Acestridium triplax, Rodriguez & Reis, 2007

Rodriguez, Mónica S. & Reis, Roberto E., 2007, A new species of Acestridium Haseman, 1911 (Loricariidae: Hypoptopomatinae) from the Eastern Amazon basin, Brazil, Neotropical Ichthyology 5 (4), pp. 429-434 : 429-432

Acestridium triplax , new species

Fig. 1

Holotype. MPEG 13355 , 55.5 mm SL, female, Brazil, Pará , Município de Juruti , igarapé Mutum , tributary to rio Aruã , a tributary to rio Branco , a tributary to rio Aripiuns, itself a tributary to the lower rio Tapajós, 2º36’45"S 56º11’37"W, 11 Dec 2006, L.F. Montag.

Paratypes. Brazil, Pará , Juruti : MCP 41745, 9 + 1 c&s, 41.8-52.8 mm SL (2, 51.7-52.8 mm SL) and MPEG 12461 , 9 , 28.9-50.4 mm SL (1, 50.4 mm SL), same data as holotype. MPEG 10123 , 5 , 27.8- 42.7 mm SL and MPEG 10124 , 10 , 27.6-49.5 mm SL (2, 46.6-49.5 mm SL), same locality as holotype, 3 Mar 2006, L.F. Montag. MPEG 12476 , 5 , 37.4-46.8 mm SL (1, 46.8 mm SL), same locality as holotype, 6 Aug 2004, W. Wosiacki. AMNH 240826 , 10 , 41.1- 51.8 mm SL (2, 50.9-51.8 mm SL), igarapé Socó Barroso, 2º27’05"S 56º11’35"W, 12 Dec 2006, L.F. Montag. MPEG 10122 , 3 , 48.9- 49.1 mm SL (2, 46.9-47.5 mm SL), igarapé Socó Barroso, 2º27’05"S 56º11’35"W, 4 Mar 2006, L.F. Montag. MPEG 12462 , 5 , 34.8 - 50.0 mm SL (1, 50.0 mm SL), rio São Francisco, 2º10’44"S 56º04’37"W, 14 Dec 2006, L.F. Montag. AMNH 240827 , 10 , 32.4- 50.2 mm SL (1, 48.5 mm SL), creek at 2º36’45"S 56º11’37"W, 2 Aug 2004, W. Wosiacki. MCP 41746, 8 + 2 c&s, 30.6-52.0 mm SL (2, 49.1-50.8 mm SL), creek at 2º36’45"S 56º11’37"W, 3 Sep 2002, W. Wosiacki. MPEG 12614 , 8 + 2 c&s, 41.0- 53.5 mm SL (1, 53.5 mm SL), creek at 2º36’45"S 56º11’37"W, 8 Sept 2002, W. Wosiacki. ANSP 187168 , 10 , 36.7-55.4 mm SL (2, 50.6-55.4 mm SL) and MPEG 12492 , 4 , 23.8-28.9 mm SL and MPEG 12493 , 4 , 27.4-52.3 mm SL (1, 52.3 mm SL), creek at 2º36’45"S 56º11’37"W, 9 Sep 2002, W. Wosiacki. ANSP 187167 , 11 , 29.2 -51.0 mm SL (3, 46.5- 51.0 mm SL) and MPEG 12451 , 10 , 32.3-49.1 mm SL (3, 47.1-49.1 mm SL), creek at 2º34’52"S 55º54’11"W, 14 Dec 2006, L.F. Montag. MPEG 12467 , 10 , 35.4 - 48.0 mm SL (3, 45.3-48.0 mm SL), creek at 2º33’40"S 55º54’11"W, 15 Dec 2006, L.F. Montag. MPEG 12468 , 10 , 34.1-53.3 mm SL (1, 53.3 mm SL) and MPEG 12475 , 7 , 34.1- 52.3 mm SL (1, 52.3 mm SL), creek at 2º27’54"S 56º00’25"W, 12 Dec 2006, L.F. Montag. MPEG 12453 , (1, 53.4 mm SL), creek at 2º27’54"S 56º00’25"W, 13 Nov 2006, L.F. Montag .

Diagnosis. The new species differs from all other species of Acestridium in having one series of middle abdominal plates between the lateral abdominal plates (vs middle abdominal plates absent in remaining species). It is also distinguished from A. dichromum (the most similar species) in having a small and rounded snout-tip spatulate projection (vs large and oval spatulate projection; Fig 2 ), a thick pectoral-fin spine without serrae or slightly serrated (vs very thick and strongly serrated in the inner face; Fig. 3), posterior most lateral abdominal plate small (vs large; Fig. 4 ), and preanal plate approximately rectangular, with short lateral projections (vs preanal plate cross-shaped, with large lateral projections; Fig. 4 ).

Description. Morphometric data given in Table 1. Head, trunk, and caudal peduncle strongly depressed, elongate, and narrow. Body width 14.9-20.8% of postdorsal length (mean= 17.8%). Dorsal surface of body straight or slightly concave from snout tip to eye, straight or slightly convex from eye to dorsal fin, straight from dorsal fin to penultimate plate of caudal peduncle.

Body covered with bony plates except for ventral part of head and very narrow area surrounding pelvic-fin base and anus. Body plates uniformly covered by small odontodes, arranged in longitudinal rows.

