Redescription and Geographical Distribution of the Endangered Fish Ossubtus xinguense Jégu 1992 (Characiformes, Serrasalmidae) with Comments on Conservation of the Rheophilic Fauna of the Xingu River

The monotypic species Ossubtus xinguense was originally described based on scarce material putatively divided into juveniles and adults. Ossubtus xinguense has a restricted distribution and was previously known only from a few rapids downstream of the city of Altamira, in the Volta Grande stretch of the Middle Xingu River. Until recently, the species was rare in museums because its habitat (large rapids) is difficult to sample. Large-scale collecting efforts targeting rapids throughout the Xingu River basin have yielded an abundance of new material. Based on an analysis of the type series and freshly preserved specimens, we redescribe O. xinguense and provide detailed osteological descriptions along with comments about its relationships within Serrasalmidae. Furthermore, we expand the geographical distribution of the species and discuss its conservation status.


Introduction
The monotypic genus Ossubtus was established by Jégu [1] to include new species O. xinguense. The original description was based on 15 specimens from rapids in the Xingu River near Altamira city. At that time, the species was thought to be rare in nature and restricted to the vicinity of the type locality [2]. Ossubtus xinguense is endemic to the Xingu basin and inhabits rapids with rock outcrops covered by macrophytes of the family Podostemaceae [1], [2]. Those habitats are severely threatened by the recent completion of major construction on the Belo Monte Dam complex on the Xingu River. The poorly known O. xinguense is easily diagnosed from all other serrasalmids by having mouth subinferior to inferior (versus mouth terminal or upturned). The profile of the head and snout in O. xinguense resembles that of the rodent capybara, inspiring its Brazilian common name, 'pacu-capivara' .
Ossubtus xinguense is related to the herbivorous Serrasalmidae named 'pacus' [3], and hypothetically nested within a monophyletic clade comprised exclusively of pacus from rapids and waterfalls, the so-called 'Myleus' clade sensu Ortí et al. [4]. That clade also includes Myleus, Myloplus, Tometes and Mylesinus, however, the monophyly of those genera is not supported [4]. Jégu [5], in his unpublished PhD dissertation on serrasalmid relationships, proposed a monophyletic clade formed by Myleus-Tometes-Mylesinus-Ossubtus based on 12 synapomorphies highlighting the morphology of the jaws and olfactory fossa: dorsal and lateral processes of premaxilla slender and thin; premaxillary teeth in two rows contacting each other; incisiform teeth specialized for cutting aquatic macrophytes; and olfactory fossa wide (versus the sister clade Myloplus with premaxillary teeth in two rows separated from each other; molariform teeth specialized for crushing seeds; and a narrow olfactory fossa). The clade sensu Jégu [5] is further divided into Myleus-Tometes group with robust incisiform teeth strongly attached to jaws versus Mylesinus-Ossubtus group with fragile and weakly attached incisiform teeth [6]. Furthermore, in the original description, Jégu [1] noticed that the first two labial premaxillary teeth of O. xinguense are weak and canine in shape, thereby differing from the remaining premaxillary teeth (Fig 1A). That condition was previously observed only in juvenile specimens of Mylesinus and Tometes up to 45 mm standard length [7], [8], [9]. In the other pacu genera, the first two labial teeth resemble the remaining premaxillary teeth (Fig 2A-2D). Jégu [5] treated the anterior labial caniniform teeth in O. xinguense as an autapomorphy. However, while examining these two first teeth in O. xinguense the authors noticed a pattern different from original description, rendering their attribution as caniniform obsolete.
The aim of this study, based on an analysis of the type series and additional freshly preserved specimens, is to redescribe O. xinguense with attention to anatomical novelties (particularly osteological ones) and allometry between juveniles and adults as well as males and females. In addition, we provide important updates on the geographical distribution of this endangered and poorly known species of rheophilic fish, and cast perspectives on its conservation.

