A new spiny species of Hypostomus Lacépède ( Loricariidae : Hypostominae ) from thermal waters , upper rio Paraná basin , central Brazil

A new species of Hypostomus is described from the rio Quente, rio Paranaíba drainage, in the upper rio Paraná basin in central Brazil. The rio Quente is a peculiar small and shallow fast water stream with water temperatures around 34oC. The new species is distinguished from all congeners by having the five lateral series of plates with hypertrophied odontodes not aligned in rows and more conspicuous on flanks of caudal peduncle of the larger specimens. The new species is only known from the rio Quente.


Introduction
The genus Hypostomus Lacépède bears the greatest diversity within the Loricariidae, with about 130 valid species (Eschmeyer, 2012;Zawadzki et al., 2014).The phylogenetic relationships among these species are still unclear and the monophyly of the genus is subject of many morphological and molecular studies (e.g., Montoya-Burgos, 2003;Armbruster, 2004).The rio Paraná basin shelters more than 30 of these species, most of them described in the early twentieth century which renders taxonomic questions such as H. fluviatilis (Schubart), H. meleagris (Marini, Nichols & La Monte), H. scaphiceps (Nichols), and H. topavae (Godoy).
The upper rio Paraná basin is a well-sampled ecoregion (sensu Abell et al., 2008) and has one of the better known ichthyofaunas in Brazil (Langeani et al., 2007).However, some of its unexplored areas still reserve undiscovered taxa, mainly on its headwaters.The rio Quente, a tributary of the rio Piracanjuba, rio Paranaíba drainage, upper rio Paraná basin, located in the southern Goiás State, central Brazil, can be considered one of these underexplored areas.This is a natural stream of warm water that starts to flow at 700 m above sea level, in the upper part of the western flank of the Caldas Novas dome.The springs of thermal waters rise to the surface at temperatures around 50ºC from a series of open spaces in the basal layers of quartzite of the Paranoá Group, and remain relatively warm downstream (D'el-Rey Silva et al., 2008).According to these authors, the rio Quente is unique, and there is no other natural spring of thermal waters in the region.
Recent samples in the rio Quente revealed the presence of a peculiar spiny species of Hypostomus which does not pertain to any of the known species for the genus and is herein described.

Material and Methods
Measurements and counts were taken according to Boeseman (1968) and Weber (1985).All measurements were taken point to point with digital calipers to the nearest 0.1 mm.Body measurements are given as percents of the standard length (SL), except when noted; subunits of the head are given as percents of head length (HL).Vertebrae count included the five vertebrae from the Weberian apparatus, and the compound caudal centrum was counted as a single element.Body plate nomenclature was based on Schaefer (1997) with modifications by Oyakawa et al. (2005).In the description, the mode of each count is given in parentheses after the respective count.Specimens were cleared and stained (c&s) according to Taylor & Van Dyke (1985).
Institutional abbreviations are: AMNH, American

