Baryancistrus xanthellus, new species

Figs. 1 and 2

Holotype. INPA 33849, 214.2 mm SL, Brazil, Pará, Senador José Porfírio, rio Xingu in the vicinity of the cachoeira Buraco do Inferno, 03º27’07”S 51º41’53”W, 18 Mar 1997, J. Zuanon.

Paratypes. Brazil, Pará, rio Xingu: ANSP 191482, 3, 183.8- 188.7 mm SL, MCP 45752, 2, 191.0- 196.3 mm SL, MNRJ 37863, 2, 186.2- 191.5 mm SL, Arroz Cru, 03º25’16”S 51º55’08”W, 1 Oct 1990, L. Rapp Py-Daniel & J. Zuanon. INPA 3942, 7, 78.3-184.8 mm SL (4 not measured), Furo do Tucum Seco, 1 Oct 1990, L. Rapp Py-Daniel & J. Zuanon. INPA 3955, 27, 48.9-126.2 mm SL, 1 c&s, 82.0 mm SL, (19 not measured), INPA 3963, 3, 110.3- 145.1 mm SL (1 not measured), Cachoeira do Kaituká, 03º33’47”S 51º51’20”W, 9 Oct 1990, L. Rapp Py-Daniel & J. Zuanon. INPA 4057, 1, 223.3 mm SL, Ilha de Babaquara, 03º12’43”S 52º12’12”W, 4 Oct 1990, L. Rapp Py-Daniel & J. Zuanon. INPA 4058, 1, 224.0 mm SL, Praia do Pedral, 03º15’16.7”S 52º13’12”W, 7 Oct 1990, L. Rapp Py-Daniel & J. Zuanon. INPA 31414, 9, 49.8-210.2 mm SL (4 not measured), Comunidade do Maia, 03º30’44”S 51º44’43”W, 9 Nov 2008, L. Rapp Py-Daniel & R. R. Oliveira. INPA 31423, 1, 48.3 mm SL, Cachoeira do Landi, 03º35’01”S 51º49’21”W, 8 Nov 2008, L. Rapp Py-Daniel & R. R. Oliveira. INPA 31473, 6, 51.2- 171.8 mm SL, (2 not measured), Arroz Cru, 03º25’20”S 51º57’22”W, 6 Nov 2008, L. Rapp Py-Daniel & R. R. Oliveira. INPA 31772, 1, 55.3 mm SL, Ilha do Davi, 03º32’39”S 51º57’29”W, 10 Nov 2008, L. Rapp Py-Daniel & R. R. Oliveira. INPA 31799, 2, 57.8-189.3 mm SL, Gorgulho da Rita, 03º20’14”S 52º11’18”W, 7 Nov 2008, L. Rapp Py-Daniel & R. R. Oliveira. INPA 33848, 1, 212.8 mm SL, Cachoeira Buraco do Inferno, 03º27’07”S 51º41’53”W, 17 Mar 1997, J. Zuanon. INPA 33850, 5, 88.4-162.6 mm SL, 1 c&s, 88.4 mm SL, Corredeiras do Arini, 03º24’15”S 51º41’53”W, 6 Sep 1997, J. Zuanon. INPA 33851, 1, 65.1 mm SL, Furo do Ramiro, 03º15’21”S 52º05’06”W, 15 Sep 1997, J. Zuanon. INPA 33852, 1, 175.2 mm SL, Ponta da Ilha da Bela Vista, 03º24’22”S 51º43’03”W, 29 Sep 1996, J. Zuanon. INPA 33941, 1, 110.9 mm SL, Costa Junior, 03º29’28”S 52º19’07”W, 12 Oct 1996, J. Zuanon. MPEG 19132, 2, 171.3-183.0 mm SL, Ilha de Babaquara, 5 Oct 1990, J. Zuanon. MZUSP 106738, 3, 173.3- 194.7 mm SL, Ilha de Kaituká, 03º33’47”S 51º51’20”W, 8 Oct 1990, L. Rapp Py-Daniel & J. Zuanon. ZUEC 4485, 2, 46.6-52.5 mm SL, just above Senador José Porfirio, 02º41’7.7”S 52º00’30.9”W, 13 Sep 1997, J. Zuanon. Rio Iriri: INPA 30767, 1, 146.4 mm SL, ROM 88756, 2, 137.2- 191.2 mm SL, Ilha do Curapé, 04º06’53”S 53º22’28”W, 17 Aug 2008, H. López-Fernández et al. INPA 31081, 2, 184.4-207.0 mm SL, ROM 88757, 1, 172.5 mm SL, Cachoeira Grande do Iriri, 03º51’34”S 52º41’25”W, 23 Aug 2008, H. López-Fernández et al. INPA 31095, 2, 176.6- 188.6 mm SL, ROM 88755, 2, 103.8- 187.5 mm SL, above mouth of rio Novo, 04º28’11”S 53º41’38”W, 21 Aug 2008, H. López-Fernández et al. INPA 31169, 1, 140.8 mm SL, ROM 88754, 1, 131.1 mm SL, Cachoeira Grande do Iriri, 03º50’35”S 52º44’03”W, 24 Aug 2008, H. López-Fernández et al. INPA 32452, 1, 46.3 mm SL, Barinha, 04º09’05”S 53º23’28”W, 19 Aug 2008, H. López-Fernández et al .

