Revision of the genus Macropsobrycon Eigenmann, 1915 (Characidae: Cheirodontinae: Compsurini)

Neotropical Ichthyology, 9(2):299-312, 2011 Copyright © 2011 Sociedade Brasileira de Ictiologia 299 Revision of the genus Macropsobrycon Eigenmann, 1915 (Characidae: Cheirodontinae: Compsurini) Fernando C. Jerep1 and Luiz R. Malabarba2 The cheirodontine genus Macropsobrycon is redefined and considered monotypic. The type species, M. uruguayanae, is redescribed based on samples from the entire known geographical distribution of the species. Sexually dimorphic characters of M. uruguayanae are further described and the relationships of the species with the remaining Compsurini are discussed. O gênero Macropsobrycon de Cheirodontinae é redefinido e considerado monotípico. A espécie-tipo, M. uruguayanae, é redescrita com base em amostras de toda a distribuição geográfica conhecida da espécie. Caracteres de dimorfismo sexual secundário de M. uruguayanae são descritos e as relações da espécie com outros Compsurini são discutidas. Key words: Neotropical, Sexual dimorphism, Systematics, Taxonomy. 1Pontifícia Universidade Católica do Rio Grande do Sul, Laboratório de Sistemática de Vertebrados, Setor de Ictiologia, Museu de Ciências e Tecnologia. Av. Ipiranga 6681, 90619-900 Porto Alegre, RS, Brazil. [email protected] 2Universidade Federal do Rio Grande do Sul, Laboratório de Ictiologia, Departamento de Zoologia e Programa de Pós-Graduação em Biologia Animal, IB, UFRGS, Porto Alegre, RS, Brazil. [email protected] Introduction Historical review. Eigenmann (1915) described Macropsobrycon, containing a single species, M. uruguayanae, in the subfamily Cheirodontinae. In the Eigenmann’s early concept of this subfamily (now corresponding to Aphyocharacinae + Aphyoditeinae + Cheirodontinae + Paragoniatinae + some Tetragonopterinae characid genera sensu Mirande, 2010), the Cheirodontinae diagnosis was presented in a topic “Generalized type of the subfamily” in which he referred the conspicuous small body size, the single tooth series, and the teeth with “lateral notches” [cusps] to these characids. Actually, the multicuspid teeth and the presence of one tooth series in the premaxilla and dentary have been the main characters used in subsequent years to diagnose the Cheirodontinae among characids (Eigenmann, 1915). In the end of the description of M. uruguayanae, notably Eigenmann considered that its conical teeth would “place it outside the subfamily”, but also stated that its relationships [to Cheirodontinae] are “unmistakable”. He further considered Macropsobrycon related to Parecbasis Eigenmann, Aphyodite Eigenmann, Leptobrycon Eigenmann, and possibly to some Megalamphodus Eigenmann, all genera currently removed from Cheirodontinae (Malabarba, 1998), differing from Parecbasis by having fainter dentition and incomplete lateral line, from Aphyodite by having the caudal fin naked and well developed pseudotympanum, and from Leptobrycon by having the anal fin longer. Although consistent, Eigenmann’s description was incomplete in some aspects as the color pattern of the dorsal and anal fins, and neither mentioned the complex caudal-fin structures present on mature males. He still affirmed that scales were “apparently absent from caudal” causing uncertainty about the presence of them on the caudal fin. Géry (1960), reviewing the morphological affinities of some Cheirodontinae, placed Macropsobrycon in an “intermediate section” of the subfamily, jointly with 13 genera characterized by the presence of slender conical or tricuspid teeth, fourth infraorbital not developed, and presence of an adipose fin. In this work, Géry split this “section” in three smaller groups, situating Macropsobrycon with some other “degenetrate or specialized species” of the genera Parecbasis, Leptobrycon, Aphyodite, and Thrissobrycon Böhlke, based on the shared presence of an elongated body, conical teeth reduced in number and size (at least on the maxilla), and a thicker and blade-like maxilla. Géry still briefly distinguished Macropsobrycon from these other genera by the presence of a superior mouth, postorbitals (infraorbitals 4 and 5) not developed, fontanels short, lateral line incomplete, caudal