Hisonotus megaloplax, Carvalho, Tiago P. & Reis, Roberto E., 2009

Carvalho, Tiago P. & Reis, Roberto E., 2009, Four new species of Hisonotus (Siluriformes: Loricariidae) from the upper rio Uruguay, southeastern South America, with a review of the genus in the rio Uruguay basin, Zootaxa 2113 (1), pp. 1-40 : 18-22

Hisonotus megaloplax , new species

Figure 11 , Table 4

Hisonotus sp. - Câmara & Hahn (2002) [listed].

Hisonotus sp. 7 .- Reis & Carvalho (2007: 84) [listed].

Holotype. MCP 42577, 42.8 mm SL, female, Brazil, Rio Grande do Sul, Coxilha, arroio Caraguatá on secundary road to highway BR-153 between Passo Fundo and Ipiranga, 28°07’55”S 052°20’56”W, 29 Apr 2007, T. P. Carvalho, A. R. Cardoso & C. A. Cramer.

Paratypes. All from Brazil, Rio Grande do Sul, rio Uruguai basin, rio Passo Fundo drainage , ANSP 187474 , 4 , 30.9 – 39.6 mm SL ; MCP 41352, 6 , 23.7 – 47.1 mm SL, collected with the holotype . MCP 31765, 2 , 18.2 – 38.6 mm SL, Passo Fundo, rio Passo Fundo on highway BR-285, 28°14’55”S 052°18’39”W, 3 Jan 2003, W. Bruschi Jr. & J. P. Silva . MCP 31779, 9 + 3 c&s, 30.7–43.9 mm SL, Passo Fundo, rio Passo Fundo, downstream of Corsan dam, 28°15’15”S 052°18’52”W, 3 Jan 2003 W. Bruschi Jr. & J. P. Silva .

Diagnosis. Hisonotus megaloplax differs from its congeners by the arrangement of abdominal plates, which is composed by greatly enlarged lateral abdominal plates, which contact each other at the belly midline, without a median abdominal series or with a single triangular plate located posteriorly ( Fig. 6C ). All other species of Hisonotus , with the exception of Hisonotus montanus , possess a complete median series between the lateral abdominal plates or a naked area between them. This feature is polymorphic in H. montanus , some individuals having enlarged lateral abdominal plates contacting at the midline and leading to misidentification. Hisonotus megaloplax is further distinguished from Hisonotus montanus by the caudal-fin color pattern which is almost completely dark vs. color pattern composed by vertical dark stripes, and by the infraorbital canal entering infraorbital series via sphenotic vs. infraorbital canal entering infraorbital series via compound pterotic.

Description. Morphometrics and meristics in Table 4. Adult size moderate to large for members of this genus (larger individual surpassing 47 mm SL). Body robust, somewhat elongate, without conspicuous keels. Caudal peduncle round in cross section. Dorsal body profile slightly concave from tip of snout to nares, convex from that point to parieto-supraoccipital tip. Straight and posterodorsally inclined from posterior end of parieto-supraoccipital to dorsal fin origin. Dorsal-fin base straight and posteroventrally sloped, almost straight from posterior end of dorsal-fin base to caudal-fin origin. Ventral profile almost straight from snout tip to anal-fin origin. Concave at anal-fin base, straight from that point to origin of caudal fin. Greatest body depth at dorsal-fin origin. Least body depth just posterior to middle of caudal peduncle. Posterior profile of caudal-fin margin concave. Head and snout broad, snout rounded in dorsal view, body progressively narrowing posterior of pectoral-fin insertion. Snout region anterior of nares slightly depressed; interorbital region straight to slightly convex. Upper margin of orbit not elevated. Eye dorsolaterally positioned. Iris operculum present.

Pectoral fin I,6. Posterior margin almost straight, when depressed tip extending beyond middle of pelvic fin. Posterior margin of pectoral-fin spine smooth. Pectoral-fin axillary slit present in juveniles, but absent in larger specimens. Pelvic fin i,5. Tip of depressed fin not reaching anal-fin origin in females, but extending just to that point in males. Dorsal II,7, its origin located slightly posterior of vertical through pelvic-fin origin. Dorsal-fin spinelet laterally extended. Anal fin i,5. First anal-fin pterygiophore exposed anterior to anal fin. Adipose fin absent. Caudal fin i,14,i.

Body almost entirely covered by plates except for region overlying opening of swim bladder capsule, area between pectoral girdle and lower lip, region around anus, and base of paired fins. Rostral plate with posterior notch articulation with mesethmoid. Rostral plate thickened, with odontode-free band between dorsal and ventral series of odontodes. Area devoid of odontodes narrow at medial portion and wider at lateral portions ( Fig. 12 ). Prenasal plates present and not reduced. Three rows of predorsal plates. Lateral plate series formed by 22–24 plates ( Tab. 2). Lateral line incomplete, with gap on line of pores along midlength of body. Median plate series truncated, not reaching posterior end of caudal peduncle ( Fig. 5C ). Abdominal plates composed by enlarged lateral plate series, contacting each other at midline of belly. Lateral abdominal plates forming regular series of three to five plates, two to four of them contacting at midline. Pre-anal shield region formed by large size plates ( Fig. 6C ). Coracoid and cleithrum exposed and covered by odontodes, except for medial region of cleithrum between arrector fossae opening and symphysis.

