Hypostomus rhantos, Jonathan W. Armbruster, Leigh A. Tansey & Nathan K. Lujan, 2007

Jonathan W. Armbruster, Leigh A. Tansey & Nathan K. Lujan, 2007, Hypostomus rhantos (Siluriformes: Loricariidae), a new species from southern Venezuela., Zootaxa 1553, pp. 59-68 : 60-67

Hypostomus rhantos New Species

Figs. 1-2 and 3b

Holotype: MCNG 55352 , 157.2 mm SL, Venezuela, Amazonas, Río Manapiare-Río Ventuari-Río Orinoco Drainage, Rio Parucito at Raudales Salomon, 2.7 km NE of San Juan de Manapiare , 05.34637°, -066.03347°, D.C. Werneke, N.K. Lujan, O. León, 16 April 2004.

Paratypes: 63 specimens. All collections Venezuela, Amazonas, Río Orinoco drainage: ANSP 160774 , 11, 76.7-103.5 mm SL, Morichal 26.9 km from Puerto Ayacucho, along Puerto Ayacucho - Caicara highway , B. Chernoff et al., 15 November 1985 ; ANSP 162365 , 2, 134.4-139.0, Backwater of Rio Orinoco behind sand playa ca. half hour upstream from Isla Temblador , 03°04’N, 066°28’W, B. Chernoff et al., 10 March 1987 ; ANSP 185240 , 4, 56.8-124.5, AUM 39273 , 3 c &s, 5, 53.5-149.7, MCNG 55353 , 5, 52.0-154.5, UF 164255 , 2, 54.4-145.5, Same data as holotype ; AUM 39308 , 1, 124.0, MCNG 55354 , 2, 94.7-99.2, RMNH 35500 , 1, 99.0, Rio Manapiare, tributary of Rio Ventuari, 14.5 km NW of San Juan de Manapiare , 05.42863°, -066.13616°, N.K. Lujan, M.H. Sabaj, L.S. de Souza, and D.C. Werneke, 12 April 2004 ; AUM 39235 , 1, 52.9, Rio Ventuari, beach across the river from Picua Village, 34 km ENE of Macuruco, 104 km E of San Fernando de Atabapo , 04. 11534°, -066.76457°, M.H. Sabaj, N.K. Lujan, D.C. Werneke, L.S. de Souza, and O. León, 5 April 2004 ; AUM 39874 , 1, 145.3, Rio Ventuari at mouth of Cano Camoni, 145 km NNE of Macuruco, 189 km NE of San Fernando de Atabapo , 05.05588°, -066.32742°, M.H. Sabaj, N.K. Lujan, D.C. Werneke, L.S. de Souza, and O. León, 8 April 2004 ; AUM 40070 , 1, 123.4, Rio Manapiare, tributary of Rio Ventuari, 20 km NW of San Juan de Manapiare , 05.45272°, -066.17682, D.C. Werneke, N.K. Lujan, and L.S. de Souza, 12 April 2004 ; AUM 39506 , 1, 168.7, Rio Ventuari at Raudales Tencua, 56 km ESE of San Juan de Manapiare , 05.04968°, -065.62722, D.C. Werneke, N.K. Lujan, O. León, A. Luna, and R. Pajua, 20- 21 April 2004 ; AUM 39216 , 1, 93.1, Rio Ventuari, mouth , 3.99528°, -067.04250°, N.K. Lujan and O. León, 15 April 2004 ; AUM 41496 , 1, 147.5, Rio Manapiare at mouth of Cano Yutaje, tributary of Rio Ventuari, 14 km NW of San Juan de Manapiare , 05.43667°, -066.11261°, M.H. Sabaj, L.S. de Souza, D.C. Werneke, and N.K. Lujan, 11 April 2004 ; AUM 41418 , 1, 164.4, MCNG 55355 , 1, 154.5, Rio Ventuari, bedrock outcrops, 83 km ENE of Macuruco, 153 km ENE of San Fernando de Atabapo , 04.25346°, -066.34466°, N.K. Lujan, D.C. Werneke, M.H. Sabaj, L.S. de Souza, and O. León, 6 April 2004 ; AUM 41336 , 1, 70.2, Cano Guavialito, tributary of Rio Manapiare, tributary of Rio Ventuari, near Alto Guaviarito, 17.5 km NW of San Juan de Manapiare , 05.44135°, -066.16294°, L.S. de Souza, D.C. Werneke, N.K. Lujan, and M.H Sabaj, 13 April 2004 ; AUM 41440 , 1, 66.3, ANSP 185241 , 1, 66.9, Cano Guavialito, tributary of Rio Manapiare, tributary of Rio Ventuari, directly off of Rio Manapiare, 17.5 km NW of San Juan de Manapiare , 05.44010°, -066.16175°, M.H. Sabaj, L.S. de Souza, D.C. Werneke, and N.K. Lujan, 13 April 2004 ; AUM 41496 , 1, 147.5, Rio Manapiare, tributary of Rio Ventuari, at mouth of Cano Yutaje, 14 km NW of San Juan de Manapiare , 05.43667°, -066.11261°, M.H. Sabaj, L.S. de Souza, D.C. Werneke, and N.K. Lujan, 11 April 2004 ; AUM 42100 , 8, 50.6-176.4, Rio Orinoco, beach and bedrock outcropping, 50 km E of San Fernando de Atabapo , 03.97029°, -067.25506°, N.K. Lujan, D.C. Werneke, M.H. Sabaj, M. Arce, R. Betancur, and T.E. Wesley, 2 March 2005 ; AUM 42114 , 5, 135.6-159.1, Rio Orinoco, 117 km W of La Esmeralda , N.K. Lujan, M. Arce, T.E. Wesley et al., 03.28998°, -066.60004°, 29 March 2005 ; AUM 42121 , 3, 115.0-126.0, Rio Orinoco, 33.9 km E of La Esmeralda, Punto Piaroa , 03.14744°, -065.85381°, N.K. Lujan, M. Arce, T.E. Wesley et al., 28 March 2005 ; AUM 42164 , 1, 195.8, Rio Orinoco, bedrock outcrop, 52.9 km SE of San Antonio, 102 km W of La Esmeralda , 03.10036°, -066.46277°, N.K. Lujan, D.C. Werneke, M.H. Sabaj, O. León, M. Arce, R Betancur, and T.E. Wesley, 4 March 2005 ; AUM 42220 , 1, 136.7, Rio Orinoco, near Puerto Ayacucho on a beach called Playa Bagre , 05.65642°, -067.63103, N.K. Lujan, M. Arce, and T.E. Wesley, 13 April 2005.

