Taxonomic revision of the genus Atopomesus Myers, 1927 (Characiformes: Characidae), with comments on its phylogenetic relationships

The monotypic genus Atopomesus , a poorly known member of the family Characidae, is re-described. Atopomesus pachyodus is endemic to the Rio Negro basin, where it feeds on aquatic insect larvae, and seems to occur mostly in riverine sand banks and beaches. A. pachyodus was originally described as a member of the Characidae subfamily Cheirodontinae, but has been considered as an Characidae incertae sedis based on relatively superficial studies of its dentition and external morphology. However, according to the morphological analyses presented herein, a close relationship between A. pachyodus and the members the subfamily Aphyoditeinae is hypothesized. The inclusion of A. pachyodus in the Aphyoditeinae is based on the shared presence of a long posteriorly directed spiniform process on the posteroventral margin of the orbitosphenoid, the elongation of the transverse process of the neural arch of the third vertebra, and the elongation of the posterior process of the parasphenoid.


Introduction
In July 1927, GeorGe S. MyerS proposed the new Characidae genus Atopomesus.This new genus was described based on specimens deposited in the Indiana University, collected by Carl TerneTz during an expedition to northern Brazil.The type material of Atopomesus pachyodus MyerS 1927 was collected in the Rio Negro drainage, Amazon basin, nearby Cucuí Municipality, Amazonas State, Brazil, on the Colombian border.Atopomesus is still currently considered as a monotypic genus diagnosed by MyerS (1927) by being "unique in its massive retrorse dentition".
MyerS (1927) considered A. pachyodus as a member of the Cheirodontinae, a subfamily of the Characidae established by eiGenMann (1909,1914,1915,1917), and Géry (1960) reported A. pachyodus as an "Aphyocharacidi" member, an artificial group belonging to the subfamily Cheirodontinae.Later on, in its compilation of the Characiformes of the world, Géry (1977) considered this species as a member of the Aphyoditeina, an artificial group belonging to the Grundulini, according to him, a tribe of small fish species belonging to the subfamily Cheirodontinae, primarily diagnosed by their dentition.In a revision of the subfamily Cheirodontinae, Malabarba (1998) reported that A. pachyodus lacks one of the diagnostic synapomorphies of the Cheirodontinae, the possession of pedunculated teeth that are largely expanded and compressed distally, and, consequently, considered A. pachyodus as incertae sedis within the Characidae.Recently, in a more comprehensive morphological phylo geny of the Characidae, Mirande (2009Mirande ( , 2010) ) reached the same conclusions reported by Malabarba (1998Malabarba ( , 2003)), maintaining the genus Atopomesus as incertae se dis in Characidae.
Although the cited studies have addressed the relationships of A. pachyodus with other characids, their hypothesis were based only on a superficial analysis of its dentition and external morphology.Thus, the present study aims to redescribe A. pachyodus, a poorly known species, and discuss its phylogenetic relationships within the Characidae, based on a wider comparative osteological analysis.Fink & WeiTzMan (1974), with the addition of the following measurements: snout to anus -measured from the snout tip to the anterior limit of the anus; dorsal-fin base length -measured from the dorsal-fin origin to the last dorsal-fin ray; analfin base length -measured from the anal-fin origin to the last anal-fin ray; dorsal-fin origin to adipose-fin origin; head depth -measured at vertical through the posterior end of supraoccipital spine; mouth length -measured as the internal distance between the anterodorsal region of the maxillary bones.Measurements were taken point to point with a digital caliper.The standard length (SL) is expressed in mm and all other measurements are expressed as a percentage of SL, except subunits of head, that are expressed as percentage of the head length (HL).Counts are followed by their occurrence in parentheses; asterisks indicate the counts of the holotype (reported values for the holotype were taken from the original description, high definition photographs and a radiograph graciously provided by the California Academy of Sciences Ichthyology Section).

