Description of a new species of Parancistrus ( Siluriformes : Loricariidae ) from the rio Xingu , Brazil

Parancistrus nudiventris, new species, is described and compared with the congener P. aurantiacus. The new species has only been recorded from rio Xingu and can be distinguished from P. aurantiacus by having a naked abdomen (plated in P. aurantiacus), the presence of bluish dots in living specimens or spotted in preserved specimens (uniformly dark or clear brown or mottled, never spotted in P. aurantiacus), larger interbranchial distance (39-56% in HL vs. 24.9-39.5% in P. aurantiacus), narrower interorbital distance (26.8-38% in HL vs. 38.5-43.1% in P. aurantiacus). Parancistrus nudiventris also has buccal teeth more conspicuous than in P. aurantiacus. Main skeletal differences include the presence of a strong condyle on the lateral ethmoid for articulation with the metapterygoid in P. nudiventris (not seen in P. aurantiacus); anguloarticular processes short in P. nudiventris (long in P. aurantiacus); opercle with odontodes, partly exposed in P. nudiventris (completely embedded in skin in P. aurantiacus).


Introduction
Recent expeditions to the rio Xingu, the last eastern large tributary of the rio Amazonas, Brazil, uncovered specimens of a new species of the loricariid genus Parancistrus, described herein.In the most recent revision of the genus (Rapp Py-Daniel, 1989), two previously described species of Parancistrus (P.nigricans and P. vicinus) were placed in synonymy with the type species, P. aurantiacus.Many specimens of Parancistrus are often misidentified at the generic level (as Baryancistrus or Oligancistrus or in other genera), and, more frequently misidentified at species level.Currently, a considerable part of the loricariid input in the international aquarium trade originates from the rio Xingu and misidentification is becoming a big issue in Brazilian export regulations for aquarium trade purposes.
Parancistrus aurantiacus has been recorded from the rio Ucayali and rio Araguaia, from the Amazon (without a precise locality) (Castelnau, 1855) and from the rio Tocantins (Rapp Py-Daniel, 1989).Recent collections also yielded more specimens of P. aurantiacus from the rio Araguaia.The new species of Parancistrus has been collected only in the rio Xingu.
Herein we describe this new species of Parancistrus and provide some skeletal and ecological data.

P R O O F S
tip of spine; pectoral spine length -from base to tip of spine; first pelvic fin ray -whole extension of first unbranched ray; first and second anal fin rays -extension of first and second unbranched rays; and interbranchial length -shortest distance between gill openings ventrally.Institutional abbreviations are as listed in Leviton et al. (1985).Osteological nomenclature follows Schaefer (1987), Armbruster (2004), and unpublished dissertations of Schaefer (1986), Armbruster (1997), andRapp Py-Daniel (1997).
Parancistrus nudiventris was observed during diurnal and nocturnal snorkeling sessions by one of us (JZ) at several rapids of the rio Xingu near Altamira, Pará State.The underwater observations included activity period, foraging tactic, microhabitat characteristics, and type of diurnal shelters.The dietary data were obtained through analyses of stomach contents of two preserved specimens.

Parancistrus Bleeker, 1862
Diagnosis.Parancistrus can be characterized by the presence of a membranous connection between the dorsal and adipose fins, cephalic and body scutes not carinate and a large gill opening.Armbruster (1997) additionally diagnosed the genus based on posterior deflection of the hyomandibula and presence of fleshy folds along the dorsal-fin base of nuptial males.Parancistrus can be further distinguished from Ancistrus and Chaetostoma by having a plated snout and from all other genera of the Ancistrini (sensu Armbruster, 2004), except Baryancistrus, Oligancistrus, and Spectracanthicus, by the connection between dorsal and adipose fins; from Baryancistrus, Oligancistrus, and Spectracanthicus, by the large gill openings.

P R O O F S
Basipterygia anterior processes curved.Basipterygia connected by three sutures (additional suture between mesial anterior basipterygia processes).Lateral line complete, not entering supracaudal plates.
Color in alcohol.Body and fins evenly dark brown covered by minute yellowish spots.Spots more concentrated and conspicuous on dorsal and caudal fins.Head and abdomen poorly spotted.
Color in life.Overall body color dark grey to olive, with scattered small bluish white dots (Fig. 2).
Sex dimorphism.Sexual dimorphism in Parancistrus nudiventris is expressed by the presence of strong spines on the pectoral fin spine, the cheek area and body plates of sexually mature males.Fleshy folds were not observed in the examined material despite the presence of large mature males in the type series.

Geographic distribution. All specimens examined of
Parancistrus nudiventris are from the rio Xingu (Fig. 3).