Materials and Methods
Redescription and analysis based on the holotype, four paratypes, and more than 200 additional specimens ranging from 36.4 to 228.6 mm standard length (SL). Measurements and counts follow Jégu [1] and Andrade et al. [10]. Body measurements are given as percents of SL, and head subunits as percents of head length (HL). Counts are followed by the frequency in parentheses with the value observed in holotype indicated by an asterisk. Osteological descriptions based on two juvenile specimens cleared and stained (c&s) according to Taylor and Van Dyke [11], three adult specimens skeletonized (skel.), and radiograph of one paratype. Counts of vertebrae included the Weberian apparatus as four and the fused PU1+U1 caudal vertebrae as one. Osteological nomenclature follows Weitzman [12] with modifications proposed by Mattox et al. [13] and the addition of abdominal serrature composed of midventral spines oriented caudally (characteristic of Serrasalmidae taxa). Morphological comparisons employ previous observations in studies by Jégu [5], Machado-Allison [14], Dahdul [15], and Ota [16].
Variation in body shape was evaluated using principal components analysis (PCA) on 33 linear measurements according to Strauss [17]. Principal components were computed from the covariance matrix of log-transformed data according to Jolicoeur [18]. The first principal component (PC1) was interpreted as a general size factor (i.e., overall body size correlated significantly and positively with PC1) [19]. Analysis of covariance (ANCOVA) was used to compare the allometric trajectory of body shape between groups; analysis of variance (ANOVA) compared the means of body shape with respect to size class and sex. Institutional

Results
Ossubtus  Ossubtus.  Fig 2D), and labial row of premaxillary teeth in contact with lingual row (versus labial and lingual rows of premaxillary teeth separated by gap). It can be distinguished from Myleus, Tometes and Mylesinus by having four teeth (versus five or more) on dentary, and the first two labial premaxillary teeth with crown reduced, narrow (versus first two labial premaxillary teeth with crown well-developed, width approximating tooth base). It also differs from Myleus and Tometes by having incisiform teeth very fragile, much flattened anteroposteriorly, and weakly attached to jaws (versus incisiform teeth robust and strongly attached to jaws). Ossubtus

Diagnosis
Same as generic diagnosis.

Description
Morphometric data presented in Table 1. Medium sized serrasalmid, largest examined specimen 228.6 mm SL. Body laterally compressed, profile sub-ovoid (Figs 3-5). Predorsal profile steep. Dorsal profile of head markedly convex from upper lip to vertical through anterior nares, gently straight or nearly concave from that point to distal margin of supraoccipital spine, and slightly convex from that point to dorsal-fin origin. Greatest body depth at dorsal-fin origin, means 56.7% of SL in adults and 53.8% SL in juveniles. Dorsal-fin base straight to gently convex. Body profile straight from dorsal-fin terminus to adipose-fin origin. Ventral profile of head straight to gently convex. Ventral body profile distinctly convex. Anal-fin base convex. Caudal peduncle short, upper and lower profiles concave. Mouth subterminal to subinferior in juvenile specimens up to 50 mm SL (Fig 5B), and markedly inferior in larger specimens. Snout strongly rounded. First branchial arch with gill rakers elongated and recurved. Gill rakers in upper branch 10 (1), 11 (6), or 13 (2), and in lower branch 13 (1), 14 (5), or 15 (4); one gill raker at cartilage between upper and lower branches.
Osteology. Neurocranium. overall neurocranium shallow and elongated with slender bones; long axis set at about a 45°angle from longitudinal axis of body (Figs 6 and 7).
Olfactory region: composed by mesethmoid, lateral ethmoid, vomer, and nasal (Fig 6). Mesethmoid triangular in frontal view, profile gently curved with ventral portion (lateral wings) finishing vertically; posteriorly directed keel large with anteroventral portion pointed, surpassing lateral wings and vomer. Mesethmoid with posterolaterally and posteroventrally    Orbital region: composed by parasphenoid, frontal, pterosphenoid, and orbitosphenoid ( Fig  6). Parasphenoid long with ventral aperture forming two thin projections somewhat parallel  across ventral margins of prootic and basioccipital. Parasphenoid contacts prevomer anteriorly, prootic dorsally and basioccipital posteriorly. Frontal large, subrectangular, length approximately 50% of neurocranium length; lateral margin of posterior half with deep notch receiving sphenotic; superficial surface with foraminas and grooves. Cranial fontanel elongated, narrower at extremities; portion anterior to epiphyseal bar almost two times longer than posterior portion. Pterosphenoid laminar, laterally articulated with sphenotic. Orbitosphenoid deep, composed of two laterally compressed bony lamellae projecting posteroventrally, but not contacting parasphenoid; anterior process enlarged with anterodorsal surface in contact with internal surface of frontal.