Head without conspicuous crests. Odontodes on parieto-supraoccipital tip slightly larger than those of surrounding areas, mostly in smaller specimens. Head, fin rays, and body plates covered with odontodes, these larger on anterior surface of all fin spines, and on ventral margin of rostrum, slightly enlarged in dorsal margin of rostrum. Anteroventral margin of compound pterotic with median-to-large size perforations. Infraorbital canal entering infraorbital series via sphenotic. Lips roundish and papillose, posterior margin of lower lip fimbriate. Maxillary barbel present.

Premaxillary and dentary teeth bifid, slender proximally and flattened distally; major (medial) cusp round; minor (lateral) cusp minute pointed. Accessory patch of teeth absent on dentary and premaxilla.

Hypurals 1–2 and 3–5 almost completely fused, with median notch on anterior portion caudal-fin skeleton. Total vertebrae 28 (2 c&s).

Color. Ground color of dorsal and lateral surfaces dark gray to almost black. Dorsal and lateral portions of head darker than body, except for yellowish area in ventrolateral region of head contrasting with dark rounded blotches, and lighter spots on parieto-supraoccipital. Region anterior to nares lighter than surrounding areas, forming narrow paired longitudinal light stripes from snout tip to posterior end of parieto-supraoccipital, bifurcating and becoming inconspicuous from that point, and completely disappearing at vertical bellow dorsal-fin origin. Ventral surface of body pale yellow, with scattered chromatophores grouped on base of pectoral fin, forming dark blotches. Unbranched rays of pectoral, pelvic, dorsal, and anal fins mostly unpigmented, except for dark transverse bars, forming striped pattern. Branched rays of these fins mostly hyaline except for unconpicous dark transverse bars. Caudal fin mostly dark gray pigmented, except for hyaline tranverse bands on posterior portion of upper rays. Unbranched rays of caudal fin with transverse dark bands. In life, ground color of dorsum and flanks dark green.

Sexual dimorphism. Characterized by the urogenital papilla, positioned just behind the anal opening in males. Adult males also possess a developed fleshy flap along the dorsal margin of the first thickened pelvicfin ray, that is absent in females. The flap is slightly wider basally and progressively narrows distally. A fleshy flap also present on medial portion of first and second branched rays of pelvic fin. In juvenile males, flaps smaller or absent. Males have a longer pelvic fin that extends up to the anal-fin origin, with pelvic fin falling short of anal-fin origin in females.

Distribution and habitat. Hisonotus megaloplax is known only from the headwaters of the rio Passo Fundo drainage, tributary to the upper rio Uruguay basin ( Fig. 8 ). The arroio Caraguatá at the type-locality is a small watercourse with slow to median flow of clear to brown waters running over stones, vegetal litter, and sandy bottom. The species inhabits marginal submerged vegetation composed mainly by grass. See Câmara & Hahn (2002) for detailed description of arroio Caraguatá and its fish fauna. Recent collection efforts failed to capture this species in the headwaters of rio Passo Fundo, near the city of Passo Fundo. The construction of a dam (Barragem da Fazenda da Brigada) and the pollution caused by sewer rejects from the urban area of Passo Fundo seems to be the cause of the disappearance of this species in that region.

Etymology. The species epithet megaloplax , from the Greek megalos, big and plax, plate, referring to the large lateral abdominal plate series present in the new species. A noun in apposition.

Figures

FIGURE 5. Trunk plates of Hisonotus species in lateral view of left side. A—H. iota, MCP 40029, 23.3 mm SL. B—H. leucophrys, MCP 41354, 40.8 mm SL. C—H. megaloplax, MCP 31779, 41.3 mm SL. D—H. montanus, MCP 41459, 37.9 mm SL. Scale bar is 4 mm.

FIGURE 6. Arrangement of abdominal plates of Hisonotus species. Anterior portion toward top. A—H. iota, MCP 40029, 28.0 mm SL. B—H. leucophrys, MCP 41354, 40.8 mm SL. C—H. megaloplax, MCP 31779, 37.8 mm SL. D—H. montanus, MCP 41459, 37.9 mm SL. Scale bar is 2 mm.

FIGURE 8. Drainage map showing distribution of Hisonotus species in the upper rio Uruguai basin. Some symbols represent more than one collecting locality. Open symbols represent type locality. Hisonotus iota (square); Hisonotus leucophrys (star); Hisonotus megaloplax (triangles); Hisonotus montanus (asterisk); and Hisonotus aky (circles).

FIGURE 11. Holotype of Hisonotus megaloplax, MCP 42577, female, 42.8 mm SL. Arroio Caraguatá on secundary road to highway BR-153 between Passo Fundo and Ipiranga, Coxilha, Rio Grande do Sul, Brazil.

FIGURE 12. Scanning electron micrograph of snout anterior rostral margin of Hisonotus megaloplax, MCP 41352, 40.9 mm SL. A—anterior view of rostrum; B—magnification of medial portion.