Measurements and counts follow
Six specimens (MZUSP 29613, 17.3, 20.4, 22.7, 25.3, 28.3, and 32.4 mm SL) of A. pachyodus were cleared and stained (CS) for cartilage and bone using the method of Taylor & van dyke (1985).The analyses of the hyobranchial apparatus was based only in CS specimens.Vertebrae and supraneural bones counts were made in CS specimens and also in radiographs of all alcoholpreserved individuals.Total vertebral counts include four centra of the Weberian Apparatus and a single element for the terminal centrum.Osteological terminology follows WeiTzMan (1962) with modifications adopted by zanaTa & vari (2005).Myological observations were made in a specimen (MZUSP 29613, 24.8   Diagnosis.As for the genus. Description.See Fig. 1 for general appearance.Morpho metric data of holotype and 51 non-type specimens is given in Table 1.Body laterally compressed.Greatest body depth at dorsal-fin origin.Dorsal profile of body convex from tip of the upper jaw to the anterior edge of nostril; slightly convex from snout to dorsal-fin origin;    Teeth whitish, translucent to opaque, with brown distal portions.Premaxillary teeth in one row (Fig. 3), with seven aligned unicuspid teeth, gradually decreasing in size posteriorly; first three to four anterior premaxillary teeth incisiform, extremely truncate, anteroposteriorly com pressed with rounded crest, posteromedially directed; last two or three teeth conical.Maxilla with 2(4), 3(4), 4(1), 5(27), 6*(9), 7(6) or 8(1) long and slender conical teeth (Fig. 3); toothed region straight, restricted to anterior region of maxilla.Dentary trapezoid, with 9(2), 10(5), 11(20), 12(22) or 13(2) unicuspid teeth (Fig. 3); first medial teeth incisiform, largest and robust, gradually decreasing posteriorly to the posterior most minute and slender conical teeth.

Coloration in alcohol.
Overall ground coloration yellowish (Fig. 1).Dorsal and anterior head portion with sparse dark chromatophores.Iris silver, with sparse dark chromatophores, more concentrated at upper region.Few dark-brown chromatophores sparsely distributed over body, more concentrated in mid-lateral region of body.All fins hyaline.
Distribution.All known samples of Atopomesus pach yodus were collected throughout the Rio Negro drainage, Amazon basin (Fig. 6).GouldinG et al. (1988) cited this species as endemic to an area that centers on the Rio Negro basin.
Ecological notes.The analysis of the stomach contents of two of the six CS individuals revealed a diet based mainly on aquatic insect larvae (mostly Chironomidae).
Of the six examined individuals, four had the stomach empty and only two presented some food items.This species is known only from blackwater rivers with high acidity and low nutrient concentrations, occurring in marginal areas over sand banks, where it was frequently captured.GouldinG et al. (1988) reported A. pachyodus feeding on Diptera larvae and plant remains.

