Description of five new species of Acestrocephalus Eigenmann and redescription of A. sardina and A. boehlkei (Characiformes: Characidae)

Acestrocephalus maculosus, A. stigmatus, A. nigrifasciatus, A. acutus, and A. pallidus are described as new species and A. sardina and A. boehlkei are redescribed. Acestrocephalus ginesi is considered synonymous with A. sardina. Specimens of Acestrocephalus anomalus, a valid trans-Andean species, were not available for study. Acestrocephalus stigmatus, A. maculosus, and A. boehlkei have a dark humeral blotch not present in the remaining species and the species within the two species groups thus characterized are told apart by meristic and morphometric data as well as morphological structures associated with the pseudotympanum.


Introduction
The genus Acestrocephalus Eigenmann was included with Galeocharax Fowler and Cynopotamus Valenciennes in the subfamily Cynopotaminae by Menezes (1976).Although not using a truly cladistic approach Menezes, based on a morphological character survey, considered the group monophyletic.In a subsequent contribution Lucena & Menezes (2003) did not recognize Cynopotaminae and the three genera formerly representing the subfamily were included in the characid subfamily Characinae.The phylogenetic relationships of all the characin genera have yet to be studied and as reported by Lucena & Menezes (2003) monophyly of the group is uncertain.Preliminary examination of the genera within the Characinae indicates that certain characters used by Menezes (1976) to define Cynopotaminae, including presence of ctenoid (actually spinoid) scales, shape, number and arrangement of premaxillary and dentary teeth and structure of the gill-rakers might be exclusive to Acestrocephalus, Galeocharax and Cynopotamus, suggesting that these genera are much more closely related among themselves than to any other genus in the Characinae.
Acestrocephalus is presently represented by four species: A. anomalus (Steindachner), A. sardina (Fowler), A. boehlkei Menezes, and A. ginesi Lasso & Taphorn.Five new species were discovered in material recently collected in expeditions carried out in different regions of Brazil.Additional specimens of A. sardina and A. boehlkei were also obtained.The type specimen of A. sardina, a juvenile (31.4 mm SL) not previously seen by me was examined and photographed by my colleague Heraldo Britski.
Recognition of new species and new data from A. sardina and A. boehlkei allowed for a better characterization of the species in the genus, with exception of A. anomalus as new material and previously examined specimens were unavailable.
Herein, the five new species of Acestrocephalus are described and A. sardina and A. boehlkei are redescribed.
Counts, measurements, and presentation of data follow Fink & Weitzman (1974) and Menezes (1976Menezes ( , 1977)), unless otherwise stated.The number of scales above and below lateral line and around caudal peduncle correspond to the number of horizontal scale rows.The number of gill-rakers includes only developed elements, not rudiments in both the upper and lower gill-arches.Examination of mature males under the stereomicroscope did not reveal the presence of gill glands on the first gill-arch.Measurements were taken whenever possible on the left side of the specimens, using dial calipers, recorded to a tenth of a millimeter and expressed as percentages of standard length (SL), except for subunits of the head, expressed as percentages of head length (HL).Data from Acestrocephalus anomalus and A. boehlkei in part were extracted from Menezes (1976Menezes ( , 1977)).All measurements were taken point to point.
Meristic and morphometric data of males and females were treated separately to assess possible differences between sexes, but none were found.Immature as well as males and females were recognized by examination of gonads under the stereomicroscope.Vertebral counts were taken from radiographs and include the four vertebrae of the Weberian apparatus and the terminal "half centrum".
To show the arrangement of the muscles around the pseudotympanum the skin and adipose tissue covering the muscular hiatus were carefully removed and all muscles exposed (Fig. 1).In the schematic drawings prepared for each species muscles and other structures are represented as they appear after removal of skin and adipose tissue.The structure of the pseudotympanum does not seem to vary ontogenetically as no major differences were found in specimens of different sizes in all species examined.
In the descriptions, counts pertaining to the holotype are presented first, followed in parentheses by the mean of the sample, range, and the total number of specimens counted.The statistical procedure was standardized for comparative purposes even though samples of some species are represented by very few specimens.
Diagnosis.Acestrocephalus can be distinguished from the remaining genera of the Characinae sensu Lucena & Menezes (2003) by the following unique combination of morphological characters extracted from Menezes (1976: 37) with the addition of new ranges of meristic and morphometric characters for the genus due to the values of the new species described herein.Body comparatively small (SL 31.4-135mm); anterior dorsal region not elevated; dorsal body profile regularly curved from tip of snout to caudal base; lower part of antorbital only in contact with maxilla; first infraorbital relatively short, high on its median part; nasal bone tubular; cleithrum not notched, just with a slight sinuosity along its ventral edge; ectopterygoid with a median ventral bony crest; supraoccipital spine short; dentary with two rows of teeth, anterior teeth of external row much larger than posterior ones; first and third anterior teeth canine-like, more developed than the other two; posterior dentary tooth row with 20-40 small conical teeth slightly curved posteriorly, their number tending to increase with an increase in standard length; inner row of teeth on dentary formed by 7-14 small conical teeth; scales comparatively large and numerous, perforated