naked, pseudotympanum Revision of the genus Macropsobrycon 300 and some interhaemals (caudal-fin procurrent rays) present; and described the shape of the Macropsobrycon premaxilla and mesethmoid (dermethmoid in that paper) bones as identical to those of the representatives of the “tricuspid cheirodontines” group, where the premaxilla is asymmetric with a high ascending process and low horizontal process, and the mesethmoid pointed. Géry (1965) also included Macropsobrycon in his phenetic diagram with the representatives of the sub-tribe Aphyoditeini, suggesting that the genus could be the “junction” between the subgroup formed by Prodontocharax Eigenmann & Pearson, Hyphessobrycon stigmatias Fowler, and Microschemobrycon Eigenmann; and the subgroups formed by Aphyodite, Brittanichthys, Leptobrycon, and Parecbasis Géry. Later on, Géry (1972) proposed an updated phenetic diagram for the Aphyoditeini, where Macropsobrycon was grouped with Aphyodite representing a major group with Brittanichthys, Leptobrycon and Thrissobrycon. In this work, Géry also emphasized the differences between Aphyocharax melanotus [sic] and Macropsobrycon uruguayanae, disagreeing with Eigenmann´s (1915) suggestion about a possible close relationship between these species, and proposed the new combination Microschemobrycon melanotus (Eigenmann). The genus remained monotypic up to Géry (1973) description of Macropsobrycon xinguensis in a revisionary study of the “Aphyoditeina” species from the Amazon basin. Géry (1973) quoted several differences between M. uruguayanae and M. xinguensis, like, for instance, the absence of a humeral hiatus in his new species, as well as the presence of partially scaled caudal-fin lobes, tricuspid teeth (vs. conical), dorsal-fin in front of mid-body, number of branched anal-fin rays (17-18 vs. 19-22 in M. uruguayanae), number of perforated lateral line scales (8 vs. 5-6 in M. uruguayanae), and a different caudal color pattern. The placement of the new species in Macropsobrycon was considered by Géry a “conservative solution”, taken “provisorily”, once both species have similar characters like feeble teeth, toothless maxilla and incomplete pored lateral line. Géry (1977) arranged the Cheirodontinae in two major groups: the Cheirodontinae sensu stricto, and the “allied genera”. The group represented by the “allied genera” was split in three tribes: Grundulini, Henochilini, and Probolodini. His former Aphyoditeina, including the Macropsobrycon species, was now treated as the “Aphyoditegroup”, inside the tribe Grundulini together with the Grundulusgroup and the Pristella-group. Reprodutive biology. Little was known about the biology and reproductive characters of Macropsobrycon up to Burns et al. (1997) analysed histological sections of Macropsobrycon uruguayanae, as well as of some other cheirodontines, and found M. uruguayanae to be inseminating, presenting elongated spermatozoa. Burns et al. (1998) carried out transmission electron microscopy on some inseminating characid species belonging to the subfamilies Aphyocharacinae, Glandulocaudinae (now part of Stevardiinae sensu Mirande, 2010), and Cheirodontinae, represented by M. uruguayanae. The same authors described briefly the spermatozoa ultrastructure of M. uruguayanae, and through new ultrastructural characters supported Malabarba’s (1998) hypothesis of independent development of insemination in the Glandulocaudinae and in the inseminating Cheirodontinae (now Compsurini). Later on, the sperm ultrastructure of Macropsobrycon uruguayanae was described in detail by Oliveira et al. (2008), and its reproductive biology and gill gland development by Azevedo et al. (2010), making M. uruguayanae the best known cheirodontine in terms of reproductive aspects to date. Systematics and relationships. In a phylogenetic study, Malabarba (1998) defined Cheirodontinae sensu stricto and demonstrated cladistically that in the former systematic arrangement of the subfamily the species were artificially grouped, and several genera and species were then removed from that taxon. Macropsobrycon uruguayanae, sharing the main synapomorphies with the other members of the subfamily, was kept in the more restricted