Hemigrammus arua, a new species of characid (Characiformes: Characidae) from the lower Amazon, Brazil

A new Hemigrammus species is described from tributaries of the igarapé Juruti Grande and rio Arapiuns, lower rio Amazonas, Pará State, Brazil. The new species can be easily diagnosed from all its congeners, except from Hemigrammus stictus (Durbin), by possessing a single, large humeral spot which extends longitudinally from the fifth or sixth to the posterior margin of eighth to tenth, lateral line scales. It can be distinguished from Hemigrammus stictus by possessing a pronounced upper, anteriorlyoriented extension in the humeral blotch, conferring an inverted-comma shape to it, and by displaying a distinct life color pattern.


Introduction
Among the Characidae currently assigned as "incertae sedis" within the family, Hemigrammus Gill is one of the most species-rich, with 49 species presently considered valid (Lima et al., 2003;Britski & Lima, 2008;Marinho et al., 2008). None of the features used to diagnose Hemigrammus (i.e., five or more teeth on the inner premaxillary series, incompletely pored lateral line, and scaled caudal-fin basis) is unique to the genus. The last broad survey on the genus was published more than thirty years ago (Géry, 1977). Since then, there was a considerable increase in the number of known species in the genus (38 to the present 49) but no major treatment of its taxonomy. The phylogenetic relationships of Hemigrammus are, as most genera that were formerly assigned to the Tetragonopterinae (sensu Géry, 1977), poorly understood, and its purported monophyletic nature is also highly questionable. For example, Weitzman & Palmer (1997: 225-226, 237) remarked that the type species of Hemigrammus, H. unilineatus, might be related to a putative monophyletic group within Hyphessobrycon, the so-called "rosy tetra clade", which, if confirmed, would result in a drastic taxonomic rearrangement within Characidae.
During a broad biological survey conducted at Juruti municipality, a locality lying on the right bank of the rio Amazonas, slightly above the mouth of the rio Tapajós on the lower Amazon, a distinctive undescribed species of Hemigrammus was found. The aim of the present contribution is to describe this new species.

Material and Methods
Counts and measurements were taken according to Fink & Weitzman (1974) and Menezes & Weitzman (1990), except for counts of the horizontal scale rows below the lateral line that were counted to the pelvic-fin insertion. Horizontal scale rows between dorsal-fin origin and lateral line does not include scale of median predorsal series situated just anterior to first dorsal-fin ray. In the descriptions, the frequency of each count is given in parentheses after the respective count. An asterisk indicates counts of the holotype. Counts of supraneurals, vertebrae, procurrent caudal-fin rays, unbranched dorsal and anal fin rays, branchiostegal rays, gill-rakers, maxillary, and dentary teeth were taken only from cleared and stained paratypes (c&s), prepared according to Taylor & Van Dyke (1985). Vertebrae of the Weberian apparatus were counted as four elements and included in the vertebral counts, and the fused PU1+U1 of the caudal region as a single element. In the list of material examined, the number of whole specimens in the lot is followed by the number of those cleared and stained (if any). Jaw bones were cleaned from soft tissues for SEM images by a short (less than 10 minutes) immersion in weak (less than 1 %) sodium hypochlorite solution and after that air-dried. Institutional abbreviations follows Reis et al. (2003). possessing a humeral spot roughly rectangular, higher than wide, with a pronounced upper, anteriorly-oriented extension, conferring an inverted-comma shape to the blotch (vs. humeral spot roughly rounded, not presenting an inverted-comma shape), and life color pattern presenting unpaired fins orangereddish (vs. caudal fin and caudal peduncle bright red in life, remaining fins translucent).

Color in alcohol.
Ground color tan, slightly darker dorsally. Guanine pigmentation present on opercular and infraorbital bones. Lower lip, snout, and top of head densely covered by small dark chromatophores, resulting in overall dark pigmentation. Upper half of opercle and fifth infraorbital with large, scattered dark chromatophores. Anterior two-thirds of eye dark pigmented. Gular area clear to tan.
Mid-dorsal and dorsal portion of first lateral scale rows with dense concentration of dark chromatophores, concentrated mainly on scale margins. Large, distinctive blotch, situated immediately above lateral line scales row, extending longitudinally from the fifth or sixth to the posterior margin of eighth to tenth lateral-line scales. Blotch roughly rectangular, higher than wide, with a pronounced upper, anteriorly-oriented extension, giving an invertedcomma shape to the blotch. Thin, subjacent dark midlateral stripe, extending along midline from blotch terminus to anterior portion of caudal peduncle. Scales of dorsal and lateral surfaces of body possessing numerous dark chromatophores, especially over midline. Subjacent dark chromatophores forming thin lines over myomere margins above anal fin base. Scales on abdominal region with few dark chromatophores. Narrow, subjacent stripe of dark pigment located dorsal to anal-fin base, extending approximately along region where hypaxial musculature and muscles of anal fin meet, sub-parallel to anal-fin base. Dark, subjacent pigmentation at base of anal-fin rays, forming a thin longitudinal line. Outer caudal-fin rays with margins dark pigmented, branched rays with few dark chromatophores along dorsal and ventral margins. Inner caudal-fin rays with distal portion more pigmented than remaining fin. Anal-fin rays with scattered chromatophores on interradial membranes. Dorsal, pectoral and pelvic-fin rays with dark pigment along anterior and posterior margins. Adipose fin with scattered dark chromatophores.