Diagnosis: Hypostomus rhantos is unique among Hypostomus (except for H. micromaculatus ) by having extremely small spots (see especially Fig. 2). Hypostomus rhantos is a member of the H. plecostomus group, but is not a member of the H. cochliodon subgroup (Armbruster, 2004). Hypostomus rhantos can be separated from the H. emarginatus species group by having a dark brown base color (vs. light tan), by having a small buccal papilla (vs. large), and by lacking hypertrophied odontodes on the lateral plates of nuptial males (vs. hypertrophied odontodes present); and from the H. cochliodon species subgroup of the H. plecostomus group by having viliform teeth (vs. spoon-shaped). Hypostomus rhantos can be separated from all other members of the H. plecostomus group (including species of the H. cochliodon subgroup) except H. micromaculatus by having extremely small spots (greater than 15 on the first plate in the dorsal series in H. rhantos vs. five or fewer). Hypostomus rhantos can be separated from H. micromaculatus by having all of the spots round and evenly distributed (vs. spots longitudinally oval and restricted to rows Figs. 1-2 vs. Fig. 4), keels of lateral plates well-developed (vs. weak), a ridge present on the pterotic that is contiguous with the supraorbital ridge (vs. ridge absent, Fig. 3), and by having a fully plated abdomen (vs. abdomen partially plated or naked). In addition, smaller specimens of H. rhantos have spots on the dorsal fin whereas small specimens of H. micromaculatus have the dorsal fin entirely dark.

Description: Morphometric data given in Table 1. Largest specimen 195.8 mm SL. Head and nape forming arch from tip of snout to origin of dorsal fin. Body depth decreasing from origin of dorsal fin to dorsal procurrent caudal spines then increasing to caudal fin. A rounded ridge present from anterodorsal corner of orbit, running ventral to nares, and ending slightly anteroventral of anterior nare. Longitudinal ridge of raised bone and slightly larger odontodes present on pterotic-supracleithrum beginning at posterodorsal corner of orbit and contiguous with supraorbital ridge. Space between orbits concave such that supraorbital ridge higher than medial surface of head. Supraoccipital convex medially with slight crest.

Nares separated by flap of skin held erect in life. Dorsal, middorsal, median and midventral plate rows complete from head to caudal fin, ventral plate row begins at insertion of pelvic fin and continues to caudal fin. Lateral plates with short, median keels with enlarged, dull odontodes. Keels on first two plates of dorsal row and sometimes first three plates forming line from supraoccipital to posterolateral corner of nuchal plate, not confluent with keel on dorsal plate row beginning on fourth plate. Base of caudal fin covered in elongate, roughly triangular plates. Entire ventral surface of head and body (except region around insertion of pelvic fin) covered in small platelets. Platelets on abdomen increase in number with standard length. Head covered in small plates. Frontal, nasal, sphenotic, infraorbitals, opercle, pterotic-supracleithrum, suprapreopercle, and supraoccipital supporting odontodes. Platelets that cover anteroventral corner of opercle slightly separated from opercle allowing plates to be marginally everted (angle of eversion less than 30°).

