A new sexually dimorphic Hyphessobrycon from the western Amazon basin (Characiformes: Characidae) Author Lima, Flávio C. T. 0000-0002-7636-5431 Museu de Zoologia da Universidade Estadual de Campinas “ Adão José Cardoso, Caixa Postal 6109, 13083 - 863 Campinas, São Paulo, Brazil. & fctlima @ gmail. com; http: // orcid. org / 000 - 0002 - 7636 - 5431 fctlima@gmail.com Author Bastos, Douglas A. 0000-0001-7718-7764 Instituto Nacional de Pesquisas da Amazônia, Programa de Coleções, Coleção de Peixes. Av. André Araújo, 2936, 69060 - 001 Manaus, Amazonas, Brazil. & avizdoug @ gmail. com; https: // orcid. org / 0000 - 0001 - 7718 - 7764 avizdoug@gmail.com Author Rapp Py-Daniel, Lúcia H. 0000-0002-0954-7366 Instituto Nacional de Pesquisas da Amazônia, Programa de Coleções, Coleção de Peixes. Av. André Araújo, 2936, 69060 - 001 Manaus, Amazonas, Brazil. & luciarpd 60 @ gmail. com; https: // orcid. org / 0000 - 0002 - 0954 - 7366 luciarpd60@gmail.com Author Ota, Rafaela P. 0000-0002-5333-8748 Departamento de Biologia Estrutural e Funcional, Instituto de Biociências, Universidade Estadual Paulista Júlio de Mesquita Filho, Rua Prof. Dr. Antônio Celso Wagner Zanin, 250, 18618 - 689 Botucatu, São Paulo, Brazil. & rafaelapota @ gmail. com; https: // orcid. org / 0000 - 0002 - 5333 - 8748 rafaelapota@gmail.com text Zootaxa 2022 2022-03-18 5116 2 253 266 journal article 20177 10.11646/zootaxa.5116.2.4 638268d9-8e4c-4258-9cdf-0bc4f625fd82 1175-5326 6372795 2CC29978-0AAC-402F-9B1A-7D363343F8BC Hyphessobrycon bayleyi , new species ( Figs. 1–4 ) Hemigrammus marginatus (not Ellis): Géry, 1964: 27 , fig. 4 (photo in life), 32 ( Peru , Loreto , “surroundings of Iquitos”; specimen examined); Lüling, 1981: 175 (upper photo), 176 ( Peru , Ucayali , Pucallpa, “Weisswasserbach” at road to Tournavista; specimen not examined); Ortega & Vari, 1986: 8 (listed for Peru ; based on Géry, 1964 ); Bogotá-Gregory & Maldonado-Ocampo, 2006: 67 (listed, Amazon basin, Colombia ; see Discussion); Bejarano et al. , 2006: 362 ( Colombia , Caquetá , Río Mesay, Rio Caquetá basin; specimens examined); Maldonado-Ocampo et al ., 2008: 167 (in part; Amazon basin, Colombia ; based on Bogotá-Gregory & Maldonado-Ocampo, 2006 ); Ortega et al. , 2010: 36 ( Peru , Amazon basin; listed, based presumably on Ortega & Vari, 1986 ); Barriga, 2012: 107 ( Ecuador , Río Napo basin); Meza-Vargas et al ., 2021: 18 ( Peru , Departamento Loreto ). Hemigrammmus sp.: Claro-García et al. , 2013: 1413 , 1416, 1423 ( Brazil , Acre , rio Acre basin; photo; specimens examined). Hyphessobrycon diancistrus (not Weitzman): Lima et al. , 2013: 278–279 (in part; Brazil , middle rio Madeira basin; see Remarks). Moenkhausia intermedia (non Eigenmann): Galvis et al. , 2007: 384 , fig. 158 ( Colombia , Amazonas, Puerto Nariño , Río Loreto Yacu; photo in life; specimen not