group of the Cheirodontinae as part of a new inseminating tribe, Compsurini. On the other hand, M. xinguensis was considered species incertae sedis in Characidae, since it does not present all of the synapomorphies of the subfamily (Malabarba, 1998; Reis et al., 2003). The genus Macropsobrycon was further diagnosed by Malabarba (1998) based on four characters: (1) presence of a large space bearing hypertrophied tissue between the twelfth and thirteenth caudal-fin rays; (2) small and flexible spines present along the proximal portion of the lower lobe principal caudal-fin rays; (3) jaw teeth elongated and conical or tricuspid [modified herein, see diagnosis of Macropsobrycon]; and (4) the dorsal-fin strongly blackpigmented along the mid-length of the second unbranched and first 5 branched rays [modified herein, see diagnosis of Macropsobrycon], and weakly pigmented along their distal portion. Secondary sexual characters of M. uruguayanae have been briefly discussed by Malabarba & Weitzman (1999, 2000) and Malabarba et al. (2004) in comparison to Acinocheirodon melanogramma Malabarba & Weitzman, Kolpotocheirodon theloura Malabarba & Weitzman, and Kolpotocheirodon figueiredoi Malabarba, Lima & Weitzman, respectively. Mirande (2009, 2010) proposed a hypothesis of relationships among characid representatives and a new classification. Although the genus Macropsobrycon was kept in Cheirodontinae, none of its species were included in his analysis. Most recently, Javonillo et al. (2010) developed a phylogenetic study within the Characidae based on molecular sequenced data of a number of genera and species. The results showed M. uruguayanae as part of the Cheirodontinae in accordance with previous phylogenetic analysis based on morphological data, and M. xinguensis as sister group of Hyphessobrycon herbertaxelrodi Géry, inside a large clade including several incertae sedis Characidae genera distant from the Cheirodontinae clade. F. C. Jerep & L. R. Malabarba 301 Despite of the works regarding the relationships and reproductive biology of Macropsobrycon uruguayanae, the range of its geographical distribution, its morphological variation, and the secondary sexual dimorphism on its caudal fin, were never assessed. Macropsobrycon uruguayanae was described based on six specimens collected by Haseman in the rio Uruguay basin [Uruguaiana and Cacequi, Rio Grande do Sul State], nevertheless the species has been collected in an wider geographical range, encompassing the rio Negro basin (a tributary on the lower part of rio Uruguay), and the laguna dos Patos system in Rio Grande do Sul, Brazil and Uruguay. It is often misidentified as the Heterocheirodon species, which present the same geographical distribution and similar body shape. Herein the genus Macropsobrycon is redefined, and its type species M. uruguayanae redescribed based on specimens from all its geographical range, which allowed the study of the morphological variation among populations from different drainages. The secondary sexual characters found in the caudal fin of mature males are detailed reviewed and firstly described. Material and Methods Counts and measurements were taken according to Fink & Weitzman (1974), whenever possible on the left side of the specimens. Measures were taken point-to-point straight-line with a 0.01 precision caliper. Head length is defined as the distance between the tip of the snout and the posterior margin of subopercle (Bührnheim & Malabarba, 2006). Total vertebrae number includes the four vertebrae of the Weberian apparatus, and the terminal “half centrum” (Malabarba & Weitzman, 1999). Gill rakers were counted on first branchial arch, and the gill raker located at the junction of the ceratobranchial and the epibranchial was referred to the epibranchial count. The counts of vertebrae, supraneurals, teeth, and gill rakers were based on radiographs and cleared and stained (c&s) specimens following Taylor & van Dyke (1985) protocol. Scanning Electron Microscope (SEM) images were obtained from the jaws. Drawings of the anal fin, caudal fin, and scales were prepared in a stereomicroscope with camera lucida. Sex identification of specimens was based on Gonçalves et