New Myloplus from Essequibo River basin, Guyana, with discussion on the taxonomic status of Myleus pacu (Characiformes: Serrasalmidae)

A new species in the serrasalmid genus Myloplus is described from the middle Mazaruni River, Essequibo River basin, Guyana. The new species is similar to Myleus pacu and Myloplus planquettei in its silver to purplish black coloration and its overall morphology, and has a putative syntopic distribution with those species. The new species is, however, readily distinguishable from the other two by meristic counts of the unpaired fins and by differences in color pattern, primarily in males. Additionally, we provide comments on Myleus pacu and other species that have been confounded under that name.

In this paper, we report and describe a previously unknown species of Myloplus from the Mazaruni River, a tributary of the Essequibo River basin in Guyana. This species was initially identified as Myleus pacu (Jardine, 1841) because of its silver to purplish black coloration and its supposedly syntopic distribution with that species. However, this previously unknown species has molariform teeth and premaxillary rows separated by a clear gap characteristic of the genus Myloplus, whereas Myleus is characterized by two adjacent rows of incisiform teeth in contact with each other (Andrade et al., 2018b). Additionally, we provide some comments on characters distinguishing Myleus pacu and other species that have been confounded under that name.

Material and Methods
Measurements and counts follow  with addition of caudal-peduncle length. Standard length (SL) is expressed in millimeters and all other measurements are expressed as percentage of SL, except for subunits of head, which are expressed as percentage of head length (HL). Counts are given in the description followed by frequency of each count in parentheses; counts observed in the holotype are indicated by an asterisk. Osteological terminology follows Weitzman (1962) with modifications based on Mattox et al. (2014). Osteological descriptions, vertebral and supraneural counts were based on radiographed specimens. Vertebrae of the Weberian apparatus are counted as four elements and the compound caudal centra (PU1+U1) as one element.

Coloration in alcohol.
Background coloration yellowish silver (Figs. 1a-c). Flank homogeneously yellowish silver, countershaded (females, Fig. 1c) or with black coloration pattern (males, and see coloration under Sexual dimorphism for details), mainly concentrated on dorsal half of flank (Figs. 1a-b). Eye with well-marked vertical black bar, anterior and posterior portions of sclerotic clear to light yellow. Infraorbital series lacking pigmentation or with very few dark chromatophores on 1st, 2nd and/or 3rd infraorbitals. Opercle silver, anteromedial portion with very few dark chromatophores. Belly silver or light brown. Fins hyaline to light brown with few chromatophores scattered on rays and hyaline membrane between rays. Dorsal fin generally light brown with discrete black pigmentation distally on anterior rays. Adipose light brown to clear or with a very thin brown edge. Anal rays with dark chromatophores along base and extending to middle portion of rays or uniformly light brown. Caudal fin yellow to light brown, distally whitish gray.

Coloration in life.
Background coloration silver to purplish silver, especially in males (Fig. 3a); females silver to bluish silver (Fig. 3b). Dorsal portion of head brownish black, ventrally and posterior to eye light silver. Upper portion of 3rd and 4th infraorbital and operculum light olive green. Eye with conspicuous vertical black bar, anterior and posterior portions of sclerotic bright yellow. Flank homogeneous silvery or with evident scattered pattern of black scales, mainly on upper flank. Belly silver or light black and metallic purple. Pectoral, pelvic and adipose hyaline to light yellow. Dorsal, anal and caudal fins yellow-orange to bright or brownish red.
Sexual dimorphism. Males and females are distinguished mainly by differences in coloration. Background coloration of dorsal region of males distinctly brownish red (Figs. 1a-b, 3), ventrally purple to purplish silver with a dark purple, almost black, diffuse band on the ventral-most quarter of the flank. Females are primarily silver with a distinct bluish veneer in life (Figs. 1c, 3). Male flank with a whitish silver medial region above and below the lateral line, bordered by deep black scales, most of which are overlaid with a metallic green sheen; perforated scales and lateral line pigmented black at center. Males also recognized by the presence of elongated middle anal fin rays, forming a lobe with apex centered on 16th branched ray. Females with first anal fin ray clear, longest, decreasing in size towards posterior portion of fin. Males lack stiff hooks on distalmost lepidotrichia of anal-fin rays and filamentous extensions on dorsal-fin rays.
Geographical distribution. Myloplus taphorni is currently known only from two tributaries of the middle reaches of the Mazaruni River, Essequibo River basin, Guyana (Fig. 4).
Ecological notes. The species was collected in black water channels (Fig. 5) between 25 and 75 m wide. Water characteristics at the two sites were similar, with temperature between 26.7-29 ºC, pH of 5.1, extremely low conductivity between 4-12 µS/m and secchi depth between 65-85 cm. Both sites had bottom consisting of a mixture of sand and coarse gravel, likely a semi-natural substrate transformed at least partially by ongoing or past gold mining activities in the area (see Discussion). Specimens were collected near large rocks or submerged woody debris at depths of 1 m or deeper.
Etymology. The species is named in honor of American ichthyologist Dr. Donald C. Taphorn in recognition of over four decades of continuing contributions to Neotropical ichthyology, his expansive role in training South American ichthyologists (including the authors), and for his participation in the expedition to the middle Mazaruni during which the new Myloplus was collected.