Hypostomus yaku, new species
Figs Diagnosis.The new species is distinguished from all congeners by having the five lateral series of plates with hypertrophied odontodes, not aligned in longitudinal rows, more conspicuous on flanks of caudal peduncle of the larger specimens (vs.lateral series of plates lacking hypertrophied odontodes or with aligned hypertrophied odontodes only along the longitudinal keels) (Fig. 2).
Description.Morphometric and meristic data in Table 1.
Largest specimen examined 70.8 mm SL.Body deepest at dorsal-fin origin.Dorsal profile of body in lateral view ascendant from tip of snout to nares; convex from nares to dorsal-fin origin; slightly descending to caudal-fin base.
Ventral profile of body in lateral view almost straight from tip of snout to pelvic-fin origin; posterodorsally inclined from pelvic-fin origin to caudal-fin base.Caudal peduncle ovoid in cross-section.Anterior profile of snout markedly rounded in dorsal view.
Posterior process of supraoccipital bordered by one or two predorsal plates.Opercle rectangular, with 40-70 small to well-developed odontodes, mainly in ventral margin.Preopercle without odontodes, partially covered by four platelets.Preopercle branch of laterosensory system entering from suprapreopercle, passing through preopercle, and reaching small single canal-bearing plate (cp1).Three cheek plates with large and strong odontodes between opercle and canal-bearing plate; cheek plates not evertible.Eye small, 2.0-2.5 times in interorbital width, dorsolaterally placed; iris operculum present, well developed; space between orbits concave.Nares separated by flap of skin.Oral disk elliptical, wider than long and papillose.Papillae at upper lip ovoid and larger, rounded and smaller at lower lip, decreasing in size towards margins in both lips.Outer margin of upper lip with small lateral isolated patch with few odontodes.Lower lip with fringed margin, not reaching transverse line through gill openings and scapular bridge.Maxillary barbel shorter than orbital diameter, proximally linked to lip, ornamented with small papillae.Angle between contralateral dentaries approximatelly 170º.Premaxillary teeth 27 to 69 (43); dentary teeth 31 to 72 (49).Teeth bicuspid; newly emerged ones with medial cusp well developed, considerably larger than lateral cusp; most teeth in functional series with subequal cusps, some with symmetrical cusps (Fig. 3).
Tip of snout dorsally covered by small plates and odontodes, anteroventral portion completely naked, except for two lateral patches of very small plates with odontodes.Odontodes in most anterior portion of snout markedly rounded.Ventral surface of head naked; sometimes few odontodes near opercular opening.Five complete lateral series of plates.Dorsal plate series with six to seven (seven) plates delimiting a naked area along dorsal-fin base.Median series with 23-25 (24) perforated plates.Ventral plate series starting at vertical through pelvic-fin origin, with two to three ventral plates along anal-fin base, and 11 to 13 ventral plates from end of anal-fin base to caudal-fin origin.Lateral series of plates with poorly-developed longitudinal keels: from dorsal-fin origin to end of caudal peduncle on dorsal series; from compound pterotic to end of caudal peduncle on mid-dorsal and median series, and to vertical through second or third branched dorsal-fin ray on mid-ventral series; keels absent on ventral series.Lateral plates covered by irregularly arranged odontodes; each plate with two to four hypertrophied straight odontodes posteriorly oriented, more conspicuous on flanks at posterior portion of caudal peduncle (Fig. 2); odontodes with tips slightly curved laterally in larger specimens.Abdomen naked, except for lateral platelets between pectoral-and pelvic-fin origins, and at pectoral girdle region; platelets absent in juveniles.Dorsal fin with seven branched rays, posterior border convex; tip of adpressed rays extending almost to vertical through end of last anal-fin ray, not reaching adipose-fin spine; well-developed V-shaped spinelet present, locking mechanism functional; tip of first and last proximal radials contacting neural spine of seventh and 15 th or16 th vertebrae, respectively.Adipose fin triangle-shaped; distal border not free from caudal peduncle surface, extending in oblique descendent line (Fig. 2); adipose-fin spine markedly straight; one or two pre-adipose azygous plates.Pectoral fin with six branched rays; spine strong and distally covered with welldeveloped odontodes; tip of adpressed pectoral-fin rays reaching posteriormost portion of pelvic-fin base.Cleithrum exposed projection truncated posteriorly.Pelvic fin with thin and flexible spine, and five branched rays; tip of adpressed pelvic fin surpassing end of anal-fin base.Anal fin with thin and flexible unbranched ray, and four branched rays; first proximal radial not exposed ventrally; tip of first and last proximal radials contacting hemal spine of 14 th and 18 th vertebrae, respectively.Caudal fin concave, with two outer unbranched rays and 14 or 15 (one specimen) inner branched rays; lower lobe longer than upper one; four to five dorsal and two to four ventral procurrent rays.Vertebrae 27-28.
Color in alcohol.Ground color of dorsal and lateral surfaces homogeneously brown in head and trunk; ventral surface light brown, lighter along head.Some specimens with slightly darker chromatophores forming dots on head, smaller than eye diameter, and four inconspicuous dorsal dark brown saddles on trunk: two anteriormost along dorsal-fin base; third along adipose fin and slightly ahead; fourth shorter at end of caudal peduncle.All fins with hyaline membranes and medium to dark brown rays forming inconspicuous transverse bars.
Ecological notes.Specimens were sampled in warm water, with temperature around 34ºC.The type locality, where specimens were more abundant, is about three meters wide and 1.0-1.5 m deep, with moderate flow and bottom with sand, pebbles and rocks; specimens of H. yaku were associated predominantly with the rocks.Other species collected with Hypostomus yaku are: Characidium xanthopterum Silveira, Langeani, da Graça, Pavanelli & Buckup, Characidium aff.zebra, Knodus moenkhausii (Eigenmann & Kennedy), an undescribed species of a putatively new genus of the Characidae, and the exotic Xiphophorus helleri Heckel and Oreochromis niloticus (Linnaeus).The low species richness may be related to water temperature, since life in thermal water demands some physiological specializations.
Hypostomus yaku seems to be restricted to warm waters, such as areas inside the Rio Quente Resort and other sites in the urban area of the Municipality of rio Quente.Samples made in other nearby streams with lower water temperature did not yield specimens of the species.Although very abundant in the type locality, this restricted occurrence to areas with anthropic influence (e.g., urban expansion, tourism, organic pollution) requires special attention to the conservation of Hypostomus yaku, with frequent monitoring of the populations of the species.Etymology.In the specific name yaku, from the Tupi Guarani language, dialect Mbyá, y means water, and raku means hot, warm, a reference to the name of the rio Quente, a warm water river.A noun in apposition.