examined). Holotype : ZUEC 17121 ( male , 33.4 mm SL): Brazil , Amazonas , Benjamin Constant , igarapé do Palhau , Ramal do Incra , 4º24’28’’S , 70º1’24’’W ; F.C.T. Lima , C.R. Moreira , G.N. Salvador & N. Flausino Jr. , 16 Nov 2017 . Paratypes : Brazil , Amazonas. ZUEC 15501 (9, 2 females , 34.1–36.0 mm SL, 5 males , 31.1–36.0 mm SL); ANSP 206907 (2, 1 female , 34.7 mm SL, 1 male 32.4 mm SL); LBP 26061 (1, male, 31.4 mm SL, alcohol-preserved); FMNH 141250 (2, 1 female , 33.5 mm SL, 1 male , 32.8 mm SL); MCZ 173977 (2, 1 female , 35.3 mm SL, 1 male , 31.8 mm SL): same data as holotype . ZUEC 15040 (3, 2 females , 32.5–34.2 mm SL, 1 male , 29.1 mm SL): Benjamin Constant, igarapé Cajarizinho (trib. rio Solimões), 4º26’41’’S , 69º59’56’’W ; F.C. T . Lima, C. R . Moreira, G.N. Salvador & N. Flausino Jr., 17 Nov 2017 . ZUEC 16963 (5, 2 males , 27.2–28.5 mm SL, 1 female , 32.8 mm SL, 2 sex undetermined, 24.9–27.8 mm SL): Benjamin Constant, igarapé da Prosperidade, road Benjamin Constant/Atalaia do Norte, 4º26’17’’S , 70º5’22’’W ; F.C. T . Lima, A.A. Acosta & A. Camacho, 11 Aug 2018 . ZUEC 15475 (6, 2 females , 35.4–37.2 mm SL, 4 males , 31.6–33.3 mm SL, 2 C&S, 32.8–35.4 mm SL); MCP 54775 (2, 1 female , 34.7 mm SL, 1 male , 32.0 mm SL); MNRJ 51459 (2, 1 female , 34.1 mm SL, 1 male , 32.8 mm SL); CAS 246907 (2, 1 female , 36.0 mm SL, 1 male , 33.6 mm SL): Atalaia do Norte, igarapé do Adolfo, road Benjamin Constant/Atalaia do Norte, 4º26’5’’S , 70º7’19’’W ; F.C. T . Lima, C. R . Moreira, G.N. Salvador & N. Flausino Jr., 11 Nov 2017 . ZUEC 15569 ( 2 females , 33.6–33.9 mm SL): Atalaia do Norte, igarapé do Pixaim (trib. rio Javari), 4º21’46’’S , 70º12’5’’W ; F.C. T . Lima, C. R . Moreira, G.N. Salvador & N. Flausino Jr., 10 Nov 2017 . ZUEC 15432 (2, 1 male , 29.7 mm SL, 1 female , 32.2 mm SL): Atalaia do Norte, igarapé do Tampinha (trib. rio Javari), 4º21’53’’S , 70º11’52’’W ; F.C. T . Lima, C. R . Moreira, G.N. Salvador & N. Flausino Jr., 13 Nov 2017 . ZUEC 15101 (5, 1 male , 31.7 mm SL, 4 sex undetermined, 25.4–27.0 mm SL, 1 c&s, 27.0 mm SL): Atalaia do Norte, igarapé Boa Vista (trib. rio Itaquaí), above the mouth of rio Quixito, 4º26’37’’S , 70º14’11’’W ; F.C. T . Lima, C. R . Moreira, G.N. Salvador & N. Flausino Jr., 15 Nov 2017 . INPA 56395 (2, 1 male , 30.2 mm SL, 1 sex undetermined, 24.4 mm SL): Atalaia do Norte, pond behind comunidade Cachoeira, rio Itaquaí (trib. rio Javari), 4°29’7.33’’S , 70°16’45.26’’W ; J. Zuanon, G. Torrente-Villara & D. Bastos, 26 Aug 2017 . INPA 56064 (1, male, 27.7 mm SL): Atalaia do Norte, Aldeia São Pedro, below comunidade Palmari (rio Javari basin), 4°17’55.36’’S , 70°16’16.25’’W ; J. Zuanon & D. Bastos, 22 Aug 2017 . INPA 56018 (1, sex undetermined, 25.5 mm SL): Atalaia do Norte, stream tributary of rio Javari at comunidade Palmari, 4°17’37.61’’S , 70°17’29.51’’W ; J. Zuanon, G. Torrente-Villara & D. Bastos, 21 Aug 2017 . INPA 55237 (18, sex undetermined, 22.1–26.3 mm SL): Atalaia do Norte, Igarapé da Pousada (trib. rio Itaquaí), Lago do Contrabando, near comunidade Santa Cruz (rio Javari basin), 4°21’46.17’’S , 70°16’13.26’’W ; J. Zuanon & D. Bastos, 14 Aug 2017 . INPA 56509 (1, sex undetermined, 21.9 mm SL): Rio Quixito, above its mouth at rio Itaquaí, 4°26’9.82’’S , 70°20’10.93’’W ; J. Zuanon, G. Torrente-Villara, D. Bastos, F. Rocha, R . Ribeiro, A. Cella-Ribeiro & S. Hashimoto, 28 Aug 2017 . INPA 57267 (3, males, 27.6–30.8 mm SL, 1 C&S, 30.4 mm SL): São Paulo de Olivença, igarapé Taboca, tributary of rio Camatiã, Nossa Senhora de Nazaré community, 3°39’04”S 69°16’51”W ; L.H. Rapp Py-Daniel, A.L.C. Canto & A. Negrão, 31 Jan 2008 . INPA 49142 (2, females, 30.0– 30.7 mm SL): Japurá, stream trib. rio Japurá, 1°50’56”S 69°01’43”W ; P.M. Ito & R . Collins, 6 Sep 2014 . INPA 49254 (5, females, 27.7–38.9 mm SL): stream trib. rio Japurá, 1°42’52”S 69°07’40”W ; P.M. Ito & R . Collins, 8 Sep 2014 . ZUEC 15388 (1, sex undetermined, 28.5 mm SL): Tefé, Igarapé Açu (trib. rio Solimões), Ramal do Emade, 3º25’23’’S , 64º37’6’’W ; F.C. T . Lima, G.N. Salvador, N. Flausino Jr. & J.A. Oliveira, 26 Nov 2017 . Peru , Loreto . UF 242746 (1, female, 35.9 mm SL): Iquitos, Quebrada San Pedro, road Iquitos/Nauta, 4º18’14’’S , 73º31’16’’W ; W.G. R . Crampton, R .E. Reis, H. Ortega & F.C. T . Lima, 7 Jan 2004 . MHNG 1576.059 (1, sex undetermined, 30.3 mm SL); MHNG 2180.078 (1, sex undetermined, 28.0 mm SL): Jenaro Herrera, Quebrada Copal; P. de Rham, 18 Oct 1977 . MUSM 66974 (2, 1 female , 37.7 mm SL, 1 male , 30.9 mm SL): Jenaro Herrera, Quebrada Copal, km 15 at the road Jenaro-Herrera-Colonia Angamos, 4º56’22’’S , 73º30’25’’W ; H. Ortega, W.G. R . Crampton, R .E. Reis & F.C. T . Lima, 10 Jan 2004 . Colombia , Amazonas. ICNMHN 17244 (8, 1 sex undetermined, 28.4 mm SL, 7 males , 34.1–38.0 mm SL): Quebrada Jotabeyá, trib. Río Apapóris, 0º37’20’’S , 70º13’32’’W ; F. Arbeláez, 27–30 March 2009 . ICNMHN 5066 (167, 102 females , 32.7–43.0 mm SL, 65 males , 32.2–36.8 mm SL): Leticia, Río Puré, 2º7’5’’S , 69º37’50’’W ; J.D. Lynch, Jan 2000 . Not types: Brazil , Amazonas. MHNG 2180.074 (1, sex undetermined, 27.0 mm SL): rio Purus , Boca do Tapauá , 5º38’S , 63º12’W ; H. R . Axelrod, H . Schultz & F. Terofal , Nov 1963 . Brazil , Acre : MCP 37745 (7, 23.7–26.9 mm SL): Sena Madureira , igarapé Taquari (trib. rio Antimari , rio Acre basin), at road BR-364, 9º27’49’’S , 68º22’43’’W ; R . E. Reis , P.A. Buckup , F. Langeani & F.C. T . Lima, 21 Jul 2004 . MCP 37771 (10 of 27, 26.8–28.4 mm SL): Bujari , Igarapé Marizinho (trib. rio Antimari , rio Acre basin), at road BR-364, 9º36’41’’S , 68º14’40’’W ; P.A. Buckup , J.F. Pezzi da Silva , P. Lehmann & V . A. Bertaco , 20 Jul 2004 . MZUEL 6886 (2, 