al. (2005) and Bührnheim & Malabarba (2006), where males and females are recognized by dissection, or by the presence of sexual dimorphism, like anal and caudal-fin hooks, in specimens with similar size in the same sample. Specimens smaller than the smallest specimen showing sexual dimorphism in each lot are termed as unsexed, as well as specimens belonging to lots where sexual dimorphism is absent. The redescription and the statistical and histological analysis were based on specimens from museum collections listed in the material examined, which abbreviations are: ANSP - Academy of Natural Sciences of Philadelphia, Philadelphia, USA; CAS - California Academy of Sciences, San Francisco, USA; FMNH - Field Museum of Natural History, Chicago, USA; MCP - Museu de Ciências e Tecnologia da PUCRS, Porto Alegre, Brazil; MZUSP - Museu de Zoologia da Universidade de São Paulo, São Paulo, Brazil; UFRGS - Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil; and USNM - National Museum of Natural History, Smithsonian Institution, Washington D.C., USA. Other abbreviations are given along with the number of specimens: m - morphometric and meristic. Statistical analyses. Principal Component Analysis (PCA) was used to investigate morphometric variation among Macropsobrycon uruguayanae populations from three river basins: rio Uruguay basin, rio Negro basin (a tributary of the lower rio Uruguay), and laguna dos Patos basin. The program PAST version 2.04 (Hammer et al., 2001) was used for PCA analysis, the measures were logarithmically transformed. Considering that generally the first principal component mostly accounts for size variation (Strauss, 1985), a linear correlation was performed between the PC1 scores and the standard length values of the analysed specimens to test this hypothesis. A simple Multivariate Analysis of Variance (MANOVA) on the PC2 and PC3 scores, grouped by drainage, was also performed. The morphometric variable “dorsal-fin length” was not included on the PCA analysis once this data was missing for several specimens with the dorsal fin broken. Results Macropsobrycon Eigenmann, 1915 Macropsobrycon Eigenmann, 1915: 16 [key to Cheirodontinae species]; 48 [diagnosis; type species by original designation and monotypy: Macropsobrycon uruguayanae Eigenmann, 1915:]. -Géry, 1960: 2, 5, 9, 14 [placement in an unnamed group of degenerated of very specialized species with reduced conical teeth]. -Géry, 1965 [listed in a new subtribe of Cheirodontidi [sic], the Aphyoditeini]. -Géry, 1972: 15-18 [inclusion in Aphyoditeini]. -Géry, 1977: 594 [in key to Aphyoditeina genera and species]. -Malabarba, 1998: 218 [phylogenetic diagnosis and relationships to cheirodontine genera; included in the new tribe Compsurini]. -Malabarba & Weitzman, 1999: 424 [comparison with Acinocheirodon melanogramma]. -Malabarba et al., 2003: 217 [listed in Cheirodontinae]. -Mirande, 2009: 8, 11 [listed as Cheirodontinae]. -Javonillo et al., 2010: 505-507 [relationships within Characidae]. -Mirande, 2010: 528, 531532 [listed as Cheirodontinae]. Diagnosis. Distinguished from other genera of the Compsurini by the jaw teeth elongate and conical or rarely bicuspid. Macropsobrycon is further diagnosed from all characid species except the Cheirodontinae by the presence of cheirodontine synapomorphies (see discussion), and from all cheirodontine genera by the following autapomorphies: (1) a small and elongate black spot is present on the proximal half of the second unbranched dorsal-fin ray, corresponding Revision of the genus Macropsobrycon 302 to the distal half of the first unbranched dorsal-fin ray, sometimes extending over the first unbranched ray and corresponding region of the first branched ray [modified from Malabarba, 1998; originally stated as, black pigmentation along the mid-length of the second unbranched and first 5 branched rays]; (2) presence of a small black spot on the base of unbranched anal-fin rays; (3) Ppresence of a hypertrophied caudal-fin scale on ventral lobe of mature males; (4) 12th and 13th principal caudal-fin rays of males with robust retrorse hooks anteriorly directed at their branched portion; (5) 14th to 18th principal caudal-fin rays of males with feeble