Discussion
Hypertrophied odontodes on flanks in the Hypostominae are mainly found in species of Aphanotorulus Isbrücker & Nijssen, Peckoltia Miranda Ribeiro [as in some specimens of P. brevis (La monte)], Squaliforma Isbrücker & Michels, and even in some keeled species of Hypostomus.However, Hypostomus yaku is clearly distinct from these genera by lacking conspicuous hypertrophied cheek odontodes on evertible plates of preopercular region [vs.presence in Peckoltia, as suggested by Armbruster (2008)]; and by lacking numerous hypertrophied papillae into the mouth, and by having the caudal peduncle ovoid in cross-section (vs.presence of hypertrophied papillae into the mouth and contour area of caudal peduncle trapezoid due to a strong angle along the mid-ventral series of plate in Aphanotorulus and Squaliforma).Among Hypostomus species, hypertrophied odontodes on flanks are also observed in H. carinatus Kner, H. commersoni Valenciennes, H. delimai Zawadzki, de Oliveira & Debona, and H. hoplonites Rapp Py-Daniel.However, these odontodes are relatively smaller and longitudinally aligned on the keels along the lateral series of plates, instead of larger odontodes randomly distributed on the flank, neither aligned nor restricted to the longitudinal keels, as found in Hypostomus yaku.
Hypostomus yaku is morphologically more similar to two widespread species inhabiting small, fast flowing waters and rocky streams in the upper rio Paraná basin: H. nigromaculatus (Schubart) and H. paulinus (Ihering).Hypostomus nigromaculatus is a small-sized species, with larger mature males usually reaching around 90.0 mm SL (NUP 10744), and differs from H. yaku by having conspicuous dark spots, increasing in size backwards, deeper head, and short, curved pectoral-fin spine usually enlarged on distal region, having a club-shaped aspect.Hypostomus paulinus is the most similar species to H. yaku.Nogueira et al. (2010) have cited it as a restricted-range species, however H. paulinus can be found in most rocky headwater tributaries of the upper rio Paraná with shallow and fast flowing waters.Although being considered as a small species, mature specimens of H. paulinus from the rio Piracicaba, its type locality, can reach up to 135.0 mm SL (holotype, BMNH 1905.6.9.4); this is twice the size and considerably longer than the largest specimen (holotype) of the new species.Differing from H. yaku, hypertrophied odontodes are lacking even in the largest specimens of H.

Fig. 2 .
Fig. 2. Detail of the hypertrophied odontodes on the lateral plates of the posterior portion of caudal peduncle in dorsal and lateral views, DZSJRP 15735, holotype, 70.8 mm SL.

Fig. 3 .
Fig. 3. Dentary teeth from the same specimen of Hypostomus yaku, paratype, DZSJRP 16421, 60.5 mm SL, showing the different size of the medial cuspid according to time of emergence.a) new emerging tooth; b) worn tooth.Scale bar = 0.05 mm.

Table 1 .
Morphometric and meristic data of the holotype and 29 paratypes of Hypostomus yaku.Means are presented for measurements, and modes for counts.The range includes the holotype.SD = Standard deviation.