Life coloration.
Based on a photograph of the holotype (MPEG 14755) and paratypes (MPEG 14756), taken immediatelly after fixation (Fig. 3). Overall ground color brown. Opercular area, facial bones, and abdominal area above pectoral-fin basis bright golden, with a greenish hue. Gular and ventral area below pectoral-fin basis silvery. Dorsal, adipose, caudal, and anal fins, intense orange-reddish, except for their relatively faded distal portions. Pelvic fins yellowish, except in the holotype, which possess orange-reddish pelvic fins. Pectoral fins translucent.
Sexual dimorphism. Not detected. Bony hooks on anal and pelvic fins of mature males, a common dimorphic feature among characids (Malabarba & Weitzman, 2003) were not found.
Geographical distribution. Known from the headwaters of the rio Arapiuns (rio Branco and rio Aruã), rio Tapajós basin, and from the adjacent headwaters of the igarapé Juruti Grande, a right margin tributary of the rio Amazonas, Pará State, Brazil (Fig. 5).
Ecological notes. The type locality, lago São Francisco do Alto Aruã, is, in spite of its name, not a lake but actually a large river pool (about 200 meters wide). Satellite images of the area suggest that this portion of the rio Aruã is a valley of an ancient ria-lake which is being filled by sediments, as evidenced by the presence of extensive floodplains upstream  the lake Aruã (see Klammer, 1984, for a discussion on this type of landscape feature in the Central Amazon region). Specimens of Hemigrammus arua were collected in relatively shallow (1-1.5 meters deep) marginal areas, with abundant aquatic macrophytes, both emergent and non-emergent. Identifiable itens found in gut contents of four specimens (MPEG 14757. 22.0-27.7 mm SL) were insect larvae (both aquatical and terrestrial forms), winged insects, and vegetal remains.
Etymology. Arua, after lago São Francisco do Alto Aruã, the type locality. A noun in apposition.

Discussion
Hemigrammus arua is herein assigned to this genus based on the possession of the features used to diagnose it (i.e., five or more teeth on the inner premaxillary series, incompletely pored lateral line, and scaled caudal-fin basis). As noticed in the Introduction, the relationships within Characidae, and particularly among many of the genera formerly assigned to the Tetragonopterinae and currently considered incertae sedis in the family (see Lima et al., 2003), are poorly known. The distinction of Hemigrammus from several related genera is based in a single character state, e.g., the naked caudal-fin basis in Hyphessobrycon, the completely pored lateral line in Moenkhausia, the high number of maxillary teeth in Parapristella, the absence of the outer row of premaxillary teeth in Petitella, the absence of adipose fin in Hasemania, or the elongated lower caudal-fin lobe in Thayeria. The typological definitions of these and another genera formerly assigned to the subfamily Tetragonopterinae (sensu Géry, 1977) were heavily criticized by Weitzman & Fink (1983), but unfortunately, our knowledge of the relationships of these genera have not advanced substantially ever since. As a result, the monophyly of Hemigrammus remains doubtful, and the intrarrelationships within the genus are still unknown. The only attempt to delimit groups within Hemigrammus, though admittedly artificial ones, was by Géry (1977: 490). This author, based on Ellis (in Eigenmann, 1918), split Hemigrammus into five groups, based on color pattern. Hemigrammus arua fit the group "b" from Géry (1977), the so-called Hemigrammus bellottii group, which includes species possessing a humeral spot but lacking a caudal peduncle spot. Among the species included in that group, Hemigrammus arua resembles more closely H. stictus, with which it shares a humeral spot situated posteriorly than the usual condition in most characids. The humeral spot in characids generally lies over the second to the third, and extends from the fourth to the fifth, lateral line scales. In contrast, Hemigrammus arua and H. stictus possess a humeral spot lying over the fifth to the sixth, and extending longitudinally into the eighth to the tenth, lateral line scales. A similar, putatively non-homologous condition, is only found, among the Characidae formerly assembled under the subfamily Tetragonopterinae, in Jupiaba polylepis (Günther), which possess a humeral spot lying over the sixth, and extending to the ninth, lateral line scales (Zanata, 1997). This "dislodged" humeral spot lies at the same position of the second humeral spot in the Hemigrammus species that possess that feature (e.g., H. ocellifer). We suggest that the dislodged humeral spot of Hemigrammus arua and H. stictus might be homologous to the second humeral spot found in some other Hemigrammus species, the condition displayed by these species having evolved through the loss of the first, more anterior, humeral spot.
Hemigrammus arua was collected in an area of terra firme near the extensive floodplain areas of the lower rio Amazonas. Other fish species recently described from the same general area are Apistogramma arua Römer & Warzel and Acestridium triplax Rodríguez & Reis. We are aware of at least two more undescribed fish species, both belonging to the genus Hyphessobrycon, occurring in the area. Due to the difficulty to reach terra firme areas, this type of habitat is comparatively much less sampled than floodplain and main channel riverine habitats. Vast expanses of terra firme habitats across central and western Amazon basin remain to be explored ichthyologically, and it is expected that much of the future ichthyological discoveries in the area will be concentrated in this type of habitat.