28.9–29.4 mm SL): Bujari , Igarapé Fumaça (trib. rio Antimari , rio Acre basin), at road BR-364, 9º34’23’’S , 68º16’50’’W ; O.A. Shibatta & A. Claro-Garcia , 6 Oct 2010 . Peru , Loreto . NRM 26870 (1, sex undetermined, 22.0 mm SL): quebrada Pintoyacu ( Río Itaya drainage), at road Iquitos-Nauta ; S.O. Kullander et al ., 3 Jul 1986 . NRM 26943 (1, sex undetermined, 25.4 mm SL); NRM 30579 (12, sex undetermined, 22.8–30.9 mm SL): Quebrada Tocón Grande , km 33 of Iquitos-Nauta road, c. 4º2’S , 73º26’W ; S.O. Kullander et al ., 4 Jul 1986 . MHNG 2180.084 (1, 38.3 mm SL): “surroundings of Iquitos ”; H. R . Axelrod , 27 Dec 1963 . MHNG 2225.01 (2, 23.4–27.3 mm SL): Jenaro Herrera , Quebrada Copal (trib. Río Ucayali ), c. 4º56’S , 73º30’W ; P. de Rham , 1 Sept 1981 . NRM 15713 (2, sex undetermined, 22.3–24.1 mm SL): Jenaro Herrera , Quebrada Copal , km 15 at the road Jenaro-Herrera-Colonia Angamos , 4º56’22’’S , 73º30’25’’W ; S.O. Kullander et al. , 29 Aug 1983 . MHNG 2180.089 (6, sex undetermined, 21.5–26.4 mm SL): Río Huytoyacu (trib. Río Paztaza ) at Nuevo Progresso , c. 3º37’S , 76º25’W ; C. Meyer , 20 Jul 1975 . NRM 15714 (1, sex undetermined, 19.8 mm SL): Colonia Angamos , quebrada trib. Rio Galvez (trib. Rio Yavari ), 20 min. upstream from Colonia Angamos , c. 5º10’40’’S , 72º53’30’’W ; S.O. Kullander et al ., 31 Jul 1984 . NRM 26920 (26, sex undetermined, 15.0– 24.5 mm SL): Loreto , Colonia Angamos , quebrada trib. Rio Galvez (trib. Rio Yavari ), 25 min. upstream from Colonia Angamos , c. 5º10’40’’S , 72º53’30’’W ; S.O. Kullander et al. , 31 Jul 1984 . Colombia : MPUJ 13846 (1, male, 31.7 mm SL): Caquetá , Solano , caño Trocha , trib. Río Rutuya , rio Caquetá basin, 0º20’59’’N , 74º45’18’’W ; J. Maldonado-Ocampo et al. , 18 Apr 2018 . ICNMHN 4271 (2, sex undetermined, 21.4–21.8 mm SL): Caquetá , Río Mesay , c. 0º4’27’’N , 72º27’5’’W ; I. Bejarano & M.P. Blanco , July–Sept 2000 . ICNMHN 13765 (4, 1 male , 31.4 mm SL, 3 females , 31.5–35.2 mm SL): Amazonas , creek 45 minutes above Puerto Nariño , c. 3º45’S , 70º25’W ; H. Hanima , H. Boschung & R . Beckham , 3 Jan 1972 . ICNMHN 18712 (1, male, 34.8 mm SL): Amazonas , Puerto Asis , Vereda La Rosa , Quebrada Cadenas ( Río Putumayo basin), 0º26’16’’N , 76º17’10’’W ; M. Murcía , 26 Jul 2014 . ICNMHN 17233 (7, 24.0– 32.8 mm SL, largest specimen a mature male): Amazonas , Rio Apapóris , above Raudal Jiri-Jirimo , 0º2’35’’S , 70º56’53’’W ; F. Arbeláez , 19–21 March 2009 . CIACOL 3183 (26, sex undetermined, 24.4–38.4 mm SL): Amazonas , caño Hormiga , near village of Buenos Aires , c. 3º10’S , 69º59’W ; A.A. Santos & E.C. Agudelo , 21 Feb 2018 . Ecuador : FMNH 102587 (13, 13.5–34.7 mm SL): Napo , trib. Río Tarapuy (trib. Río Cuyabeno , Río Napo basin), c. 0º10’S , 75º57’W ; D.J. Stewart et al. , 2 Dec 1983 (picture only). Tentative identifications (see