and flexible spinelets along their proximal length, and harder and anteriorly directed spinelets along its distal length; (6) 12th and 13th principal caudal-fin rays of males distant from each other, with the fin membrane between these contiguous rays bearing hypertrophied tissues. Macropsobrycon uruguayanae Eigenmann, 1915 Figs. 1-7 Macropsobrycon uruguayanae Eigenmann, 1915: 48 [original description; dentition figured; type locality Uruguayana (Uruguaiana) and Cacequi, Rio Grande do Sul, Brazil, rio Uruguay drainage]. -Henn, 1928: 59 [type material listed from Carnegie Museum]. -Ibarra & Stewart, 1987: 54 [type material listed from Field Museum of Natural History]. - Ringuelet, 1967 [listed as freshwater fish from Argentina]. -Géry, 1972: 15-18 [inclusion in Aphyoditeini]. -Géry, 1973: 108-109 [compared to Macropsobrycon xinguensis]. -Géry, 1977: 594 [in key to Aphyoditeina genera and species]. - Malabarba 1989: 136 [listed to laguna dos Patos drainage]. -Burns et al., 1997: 434 [listed as inseminating species]. - Malabarba, 1998: 218 [phylogenetic diagnosis of the genus and placement in Compsurini]. -Burns et al., 1998: 242 [short description of sperm ultrastructure]. -Malabarba & Weitzman, 1999: 416, 427 [comparison with Acinocheirodon melanogramma]. -Malabarba & Weitzman, 2000: 270, 279-280 [comparison with Kolpotocheirodon theloura]. -López et al., 2003: 31 [listed from Argentina]. -Malabarba et al., 2003: 217 [listed in Cheirodontinae]. -Quagio-Grassiotto et al., 2003: 41 [sperm morphology compared to other characiforms]. -Menni, 2004: 78 [listed from Argentina]. -Malabarba et al., 2004: 324-325 [comparison with Kolpotocheirodon figueiredoi]. -Malabarba, 2007: 37 [listed in Cheirodontinae]. -Pecio et al., 2007: 458 [sperm morphology compared along with other inseminating characids]. -Oliveira et al., 2008: 691697 [description of sperm ultrastructure and comparison to inseminating characids]. -Bührnheim et al., 2008: 671 [referred as bearing conical teeth]. -Miquelarena et al., 2008: 78 [listed from Argentina]. -Javonillo et al., 2010: 505-507 [relationships within Characidae]. -Azevedo et al., 2010: 87-96 [reproductive biology and gill gland development]. Diagnosis. Same as the genus. Description. Morphometrics given in Table 1. Body slightly elongate and compressed, deepest at dorsal-fin origin. Dorsal profile convex from snout tip to dorsal-fin origin, straight or slightly convex from that point to adipose-fin origin. Ventral profile convex from snout tip to pelvic-fin insertion, straight or somewhat concave up to anal-fin origin, straight to convex on anal-fin base. Caudal-peduncle profile slightly concave dorsally and ventrally. Snout short, mouth superior above horizontal line projected through pupil dorsal border and below horizontal line projected through orbit dorsal edge. Premaxilla slender with 3(1), 4(1), 5(11), 6(14) or 7(4) conical teeth, aligned and similar in size; ascending process wide and square shaped. Maxilla toothless, wide, arched, with distal tip reaching or surpassing vertical line projected through orbit anterior edge. Dentary with 4(2), 5(8), 6(13), 7(4), 8(2) or 10(2) Holotype N Min Max Mean SD Standard length (mm) 36.1 121 28.0 40.6 32.9 - Percents of Standard length Head length 23.5 121 21.3 27.3 24.2 1.1 Bony head length 20.3 121 19.0 24.1 21.7 0.8 Snout-anal fin distance 62.9 121 59.2 65.9 62.6 1.5 Snout-dorsal fin distance 53.4 121 50.3 56.9 54.2 1.3 Snout-pelvic fin distance 46.6 121 42.4 49.8 45.6 1.3 Snout-pectoral fin distance 25.4 121 24.5 29.5 26.8 1.0 Dorsal-fin base length 12.1 120 10.4 14.2 12.4 0.7 Anal-fin base length 28.9 121 22.7 28.9 26.0 1.4 Length of caudal peduncle 14.6 121 13.9 18.6 16.2 1.0 Depth of caudal peduncle 11.7 121 9.7 13.5 11.7 0.8 Body depth at dorsal-fin 31.8 121 27.3 35.2 30.9 1.4 Dorsal-fin length 25.6 113 23.3 30.1 26.5 1.2 Pelvic-fin length 14.7 120 14.0 17.7 15.9 0.8 Pectoral-fin length 23.3 120 19.5 25.9 22.4 1.3 Percents of Head length Snout length 23.6 121 20.2 28.6 23.8 1.6 Upper Jaw length 36.6 121 29.3 38.5 34.5 1.8 Horizontal orbit diameter 37.4 121 30.8 42.9 36.4 2.8 Interorbital width 25.1 121 23.6 32.2 27.4 1.5 Table 1. Morphometric data of Macropsobrycon