lateral-line scales 67-79, 10-15 above and 9-13 below lateral line; anal fin comparatively short, with iv-v,25-36 rays, its origin situated on vertical line always crossing behind middle of dorsal-fin base length; pectoral fin with i,11-16 rays.Böhlke, 1984: 32 (catalog); Eschmeyer, 1998Eschmeyer, : 1507 (catalog) (catalog).Cyrtocharax sardina ;Fowler, 1950: 312 (listed).Cynopotamus (Acestrocephalus) sardina; Géry, 1972: 28 (di-agnosis in key).Acestrocephalus sardina; Menezes, 1976: 39 (in  Diagnosis.Acestrocephalus sardina is most similar to A. nigrifasciatus, both having identical color patterns consisting of a triangular dark blotch on caudal-fin base, anteriorly continuous with a longitudinal dark stripe extending along lateral line (Figs. 3 and 12) not found in any other species of the genus.In addition, both have the eye diameter greater than their congeners (Fig. 4).Acestrocephalus sardina can be distinguished from A. nigrifasciatus by having fewer horizontal scale rows above lateral line (10-12 vs 13-14) and below (10-12 vs 12-13).See Tables 1 and 2. Acestrocephalus nigrifasciatus also has the muscular hiatus over the anterior part of the swim bladder which is longer than and narrower than A. sardina and the fibers of the obliquus inferioris muscle, visible in the latter are entirely covered by the obliquus superioris muscle in the former (compare Figs. 5a and 5e).boehlkei.Acestrocephalus nigrifasciatus, A. pallidus, and A. acutus.The figure is not intended to show species differences therefore regression lines were not drawn and regression analyses were not performed.The primary purpose is to express the tendencies of the number of teeth on the maxilla to increase according to an increase in standard length.For the species represented by large number of specimens the data points are widely scattered.A. boehlkei, A. pallidus, A. nigrifasciatus, and A. acutus.The figure is not intended to show species differences therefore regression lines are not drawn and regression analyses were not performed.The primary purpose is to express the tendencies of the number of teeth on the posterior row on the dentary to increase according to an increase in standard length.For the species represented by large number of specimens the data points are widely scattered.but very few sexually mature males represented in examined sample.Pectoral-fin rays i,13 (anterior unbranched ray i, n=76) branched rays mean=13.6,range 12-15, n=76.Posterior tips of longest pectoral-fin rays reaching slightly beyond pelvic-fin origin.Pelvic-fin rays i,7, n=76.No hooks on pelvic-fin rays of sexually mature males.Distal tips of longest pelvic-fin rays extending to or slightly beyond anterior border of anus.Principal caudal-fin ray count 10/9 in all specimens, n=76.
Premaxilla with two slightly enlarged inner conical teeth, an outer row with one anterior canine followed by row of small conical teeth and one posterior canine.Anterior canine outside anterior border of lower jaw when mouth closed.Outer row small conical teeth 8 (mean=7.7,range 7-9, n=75).Maxillary teeth 44 (mean=43.6, range 40-53, n=75).Maxillary teeth conical with tendency to increase in number according to increase in standard length (Fig. 6).Dentary with outer row of conical teeth of which four anterior most spaced and first and third larger than third and fourth, these followed by posterior row of close-set smaller conical teeth that show tendency to increase in number with increase in standard length (Fig. 7).Posterior dentary teeth 28 (mean=29.4,range 26-33, n=75).Teeth in inner row on dentary conical, very small, slightly inclined proximally to four anterior spaced larger conical teeth of outer row.Inner row dentary teeth 9 (mean=9.8,range 7-12, n=75).
Muscular hiatus of pseudotympanum (Fig. 5a) relatively large, dorsally limited by lateralis superficialis muscle, posteriorly by large exposed portion of second pleural rib, anteriorly by small exposed portion of first pleural rib, anteroventrally by large portion of the obliquus superioris and posteroventrally by small portion of obliquus inferioris muscle.Small opening anterior to first pleural rib visible.
Color in alcohol.Body pale to dark yellow, slightly darker dorsally than ventrally due to presence of dark chromatophores.Dark chromatophores on dorsal and lateral parts of trunk mostly concentrated along free border of scales.Dorsal part of head from tip of snout to supraoccipital region darker than remaining dorsal surface of body.Premaxilla, median part of maxilla and lower jaw densely pigmented with dark chromatophores.Scattered dark chromatophores on infraorbital and opercular bones.Lateral dark body stripe extending from upper opercle to caudal peduncle where fused with triangular dark blotch on caudal base, dark color extending to base of principal caudal rays.Lateral body stripe narrower anteriorly and posteriorly and sometimes obscured by guanine.Anterior part of lower jaw bordered with dark pigmentation.Mental area of lower jaw with diffuse dark blotch in some specimens.Dorsal-fin origin with black dot extending to base of first unbranched dorsal-fin ray.Pectoral, pelvic, dorsal, and caudal fins pale, with few scattered dark chromatophores.
Distribution.This species is known from tributaries of the rio Madeira, Mato Grosso, rio Jutaí and rio Negro, Amazonas, Brazil and of the Orinoco Basin, Venezuela (Fig. 8).
Remarks.Menezes' redescription of A. sardina (1976) was based on the examination of two specimens, one of them from rio das Mortes near Xavantina, Mato Grosso.It was mentioned that no differences had been found among them and a small dark blotch at the humeral region was described for the species.Actually, this spot is present only in the specimen from rio das Mortes considered to belong to A. stigmatus herein described.A dark humeral spot is never present in specimens of A. sardina.At that time it was predicted that the taxonomic status of A. sardina would be cleared up only after a study based on a large series of specimens.