Otic region: composed by prootic, sphenotic, parietal, intercalar, pterotic, and epiotic ( Fig  6). Prootic quadrangular, with circular aperture for myodome passage. Sphenotic narrow with concave margin contributing to orbit, finishing posterolaterally with long pointed spine for origin of dilator operculi muscle. Parietal short and wide, increasing in width laterally; dorsal process well developed with surface sculptured by grooves. Intercalar with well-developed posterolateral portion, located in posterior region of neurocranium. Pterotic with short process ending in two truncated lobes posteriorly directed. Epiotic with lateral arm extending towards posterior margins of parietal and pterotic, dividing posttemporal fossa into dorsal and ventral portions (Fig 6).
Infraorbital series: composed by antorbital, infraorbitals 1-4, and supraorbital (Fig 7). Antorbital with ventral half larger, directed anteroventrally and reaching fourth infraorbital; dorsal half narrower, nearly vertical. First infraorbital well developed, overall size slightly smaller than circular orbit; anterodorsal margin (anterior to contact with antorbital) with shallow, irregular crenations; laterosensory canal obliquely oriented near center of bone. Second infraorbital smaller, wing shaped (ventral portion expanded), obliquely oriented with laterosensory canal slightly removed from dorsal margin. Third infraorbital vertically elongated with laterosensory canal close to anterior margin. Fourth infraorbital smallest, with anterodorsal portion expanded, forming obliquely oriented "Y" with irregular margin sutured to frontal; laterosensory canal restricted to posteroventral portion. Supraorbital subrectangular, contributing to orbit rim but not contacting antorbital, leaving small gap in circumorbital series.
Jaws: premaxilla high with wide surface; interdigitations lacking at symphyseal sututre, Ascending premaxilla process elongated, slender, and oblique. Lateral premaxilla process short, subrectangular. Premaxilla with two rows of fragile, weakly inserted, incisiform teeth. Teeth visible outside mouth, labial row in internal contact with lingual row. Premaxilla labial row with five teeth and lingual with two teeth. First and second teeth of labial row with poorlydeveloped edges; three remaining teeth well developed, high, trilobed and spatulate (Fig 1A). First premaxilla lingual teeth bilobed and second trilobed (Fig 1B). Teeth in juvenile specimens up to 70 mm SL with narrow cusps, higher, with main cusp resembling spearhead; teeth of adults with rounded cusp edges, lower, with main cusp resembling flattened spoon. Maxilla edentulous, narrow, with middle expansion connected to posterior arm of premaxilla. Paired symphyseal teeth absent from dentary. Posterodorsal margin of dentary convex with apex not reaching horizontal through tip of fourth tooth; posterior margin sloped at 45°angle with long axis of lower jaw. Dentary with four teeth, first trilobed, second to fourth bilobed (Fig 8A). Dentary teeth with posterior cusp of each tooth inserted externally into groove of anterior cusp of next tooth. Symphyseal dentary teeth absent. Retroarticular elongated, lenticular, reaching ventral margin of lower jaw but slightly removed its posteroventral tip completed by dentary. Anguloarticular elongated, articulated with quadrate by thin cartilage. Cononomeckelian comma shaped. Dentary symphysis with three bony lamellae oriented obliquely to long axis of bone (Fig 8B).
Weberian apparatus and associated centra: composed by compound centra 1-4, neural arches 3 and 4, neural spine of vertebra 4, intercalarium, scaphium, inner and outer arms of os suspensorium, tripus, claustrum, and neural complex. Centra 1 and 2 of approximately equal size (Fig 6). Lateral process of centrum 2 well-developed, longitudinally elongated, articulated ventrolateraly with centra 3. Neural arches 3 and 4 joined by thin suture; neural arch 3 small; neural arch 4 bearing elongated neural spine of 4th vertebra (Fig 6). Intercalarium thin, scaphium short, composed by concha scaphium and ascending process. Inner arm of os suspensorium as robust process posteriorly directed and parallel to dorsal surface of swimbladder. Outer arm of os suspensorium short and dorsoventrally flattened. Tripus aligned with intercalar and lateral process of 3rd centrum with ventral margin straight for centrum (Fig 6). Claustrum with short ascending arm, and bearing triangular flange along dorsal margin. Neural complex with posterior lamelar portion narrow (Figs 6 and 7).