Remarks): All from Brazil , Rondônia . UFRO-I 4892 (2 of 5, 23.5–24.6 mm SL): Porto Velho , rio Madeira , Cachoeira de Teotônio , 8º51’32’’S , 64º3’46’’W ; J.L.O. Birindelli et al. , 24 Oct 2009 . UFRO-I 9026 (5 of 12, 22.7–23.1 mm SL): Porto Velho , rio Madeira , at mouth of igarapé Jatuarana , 8º50’33’’S , 64º2’55’’W ; E. Silva et al ., 8 Sept 2009 . Diagnosis. Hyphessobrycon bayleyi can be distinguished from all congeners, except from Hy. diancistrus and Hy. otrynus by presenting two symmetrical dark blotches at the distal portion of the caudal-fin lobes (vs. absence of two symmetrical dark blotches at the distal portion of the caudal-fin lobes in the remaining congeners). The new species can be distinguished from Hy. diancistrus and Hy. otrynus by the presence, in mature males, of typically a single large hook at last unbranched anal-fin ray (vs. presence in mature males typically of two large hooks, the first at last unbranched anal-fin ray, the second at first branched anal-fin ray). It can be additionally diagnosed from Hy. otrynus by presenting a patch of dark pigmentation on dorsal fin (vs. dorsal fin without any patch of dark pigmentation), and from Hyphessobrycon diancistrus by presenting a higher body depth (22.4–32.5 % SL, mean 28.4% SL, n = 63 vs. 19.0–26.8 % SL, mean 22.9 % SL, n = 76), and by a lower number of anal-fin branched rays (18–22, n = 63 vs. 12–16, n = 76). Hyphessobrycon bayleyi can be distinguished from two non-congeners which share the same color pattern, Hemigrammus durbinae and He. marginatus by presenting a patch of dark pigmentation on dorsal fin (vs. dorsal fin without any patch of dark pigmentation), by presenting anterior portion of anal fin with relatively broad dark stripe (vs. two parallel narrow stripes in He. durbinae , and a single narrow stripe in He. marginatus ), and by the presence, in mature males, of a large hook at last unbranched anal-fin ray (vs. absence of hooks of any size in males of He. durbinae and He. marginatus ). See the Discussion, for additional comparisons with these species. Description. Morphometric data summarized in Table 1 . Body compressed, moderately slender; greatest body depth anterior to dorsal-fin origin. Dorsal profile of head slightly convex from tip of snout to anterior naris; straight to gently concave from latter point to tip of supraoccipital spine. Dorsal profile of body slightly convex from tip of supraoccipital spine to dorsal-fin origin; posteroventraly slanted and straight from latter point to adipose-fin origin and slightly concave along caudal peduncle. Ventral profile of body slightly convex from tip of dentary to anal-fin origin, posteroventraly slanted along anal-fin base. Ventral profile of caudal peduncle slightly concave. TABLE 1. Morphometric data of Hyphessobrycon bayleyi . SD = Standard deviation.