uruguayanae. SD = standard deviation. F. C. Jerep & L. R. Malabarba 303 Fig. 1. Macropsobrycon uruguayanae, a - ANSP 168826, 32.8 mm SL, Uruguay, Depto. Cerro Largo, pool of rio Negro; b - specimen alive, not preserved; c - holotype, FMNH 57910, 36.0 mm SL, Brazil, Cacequy, rio Uruguay basin. Revision of the genus Macropsobrycon 304 aligned teeth; teeth conical, rarely bicuspid; four medial teeth larger with similar size, following teeth smaller decreasing in size laterally (Fig. 2). Dorsal-fin origin slightly posterior to vertical through middle of standard length. Dorsal-fin rays ii,8-10 (mean = 9.0, n = 120). Adipose-fin origin at vertical line through last analfin ray insertion. Anal-fin origin slightly anterior to last dorsalfin ray insertion; anterior rays longer than posterior ones; distal profile more convex on females than on males. Anal-fin rays iii-v,17-22 (mean = 23.8, n = 121); last unbranched up to nineth anal-fin branched ray of mature or maturing males bearing small retrorse hooks (Fig. 3). Hooks placed from middle length to near the distal tip of anal-fin rays, attached at posterior margin and usually posterior branch in branched portion, bent over lateral surface, and directed anteriorly to anal-fin frontal margin. One or two hooks per anal-fin ray segment. Pectoral-fin rays i,10-12 (mean = 10.6, n = 121); longest rays surpassing pelvic-fin insertion. Pelvic-fin insertion anterior to vertical line through dorsal-fin origin. Pelvic-fin rays i,7; all of them bearing 1-2 unilateral ventromedial antrorse hooks per segment on mature males. Caudal fin with 18-20 (mean = 19.0, n = 121) principal rays. Principal caudalfin rays 12th and 13th bearing recurved strong hooks anteriorly directed on branched portions, usually dorsally; principal caudal-fin rays 14th to 18th bearing posteriorly directed feeble spinelets along proximal portion, and anteriorly directed bony hooks along midand distal portions of fin rays on mature males (Fig. 4). Lateral line incomplete, with 3-8 (mean = 6.1, n = 91) perforated scales. Total scales at lateral line series 30-37 (mean = 33.6, n = 120) scales. Scale rows between dorsal-fin insertion and lateral line 5; scale rows between lateral line and pelvicfin insertion 2-4 (mean = 3.2, n = 121). Predorsal scales 10-16 (mean = 12.9, n = 119); postdorsal scales 7-10 (mean = 8.2, n = 121); scales rows around caudal peduncle 12-14 (mean = 13.8, n = 121). Last scales of lateral scale rows with higher radii number on adult males than other body scales. Last scale of scale row below lateral line larger and posteriorly hypertrophied on mature males, with distal margin reaching or slightly overlapping most proximal caudal-fin ray spinelets (Figs. 5-6). Mature males with gill gland on first branchial arch (Fig. 7). Gill rakers of first branchial arch 1.5 times longer than rakers from remaining branchial arches, not bearing denticles, 3(2), 4(2) on hypobranchial, 13(3), 14(1) on ceratobranchial, 7(4) on epibranchial. Supraneurals 4(1), 5(15), 6(74), 7(115), 8(28), 9(2). Total vertebrae 29(1), 30(35), 31(179), 32(35). Precaudal vertebrae 12(4), 13(227), 14(20). Pseudotympanum as large muscular hiatus between first and second pleural ribs, limited dorsally by the lateralis superficialis muscle, posteriorly by naked anterior face of second pleural rib, posteroventrally by obliquus inferioris muscle, anteroventrally by obliquus superioris muscle. Color in alcohol. Overall body color varying from light yellow in fresh preserved specimens to pale brownish on old preserved specimens (Fig. 1a,c). Head black to dark brown dorsally. Region of infraorbitals 3-5 and opercular apparatus silver on freshly preserved specimens and yellow on older ones. Dark melanophores surround nares, lips, and anterior margin of orbit. Mental region dark due to high concentration of melanophores. Humeral spot absent. Dorsal and dorsolateral portion of body with melanophores distributed Fig. 2. Macropsobrycon uruguayanae, MCP 20900, 27.9 mm SL; Scanning Electron Microscopy (SEM) images showing a - right side premaxilla, b - maxilla , and c - dentary, with d - detail of the teeth. F. C. Jerep & L. R. Malabarba 305 at posterior margin of the scales. Thin