Acestrocephalus ginesi Lasso & Taphorn is herein considered a junior synonym of A. sardina.Five paraypes from Río Negro, near San Carlos de Río Negro, Venezuela (MZUSP 53499) and three specimens collected in the Apure State, near the type locality were studied and proved to be morphologically indistinguishable from specimens of the population samples representing A. sardina.
Description.Morphometrics of holotype and additional specimens presented in table 8. Meristic and morphometric data based on all examined lots of this species because no statistical differences found among them.Body moderately large (SL=31.5-135.0mm).Body form, dorsal and ventral body profiles, shape of snout and mouth and extension of maxilla as in A. sardina.Dorsal-fin rays ii,9 in all specimens, n=21, including holotype.Posterior most ray unbranched, n =21.Adipose fin present.Anal-fin rays v,34 (iv or v, usually v unbranched, branched rays mean=34.5, range 33-36, n=21, posterior ray split to its base and counted as 1).Slightly developed anterior anal-fin lobe including anterior unbranched rays and first 9-10 branched rays.Anal fin of three sexually mature males with bilateral hooks on posterior part of anterior branched rays.In a specimen 87.5 mm SL (MZUSP 38699) hooks present on anterior 11 branched rays and number of hooks per ray vary considerably: first branched ray with 9, third with 17, sixth with 24 and eleventh with 8. Pectoral-fin rays i,14 (anterior branched ray i in all specimens, n=21), branched rays mean 14.6, range13-16, n=21.Posterior tips of longest pectoral-fin rays reaching beyond pelvic-fin origin.Pelvic-fin rays i,7, n=21.No hooks on pelvic-fin rays of sexually mature males.Distal tips of longest pelvic-fin rays extending to or slightly beyond posterior border of anus, but not reaching anal-fin origin.Principal caudal-fin ray count 10/9, n=21.
Arrangement of muscles limiting muscular hiatus of pseudotympanum and degree of exposition of first and second pleural ribs as in A. sardina (compare figs.5a and 5b).
Color in alcohol.Body light brown to pale yellowish, darker dorsally.Dorsal head and snout more densely pigmented with dark chromatophores compared to lateral part of head.Lateral body stripe broader from below dorsal-fin origin to below adipose-fin origin, narrower from dorsal-fin origin anteriorly   Distribution.Upper Amazon River basin in tributaries of Río Napo, Ecuador and of Río Putumayo, Peru (Fig. 8).
Remarks.Lasso & Taphorn (1992) reported A. boehlkei from the Ventuari River, Apure River system, in the upper Río Orinoco, but emphasized that the record should be confirmed.Lasso et al. (2004), in referring to this record added that the specimens probably belong to A. ginesi Lasso & Taphorn (= A. sardina) or to a species not yet described.Lasso & Taphorn (2000) compared meristic data with specimens from the Río Ventuari to data taken from specimens of A. ginesi (= A. sardina) and suggested that the former have more branched anal-fin rays and lateral-line scales than the latter (35-38 vs 29-36 and 73-76 vs 63-71 respectively).In two specimens (MZUSP 37268) from the Río Suripa, a tributary of Río Apure, Barinas, Venezuela, the number of anal-fin rays (36 and 37) and maxillary teeth (35 and 38) are outside the range of values for A. sardina (28-33 and 40-53 respectively) and differ from A. boehlkei by having more posterior dentary teeth (32 and 34 vs 20-33 in A. boehlkei) and fewer small conical teeth in the outer row on the premaxilla (8 and 9 vs 9-12 in A. boehlkei).This strongly suggests that the specimens from the Apure River system in Venezuela, tentatively identified as A. ginesi (= A. sardina) and A. boehlkei probably belong to an undescribed species.Diagnosis.Acestrocephalus maculosus has a dark blotch at the humeral region, a character shared only with A. boehlkei and A. stigmatus.In the diagnosis of A. boehlkei the differences among these species are discussed.Acestrocephalus maculosus can be easily distinguished from A. stigmatus by the number of anal-fin rays (25-27 vs 29-31), and total number of gill-rakers (7-8 vs 5-6).See Tables 4 and 9.The general structure of the pseudotympanum of A. maculosus (Fig. 5c) is quite peculiar and different from that of A. stigmatus (Fig. 5d) and all the other species of Acestrocephalus examined (Figs.5a, b, e, f and g).In A. maculosus the muscular hiatus is very reduced as the result of the great development of the obliquus superioris muscle covering the anterior portion of the swim bladder.As a consequence, the opening anterior to first pleural rib, this and the second pleural rib as well as the obliquus inferioris muscle are not visible.
Obliquus superioris muscle greatly developed, reducing muscular hiatus of pseudotympanum to small slit and completely covering cavity anterior to first pleural rib, this, second pleural rib and obliquus inferioris muscle (Fig. 5c).
Color in alcohol.Body color light brown to pale yellow, head and trunk profusely covered with dark chromatophores.Chromatophores on head mostly concentrated dorsally, as well as snout, anterior border of lower jaw, maxilla, circumorbital bones and opercle.Dark chromatophores especially abundant along free edge of scales in both dorsal and anterior ventral parts of trunk.Dark irregularly shaped blotch at humeral region, slightly longer than deep.Dark lateral stripe extending from posterior part of humeral dark blotch to caudal peduncle joining oblong dark blotch at caudal-fin base, dark color extending to bases of middle caudal-fin rays.Lateral body stripe wider from dorsal-fin origin to adipose-fin origin, narrower from dorsal-fin origin anteriorly to humeral blotch and on caudal peduncle.Mental area of lower jaw with diffuse dark blotch.Dorsal-fin origin with black spot extending to base of first unbranched dorsal-fin ray.Fins pale with scattered dark chromatophores more abundant on dorsal and caudal fins.
Distribution.Acestrocephalus maculosus is known only from tributaries of the upper Tocantins (Fig. 8).
Etymology.The species name maculosus, adjective, comes from Latin meaning spotted.It is in reference to the dark spots and stripes on the body of this species.