Axial skeleton: composed by vertebral centra, neural arches, haemal spines, ribs, neural spines, parapophyses, and supraneurals. All vertebrae with neural arch and dorsal neural spine. Vertebral centra composed by 13 precaudal vertebrae (5th to 17th vertebrae) and 21 caudal vertebrae (18th to 38th vertebrae) including three pleural vertebrae and compound caudal centrum. Precaudal vertebrae lack haemal spines and bear parapophyses articulating with ribs. Caudal vertebrae have haemal arch with ventral haemal spine. Eighteenth and 19th vertebrae lack haemal spine, but with closed haemal arch bearing tiny ribs. Haemal spines of 20th and 21st vertebrae short.  posterior portion lamelar, approximately half of anterior dorsal portion laterally overlapped by supracleithrum. Postcleithrum 2 vertically elongated with posterior portion lamelar, and anterior portion overlapped by cleithrum. Postcleithrum 3 elongated and pointed, extending beyond ventral margin of pectoral fin. Scapula with anterior process narrow, not covering space between scapula, cleithrum and coracoid, filled with thin connective tissue. Coracoid contacting cleithrum bilaterally with ventral margin curved. Mesocoracoid vertical with dorsal portion articulated to horizontal process of cleithrum, contacting coracoid ventrally.
Pelvic girdle: composed by basipterygium, and pelvic fin rays i,7 (Fig 11). Basipterygium elongated with ischiatic process long and pointed, nearly reaching first third of outermost pelvic fin rays.
Anal fin: composed by proximal-middle radials, and anal-fin rays iii-iv, 22-25. Proximalmiddle anal fin radials 25, anterior three each with lateral lamelar process. Males with second pointed lobe formed by elongated middle rays, each one sometimes with pair of stiff laterally curved hooks near distal tip (Fig 12, see more details under Sexual dimorphism).
Color in alcohol. Background color silvery brown, head and upper flanks darker. Most specimens with lower flank light brown (Figs 3, 4 and 5B). Ventral surface of head, opercular region, supracleithrum, abdomen, and anal-fin base pale yellow. Irregular brownish blotches scattered over flank, mainly in mature males (Fig 4A). Larger adults with large diffuse blotch formed by few scattered melanophores on anterior-medial portion of flank (Fig 4). In juveniles, dorsal fin with concentration of melanophores on anterodistal margin, humeral blotch shaped as irregular inverted triangle, faint in smallest preserved specimens. Juveniles up to 60 mm SL with adipose fin pigmented (Fig 5B).
Color in life. Similar to that described for preserved specimens (Fig 5A), except ventral surface of body more pale. Adipose fin darker in specimens up to 60 mm SL, becoming hyaline with growth. Triangular humeral blotch iridescent turquoise, observable in specimens up to 130 mm SL.
Sexual dimorphism. Mature males of O. xinguense are recognized in specimens with 150 mm SL or more by exhibit additional anal-fin lobe formed by branched rays 12-14 (Fig 4A), whereas females have anal fin with falcate distal margin (Figs 3 and 4B). Some males with 180 mm SL or greater have pair of stiff, laterally divergent hooks near distal tip of each anal-fin ray in additional lobe (Fig 12). Stiff hooks on rays of additional lobe were observed in 36 of 52 mature male specimens greater than 180 mm SL. Dorsal-fin rays extended by modest filaments in 11 male specimens greater than 160 mm SL. All mature specimens with dark blotches on flanks (described under Color in alcohol), but blotches more intense in males.
Geographic distribution. Ossubtus xinguense is endemic to the Xingu River basin, and confirmed from the rapids of Volta Grande do Xingu and the lower Rio Iriri, near its confluence with the Rio Xingu (see Material Examined). In addition, local fishermen report O. xinguense from the Rio Iriri Extractive Reserve (Resex do Rio Iriri at Cachoeira do Julião, 4°4 5'58"S 54°38'43"W), and from the Rio Xingu near the city São Félix do Xingu (Fig 14).