Discussion
Herbivorous serrasalmids are known to show dimorphic traits; specifically, males are often distinguished by presenting red patches scattered over the flanks, having a dorsal fin with elongated filaments, the branched rays of the anal fin prolonged forming an apex on the middle of the fin, and frequently the presence of stiff hooks on the distalmost anal lepidotrichia (Andrade et al., 2016c;Pastana et al., 2017). Males and females of Myloplus taphorni are readily distinguished by the coloration of the males, which consist of striking purplish silver flanks with conspicuous black overtones composed by scales intensely pigmented and often with a metallic green sheen, whereas females are mainly bluish silver without any evident pigmentation or markings (Fig. 3). The presence of evident marks on males of Myloplus taphorni is reminiscent of the congeners M. asterias and M. planquettei, however, the coloration pattern exhibited by males M. taphorni is unique in having many scales fully pigmented in black forming a vermiculated pattern around a distinct silvery or yellowish region that may present black spotting on and below the lateral line (Fig. 2). In turn, males of M. asterias and M. planquettei show some black aureoles or near ocellated spotting with reddish yellow inside or amorphous patches fully blackened and covering most of the flanks. Even though M. taphorni shares a general coloration pattern with M. asterias and M. planquettei and all three species are putatively syntopic, the new species is readily distinguishable on the basis of its coloration, particularly among males, and by meristic counts of the unpaired fins and/or relative length of body subunits. Additionally, Myloplus taphorni of both sexes display a conspicuous vertical black band through the eye, which contrasts sharply with a light yellowish sclerotic on the anterior and posterior portions of the eye. In M. asterias and M. planquettei the sclera is homogeneously pigmented and invariably without a vertical stripe. The vertical band crossing the eye is registered in literature in carnivorous piranhas such as species in the genera Pygocentrus Müller, Troschel, 1844 and Serrasalmus Lacepède, 1803 (Géry, 1977), and in Metynnis and Catoprion Müller & Troschel, 1844 (Rafaela P. Ota, 2019, pers. comm.). The coloration of Myloplus taphorni probably led to its initial identification as the possibly syntopic Myleus pacu. However, coloration in Myleus pacu consists of a solid terracotta red background, with males showing brownish black patches scattered on the flanks. Jardine (1841) described Myletes pacu from drawings by Robert H. Schomburgk, who illustrated a pair of specimens from somewhere in the Essequibo River basin, both predominantly red colored with the male presenting some black patches. Jardine (1841) reported the sizes for these two Myletes pacu, the male reaching 400 mm and the female about 600 mm in total length, and the presence of 43 total rays in the anal fin for both fish. Eigenmann (1912) redescribed the genus Myleus -following the observations of Jardine (1841) on the Schomburgk's drawings-, then suggested Myletes pacu, Myleus setiger Müller, Troschel, 1844 and Tometes trilobatus Valenciennes, 1850 as synonyms of Myleus pacu, and diagnosed it as species having a marked ventral keel as juveniles and becoming less evident with growth. Jégu, Santos (2002) revisited the taxonomic status of Myleus based on analyses of type-specimens and examined additional material and illustrations, and considered Myleus setiger as a different species from Myleus pacu. However, Jégu, Santos (2002) did not examine any specimen as large in length as those reported by Jardine (1841) at about 600 mm for Myletes pacu, with the maximum size reached by Myleus setiger at about 300 mm SL. Jégu, Santos (2002) redescribed Myleus setiger, and further mentioned that the body of Myletes pacu in Schomburgk's illustrations is very elongated and that number of rays in the anal fin described by the author is too high for any serrasalmid (43 vs. overall maximum of 38 in specimens of Myleus setiger, for example). Therefore, Myleus setiger which was in synonymy of Myleus pacu since Eigenmann (1912) was revalidated by Jégu, Santos (2002). In turn,  considered Myleus pacu as a doubtful species due to the absence of type specimens to support its validity, and the name was listed as species inquirenda in the Check list of the freshwater fishes of South and Central America 'CLOFFSCA' .