Acestrocephalus stigmatus, new species
Fig. 11 Acestrocephalus sardina; Menezes, 1976: 39 (in part, only specimen from rio das Mortes, MZUSP 10422).  . maculosus (25-27).The muscular hiatus of the pseudotympanum in A. stigmatus (Fig. 5 d) is narrower than that of A. boehlkei, but much more developed than that of A. maculosus (Fig. 5c).Description.Morphometrics of the holotype and additional specimens presented in Table 11.Meristic and morphometric data based on two lots from two widely separate localities, however no statistical differences were found between samples.Body moderately small (SL=78.5-98mm).Body form, dorsal and ventral body profiles, shape of snout and mouth and extension of maxilla as in A. sardina.Dorsal-fin rays ii,9 in all specimens, n=6, including holotype.Posterior most ray unbranched, n=6.Adipose fin present.Anal-fin rays iv,30 (unbranched rays iv, n=6, branched rays mean=30, range 29-31, n=6, posterior ray split to its base and counted as 1).Moderately developed anterior anal-fin lobe including anterior unbranched rays and first 7-8 branched rays.Two sexually mature males (MNRJ 17610) with bilateral hooks on posterior anterior branched rays.One specimen (93 mm SL) has 10 hooks on second branched ray, 9 on third, 7 on fourth and 2 on fifth.Pectoral-fin rays i,13 (anterior unbranched ray i, n=6, branched rays mean=13.3,range 13-14, n=6).Posterior tips of longest pectoral-fin rays reaching to about one third of pelvic fin length.Pelvic-fin rays i,7, n=6.No hooks on pelvic-fin rays of sexually mature males.Distal tips of longest pelvic-fin rays reaching posterior border of anus.Principal caudal-fin ray count 10/9, n=6.
Muscular hiatus of pseudotympanum longer than deep, but arrangement of associated muscles without major modifications (Fig. 5d).First and second pleural ribs slightly less exposed than in A. sardina (Compare figs.5d and 5a) and obliquus inferioris muscle visible.
Color in alcohol.Body pale yellow darker dorsally especially on head and snout.Premaxilla and anterior maxilla densely pigmented with dark chromatophores.Dark chromatophores also concentrated along lower jaw upper edge extending onto bases of anterior canines.Free edges of scales on dorsolateral trunk bordered with row of dark chromatophores.Dark blotch at humeral region irregularly shaped, deeper than long.Lateral body stripe represented by thin layer of dark chromatophores, unconspicuous in some specimens sometimes obscured by guanine pigment.Lateral body stripe extending from behind dorsal opercle across humeral dark blotch to caudal peduncle.Oblong dark blotch encompassing posterior caudal peduncle and caudal-fin base, extending to bases of middle caudal-fin rays.Mental area of lower jaw with diffuse dark patch of dark chromatophores.Dorsal-fin origin with black spot extending onto base of first unbranched ray.All fins pale with very few chromatophores except dorsal and pectoral fins where dark chromatophores are mostly concentrated along proximal half of first unbranched ray (pectoral) or along entire length of first two unbranched rays (dorsal).
Distribution.Specimens of this species are known from the rio Tocantins, rio das Mortes, a tributary of rio Araguaia (Fig. 8) and from the rio Xingu.It is sympatric with A. maculosus and A. acutus in the upper Tocantins basin.
Remarks.The two specimens from the rio Xingu drainage, not included in the type series did not show significant differences with respect to the type series, but additional specimens are needed for a more thorough evaluation.
Etymology.The species name stigmatus, adjective, is derived from the Greek word "stigma" meaning mark, spot, in reference to the dark spot found at the humeral region of this species.Diagnosis.Acestrocephalus nigrifasciatus has the same color pattern (Fig. 12) and eye size (Fig. 4) of A. sardina (Fig. 3), distinct from all other species of Acestrocephalus. A. nigrifasciatus, however, has more scale rows above (13-14 vs 10-12) and below (12-13 vs 10-12) lateral line.The muscular hiatus of the pseudotympanum in A. nigrifasciatus (Fig. 5e) is longer and narrower than that of A. sardina (Fig. 5a) and the fibers of the obliquus inferioris muscle are not visible as in the latter species.