Holotype	Range	Mean	SD	N
Standard length	33.4	26.9–38.9	–	–	63
Percentages of SL
Depth at dorsal-fin origin	27.2	22.4–32.5	28.4	1.97	63
Snout to dorsal-fin origin	49.7	48.3–55.0	51.9	1.35	63
Snout to pelvic-fin origin	44.6	43.3–48.2	45.8	1.10	63
Snout to anal-fin origin	61.4	60.3–65.8	62.5	1.29	63
Caudal peduncle depth	10.5	7.7–10.9	9.4	0.80	63
Caudal peduncle length	16.2	14.6–19.2	16.3	1.02	63
Pectoral-fin length	21.6	18.6–25.6	22.0	1.52	62
Pelvic-fin length	17.4	16.2–19.7	17.9	0.84	63
Dorsal-fin base length	12.3	11.1–15.8	12.9	0.93	63
Dorsal-fin heigth	25.4	22.7–29.4	25.8	1.38	62
Anal-fin base length	27.2	23.6–28.8	26.1	1.21	63
Anal-fin lobe length	18.3	16.7–22.7	19.3	1.33	63
Bony head length	23.7	22.8–26.5	24.7	0.80	63
Percentages of HL
Horizontal eye diameter	38.0	35.3–48.6	39.8	1.77	63
Snout length	25.3	22.5–30.3	26.0	1.79	63
Least interorbital width	34.2	28.6–36.1	32.1	1.83	63
Upper jaw length	39.2	37.1–45.8	41.7	2.08	62

Mouth terminal; jaws equal, isognathous. Distal tip of maxillary extending slightly beyond vertical through anterior margin of eye. Premaxillary teeth in two rows, outer row composed by 1(1), 2*(36), or 3(26) tricuspid teeth, central cusp longer; inner row with 4(1) or 5*(62) bi- to pentacuspid teeth teeth, central cusp longer. Maxilla with 1(2) or 2(2) small, unicuspid teeth. Dentary with 9(1), 10(2), 11(1) teeth, anteriomost four teeth larger than remaining, tri- to pentacuspidate, remaining 5 – 7 teeth unicuspid. One specimen with second inner row of 4(1) unicuspid teeth, situated on inner margin of dentary, justaposed with outer unicuspid teeth ( Fig. 3 ). Scales cycloid, with two to seven strongly marked radii, diverging from focus to posterior portion of scale; circuli well-marked anteriorly. Lateral line incomplete, slightly curved ventrally, with 7(1), 8(3), 9(12), 10*(17), 11(16), 12(8), 13(3), 14(2), or 15(1) pored scales; longitudinal series including perforated scales 29(1), 31(1), 32(1), 33(5), 34(8), 35(15), 36(22), 37*(6), 38(3), or 39(1). Scales rows between dorsal-fin origin and lateral line 5*(61) or 6(1). Scales rows between lateral line and pelvic-fin insertion 3*(61) or 4(1). Predorsal scales 10(7), 11*(40), or 12(16). Anal sheath along anal-fin base with 4 to 5 scales in a single row, covering base of first unbranched to fourth branched anal-fin rays. Circumpeduncular scales 12(9), 13*(36), or 14(18). Caudal-fin scales covering basis of upper and lower caudal-fin lobes margins, relatively large and few in number, concentrated over inner fin rays. Dorsal-fin rays ii,9*(63), first unbranched ray nearly one-third of second unbranched ray length; small ossification anterior to first unbranched ray present in four c&s specimens examined. Dorsal-fin origin at midbody or slightly behind this point. Insertion of first dorsal fin pterygiophore posterior to neural spine of 10 th (1) or 11 th (2) vertebrae. Adipose fin present. Pectoral-fin rays i,9 (5), 10(9), 11*(24), or 12(25). Pelvic-fin rays i,7*(63), its tip reaching anteriormost unbranched anal-fin rays. Anal-fin rays iv(2) – v(2), 18(4), 19(16), 20(25), 21*(14), or 22(2); last unbranched ray to fifth branched ray decreasing steeply in size, forming a distinctive anterior lobe; remaining rays decreasing very gradually decreasing in size to anal-fin terminus. Last anal-fin pterygiophore insertion behind hemal spine of 17th(4) vertebrae. Caudal fin forked, lobes slightly pointed, equal in size. Principal caudal-fin rays i,17,i (4); dorsal procurrent caudal-fin rays 10(1), 11(1), 12(1), or 13(1); ventral procurrent caudal-fin rays 9(2), 10(1), or 11(1). Vertebrae 36(4). Supraneurals 5(3) or 6(1). First gill arch with 2 (4) gill rakers on hypobranchial, 10 (4) on ceratobranchial, 1 (4) on cartilage between ceratobranchial and epibranchial, and 5 (1), 6(2) or 7(1) on epibranchial. FIGURE 1. Hyphessobrycon bayleyi : (A) ZUEC 17121, holotype, male, 33.4 mm