We would expect that the species radiated from an ancient connection between the two basins, possibly where now lies the headwaters of the Jamanxim and Curuá rivers, which are currently separated by a dry stretch of approximately 23 km in straight line.
Hopliancistrus belongs to Ancistrini by having fully eversible cheek plates with enlarged odontodes (also present in Pterygoplichthini); and by having the opercle modified into a sickle-shaped structure (a unique feature of Ancistrini). Hopliancistrus is unique by having three claw-like hypertrophied curved odontodes on the cheek plates, and short stiff odontodes on the lateral border of the snout, two exclusive features among loricariid catfishes. Enlarged to hypertrophied odontodes on the lateral border of the snout are present in several other cisandean loricariids (e.g., Delturus, Pareiorhaphis, Lasiancistrus, Pseudancistrus) as well as transandean (Dolichancistrus). Whereas odontodes are elongate, thin, and somewhat flexible in the former species, these are short and stiff in Hopliancistrus. Nevertheless, large specimens of Dolichancistrus and Chaetostoma have few very long odontodes, somewhat resembling the condition found in Hopliancistrus. However, the condition in Hopliancistrus is quite modified. Besides carrying these strong and few odontodes, Hopliancistrus use them as a protective weapon, everting them deliberately to hold and stab the aggressor.
The osteological study of the species of Hopliancistrus revealed some interesting characters that could help us understand the phylogenetic position of the genus. Hopliancistrus has a conical and deep fossa on the base of the levator arcus palatini muscle crest, positioned laterally to preopercle-hyomandibular crest, with its opening turned to the dorsal portion of hyomandibula. A similar condition of the fossa was also observed in In most loricariids, the compound pterotic has an anteroventral process located laterally to the main axis of the bone (Fig 23). In Hopliancistrus, the antero-ventral process of the compound pterotic is always present and connected dorsally to the top of the compound pterotic. The compound pterotic in Hopliancistrus is quite peculiar in comparison to other loricariids, as the bone is robust and forms almost a box, with a large chamber between the top and floor of the bone. Ventrally, the compound pterotic has the ossified Baudelot ligament extended laterally as a branched bony strut providing different openings for the internal weberian capsule (Fig 6). This huge chamber inside the compound pterotic is completely connected to the other side through a set of foramina on the internal dorsal wall of the weberian complex.
The presence of the branched bony strut on the main body of the compound pterotic, as well as the fossa on the base of the levator arcus palatini muscle crest, might be responsible for the insertion of muscles that support and move the opercle and the three hypertrophied odontodes of the cheek plate, such as the dilatator operculi muscle.
The spinelet in Ancistrini, in general, is well-developed, presenting two ventral elongate arms. Hopliancistrus has the smallest spinelet among all studied genera, with a non-functional locking mechanism. This condition is also present in Ancistrus, Araichthys, Dekeyseria and Exastilithoxus hoedemani, although with spinelet slightly more developed than in Hopliancistrus (Fig 24).
Schaefer [13] found a great variation in opercle shape among loricariids. Ancistrini was diagnosed by Schaefer [13] based on modifications of the opercle. The Ancistrini present two shapes: 1. the opercle is relatively straight with posterior lateral section reduced, Peckoltia-type (sickle-shaped, following [25]), present in the genera that have bristle-like odontodes on cheek
The distal portion of the posterohyal in Ancistrini is generally pointed and conical with the lateral wall forming a pouch [25]. In Hopliancistrus, the distal portion of the posterohyal is nearly flat, and the pouch absent. This condition is also present in Ancistrus, Araichthys, Chaetostoma dorsale, Dekeyseria, Dolichancistrus, Exastilithoxus, Lasiancistrus, Lithoxus, Peckoltichthys and Pseudolithoxus (Fig 19).
The anteroventral process of the compound pterotic in most Ancistrini is separated medially from its main body and connected only by a strut, leaving a gap between the main body of the compound pterotic and the strut [25]. However, the size of the gap is influenced by the width of the strut. In H. tricornis and H. wolverine, the strut is very narrow and bar-shaped, leaving a large gap between the connecting strut and the main body of the compound pterotic. In H. munduruku, H. xavante and H. xikrin, the strut is wide or continuous, forming a wall mesially between the compound pterotic and the anteroventral process, sometimes leaving a small gap posteriorly (Fig 6).
A morphology-based cladistic analysis including Hopliancistrus is under preparation by the first author (RRO) and depicts a well-supported clade with Pseudolithoxus as sister group to two smalls clades, one composed by (Araichthys + Hopliancistrus) and other by (Lasiancistrus + Ancistrus). These results are similar to Roxo et al. [31], with a single difference that Guyanancistrus brevispinis was recovered as sister group to Araichthys + Hopliancistrus. The relationships of theses large clades are weakly supported in Armbruster's [24,29] analyses, possibly because of the limited number of species used. As knowledge and understanding of morphological variation in loricariids improve we expect more congruent results on morphological and molecular approaches.