Acestrocephalus nigrifasciatus, new species
Description.Morphometrics of holotype and additional specimens presented in Table 12.Body moderately large (SL=64.0-93.0mm).Body form, dorsal and ventral body profiles, shape of snout and mouth and extension of maxilla as in A. sardina.Dorsal fin rays ii,9 in all specimens, n=12, including holotype.Posterior most ray unbranched, n=12.Adipose fin present.Anal-fin rays iv,28 (iv or v, usually iv unbranched, branched rays mean=29, range 27-31, n=12, posterior ray split to its base and counted as 1).Moderately developed anal-fin lobe including anterior unbranched rays and first 6-7 branched rays.Anal fin of six sexually mature males with bilateral hooks on posterior anterior branched rays.One specimen (MCP 30420, 79 mm SL) has the first 15 branched rays bearing hooks.Number of hooks per ray decreasing from anterior most to posterior most rays (8 hooks on first, 17 on second, 7 on seventh, 3 on 13th and 1 on 15th).Pectoral-fin rays i,15 (anterior unbranched rays i, n=12, branched rays mean 14.6, range 14-15, n=12).Posterior tips of longest pectoral-fin rays reach-ing slightly beyond pelvic-fin origin.Pelvic-fin rays i,7, n=12.No hooks on pelvic-fin rays of sexually mature males.Distal tips of longest pelvic-fin rays reaching anterior border of anus.Principal caudal-fin ray count 10/9, n=12.
Muscular hiatus of pseudotympanum (Fig. 5f) reduced, much longer than deep, exposed first pleural rib scarcely visible, second pleural rib slightly more visible, but obliquus inferioris muscle fibers entirely covered by ventral fibers of obliquus superioris.
Color in alcohol.Similar to that of A. sardina, but dark lateral body stripe broader along entire length (compare Figs. 3 and 12) darker anteriorly; diffuse dark blotch on mental area of lower jaw less conspicuous.
Distribution.Acestrocephalus nigrifasciatus is known from the rios Arinos and Juruena, and rio Tapajós basin (Fig. 8) in the state of Mato Grosso.
Etymology.The first part of the species name niger is Latin for black and the second part fasciatus is also derived from the Latin word "fascia" meaning band or stripe, in reference to the dark lateral body stripe of this species.Both names are adjectives.

