Steindachneridion melanodermatum, Garavello, 2005

Garavello, Julio Cesar, 2005, Revision of genus Steindachneridion (Siluriformes: Pimelodidae), Neotropical Ichthyology 3 (4), pp. 607-623 : 618-619

Steindachneridion melanodermatum , new species Fig. 9

Steindachneridion sp. : Severi & Cordeiro, 1994: 73 (ref.); Garavello et al., 1997: 71 (ref.); Nakatani et al.: 2001: 304 (larval diagnosis).

Holotype. MZUSP 87983, 530 mm SL, Brazil, Paraná State, municipality of Quedas do Iguaçu, rio Iguaçu, down river of the Salto Osório dam (approx. 25 o 35´S 53 o 05´W), J. C. Garavello, A. S. Soares, J. C. Soares & A. Aparecido, 30 Oct - 6 Nov 1989.

Paratypes. MZUSP 87984 , 5 , 381 to 532mm SL, same locality and collectors as holotype; LISDEBE 1165, 2, 474 and 512 mm SL, Brazil, Paraná State, municipality of Quedas do Iguaçu, rio Iguaçu, upriver from mouth of rio Chopim (approx. 25 o 35´S 53 o 05´W), J. C. Garavello, A. S. Soares, J. C. Soares & A. Aparecido, 9-18 Jan 1990 .

Diagnosis. Steindachneridion melanodermatum differs from the remaining species of the genus by the following combination of characters: supraoccipital process remote from the anterior nuchal plate by a distance nearly twice the orbital diameter; premaxillary tooth plate slender, separated one from another by a medial groove and curved in distal region; a single oval vomer tooth plate, continuous and circumscribed by the premaxillary plates; 16 to 18 gill rakers on first branchial arch. Lips well developed; maxillary barbel short, reaching to base of unbranched pectoral-fin ray; pelvic fin short, when adpressed only slightly surpassing the anus, but distant from the anal-fin origin; caudal fin slightly notched, dorsal lobe slightly larger than ventral, each lobe nearly rounded distally. Ground color of head and trunk in alcohol dusk brown; abdomen whitish; tiny circular black or dark brown blotches irregularly scattered on the ground color of head and trunk, masking a black dotted pigmentation; caudal peduncle high, its depth 29.5 to 33.4% of HL; head depth nearly equal head width, 74.4 to 97.3%, and head width moderately small, 20.4 to 22.6% of SL.

Description. Morphometric data presented in Table 2; maximum body length 532 mm SL. Snout slender and robust; snout tip roughly elliptical in dorsal view; 12.1 to 13.3 % of SL; each pair of nostrils protruding on dorsal snout profile; anterior pair of nostrils nearly equidistant from each other as in posterior pair. Head long in dorsal view and slightly depressed in lateral view; head depth nearly long as head width, 74.4 to 97.3%; head depth 17.8 to 21.2% of SL; head length 28.3 to 29.5% of SL. Dorsal profile of head almost straight from tip of snout to supraoccipital process, slightly depressed at interorbital space; slight convex between this point and the dorsal-fin origin; distance between supraoccipital process and anterior nuchal plate separated by near two orbital diameter. Eyes dorsally placed and in midway between tip of snout and the opercular opening; orbit moderately small, 25.1 to 29.7% of interorbital distance and 8.6 to 9.5% of HL.

Opercular opening large; branchial membranes joining at narrow isthmus; eight branchiostegal rays progressively reduced in size; 16-18 gill rakers on first branchial arch. Lower region of head with arrow-like, shallow gular fold; deep grooves at lower jaw distal margin. Mouth large, terminal and with thick lips; mouth width 42.2 to 47.5% in HL; lower jaw shorter than upper jaw; posterior postcleithral process short, reaching vertical through terminus of supraoccipital process; all barbels narrow and depressed; maxillary barbel long, reaching or surpassing median length of dorsal fin when adpressed; pair of short mental and post mental barbels on each side. Tooth plates of premaxillary and dentary with villiform teeth; premaxillary tooth plates slender, with a median groove, narrowing and curved inward distally; single vomer tooth plate continuous, almost elliptical and circumscribed by the premaxillary plate on each side; a single and elongated dentary tooth plate distally sharp.