dark line extending laterally on body, from posterior region of pseudotympanum to caudal spot. Posterior portion of this line sometimes silver. Caudal region with few melanophores distributed in oblique lines along myoseptum of epaxialis and hypaxialis muscles. Ventral region with higher concentration of melanophores around pelvic-fin insertion in mature males. All fins hyaline with scattered melanophores, except dorsal fin with a small elongated spot between distal tip of first unbranched ray and midpoint of second unbranched ray. Anal fin with an elongated small spot at base of unbranched anal-fin rays. Males with higher concentration of melanophores at distal margin of anal fin than females. Caudal spot round, not reaching dorsal and ventral margins of caudal peduncle, and not extending over middle caudal-fin rays; not conspicuous on some specimens. Color in life. Overall body and head color pattern silver to white (Fig. 1b). Lateral body band silver. All the fins are hyaline with few scattered melanophores, and a small dark spot present at the dorsal and anal fin as described above. Sexual dimorphism. Mature males can be differentiated from adult females by the presence hooks on pelvic, anal and caudal-fin rays (Figs. 3-5). Males also present the the gill filaments from the first branchial developed into a gill gland Fig. 3. Macropsobrycon uruguayanae, mature male, MCP 20900, 34.2 mm SL, left side lateral view of anal fin showing anal-fin hooks shape and distribution. Scale bar = 1 mm. Fig. 4. Macropsobrycon uruguayanae, mature male, MCP 20900, 34.2 mm SL, left side lateral view of the caudal-fin lower lobe, hook distribution. Scale bar = 1 mm. Fig. 5. Macropsobrycon uruguayanae, mature male, MCP 20900, 34.2 mm SL, left side lateral view of the terminal portion of the caudal peduncle and caudal-fin base, showing modified scales and distribution of the “spinelets” on the caudal-fin rays. Scale bar = 1 mm. Revision of the genus Macropsobrycon 306 (Fig. 7), due to the fusion of adjacent gill filaments by expansion of the epithelium that covers it (Azevedo et al., 2010). The last scale of the scale row below the lateral line is also larger and more developed in males than females (Fig. 6b). Although the females also present a homologous scale relatively large when compared to other cheirodontine species, it is still smaller than that observed in males. Mature males also have a higher number of melanophores at the distal portion of the anal-fin rays forming a tenuous dark band along the distal margin of the anal fin that it is not seen in females. The ventral profile of the anal fin also differs between males and females, being less concave on males. Habitat and ecological notes. Macropsobrycon uruguayanae is usually found in natural flooded ponds and/or small lagoons with direct and active connection to rivers or streams. This kind of habitat is strongly affected along the area of distribution of this species by its extensive use to rice cultivation. The species is inseminating with reproductive biology described in detail by Azevedo et al. (2010). Distribution and geographical variation. Records of Macropsobrycon uruguayanae are known from Rio Grande do Sul State in southern of Brazil, Província de Entre Ríos in Argentina (Demonte & Arias, 2005; Miquelarena et al., 2008), and Uruguay. Its geographical distribution encompasses the rio Uruguay basin, rio Negro basin (large tributary of the lower rio Uruguay), and laguna dos Patos drainage (Fig. 8). The PCA analysis provided some morphological differences among the populations from the three studied drainages. The principal component 1 represented 72.5% of the variance, however it was found a correlation of 0.96 between its scores and the standard length values, showing to be most affected by the size. The second and third principal component represented respectively 6.2% and 4.7% of the variance. The principal component 2 showed a trend of separation between the specimens from rio Negro and from rio Uruguay basins. The individuals from rio Negro presented relatively smaller snout length and trends to bigger horizontal orbit diameter than the rio Uruguay representatives. Morphological