Profile of head triangular with round spatulate projection on tip of snout in dorsal view. Spatulate projection covered by pair of rostral plates with large odontodes. Eyes placed laterodorsally, not visible from below; iris operculum present. Snout tip with small naked area (seen in cleared and stained specimens). One rostral plate and two postrostral plates on each side. Postrostral plates and cheek plate bent ventrally and visible in ventral view, cheek plate with canal from fifth infraorbital. Posterior portion of postrostral plates not united at midline ventrally. Mouth rounded, with globular papillae on both upper and lower lips. Oral disc width between 12.9-18.7% of postdorsal length (mean= 15.8). Maxillary barbel shorter than eye diameter. Premaxilla with 22 to 30 (mean=26, n=31) bilobed teeth in functional series. Dentary with 15 to 24 (mean=20, n=31) bilobed teeth in functional series. All teeth with uneven cusps, with reddish-brown tip. Mesial cusp longer and wider than lateral one.

Abdomen fully plated except for region of pectoral girdle. One series of middle abdominal plates between lateral abdominal plates. Preanal plate wide. Posterior most lateral abdominal plate, in contact with preanal plate, narrow ( Fig.4 ). Three to seven (mean= 4.5, n=31) lateral abdominal plates on each side. One median prenucal plate and one nucal plate anterior to dorsal fin. Postdorsal plates 13 to 16 (mean=14, n=31). Thirteen to 15 (mean=14, n=31) postanal plates. Four lateral series of plates, mid-dorsal series absent. Parieto-supraoccipital bone and predorsal plates smooth, without ridges.

Dorsal-fin rays i,7. Posterior margin of dorsal fin straight or slightly rounded, with first or second branched ray longest. Tip of dorsal fin, when depressed, reaching third plate posterior to fin base. Dorsal-fin spinelet absent. Pectoral-fin rays I,6. Pectoral spine thick, without serrae or slightly serrate on inner margin, reaching beyond middle of first branched ray. Posterior margin of pectoral fin rounded, with longest first or second branched ray not reaching to pelvic-fin origin. Pelvic-fin rays i,5. Unbranched ray very thick with enlarged odontodes turned mesially. Posterior margin of pelvic fin rounded; second, third, or fourth branched ray longest, not reaching to anal-fin origin.Anal-fin rays i,5. Anal unbranched ray reaching slightly beyond middle of first branched ray. Posterior margin of anal fin rounded, with first, second, or third branched ray longest. Tip of anal fin, when depressed, reaching third or fourth plate posterior to fin base; two or three ventral plates along its base. Posterior margin of caudal fin truncate or slightly concave; principal caudal-fin rays i,12,i. Adipose fin absent.

Color in alcohol. Dorsum light brown with short stripes of dark pigmentation running between longitudinal rows of odontodes. Dark stripe starts about half distance between tip of snout and eye and runs along side of head through eye, becoming less distinct as it reaches body; stripe becomes indistinct at midbody. Some dark dots sometimes present on sides of head, below dark stripe; on maxillary barbel and upper lip. Ventral side yellowish. Body scarcely pigmented ventrally from anal to caudal fin. Snout with brown background ventrally. Upper lip and snout lightly pigmented. Except for caudal fin, all spine or unbranched rays yellowish tan with round black spots distributed regularly; interradial membranes hyaline. Caudal-fin rays with numerous dark spots which sometimes form 1-2 dark bands, spines with round black spots, distributed regularly, arranged in rows. Dark spots at base of caudal fin.

Sexual dimorphism. Males of Acestridium triplax have conspicuous urogenital papilla, immediately posterior to anal opening, not present in females.

Ontogeny. Many structures vary ontogenetically in Acestridium triplax . The general aspect of juveniles (23.8- 28.9 mm SL) resembles Sturisoma species. Young specimens are intensely pigmented, especially showing a dark stripe beginning at the snout tip and running along side of head through eye and becoming less conspicuous at midbody. In juvenile specimens the lower lip is very large with filiform, marginal papillae. Lower lip gradually forms with the upper lip the round mouth typical of Acestridium , with both lips covered by globular papillae. In the smallest specimens, the snout is short and the globular protuberance at snout tip is absent. As noted by Aquino & Schaefer (2002) for Oxyropsis , the abdominal plates are absent in the smaller specimens and have been seen in a specimen of 28.9 mm SL. In this specimen small lateral abdominal plates are at the lateralmost sector of the abdomen, and the series of middle abdominal plates is represented by small, loose platelets, not contacting each other. The prenucal and nucal plates are seen in specimens from 23.8 mm SL, the smallest we have.

Distribution and habitat. Known only from small, forest creeks near Juruti, Pará State, lower rio Tapajós drainage, Amazon River basin, Brazil. These creeks are usually shallow (0.3-1.0 m deep, 5-10 m wide), with sandy bottom, clear or slightly black water, moderate water current, and dense aquatic and marginal vegetation.

Etymology. The species epithet triplax from the Greek tri, three, and plax, plate, referring to the three series of abdominal plates present in the new species.

Figures

Fig.1. Acestridiumtriplax,MPEG 13355, holotype, 55.5mm SL, female,Igarapé Mutum,Jurutí,Amazonas river Basin,Pará, Brazil.

Fig. 2. Spatulate projection of snout. A. Acestridium triplax; B. Acestridium dichromum. RP: rostral plates. Scale: 1 mm.

Fig. 4. Abdominal and preanal plates.A. Acestridium triplax; B. Acestridium dichromum. PP: preanal plate. Scale: 1 mm.