Discussion
Atopomesus pachyodus Myers, 1927 is a small size species of the family Characidae, with a distribution restricted to the acidic blackwaters of the Rio Negro drainage, Amazon basin.It feeds on aquatic insect larvae, probably in sand banks and beaches, where this species was frequently found, according to GouldinG et al. (1988).Its mouth, subterminal and ventrally oriented, also suggests a benthic feeding behavior (WineMiller et al., 1995).
The shape of the anteriormost premaxillary and dentary teeth, resembling the teeth of some Anostomidae species (see birindelli & briTSki, 2009, for an example), could be correlated with the invertivory diet of A. pachyodus, as reported by vari (1983) for the family Anostomidae.
In fact, the odd dentition of A. pachyodus readily distinguishes it from the remaining Characidae representatives, as also its lack of gill rakers on the trailing margins of the third and fourth ceratobranchials, a characteristic hitherto reported only for the genus Boulengerella eiGenMann 1903(vari, 1995) and Acestrorhynchus eiGenMann & kennedy 1903(Toledo-piza, 2007).
Of the 51 non-type individuals analyzed, seven presented rayless pectoral fin.This paedomorphic feature was observed only in small specimens .Indeed, only two (18.6 and 19.9 mm SL) of the nine analyzed specimens smaller than 20.5 mm SL presented pectoral fin with rays.In the family Characidae, the retention of the larval rayless pectoral fin was reported for Hyphessobrycon catableptus (durbin 1909) and for species of the subfamily Characinae (luCena, 1998;luCena & MenezeS, 2003).Despite the presence of this paedomorphic feature, possibly related to ontogenetic truncations, A. pachyodus is not considered a miniaturized species, since presents none of the characteristics of a miniaturization process sensu MyerS (1958) and WeiTzMan & vary (1988).
Since its description, A. pachyodus was considered closely related to the subfamilies Cheirodontinae and Aphyoditeinae, but these hypotheses were refuted by Malabarba (1998) and Mirande (2010), based mainly on teeth morphology.Mirande (2010) reported that the strong teeth of A. pachyodus are quite different from the slender and small teeth of the Aphyoditeinae members.In fact, the teeth morphology of A. pachyodus is very different from any teeth morphology reported to these subfamilies.But, in the present analysis, other characters shared by A. pachyodus and members of the subfamily Aphyoditeinae (sensu Mirande, 2010) were found.A. pachyodus shares with Parecbasis cyclo lepis eiGenMann, 1914 and species of the genus Micro schemo brycon eiGenMann, 1915 an elongation of the posterior process of the parasphenoid, which extends posteriorly to a vertical line through the posterior margin of the epiotic in lateral view.This condition was also observed in Macropsobrycon uruguaianae eiGenMann, 1915, a Cheirodontinae member, and Thrissobrycon pectinifer bohlke, 1953, considered as an Aphyoditeinae by Gery (1977), but as an incertae sedis Characidae by Mirande (2010).In the remaining Characidae representatives here analyzed, when present, the posterior process of the parasphenoid is short, not reaching the vertical line through the posterior margin of the epiotic in lateral view.
The length of the transverse process of the third neural arch was one of the characters successfully used by zanaTa & vari (2005: 114) to establish phylogenetic relationships among various Alestidae groups.In A. pachyodus, the transverse process of the neural arch of the third vertebra extends over the scaphium, a condition also observed in all the genera of the subfamily Aphyoditeinae.This condition was also observed in T. pectinifer, Macropsobrycon xinguensis Gery, 1973, a Cheirodontinae member, and species of the genus Brit tanichthys Géry, 1965, which were considered as Aphyoditeinae representatives by Gery (1977), but treated as incertae sedis Characidae by Mirande (2010).In the remaining Characidae representatives analyzed by us, the process of the neural arch of the third vertebra is proportionally smaller, with the anterior tip falling short to the posteroventral margin of the scaphium.
Another morphological feature shared by A. pachyo dus and all the other members of the subfamily Aphyoditeinae is the presence of a long, posteriorly directed spiniform process, in the posteroventral margin of the orbitosphenoid.The joint possession of a long spiniform process as the one mentioned before was considered by MaTTox & Toledo-piza (2012: 43) as a synapomor-phy of the Heterocharacinae, a subfamily of the family Acestrorhynchidae, and was also observed by them in Cheirodon Girad, 1855 and Odontostilbe Cope, 1870, both members of the subfamily Cheirodontinae.In the present study, a long posteriorly directed spiniform orbitosphenoid process was also observed in T. pectinifer, Brittanichthys axelrodi Géry, 1965, M. uruguaianae and M. xinguensis.All mentioned shared characters of the present morphological analysis corroborate a close relationship between A. pachyodus and the other members of the subfamily Aphyoditeinae, as already proposed by Géry (1977).Consequently, the inclusion of A. pachyodus in the subfamily Aphyoditeinae (sensu Mirande, 2010) is recommended.
inclined along dorsal-fin base; straight and posteriorventrally inclined from posterior terminus of dorsal-fin base to adipose-fin origin; slightly concave along caudal peduncle.Ventral profile of body moderately convex from tip of lower jaw to analfin origin; straight and posterodorsally inclined along anal-fin base and concave along caudal peduncle.Infraorbitals one to six present; sixth smallest and third biggest of infraorbital series.Frontals in contact anteriorly to frontal fontanel.Posteroventral margin of orbitosphenoid with long posteriorly directed spiniform process (Fig.2).Posterior process of parasphenoid long, extending posteriorly to vertical line through posterior margin of epiotic in lateral view (Fig.2).Mouth subterminal, ventrally oriented.Upper jaw slightly longer than, and overhanging, lower jaw.An terior portion of snout fleshy, totally covering premaxillary teeth; papillae present along whole ventral margin of upper lip.Lower lip fleshy anteriorly, with papillae present along entire dorsal margin.

Fig. 5 .
Fig. 5. Hyobranchial apparatus of Atopomesus pachyodus, MZUSP 29613, 32.4 mm SL. a-Branchial arches, dorsal view; b-Anterior region of the first ceratobranchial, right side, image magnified four times in relation to the others; c-Upper elements of right side, ventral view; d-Hyoid arch and branchiostegals, right side, lateral view; e-Urohyal, lateral view.

Table 1 .
Morphometric data of holotype and 51 non-type examined specimens of Atopomesus pachyodus.SD = Standard deviation.