Dorsal fin moderately long, usually just barely reaching preadipose plate when depressed, consisting of small, V-shaped spinelet, fairly strong spine, and seven rays. Caudal fin forked, lower lobe longer than upper. Pectoral-fin spine strong, extending posteriorly to pelvic-fin rays when depressed ventral to pelvic fin; cleithrum with exposed process dorsal to pectoral-fin rays that tapers posteriorly to point; pectoral fin inserted on same plane as pelvic fin such that spine, when depressed parallel with body, lies on top of and in contact with pelvic fin. Pelvic-fin spine thin, flexible, reaches slightly beyond base of anal fin. Anal fin with relatively strong, unbranched first ray supporting odontodes. Adipose fin consisting of single median, unpaired preadipose plate and a stout, strong, pointed spine; adipose-fin membrane not reaching procurrent caudal-fin spines. Dorsal fin II,7, pectoral fin I,6, pelvic fin I,5, anal fin I,4, caudal fin I,14,I. Jaws weakly angled, dentaries forming angle much greater than 90°. Teeth numerous (28-45, mode 31 in premaxilla, 29-45, mode 31, in dentary, N = 48), bicuspid, median cusp moderately long, lateral cusp about one fourth length of median, stalk moderately long. Median plates 24.

Coloration: Light gray to tan when alive, becoming tan when preserved. Body densely covered with tiny spots, head spots even smaller than body spots. Spots present on all fins, generally larger than spots on body, evenly distributed on rays, spines, and membranes. Caudal fin membranes light and spotted anteriorly, fading to dark wash posteriorly. Abdomen lighter than sides, with tiny spots. Occasionally with four dorsal saddles, first below anterior dorsal-fin rays, second below and slightly behind posterior dorsal-fin rays, third below and slightly anterior to adipose-fin spine, and fourth at base of caudal peduncle; all saddles angled anteriorly, saddles one and two combine and continue to base of pelvic fin, third and fourth terminating at middle of midventral plate row. Fin spines usually lighter than rest of body. Spots relatively larger in juveniles. Juveniles with fewer spots distally on all fins, lower half of caudal fin much darker.

Range: Currently known from the Río Ventuari, a tributary of the upper Río Orinoco, and the mainstem upper Orinoco above Puerto Ayacucho to the Río Casiquiare in Amazonas, Venezuela (Fig. 5).

Ecology: Hypostomus rhantos was collected in loricariid assemblages with an average of 7.2 loricariid species per site (n=16 sites). Habitats from which H. rhantos were collected range from consolidated lateritic rocks in flow, to bedrock cracks in flow, to branches and trunks of trees in slack water.

Etymology: Rhantos is Greek for sprinkled, speckled, or spotted and refers to the tiny randomly placed spots of the species.

Discussion: Specimens of Hypostomus rhantos were analyzed in Armbruster (2004a) but were incorrectly referred to as H. micromaculatus . Hypostomus rhantos was found to be the sister to H. robinii ; however, support for this was very weak (Bremer decay index = 1), and derived from only from two homoplastic characteristics: posteromedial invagination of the fifth ceratobranchial present (character 11 state 1 from Armbruster, 2004) and a reversal to a short levator arcus palatini crest (44-1). This clade was part of a larger clade consisting of the H. cochliodon subgroup, Hypostomus plecostomus , the potentially undescribed Hypostomus similar to H. robinii from the Orinoco, and H. cordovae , with this clade being supported by a reversal to a wide posterior edge to the posterior process of the coracoid (158-0). This clade is also poorly supported with a Bremer decay index of one. Most of the relationships within Hypostomus are poorly resolved and need much further study.

Hypostomus rhantos is most similar in coloration to H. micromaculatus from Suriname. In addition to coloration, H. rhantos appears taller and wider than H. micromaculatus ; however, we do not have enough specimens available to provide confident measurements of this. There are no species that have been described or that we have examined between the Upper Orinoco and Suriname with a similar color pattern. Given the vast distance between the two species, it would be unlikely that they would be sister species. They are different in the size of the keels (relatively well-developed in H. rhantos and almost absent in H. micromaculatus ) and the amount of abdominal plating (fully plated in H. rhantos and absent or nearly so in H. micromaculatus ). Although these characteristics change a lot in loricariids, they do suggest when coupled with locality data that the two species may have small spots via convergence.