SL: Brazil, Amazonas, igarapé do Palhau (tributary of rio Javari); (B) ZUEC 15501, paratype, female, 34.1 mm SL: same locality. Color in alcohol . Overall ground color light brown. Top of head, snout and tip of dentary dark gray as a result of dense concentration of dark chromatophores. Opercle and infraorbitals silvery in specimens retaining guanine. Predorsal midline dark gray, as a result of dense concentration of dark chromatophores covering entire surface of predorsal scales and upper margin of scales from scale row situated immediately below it. Post-dorsal midline with slightly less dense concentration of dark chromatophores, imparting a dark gray coloration. Lateral line slightly delineated by dark chromatophores. A moderately developed midlateral stripe, starting as a very narrow line slightly behind humeral region, broadening from vertical through dorsal-fin origin to caudal peduncle. Midlateral stripe at caudal peduncle fused with caudal peduncle blotch, with a roughly rhomboidal shape. Anal-fin basis with a conspicuous black stripe, broader at anterior portion of anal fin, from anal-fin origin to fourth to sixth branched anal-fin ray, becoming very narrow from that point to anal-fin terminus. Anal fin with dark chromatophores aligned in rows along posterior margin of fin rays. Dorsal fin with a dark patch of pigmentation situated at central portion of fin, between last unbranched and third branched fin rays. Remaining portions of dorsal fin hyaline or with scattered dark chromatophores. Pectoral, pelvic, and adipose fins hyaline. Caudal fin with two broad, symmetrical dark blotches situated on mid-region of upper and lower caudal-fin lobes, more intensely pigmented from outer fin ray to third or fourth branched rays. Small dark chromatophores scattered along caudal-fin margin, remaining caudal fin hyaline except for dark chromatophores at basis of middle caudal-fin rays. FIGURE 2. Hyphessobrycon bayleyi : (A) ZUEC 15475, paratype, female, 37.2 mm SL: Brazil, Amazonas, igarapé do Adolfo (tributary of rio Javari); (B) INPA 56395, paratype, male, 30.2 mm SL: Brazil, Amazonas, rio Itaquaí basin; in life. Color in life. Based on pictures of 3 specimens taken in life ( INPA 56395 , 30.2 mm SL; ZUEC 15475 , 37.2 mm SL; ZUEC 15101 , 31.7 mm SL) ( Fig. 2 ), and on pictures of a specimen not preserved collected by Heiko Bleher at the rio Jandiatuba (near São Paulo de Olivença , Amazonas, Brazil ), of a specimen collected at the Río Loreto Yacu , Amazonas, Colombia (Galvis et al ., 2006: 384, fig. 158), and of a specimen collected near Iquitos ( Géry, 1964: 27 , fig. 4). Sides of head and abdominal region clear, with a silver hue. Snout and top of head dark gray, with a golden hue. Dorsal region and area above anal fin light gray, with an olive hue, slightly translucent. Midlateral stripe plumbeous, becoming darker towards caudal peduncle. Anal and dorsal fins with yellowish pigmentation, dorsal fin with whitish tip. Pectoral and pelvic fins hyaline. Caudal fin with orange (males) or red (females) pigmentation on basal and middle portions of fin lobes; caudal-fin tips white. Dark pigmentation as in preserved specimens. Sexual dimorphism. Males of Hyphessobrycon bayleyi possess a large, very distinctive hook at last unbranched anal-fin ray ( Fig. 4 ). The hook is present slightly below the midlength of the ray, and it is surrounded by a thick concentration of whitish tissue, presumably formed by club cells. Out of 102 examined dimorphic males, two specimens (ICMNHN 18712, 34.8 mm SL; ZUEC 15501, 1, 36.0 mm SL) present a second, smaller hook, at the first branched anal-fin ray, situated more distally, slightly below the branching point of the ray. Additionally, as noticed in the “Color in life” section, above, males possess an orange caudal fin, whereas in females the caudal fin is red (see also Fig. 2 ). In addition, females seem to be on