A new species of Acestridium Haseman, 1911 (Loricariidae: Hypoptopomatinae) from the Eastern Amazon basin, Brazil

Species of Acestridium Haseman, 1911 are highly distinctive, small sized, specialized loricariid catfishes, which have very narrow body, snout prolonged with a distinctive anterior spatulate projection, and usually green color in life. Acestridium was diagnosed by Schaefer (1991) based on the following characters: ten principal caudal-fin rays; plates at anterior snout margin forming conspicuous spatulate projection; head and body very slender, depressed; dorsal surface of head and trunk with longitudinal ridges; dorsal fin placed posteriorly, well beyond pelvic fin and just anterior to anal-fin origin; adipose fin absent; dermal plates anterior to first dorsal-fin proximal radial expanded, bearing ventromedial keel contacting the dorsal margin of the neural arches of the vertebrae six through twelve; and ridge of bone on the ventral surface of the lateral ethmoid contacting the metapterygoid channel reflected mesially. Currently, four species of Acestridium are described: A. discus Haseman, 1911, A. dichromum Retzer, Nico & Provenzano, 1999, A. martini Retzer, Nico & Provenzano, 1999, and A. colombiensis Retzer, 2005. In this paper, we describe a new species of Acestridium first collected by Wolmar B. Wosiaki and Luciano F. Montag in the lower Amazon River basin in Pará, Brazil, during the course of an environmental impact evaluation and subsequent environmental monitoring for a bauxite mining company. Material and Methods


Introduction
Species of Acestridium Haseman, 1911 are highly distinctive, small sized, specialized loricariid catfishes, which have very narrow body, snout prolonged with a distinctive anterior spatulate projection, and usually green color in life.Acestridium was diagnosed by Schaefer (1991) based on the following characters: ten principal caudal-fin rays; plates at anterior snout margin forming conspicuous spatulate projection; head and body very slender, depressed; dorsal surface of head and trunk with longitudinal ridges; dorsal fin placed posteriorly, well beyond pelvic fin and just anterior to anal-fin origin; adipose fin absent; dermal plates anterior to first dorsal-fin proximal radial expanded, bearing ventromedial keel contacting the dorsal margin of the neural arches of the vertebrae six through twelve; and ridge of bone on the ventral surface of the lateral ethmoid contacting the metapterygoid channel reflected mesially.
Currently, four species of Acestridium are described: A. discus Haseman, 1911, A. dichromum Retzer, Nico & Provenzano, 1999, A. martini Retzer, Nico & Provenzano, 1999, and A. colombiensis Retzer, 2005.In this paper, we describe a new species of Acestridium first collected by Wolmar B. Wosiaki and Luciano F. Montag in the lower Amazon River basin in Pará, Brazil, during the course of an environmental impact evaluation and subsequent environmental monitoring for a bauxite mining company.

Material and Methods
The morphometric variables were measured with a digital caliper (0.1 mm precision).Most measurements follow Boeseman (1971), except for pectoral-fin origin to pelvic-fin origin distance, eye to dorsal-fin origin distance, oral disk width, and counts of teeth and plates (which follow Retzer et al., 1999).Body depth at anal-fin origin is included as a new measurement.Names and counts of lateral, abdominal, and rostral plates follow Schaefer (1997).Fishes were cleared and double-stained (c&s) with Alcian Blue for cartilage and Alizarin Red for bone according to procedures of Taylor & Van Dyke (1985).In the lists of material examined, museum abbreviations and catalog numbers are presented first, followed by the number of specimens examined, number of cleared and stained specimens, size range and locality.Number of measured specimens and range of standard length (SL) between parenthesis, geographical coordinates, date of collection, and collectors are added for the new species.Abbreviations for institutions follow Reis et al. (2003).

Acestridium triplax, new species
Fig. 1 Holotype.MPEG 13355, 55.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 (23.8-28.9mm 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.

Discussion
Most loricariids have what Schaefer (1987) called the nucal plate, the exposed dorsal portion of the supraneural associated to the first dorsal-fin pterygiophore.The connecting bone usually connects the distal portion of the first rib to the lateral margins of the nucal plate and/or the transversal process of the second dorsal-fin pterygiophore.In Acestridium species there are one to three additional dermal plates anterior to the nucal plate, herein called prenucal plates.The prenucal plates are approximately hexagonal and bear ventromedial keels which contact the dorsal margin of the neural arches of the vertebrae six through twelve, being firmly articulated to each other and to the nucal plate.In Acestridium, the connecting bone links the distal portion of the first rib to the anterior most prenucal plate.Among hypoptopomatins, a prenucal plate is also present in Niobichthys.

Table 1 .
Descriptive morphometrics of holotype and paratypes of Acestridium triplax.Values are given as percents of standard length or head length.SD = standard deviation.