Body profile almost straight from the nuchal plate to dorsal-fin; dorsal-fin base slanted and profile between dorsal-fin and adipose-fin insertion slightly concave; base of adipose fin slanted; dorsal profile of caudal peduncle deep concave between adipose and caudal fin. Predorsal length 41.4 to 44.3% of SL. Ventral profile of body slanted from tip of snout to end of opercular bone; convex in abdominal region and nearly flat on pelvic and anal-fin insertions; lower caudal peduncle profile slightly concave. Caudal peduncle at vertical passing through posterior tip of anal fin approximately rounded in cross-section. Dorsal-fin rays i,7(6); the first two slightly spinous; dorsal-fin base inserted in the dorsal space at a vertical after pectoral-fin and through anal-fin origin; dorsalfin locking mechanism present. Pectoral-fin short; first pectoral ray not pungent; both margins devoid of serrae; pectoralfin length when adpressed, reaching before the vertical through dorsal-fin origin; i,9(10) rays; posterior margin truncate. Pelvic-fin rays i,5; tip of unbranched pelvic ray short and smooth; when adpressed only slightly surpassing the anus; distant from anal-fin origin. Distal margin of pelvic-fin roughly rounded. Anal-fin rays i,8 (9), posterior margin truncate; preanal distance 72.6 to 75.5% of SL. Caudal peduncle low, its depth 9 to 9.6% of SL. Caudal fin slightly notched, dorsal lobe slightly larger than ventral, both tip nearly rounded; principal caudal-fin rays, i,15(14),i.

Body and head, except orbital region covered with thick skin; ventral surfaces of pectoral and pelvic girdle entirely covered by skin; supraoccipital process short covered by thin skin, reaching vertical through posterior opercular opening. Posterior postcleithral process short and entirely covered by skin.

Color in alcohol. Ground color of head and dorsal regions of body black or dark brown; ventral region whitish; dorsal and lateral regions of head and dorsum with minute dark brown circular blotches or dots shorter than one orbital diameter, irregularly scattered on background; lower region of head and abdomen progressively whitish. Eyes dark. Dorsal, pectoral, pelvic, anal and adipose fins uniformly black or dark brown combined with small black or dark brown dots scattered on rays and membranes; adipose fin with several minute black or dark brown dots; caudal fin dark dotted on base of each lobe; distal regions uniformly dark.

Etymology. Melanodermatum after the Greek melanos = black and dermatus = skin, referring to the full dark brown ground color of body, a color pattern unique in the genus.

Distribution. Known from the rio Iguaçu at upper Paraná Basin, border of Paraná and Santa Catarina States, Brazil ( Fig. 1 ).

Notes on the rio Iguaçu and localities of S. melanodermatum . The rio Iguaçu extends almost 1,080 km, descending approximately 830 meters from its headwaters to its mouth just a little below the Iguaçu falls at the border between Brazil, Argentina, and Paraguay. Its headwaters are situated on the eastern Serra do Mar area, which is included in the First Paranaense Plateau, near the city of Curitiba. Upriver, between the localities of Engenheiro Blay and Porto Amazonas, the rio Iguaçu includes several and extensive clear water regions, running over stony beds. As mentioned by Maack (1968) this area of rio Iguaçu drainage has very old rocky beds previous to the Devonian Period constituting the Second Paranaense Plateau. Between the localities of Porto Amazonas and União da Vitória, the river includes very old beds but also several meandrous and extensive flooded areas. Below this area, it crosses the Boa Esperança mountains and reaches the Third Paranaense Plateau, were the rocky beds are younger, including rapids and some falls towards the large Iguaçu falls before meeting the rio Paraná. The collecting localities of S. melanodermatum are situated at the Third Paranaense Plateau, after the Boa Esperança mountains; in these places the river has many large falls, variable depths, but it always has a very swift water flow, running over large rocky beds. Both left and right banks have native vegetation formations of open fields (campos limpos) with sparse woodlots (capões), generally concentrated near the riparian forest, in the banks of the main channel of the river and at its main tributaries. Presently, the portion of the rio Iguaçu running over the Third Paranaense Plateau is deeply modified by the dams of several hydroelectric power plants, and S. melanodermatum may be found only between these dams, inhabiting stretches of the river where the natural flow of water is still preserved.

Figures

Fig. 9. Lateral and dorsal views of S. melanodermatum holotype. MZUSP 87983, 530 mm SL, municipality of Quedas do Iguaçu, rio Iguaçu, down river of the Salto Osório dam, Paraná, Brazil.

Fig. 1. Southeastern South America with the geographical distribution of Steindachneridion species. Each symbol may represent more than one sample.