average slightly larger than males. Distribution. Hyphessobrycon bayleyi is distributed across the western portion of the Amazon basin, in Brazil , Colombia , Ecuador , and Peru ( Fig. 5 ). The species presents a “western-central Amazon” pattern of distribution, as described by Lima & Ribeiro (2011: 160) and Dagosta & de Pinna (2019: 20 ; called by these authors as “Amazononly lowland”). FIGURE 3 . Hyphessobrycon bayleyi , INPA 57267, 30.4 mm SL: dentary, premaxillary, and maxillary, lateral view. Scale bar: 1 mm. FIGURE 4. Hyphessobrycon bayleyi , anterior portion of anal fin in a whole specimen (INPA 57267, 30.8 mm SL) and a cleared and stained specimen (INPA 57267, 30.4 mm SL), showing the large fin hook at the last unbranched anal-fin ray. FIGURE 5. Map of western portion of the Amazon basin, showing the distribution of Hyphessobrycon bayleyi (white dots). The red star indicates the type locality, the blue dot a locality from the literature ( Lüling, 1981 ; see Discussion), and the black dot a tentative identification (see Remarks). FIGURE 6. Hemigrammus boesemani , NRM 70353, female, 27.8 mm SL: French Guyana, Kourou, Crique Corossony. Ecological notes. Specimens of Hyphessobrycon bayleyi were collected in mid-sized streams that ranged from being clear water (e.g., Igarapé da Prosperidade, Igarapé Cajarizinho, Igarapé Palhau), slightly stained water (Quebrada Copal) and black water (Igarapé Boa Vista). Most streams from which the species was recorded were in “ terra firme ”, i.e., above the floodplains. Only two specimens were collected in muddy water ponds on the banks of the rio Javari (INPA 56395). The gut contents of two c&s specimens (ZUEC 15475) contained mostly insect remains (from which ants could be identified) and some vegetal matter. Etymology. The specific name honors Peter B. Bayley, a retired ichthyologist who worked at INPA during the late 1970’s and subsequently at the Illinois Natural History Survey (INHS), who sampled extensively the fishes from floodplain lakes in Central Amazon near Manaus. Among the material collected by him (distributed among INHS, MZUSP, USNM, among other collections) are large series of well-preserved small characins, that have greatly aided in our understanding of the diversity of the group in Central Amazon (e.g., Weitzman & Palmer, 1997 ; Bührnheim & Malabarba, 2006 ; Ota et al ., 2014 , 2015 ). A patronymic adjective. Remarks. We tentatively assign to Hyphessobrycon bayleyi two lots from the rio Madeira basin in Brazil . These specimens (UFRO-I 4892, and UFRO-I 9026) were part of the material identified by Lima et al. (2013: 279) in their account of Hyphessobrycon diancistrus and were the reason why these authors recorded this species as presenting a broad range of variation in anal-fin branched rays (13–21 branched rays). The examination of these lots, as well as some lots also examined by Lima et al. (2013) that truly represent Hyphessobrycon diancistrus (UFRO-I 10618, UFRO-I 10630, and UFRO-I 11539), showed that their account of Hyphessobrycon diancistrus is composite, as the specimens from lots UFRO-I 4892 and UFRO-I 9026 present a relatively higher body (22.4–26.4% SL) and higher anal-fin branched rays (18–20 rays) when compared with lots UFRO-I 10618, UFRO-I 10630, and UFRO-I 11539 (19.1–23.1% SL and 13–16 rays, respectively). The specimen depicted in Lima et al. (2013: 278) clearly belongs to first group, as evidenced by its relatively long anal fin, with approximately 21 branched rays. Unfortunately, all specimens from lots UFRO-I 4892 and UFRO-I 9026 are completely faded and are small-sized (the largest specimen reaching only 24.6 mm SL), which precludes a conclusive identification. We tentatively assign these specimens to Hy. bayleyi , noticing, however, that additional, better preserved material is necessary to ascertain the taxonomic status of the population from the rio Madeira basin.