The emerald green tetra: a new restricted-range Hyphessobrycon (Characiformes: Characidae) from the upper rio Juruena, Chapada dos Parecis, Brazil

A new species of Hyphessobrycon is described from the rio Mutum, a tributary of the rio Juruena, rio Tapajós basin, Brazil. The new taxon can be distinguished from its congeners by the presence of a well-defined and relatively narrow dark midlateral stripe on body, extending from head to the middle caudal-fin rays, presence of a humeral blotch, distal profile of the anal fin falcate in males, 13–16 branched anal-fin rays (vs. 17–26), and 11 or 12 horizontal scale rows around caudal peduncle. The new species shows polymorphism regarding the presence of the adipose fin, and a discussion on this type of polymorphism across the family and its systematic implications is presented.


INTRODUCTION
Hyphessobrycon Durbin, 1908 is one of the most species-rich genera of the Characidae, currently with more than 160 valid species (Marinho et al., 2016;Carvalho et al., 2017;Pastana et al., 2017;Faria et al., 2020;Fricke et al., 2022). It is distributed from southern Mexico to the rio de La Plata in Argentina, but most species occur in the Cis-Andean region, mainly in the Amazon basin, where it is the second most rich genus (Dagosta, de Pinna, 2019). Besides being diverse in the number of species and in body shape diversity, the group is also plenty in color variation. It ranges from the flame-colored rosy-tetra clade to the elegant Hyphessobrycon loweae-group (sensu Ingenito et al., 2013), with elongate fins, or to the contrasting colored, dark banded species such as the Hyphessobrycon heterorhabdus species-group (sensu Faria et al., 2021).
A new species was collected during an expedition to the headwaters of the upper rio Juruena, rio Tapajós basin, at Chapada dos Parecis, Brazil. The new species is remarkable in coloration and has been exported by aquarists under the name Hyphessobrycon melanostichos Carvalho & Bertaco, 2006. It is popularly known as 'emerald green tetra'. Additional specimens were recognized in the MZUSP collection and examined. The objective of the present work is to describe the new species and to evaluate its conservation status. Because the new taxon is polymorphic regarding the presence of adipose fin, we also discuss the application of this character in the systematic of characids.

MATERIAL AND METHODS
Counts and measurements follow Fink, Weitzman (1974), except for not including eyedorsal fin origin measurement and for number of horizontal scale rows below lateral line, which are counted to the pelvic-fin insertion, not including the small scale at pelvic-fin insertion, and with the addition of head depth, measured at vertical through the base of supraoccipital process. Standard length (SL) is given in millimeters (mm) and all other measurements are expressed as percentage of SL or of head length (HL) for subunits of head. In the description, counts are followed by their frequency of occurrence in 3/15 ni.bio.br | scielo.br/ni Fernando C. P. Dagosta, Thomaz J. Seren, Anderson Ferreira and Manoela M. F. Marinho parentheses, and an asterisk indicates the counts of the holotype. Number of maxillary tooth cusps, small dentary teeth, supraneurals, branchiostegal rays, gill rakers, vertebrae, unbranched anal-fin rays, and procurrent caudal-fin rays were obtained only from cleared and stained (c&s) specimens prepared according to Taylor, Van Dyke (1985). Vertebrae of the Weberian apparatus are counted as four elements and the compound caudal centra (PU1+U1) as a single element. Circuli and radii counts were taken from scale row immediately above the lateral line. The sex of specimens was confirmed by dissection and direct examination of the gonads. Diet was checked in 20 individuals from the type locality (22.3-35.0 mm SL). In the list of types and comparative material, catalog numbers are followed by the number of specimens in alcohol, their SL range, and if any, the number of c&s specimens and their respective SL range. Institutional abbreviations follow Sabaj (2020).  Travassos, 1964, H. cyanotaenia Zarske & Géry, 2006, H. fernandezi Fernández-Yépez, 1972, H. melanostichos, H. nigricinctus Zarske & Géry, 2004, H. paucilepis García-Alzate, Román-Valencia & Taphorn, 2008, H. petricolus Ohara, Lima & Barros, 2017, H. piranga Camelier, Dagosta & Marinho, 2018, H. psittacus Dagosta, Marinho, Camelier & Lima, 2016, H. scholzei Ahl, 1937 Román-Valencia &Taphorn, 2008, andH. vilmae Géry, 1966 by the presence of a well-defined and relatively narrow dark midlateral stripe on body, extending from head to the middle caudal-fin rays (vs. well-defined longitudinal stripe absent, or stripe wider than the orbit, or stripe starting approximately vertically through the origin of the dorsal fin or stripe blurred posteriorly  peduncle (vs. 14), fewer branched pelvic-fin rays (6 vs. 7), humeral blotch wider than deep, with pigmentation much more intense than the dark midlateral band, with well-defined edges (vs. humeral blotch deeper than wide, with pigmentation similar to the dark midlateral band, without well-defined edges). Another useful character in distinguishing H. comodoro from H. melanostichos is the presence of 13-15, mode 14, rarely 16 (only 3 of 30 specimens), branched anal-fin rays (vs. 16-18, mode 16).