Acestrocephalus acutus, new species
Fig. 13 Diagnosis.Description.Morphometrics of holotype and additional specimens presented in Table 13.Meristic and morphometric data based on all lots in type series.No statistical differences found among population samples studied.Body relatively large (SL=50-103.5 mm).Body form, dorsal and ventral body profiles, shape of snout and mouth and extension of maxilla as in A. sardina.Dorsal-fin rays ii,9 in all specimens, n=47, including holotype.Posterior most ray unbranched, n=47.Adipose fin present.Anal-fin rays iv,31 (iv or v, usually iv unbranched, branched rays mean=32, range 29-35, n=47, posterior ray split to its base and counted as 1).Modestly developed anterior anal-fin lobe including anterior unbranched rays and first 8-9 branched rays.Three sexually mature males with bilateral hooks on posterior anterior branched rays.One sexually mature male (MCP 15855, 85 mm SL) has hooks on first 14 branched anal-fin rays, number of hooks per ray increasing from first (5) to sixth ( 14) then decreasing from seventh (12) through tenth (5), 13th (3) and 14th (1).Pectoral-fin rays i,12 (anterior unbranched ray i, n=47, branched rays mean 12.7, range 11-14, n=47).Posterior tips of longest pectoral-fin rays reaching to about one third of pelvic-fin length.Pelvic-fin rays i,7, n=47.No pelvic-fin ray hooks on sexually mature males.Distal tips of longest pelvic-fin rays reaching anterior border of anus.Principal caudal-fin ray count 10/9, n=47.
Branchiostegal rays 4; 3 originating on anterior and one on posterior ceratohyal.
Muscular hiatus of pseudotympanum reduced, constricted by dorsal and ventral fibers of obliquus superioris muscle (Fig. 5f).Cavity anterior to first pleural rib, this as well as obliquus inferioris muscle not visible.Second pleural rib partially exposed.
Color in alcohol.Body pale yellow slightly darker dorsally due to presence of dark chromatophores along free edge of scales, lighter ventrally.Dorsal part of head and snout more densely pigmented with dark chromatophores.Lateral head and opercular bones with scattered dark chromatophores.Anterior upper edge of lower jaw also heavily pigmented with dark chromatophores.Dark lateral body stripe extending from behind upper opercle to caudal peduncle, wider from below dorsal-fin origin to origin of adipose fin, narrower from below dorsal-fin origin anteriorly and on caudal peduncle.Oblong dark blotch on anterior caudal base joining dark lateral stripe, extending posteriorly to bases of middle caudal-fin rays.Mental area of lower jaw with conspicuous dark blotch in some specimens.Black spot at origin of dorsal fin very conspicuous, anteriorly extending over two or three predorsal scales, laterally over three or four scales of first horizontal scale row.All fins pale, first unbranched dorsal and pectoral fin rays with scattered dark chromatophores.
Distribution.Acestrocephalus acutus is known from the rio Tocantins and rio Tapajós drainages.It is sympatric with A. maculosus and A. stigmatus in the upper rio Tocantins (Fig. 8).
Remarks.The specimens from rio Teles Pires in the rio Tapajós drainage, not included as type material, are morphologically undistiguinshable from the holotype and paratypes, but examination of more specimens is required for better comparison.
Etymology.The species name acutus, adjective, from Latin meaning sharp is in reference to the pointed nature of the snout of this species.18315, 28716, 28717, 21136 (paratypes), MCP15855 (paratype).
Shape of muscular hiatus of pseudotympanum, arrangement of associated muscles and remaining structures (Fig. 5g) as in A. sardina.
Color in alcohol.Body pale yellow with no distinct dark spots or blotches.Scatered dark chromatophores present on dorsal head and snout and along free edge of scales on dorsal trunk being slightly darker than remaining parts of body.Dark chromatophores on caudal-fin base scattered, visible only using stereomicroscope, not forming a blotch.Dark spot at dorsalfin origin weak and dark color on lower jaw mental area sometimes diffuse.Lateral body stripe largely consisting of guanine pigment extending from posterior section dorsal opercle to caudal-fin base.All fins pale.
Distribution.Known only from rio Madeira and rio Machado, a tributary of rio Madeira (Fig. 8).
Etymology.The species name pallidus, adjective, from the Latin for pale is in reference to the overall body color of this species.