Ecological notes.
Ossubtus xinguense is caught in the rapids of the Xingu River along with other serrasalmids such as Acnodon normani, Myloplus arnoldi, Myleus setiger and Tometes ancylorhynchus and Tometes kranponhah [10], and many other rheophilic fishes of the families Characidae, Anostomidae and Loricariidae. To capture rheophilic species, fishermen normally throw cast nets in shallow areas of the rapids. Many rheophilic fishes shelter in crevices under rocks. After throwing the cast net, the fishermen plunges into the rapids to manually close the net and prevent fish from escaping. Ossubtus xinguense lives in clear swift waters over rocky outcrops covered by Podostemaceae, a habitat type that is common in the Middle Xingu River and lower Iriri (Fig 15). While snorkeling in rapids, we observed O. xinguense hiding under rocks over sandy bottoms.
Diet analysis of 10 adult specimens chosen randomly (GEA1729, 159.4-198.9 mm SL) revealed mainly Podostemaceae and Bryophyta (Frequency of Occurrence = 100% for each food item), allochthonous leaves (FO = 80%), twigs (FO = 50%), tree bark and plants roots (FO = 40% each), aquatic macroinvertebrates (FO = 20%), and sand grains (FO = 10%). Some adult specimens exhibit accumulations of lipids between bases of dorsal-and anal-fin membranes; those specimens were in reproductive condition. In all specimens, intestines were infested with an abundance of the nematode Rondonia rondoni. This nematode is possibly a symbiont rather than a parasite (Andrade et al., in prep.). Practically all fish are parasitized by metacercariae under the skin and scattered over body, head and fins, forming black spots known as "black-spot disease". In addition, O. xinguense is parasitized by Anphira xinguensis, a gill isopod parasite exclusive to the species. Morphometric analysis. Based on our PCA analyses, juveniles, and adult males and females of O. xinguense have similar overall body shapes. On the other hand, the analyses detected allometric differences between juveniles and adults, and males are generally larger than females. The PCA performed on 216 specimens established two distinct groups (Fig 16) represented by juveniles (N = 68, 36.4-92.1 mm SL) and adults (N = 148; 65 females 114.5-187.7 mm SL, and 83 males 150.9-228.6 mm SL). The two groups were clearly separated along PC1 (x-axis) with juveniles on the left and adults on the right (Fig 16), suggesting allometric differences during the growth. Linear measurements with significant differences between The projection of individual scores on PC2 (y-axis), considered size-independent shape variation, shows full overlap between the two groups, suggesting juveniles and adults are similar in overall body shape. The measurements with the greatest loadings on PC2, such as 1st anal-fin lobe length, dorsal-fin length, postorbital distance, and fused 4th infraorbital width (Table 2), contribute to the highest variation. All other measurements are considered statistically indistinguishable with respect to growth (ANCOVA, d.f. = 214, n.s.), and therefore represent identical allometry and body shape between classes.
Overall, the PCA summarizes morphological variation in O. xinguense ( Table 2, Fig 16). The first principal component (PC1) accounts for 98.406% of total morphological variation, whereas PC2 accounts for 0.269%. PC1 correlates strongly with SL because the overall body size of the fish (represented by scores of the PC1) was significantly and positively correlated with the fish size (r = 0.987, P << 0.01). Therefore, the PC1 is interpreted as a general size factor and describes the allometric trajectory of body shape. The analysis of covariance performed between PC1 scores and log transformed SL of the species discloses that the two regressions of juveniles and adults have intercepts and slopes statistically different corroborating the separation between the two size classes (ANCOVA, F = 22.1, P << 0.01). Those results suggest strong allometric growth between juveniles and adults. PC2 is interpreted as a general descriptor of variation in body shape, free of allometry and only weakly correlated with SL (r = 0.082, n.s.). However, no differences were detected between both size classes or sexes with respect to overall shape (ANOVA, F = 2.08, n.s.). Thus, juveniles and adults, and males and females are indistinguishable with respect to overall shape excluding sexually dimorphic features.