Ecological notes.
Parancistrus nudiventris is a rheophilic species, found in rocky-bottom areas subjected to moderate to strong current (40 to 190 cm/sec).Specimens were found individually or in pairs in shelters under boulders or in narrow cracks on submerged rocks up to 2 m deep.Young specimens (up to 5 cm SL) were observed under flat rocks over the bottom, usually sharing the shelter with specimens of Baryancistrus spp., Oligancistrus sp., O. punctatissimus, Hopliancistrus tricornis, Ancistrus sp., A. ranunculus, Peckoltia vittata and Pseudancistrus aff.barbatus.Parancistrus nudiventris is a nocturnal fish that feeds by grazing over rock surface, browsing algae and other food items off the periphyton.Analysis of the stomach contents revealed the presence of algae as the most frequent food ingested, and included mainly diatoms.Other food items were cyanobacteria, plant remains, bryozoans, larvae of aquatic insects (most chironomids), microcrustaceans, and few small molluscs; sand grains and silt were also recorded.Dissected specimens had long digestive tracts, approximately 20 times the SL, characteristic of algae-feeding fishes.The consumption by P. nudiventris of food items found loosely attached to the rocks characterize this species as a comber, following the feeding-strategy classification proposed for African Rift Lake cichlids (cf.Konings, 1989;Yamaoka, 1997).Morphological and skeletal remarks.There are clear differences in body format between P. nudiventris and P. aurantiacus.Parancistrus nudiventris is slender, has larger eyes, and has a shorter caudal peduncle, whereas P. aurantiacus has a shorter and wider head and wider branchial openings.Metapterygoid channel (Howes, 1983;Schaefer, 1987) is not completely closed, with tall wall only on the anterior half of the channel in P. aurantiacus.In P. nudiventris the channel is complete and it ends on the articulation site with the lateral ethmoid (Fig. 4).Quadrate loosely sutured to metapterygoid in P. aurantiacus and just abutted in P. nudiventris.The opercle in P. aurantiacus is elongate and almost straight, embedded in skin and without any odontode.In P. nudiventris, however, the opercle is similar in shape but shows a small area with odontodes.Preopercle with larger exposed area with odontodes in P. aurantiacus than in P. nudiventris.Anterior process of basipterygium straight in P. aurantiacus and curved in P. nudiventris (Fig. 5 ).Basipterygia connected by two sutures in P. aurantiacus (the anterior and posterior sutures to the cartilage plug) and by three in P. nudiventris (additional suture between the mesial anterior basipterygia processes).
Lateral line complete, not entering the supracaudal plates on both P. aurantiacus and P. nudiventris.Table 2 summarizes the main differences described for both species.

Discussion
The genus Parancistrus can be distinguished from all other Hypostominae by the broad membranous connection between the dorsal and adipose fin associated with an extremely large gill opening.Among loricariids, and more specifically the Ancistrini, other genera have a developed dorsal fin membrane but differ from Parancistrus in other features (see diagnosis for more details).Large gill openings are present in Ancistrus ranunculus Muller et al., 1994 and Rhinelepis spp.but there is no membranous connection between the dorsal and adipose fins.

Table 1 .
Living color and detail of mouth of the holotype(INPA 15037, 152 mm SL).Evertible cheek plates with associated hypertrophied odontodes and disposed as unique block ligamentously connected to opercle.Opercular movements causes expansion of opercular block, thereby everting odontodes.Opercle with small area exposed with short odontodes.Subpreopercle not reaching border of head.Mouth wide, lips papillate.Upper lip smooth externally, but densely papillated internally.Maxillary barbels very short.Premaxillae round, reduced and ligamentously attached to each other.Dentaries short and spaced widely apart.Buccal teeth villiform, well developed, not numerous and weakly cuspidate.Gill opening wide, branchial chamber large.From mouth to anus, ventral surface completely naked, except for some scattered platelets disposed close to pectoral-fin insertion.Dorsal fin long and low, connected to adipose fin by thick membrane.Pectoral and pelvic fins well developed.Pectoral spine stiff and strong, covered by long piercing odontodes on large specimens.Anal fin extremely reduced.Caudal fin truncate to slightly emarginate.Caudal peduncle flat ventrally.Metapterygoid channel complete, ending on articulation with lateral ethmoid.Quadrate abutted to metapterygoid.Opercle elongate, almost straight, with small area with odontodes.Preopercle with exposed area with odontodes.Counts and morphometrics of Parancistrus nudiventris and P. aurantiacus (H = holotype).Holotype of P. aurantiacus is a deformed, stuffed specimen, so data not included in mean.

Table 2 .
Summary of differences between Parancistrus aurantiacus and P. nudiventris.