Fig. 6 .
Fig.6.Scatter plot of number of maxillary teeth on standard length for specimens of Acestrocephalus sardina, A. boehlkei.Acestrocephalus nigrifasciatus, A. pallidus, and A. acutus.The figure is not intended to show species differences therefore regression lines were not drawn and regression analyses were not performed.The primary purpose is to express the tendencies of the number of teeth on the maxilla to increase according to an increase in standard length.For the species represented by large number of specimens the data points are widely scattered.

Fig. 7 .
Fig. 7. Scatter plot of number of posterior dentary teeth for specimens of Acestrocephalus sardina,A.boehlkei, A.  pallidus, A. nigrifasciatus, and A.  acutus.The figure is not intended to show species differences therefore regression lines are not drawn and regression analyses were not performed.The primary purpose is to express the tendencies of the number of teeth on the posterior row on the dentary to increase according to an increase in standard length.For the species represented by large number of specimens the data points are widely scattered.
to upper opercle and on caudal peduncle, dark color sometimes obscured by guanine.Lateral stripe starts from behind upper opercle extending posteriorly to caudal-fin base where connecting to oblong dark blotch posteriorly extending to middle caudal-fin rays.Irregularly shaped, vertically elongated dark blotch at humeral region over anterior lateral body stripe, vertically above anterior pectoral fin.Mental area of lower jaw with diffuse dark blotch, sometimes unconspicuous or even absent.Small black dot at origin of dorsal-fin origin, extending to first unbranched dorsal-fin ray base.Dorsal, anal, and caudal fin dusky with scattered dark chromatophores along fin rays and membranes.Pectoral and pelvic fins pale with fewer dark chromatophores than other fins.