Discussion
Ossubtus xinguense is a strictly rheophilic fish that occurs in rapids associated with rocky substrates with crevices. Such habitats are especially difficult to sample, resulting in a scarcity of specimens in scientific collections and gaps in our fundamental understanding of the species. Based on an exhaustive analysis of newly collected material, it was possible to address questions on their taxonomic relationships, intraspecific morphological variation, distribution, and conservation status. Morphology and Taxonomic relationships.-The most important characteristics for placing O. xinguense in the Myleus clade sensu Jégu [5] are the presence of incisiform teeth, two rows of premaxillary teeth with internal contact (no gap), the lack of spines in prepelvic serrature, and the wide olfactory fossa. For species of this clade, the incisiform teeth reflect a specialization for cutting soft leaves of Podostemaceae [3]. Jégu's [5] report of caniniform teeth in the labial premaxillary row in O. xinguense is out of tune with other members of the Myleus clade. However, from analysis of several specimens of O. xinguense, including adult skeletons and cleared and stained juveniles, we notice that these teeth are actually incisiform with cutting edge poorly developed (Fig 1A). The caniniform shape in these teeth was refuted when we discovered in all specimens examined that the two anterior teeth of the labial premaxillary row are very anteroposteriorly flattened, with tooth base markedly developed laterally (Fig 1A). In adult specimens, each main cusp (most central) has scarcely developed lateral lobes, and is rounded to resemble a flattened spoon, like the remaining well-developed incisiform teeth. Therewith, all teeth are incisiform in O. xinguense, similar to the representative taxa of Myleus clade (i.e., Myleus, Tometes and Mylesinus). The two anterior labial teeth of O. xinguense were previously misunderstood as caniniform due to observations made on juvenile specimens.
The shift in mouth orientation in O. xinguense, hypothesized by Jégu [1], although not proved, was noticed here. Adults have a more inferiorly placed mouth than juveniles (Fig 5B,  45.6 mm SL). This subtle, difference results from a ventrally directed bend in the neurocranium during growth. Although the rotation of neurocranium is not statistically supported by snout length (ANCOVA, n.s.), the higher score value at PC1 for this measure (see Table 1) indicates greater differences between juveniles and adults specimens than other linear measures. In addition, O. xinguense has a shallow, elongated neurocranium (Fig 6), whereas members of the Myleus clade have a relatively deep, triangular neurocranium. The elongate neurocranium in O. xinguense is similar to the condition found in Colossoma and Piaractus. However, the neurocranium of O. xinguense is slender and light (like the neurocranium of Myleus, Tometes and Mylesinus) compared to the robust neurocranium of Colossoma and Piaractus (both seedcrushing genera). This condition is exemplified by the wide aperture of the olfactory fossa in O. xinguense, resulting from the thin mesethmoid roof. Colossoma and Piaractus have olfactory fossa with narrow aperture and stout mesethmoid roof. The wide olfactory fossa is exclusive to the Myleus clade within Serrasalmidae [5], and presumably houses a large sensory organ used to find food and perhaps mates. According to Jégu [5]: 323 (character 16), Ossubtus also shares with Colossoma and Piaractus a reduced number of branched anal-fin rays (i.e., Ossubtus with 22-25 and Colossoma-Piaractus with 20-24 versus 26-34 in other members of the Myleus clade sensu Jégu [5]). That reduction is considered a reversal in Ossubtus.
According to Jégu [5]: 365 (character 112) the presence of four infraorbitals in Ossubtus is an autapomorphy and results from the fusion of infraorbitals 3 and 4. Most species of Serrasalmidae have six or sometimes five infraorbitals (e.g., members of the piranha clade according to Machado-Allison [14]). Curiously, the reduced number of infraorbitals in Ossubtus is shared with some species of Leporinus (Anostomidae) that similarly have a subinferior to inferior mouth [49]. Thereby, the fusion of infraorbitals might be related to the downward re-orientation of neurocranium or by modifications (e.g., elongation [49]) in some bones of neurocranium in taxa with ventrally directed mouths.