Diagnosis. Baryancistrus xanthellus differs from all its congeners by having a broad light band on dorsal and caudal fin distal borders in juveniles, becoming a small dot on dorsal and caudal-fin tips in adults (vs. conspicuous band at all ages in B. chrysolomus and light bands absent in B. demantoides, B. beggini, B. longipinnis, and B. niveatus). It can also be distinguished from all congeners except B. niveatus and B. longipinnis by the presence of light spots on the whole body (vs. the presence of uniform dark coloration in Baryancistrus beggini and B. chrysolomus; presence of round spots only on the anterior part of the body until the end of dorsal-fin base in B. demantoides; presence of inconspicuous dots on the fins and rest of the body dark colored in B. longipinnis). Baryancistrus xanthellus can be further distinguished from B. demantoides, B. longipinnis, and B. niveatus by having a naked abdomen (vs. partially or completely plated in these three species). Baryancistrus xanthellus differs from B. beggini by a larger number of mandibulary teeth (around 60) on both maxillae versus 34-36 teeth in the premaxillary and 34 teeth in the dentary in B. beggini). The presence of bold yellow markings in young B. xanthellus is the reason for its popular name in the aquarium trade: “amarelinho” in Portuguese (meaning little yellow).

Description. Morphometrics and meristics in Table 1. Mediumsize loricariids, bigger specimen examined reached 224.1 mm SL. Body short and robust, deep. Profile from snout to eye strongly sloped, gently convex from eye to insertion of dorsal fin due to prominence of supraoccipital process. Profile strongly inclined from dorsal-fin insertion to caudal fin. Body deepest at region between supraoccipital and dorsal-fin insertion. Ventral surface flat and straight from snout tip to caudal-fin base. Head and eyes without crest. Anterior lateral plates gently bent; rest of body not keeled. Anterior portion of body half oval in cross section, triangular at caudal peduncle.

Head large and wide; snout round in dorsal view. Eye large and round, iris operculum not visible. Orbit slightly elevated; interorbital area almost completely flat. No ridge between eyes and nares. Supraoccipital process conspicuous, elevated and rounded posteriorly. Supraoccipital limited posteriorly by pair of large quadrangular plates, almost fused. Predorsal area reduced, with only one pair of small, separated diamond-shaped plates anterior to nuchal plate.

Measurement Baryancistrus xanthellus Baryancistrus chrysolo musHolotypeNMeanRangeSDHolotypeNMeanRangeSDStandard length (mm)214.262-46.3-224.1-219.020-4 7.4-219.0-Percents of st andard lengthPredorsal length43.46244.740.2-47.81.441.32042.239.6 -45.11.2Head length35.86237.735.3-41.01.433.72036.633.6 -39.51.6Cleithral width33.76234.431.0-37.51.534.32034.432.9 -36.50.8Thorax length25.06224.320.9-27.31.321.72024.121.0 -26.21.4Pectoral-spine len gth33.76233.025.7-38.73.334.92032.028.5 -36.12.2Abdominal length24.36224.621.5-27.71.425.12024.320.9 -26.71.6Pelvic-spine length24.26225.923.1-29.31.425.12025.023.5 -27.00.8Postanal length21.36227.821.3-30.71.731.12029.526.6 -31.51.1Anal-fin spine length10.86210.17.6-13.30.910.61910.69.1-12.00.7Dorsal spine length25.66226.622.8-32.11.926.02027.024.0 -29.71.6Dorsal-fin base length40.66240.535.4-48.02.741.22039.336.5 -43.32.0Dorsal-ad ipose distance1.3621.10-2.70.91.6201.70.5 -4.01.1Caudal penduncle d epth10.66210.79.0-12.30.811.32010.69.4-11.90.6Adipose-spine length7.8627.96.4-9.10.68.3208.27.1 -9.20.6Adipose-caudal length21.16222.419.5-24.80.925.22023.521.2 -26.11.1Body depth at dorsal-fin origin22.26222.117.1-26.81.921.22020.416.6 -22.91.8Body width at dorsal-fin origin31.96231.927.7-37.12.032.42030.627.9 -34.01.7Body width at anal-fin origin17.66216.912.6-20.51.718.72015.712.4 -19.32.1Postdorsal length22.86223.319.1-25.71.426.02025.523.4 -27.71.2Anus-anal fin len gth3.6624.12.9-5.60.64.5204.13.3 -5.70.6Body width36.56236.133.3-39.31.336.32036.335.2 -37.80.8Percents of head lengthOrb ital diameter15.46218.415.1-23.42.317.32021.617.1 -24.92.9Sn out length74.86267.260.6-74.83.164.02061.958.5 -66.52.8Internares width13.86213.09.4-15.61.614.52012.39.6-15.71.6Interorbital width37.56236.028.0-41.43.941.82036.731.4 -45.24.1Head depth57.66256.346.5-66.84.363.72054.047.1 -63.75.3Dentary len gth21.26219.516.7-24.41.623.22020.617.3 -24.92.3Premaxillary length21.36219.315.9-22.61.423.72019.916.1 -23.72.1Head width103.96290.577.2-103.97.2100.42089.37 9.1-100.46.6Eye-nostril length16.36214.79.1-17.82.215.52013.210.2 -17.92.3Interbranchial distance59.66256.551.8-60.92.458.82057.753.6 -61.22.2CountsTeeth on premaxilla606228-72742014 -91Teeth on dentary646225-76812022 -96Lateral plates in middle series246223-24242024 -25Plates between anal and caudal10629-11102010 -11Plates between dorsal and adipose0620-20.5200 -2Plates predorsal36233203