Myloplus taphorni appears to be an endemic species with a distribution likely limited to the middle reaches of the Mazaruni river basin in Guyana. Examination of a large amount of collections of Myloplus in natural history collections with large holdings from Guyana (ROM, ANSP, UMMZ) failed to reveal any specimens beyond the ones included in this paper. The Mazaruni River basin harbors a large proportion of endemic taxa, particularly in its upper section (Alofs et al., 2014). The middle reaches of the river are less explored: previously unidentified species of Cichlidae and Crenuchidae were also encountered in association with the Myloplus species described herein, and are in the process of being named (HLF, EAL et al., unpublished data). Unfortunately, the long-term conservation of this fish diversity is far from guaranteed. Nearly the entirety of the Mazaruni basin is extensively mined for gold using placer techniques that move and relocate vast amounts of river channel substrate or riparian soil. Our field observations revealed large-scale reconfiguration of the river consisting of homogenization of the substrate in main channels, a proliferation of "tailings beaches" artificially created by relocation of river substrate, and high turbidity caused by siltation from mining operations.
While most of this large-scale habitat remodeling is centered in the channel of the Mazaruni, tributaries such as the type locality area in the Kurupung River are also affected. Eping Creek, the only other locality where the new species was collected appears to not be mined at this time, but remains of mining equipment and clear reconfiguration of the banks and main channel indicate that mining must have been extensive in the past. More positively, it appears that Eping creek has, to an extent, recovered from some of the mining-related impacts on habitat, particularly in the reduction of suspended solids in the water column. Nevertheless, it is clear that even after mining stopped, the structure of the substrate remained an artificial mixture of sand and "tailings" (evenly sized pebbles and gravel resulting from the mining process) and that a majorly reconfigured channel structure remains in place. facilitating a loan of the specimens from ROM during a visit by MCA. We are especially grateful to Mary Burridge, Erling Holm, Margaret Zur and Donald Stacey (ROM) for handling the various transfers of specimens, for providing x-ray images, and especially for over a decade of exceptional curatorial and field support to HLF. We thank Donald C. Taphorn, Devya Hemraj, Mark Ram, Kavita Dookram and Jevaun Correia for help in the field, and Priya Maharaj (CSBD) for facilitating fieldwork and obtaining collecting and export permits related to this work. Thanks to Fernando Dagosta (UFGD) for providing us valuable suggestions and recommendations to improve the original manuscript. MCA is funded by Programa Nacional de Pós-Doutorado of the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -Finance Code 001 -addressed to the Programa de Pós-Graduação em Ecologia Aquática e Pesca of UFPA (PNPD/CAPES # 06/2017). HLF was funded by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada between 2008 and 2017, and various grants from the Royal Ontario Museum to perform fieldwork in Guyana between 2008 and 2016. Fieldwork in the Middle Mazaruni was funded by a UGSTSP grant to study the effects of gold mining on fish communities from the University of Guiana to EAL. Fieldwork in Guyana and Suriname and support during the preparation of this paper came from startup funds from the University of Michigan to HLF.