differences between Macropsobrycon uruguayanae from laguna dos Patos basin and rio Negro basin were evidenced by the principal component 3. The laguna dos Patos basin specimens presented somewhat larger caudal peduncle length and upper jaw length, and relatively smaller anal-fin base length than specimens from rio Uruguay basin (Fig. 9). The MANOVA analysis of the PC2 and PC3 scores did not succeed in discrimitating the three populations. Material analyzed: Macropsobrycon uruguayanae. Type material. Holotype: FMNH 57910, 36.0 mm SL, Brazil, Cacequy, 1 Feb 1909, J. D. Haseman. Paratypes: FMNH 57911, 4, 14.4-35.5 mm SL, Brazil, Cacequy, 1 Feb 1909, J. D. Haseman. FMNH 57912, 21.3 mm SL, Brazil, Cacequy, 5 Feb 1909, J. D. Haseman. Nontype material. Laguna dos Patos system, Brazil, Rio Grande do Sul: MCP 8421, 1 (m, male, 34.7 mm SL), arroio Capané at bridge on BR 290, tributary of rio Jacuí, 24 Oct 1982, C. A. S. Lucena & L. R. Malabarba. MCP 9245, 6 (2m, males, 34.8-40.6 mm SL, 3, unsexed, Fig. 7. Macropsobrycon uruguayanae, mature male, MCP 11939, 34.2 mm SL; Scanning Electron Micrograph (SEM) of the first gill arch left side, showing the gill gland delimited by the arrows head. Fig. 6. Macropsobrycon uruguayanae, mature male, MCP 20900, 34.2 mm SL, left side, lateral view, a - lateral body scale and b - hypertrophied caudal-fin scale. Scale bar = 1 mm. F. C. Jerep & L. R. Malabarba 307 39.2-40.8 mm SL, 1 c&s, 32.4 mm SL), backwater of rio Jacuí, road between Santa Maria and Vera Cruz, 16 Sep 1983, C. A. S. Lucena, L. R. Malabarba & R. E. Reis. MCP 11997, 11 (4m, males, 32.439.8 mm SL, 6m, females, 31.1-39.2 mm SL, 1, unsexed, 30.9 mm SL, 1 c&s, female, 32.4 mm SL), arroio Francisquinho on highway BR 290 between Butiá and Rio Pardo, rio Jacuí basin, 24 Oct 1982, C. A. S. Lucena & L. R. Malabarba. MCP 19410, 1 (unsexed, 22.3 mm SL), lakes on left margin of rio Jacuí, at bridge on road connecting BR 290 to Cachoeira do Sulpicaré, 22 Jan 1996, L. R. Malabarba, J. R. Burns & J. F. Pezzi. MCP 20900, 87 (6m, males, 29.3-33.0 mm SL, 2m, females, 35.5-39.6 mm SL, 66, unsexed, 10.5-36.6 mm SL, 6 c&s, males, 24.6-34.2 mm SL, 2 c&s, females 33.2-35.1 mm SL, 1 c&s, unsexed, 23.4 mm SL), stream on rio Jacuí basin, at bridge on road between São Gabriel and Tiarajupuçá, 14 Jan 1997, L. R. Malabarba, J. Alves Gomes & V. Bertaco. MCP 37579, 2 (1m, male, 36.1 mm SL, 1m, female, 33.7 mm SL), Taim, Lago do Nicola, Jun 2004, A. Garcia. UFRGS 2244, 3 (unsexed, 22.9-24.7 mm SL) Rio Grande, Estação Ecológica do Taim, 8 May 1981, R. E. Reis & J. R. Stehmann. UFRGS 2245, 2 (1m, female, 34.0, 1, unsexed, 33.2 mm SL), between Taim and Mirim, channel parallel to BR 471 near a pump building in Estação Ecológica do Taim, 21 Apr 1979, L. Jardim & L. Chomenko. UFRGS 2306, 5m (females, 30.7-36.4 mm SL), between Rio Grande and Mirim, right margin of BR 471 on Arroio Taim road, Estação Ecológica do Taim, 7 Dec 1979, N. A. Menezes et al. UFRGS 2307, 3m (females, 33.0-35.2 mm SL), same locality as UFRGS 2306, 7 Jul 1979, P. A. Buckup. UFRGS 5604, 5 (4m, males, 34.2-35.4 mm SL, 1m, female, 34.0 mm SL), Barra do Ribeiro, arroio Ribeiro, bridge on road to Barra do Ribeiro, 19 Oct 1999. USNM 268449, 3 (2m, males, 31.3-32.0 mm SL, 1m, female, 37.3 mm SL), arroio Sarandi at stream under road crossing on road between Pelotas and Jaguarão, tributary of lago Mirim North of arroio Grande, 14 Dec 1972, N. A. Menezes et al. Rio Negro basin, Brazil, Rio Grande do Sul: MCP 11932, 1 (unsexed, Fig. 8. Geographic distribution of Macropsobrycon uruguayanae, one dot can represent more than one locality. Star represents the type locality. Fig. 9. Principal Component Analysis (PCA) of Macropsobrycon uruguayanae from three different basins: laguna dos Patos basin (dots), rio Uruguay basin (filled triangles), and rio Negro basin (empty triangles); based on the 18 morphometric variables listed in Table 1, except dorsal-fin length. Ellipses correspond to 95% confidence ellipses. The largest loadings are indicated on the second and third principal components (PC2, PC3), where AbL = anal-fin base length; HOD = horizontal orbit diameter; LCP = length of caudal-peduncle; SnL = snout length; UJL = upper jaw length.