Mouth wide; lips large, covered with small round papillae except on area around maxillae. Maxillary barbel thick and short. Branchial opening moderate, wider in larger specimens. Interbranchial distance approx. 56.5% in head length.

Head and body completely covered by large plates dorsally, except dorsal-fin base. Ventral surface largely unplated from snout to anal fin; in larger specimens, presence of two or three minute round plates close to insertion of pectoral spine or urogenital opening. Twenty-two perforated median plates, 23 lateral plates; four oblong plates on caudalfin base. All plates marked with lines of odontodes. Numerous hypertrophied odontodes only on evertible cheek plates, well developed in larger specimens with largest odontode reaching posterior end of first lateral plate. Ventral border of opercle with series of strong but short odontodes on probably mature males.

Dorsal fin II,7; spinelet present and dorsal-fin locking mechanism functional. Dorsal fin long and low, reaching adipose spine on small specimens when adpressed. Dorsalfin posterior membrane covering four plates immediately behind last dorsal-fin ray, but not reaching preadipose plate. Adipose fin large and with posterior membrane slightly developed. One single plate separating dorsal from preadipose plate. Caudal fin i,14,i, emarginated. Pectoral fin I,6, large, reaching well beyond posterior end of pelvic-fin base when adpressed. Pectoral fin covered by large odontodes on (possibly mature) males. Pelvic fin i,5 reaching posterior end of anal fin base when adpressed. Anal fin i,4, very reduced. All simple first rays covered by numerous short odontodes on their free surface.

Teeth long and deeply cuspidate. Cusps round and similar in larger specimens with vertical divide between cusps; in smaller specimens cusps are largely asymmetrical, with internal cups larger. Premaxilla and dentary of similar size and disposed in parallel to anterior border of snout. Modal number of premaxillary teeth 32; dentary teeth mode 46 (range of number of mandibulary teeth in Table 1). Buccal papilla long and digitiform.

Color in life. Overall body color dark brown at dorsum and sides, paler on abdomen. Numerous bright, similar-sized yellow spots over head, body, dorsal, adipose, and caudal fins, and on dorsal surface of pectoral and pelvic fins. Yellow spots almost size of pupil in juveniles, becoming proportionally smaller, more numerous and somewhat paler in adults. Young specimens with wide yellow band on distal third of caudal and dorsal fin; dorsal-fin band larger anteriorly, becoming gradually slender posteriorly. In adults, bands on fins become reduced to spots on anterior distal ends of dorsal and caudal fins (Fig. 2). Considerable variation in density and size of spots.

Color in alcohol. Similar to pattern described for living specimens but with ground color brown to dark olive and white to cream markings (Fig. 1).

Distribution. This species has been recorded from the area called Volta Grande do rio Xingu, an area immediately above Belo Monte falls, and from rio Iriri, the larger tributary of rio Xingu (Figs. 3 and 4a).

Etymology. From the Greek xanthellus, yellow, in allusion to the bright color of the specimens. A noun in apposition.

Popular name. Known in the aquarium trade as “amarelinho” (= little yellow) and “cascudo pepita-de-ouro” (in Portuguese), or L018 and L085; and golden nugget pleco or Iriri golden nugget pleco (in English), or L177.

Ecological notes. Baryancistrus xanthellus is one of the most common species observed in the rapids of rio Xingu in the area known as “Volta Grande do Xingu” near the town of Altamira, Pará State (Fig. 4a). Young specimens of B. xanthellus occur in groups of several individuals under flat rocks at the bottom of shallow rapid stretches. Other loricariid species found sintopically are Ancistrus spp., Oligancistrus punctatissimus, and an undescribed species of Oligancistrus, young individuals of Baryancistrus aff. niveatus, Parancistrus nudiventris, and Pseudancistrus sp. Adult specimens occupy larger spaces among and under submerged boulders, together with large specimens of Baryancistrus aff. niveatus, Scobinancistrus aureatus, and S. cf. pariolispos, and Hypostomus spp. The diet of three examined specimens of B. xanthellus was composed mainly of algae, especially diatoms and loose filaments of clorophyceans such as Spirogyra, which are commonly found associated with fine sediments and sand grains in the long intestines of loricariids (intestinal length up to 16 times the body length; Zuanon, 1999). Occasional bryozoans and chironomid larvae were also found. Underwater observations indicate that the food is scraped from the surface of submerged rocks by gently combing the periphyton cover with the numerous and tightly spaced teeth (JZ, pers. obs.).