Table 1 .
Frequency distribution of scales above lateral line in the species of Acestrocephalus.
differences found among population samples studied, in spite of wide distribution of species.Body elongate, relatively large (SL= 31.4-90mm), compressed and not very deep; greatest body depth between occiput and dorsal-fin origin near middle of pelvic-fin length.Dorsal body profile nearly straight, but inclined ventrally from snout tip to base of supraoccipital spine, slightly convex between that point and dorsal-fin origin, nearly straight along dorsal-fin base, and straight or nearly so from base of dorsal-fin termination to caudal peduncle.Ventral body profile convex from tip of lower jaw to anal-fin

Table 2 .
Frequency distribution of scales below lateral line in the species of Acestrocephalus.
Fig. 3. Acestrocephalus sardina, MZUSP 81209, 93 mm SL, immature female, rio Tiquié, rio Negro drainage.Fig.4. Scatter plot of eye diameter on head length for specimens of Acestrocephalus sardina, A. boehlkei, A. pallidus, A. nigrifasciatus and A. acutus.Acestrocephalus sardina and A. nigrifiasciatus have larger eye diameters than the other species suggesting the recognition of two species groups with respect to this character (see diagnoses of A. sardina and A. nigrifasciatus).

Table 4 .
Frequency distribution of branched anal-fin rays in the species of Acestrocephalus.

Table 5 .
Frequency distribution of scales around caudal peduncle in the species of Acestrocephalus.

Table 6 .
Frequency distribution of lateral line scales in the species of Acestrocephalus.

Table 7 .
Frequency distribution of premaxillary teeth in the species of Acestrocephalus.

Table 9 .
Frequency distribution of total number of gill rakers in the species of Acestrocephalus.

Table 12 .
Morphometrics of Acestrocephalus nigrifasciatus.Standard length is expressed in mm; measurements through head length are percentages of standard length; last four entries are percentages of head length.Specimens are from: MCP 30420 (holotype), 39930, MZUSP 62847, 62845 (paratypes).
Among the species of Acestrocephalus without dark humeral blotch, A. acutus can be distinguished from A.
dark blotch vs absence of dark blotch at caudal base.The shape of the muscular hiatus of the pseudotympanum and the muscular arrangement around it in A. acutus and A. pallidus are also different (compare figs.5f and 5g).Acestrocephalus acutus differs from A. anomalus by having more teeth in the posterior row on dentary (28-36 vs 23-28) and fewer teeth in the external tooth row on premaxilla (6-8 vs 8-11).

Table 13 .
Morphometrics of Acestrocephalus acutus.Standard length is expressed in mm; measurements through head length are percentages of standard length; last four entries are percentages of head length.Specimens are from MZUSP 31650 (holotype), 82105, 40830, 40691 (paratypes), MNRJ 17612,