Sexual dimorphism and Allometric variation.-Ossubtus xinguense shares with herbivorous Serrasalmidae, such as the species of the genera Acnodon, Metynnis, Myleus, Mylesinus, Tometes, Utiaritichthys [3], and Myloplus [35] dimorphic features mainly evidenced by the anal fin wherein mature males have an additional lobe formed by elongation of the middle rays which sometimes bear divergent hooks distally (Fig 12), and dorsal fin with long and thin filaments. However, those sexually dimorphic features in O. xinguense also seem correlated to variation in physiological condition. Among males greater than 180 mm SL, 70% showed the stiff divergent hooks on anal-fin rays (Fig 12), and less of 10% of males larger than 160 mm SL showed filaments on dorsal-fin rays. The dorsal-fin extensions in O. xinguense are noticeably smaller than those found in the aforementioned genera. Some authors (e.g. [50]) express doubts on the permanence of sexually dimorphic features in serrasalmids, and consider them to be restricted to the breeding season as in other characiforms (e.g. [51], [52]). However, it is known that in Serrasalmidae the dimorphic features do not disappear once established, and remain evident in mature males outside of the breeding period.
We expected to find morphometric differences in body shape between males and females. That hypothesis was refuted by the multivariate analysis performed in this study. However, the allometric distinction between juveniles and adults [1] was supported in O. xinguense. The results may indicate ecomorphological trends associated with rheophilic behavior of O. xinguense that, like other reophilic serrasalmids, show up to 100 mm SL allometric differences between juveniles and adults (see Figure 2a in [7], and Figure 3a in [8] and [33]). Therefore, allometric bodily differences are possibly associated with skill of juveniles remain in the rapids without being carried away by the current.
Conservation.-This study establishes the downstream limit of O. xinguense as the last major rapids of the Xingu River (Fig 14), specifically Cachoeiras Tapaiúna and Itamaracá. The farthest upstream record confirmed for O. xinguense is the last large cachoeira on its left bank tributary, the Rio Iriri. Local fishermen claim that the species occurs in rapids further upstream, such as Cachoeira do Julião located in Rio Iriri Extractive Reserve (Fig 14). Although infrequently, O. xinguense is sold for human consumption at street markets in São Félix do Xingu (Fig 14), suggesting it occurs further upstream in the Xingu River as well. Based on those reports, we believe that O. xinguense is widely yet irregularly distributed in the rapids of the Middle Xingu River.
The previous scarcity of O. xinguense in fish collections suggested that the species is rare in nature (see Jégu and Zuanon [2]). However, their rarity in museums is better explained by a limited ability to effectively sample rapids. Based on underwater observations, O. xinguense commonly inhabits rocky crevices covered by aquatic macrophytes of the family Podostemaceae (Fig 15). Unlike other rheophilic serrasalmids, juveniles and adults of O. xinguense seems restricted to sheltered portions of rapids areas and seldom move between rocky outcrops with groves of Podostemaceae. Other serrasalmids (e.g. Myleus setiger, Myloplus rhomboidalis, Mylesinus spp., and Tometes spp.) are often found in sheltered portions of rapids, as well as in open swift current and calmer stretches upstream and downstream of rapids. Our observations support a stronger affinity in O. xinguense for habitats closely associated with swift rapids and ample groves of Podostemaceae. The degradation of the rapids and their conversion to reservoirs by hydroelectric dams is the main cause of loss of rheophilic diversity. Zuanon and Jégu [38] classified O. xinguense as 'Endangered' according to the World Conservation Union. Despite the expansion of the known range of O. xinguense, serious threats to the species remain: the Belo Monte Dam Complex. One dam, Pimental, will flood approximately 80 river kms of the Xingu channel; approximately 90 river kms from below the Pimental dam to above the outflow of a second dam, Belo Monte, will be dewatered [53]. The flood pulse of the dewatered stretch will be severely attenuated and likely impact stands of Podostemaceae. The irregular distribution of O. xinguense and its endemism to the Xingu Basin reinforce the species' original threat category. Therefore, we recommend an increase of studies to establish conservation units for proper management. Furthermore, the conservation of rapids upstream of the Belo Monte impact area is mandatory to assure the health of the remaining populations of rheophilic fishes in the Xingu River.