RESULTS
Description. Morphometric data in Tab. 1. Body compressed, moderately elongate. Greatest body depth at dorsal-fin origin. Dorsal profile of head convex from upper lip to vertical through posterior nostril; slightly convex from that point to base of supraoccipital spine Dorsal profile of body convex along predorsal region, slightly convex along dorsalfin base, straight from terminus of dorsal-fin base to adipose-fin origin, and slightly concave to straight from that point to origin of anteriormost dorsal procurrent caudal-fin ray. Ventral profile of head and body convex from tip of lower lip to pelvic-fin origin, slightly concave between latter point to anal-fin origin, somewhat straight to convex (see Sexual Dimorphism section) along anal-fin base, and concave from the terminus of anal fin to origin of anteriormost ventral procurrent caudal-fin ray.

Color in alcohol.
Overall ground coloration of head and body beige (Fig. 1). Some specimens retaining guanine on opercular region. Dorsal portion of head and dorsal midline of body dark. A reticulated pattern on first three to four horizontal scale rows, formed by concentration of chromatophores on posterior margin of scales. Snout, jaws and 1 st and 2 nd infraorbitals with concentration of dark chromatophores, 3 rd and 4 th infraorbitals with scattered dark pigmentation and 5 th and 6 th infraorbitals densely pigmented with dark chromatophores, continuing with dark midlateral stripe. Roughly inverted teardrop-shaped humeral blotch formed by two layers of pigmentation. Superficial layer darker and conspicuous, overlapping midlateral stripe and encompassing approximately four scales horizontally and one or two vertically. Subjacent layer with scattered pigmentation encompassing approximately three scales vertically and forming a ventral projection to the humeral spot with diffuse borders. Dark midlateral stripe on body, extending from upper half of posterior portion of eye to tip of middle caudal-fin rays. Abdominal region with few scattered chromatophores. Sparse dark chromatophores above anal fin, mainly near anal-fin base. Caudal-peduncle blotch absent. Adipose fin with scattered dark chromatophores. All fins with dark chromatophores scattered along edge of lepidotrichia. Dorsal and anal fins with dark pigmentation on interadial membranes. Some specimens with sparse dark pigmentation on pelvic-fin interadial membranes.

Color in life.
Middorsal area olive green (Figs. 1C, 2); abdominal region silvery to yellow, with some specimens with orange pigmentation in the ventral portion. Upper portion of eye yellow to red, upper-posterior region dark pigmented. First and second infraorbitals, maxilla, lower jaw, gular area and preopercle with yellow pigmentation and scattered orange chromatophores. Remaining infraorbitals mostly silvery and with 7/15 ni.bio.br | scielo.br/ni Fernando C. P. Dagosta, Thomaz J. Seren, Anderson Ferreira and Manoela M. F. Marinho sparse orange cromatophores. Some specimens with lower portion of opercle lacking guanine, exposing red branchial filaments inside branchial chamber. Bright green midlateral stripe above and below the dark midlateral stripe, thicker at region above anal-fin base. All fins vivid orange to red coloration, more intense in caudal and anal fins. Adipose fin pale hyaline to pale yellow.
Sexual dimorphism. Males with anal-fin base slightly convex (vs. somewhat straight in females). Dark midlateral stripe in males wider and blurred (vs. midlateral stripe relatively narrow and with more defined edges in females), a type of sexual dichromatism involving the larger concentration of melanophore-based pigments in males (Pastana et al., 2017). Bony hooks on fins not present.  The diet was mainly composed of resources autochthonous (91.6% of the volume of food items), mainly vegetable fragments (57.7%) and aquatic insects (32.7%). The vegetable fragments were composed of aquatic macrophyte structures and aquatic insects (fragments of adults, larvae, and pupae of Diptera and larvae of Trichoptera, and Odonata).
Specimens analyzed were sampled in a region under moderate anthropogenic pressure, which may influence the diet of fish species. Further, damming streams to road buildingchanges the taxonomic and functional of fish assemblages and limits the longitudinal dispersion (Brejão et al., 2020). The transformation from lotic to lentic environments, with an increase in the width of the canopy-opening channel, creates pelagic and benthic areas that allow the proliferation of macrophytes and algae (Brejão et al., 2020). Biological data taken from regions impacted by human action has high scientific value, but in the case of this species, it is crucial that data also be available from less impacted environments.
Etymology. The name comodoro is in reference to the Municipality of Comodoro, Mato Grosso State, where all the specimens were collected. It is also the name of a senior naval rank used in many navies, which inspired the municipality's name. A noun in apposition.

Conservation status.
Hyphessobrycon comodoro is endemic to Brazil and is a restricted-range species, a common pattern among endemic characids of the 'Chapada dos Parecis' biogeographic region (Dagosta et al., 2020). Despite such biogeographic region was considered by those authors as one of the Endemic Amazonian Fish Areas (EAFAs), i.e., regions that should be considered as conservation priorities in the basin by presenting imminent threats and a low cover of protected areas, the new species is endemic to one of the most preserved river basins draining the Cerrado biomethe rio Mutum drainage. Hyphessobrycon comodoro is so far known by three localities, but its EOO (Extent of occurrence) is likely underestimated since only the headwater of the rio Mutum basin was sampled. Most of the rio Mutum basin lies within the The emerald green tetra: a new species of Hyphessobrycon Nambikwara indigenous territory, where H. comodoro is likely to occur. Despite it has been exported in the aquarium trade it remains abundant in collection sites, which, as far as we know, are the same as those fished by the professional fishermen. Therefore, this species is assessed as Least Concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2019).

DISCUSSION
The adipose fin is variably developed in Hyphessobrycon comodoro, with few specimens lacking it (4 out of 27). Among Characiformes, absence of adipose fin is relatively uncommon and occurs in species of different lineages of the order (Mirande, 2019;Mattox et al., 2020). Among more than 6,000 living Teleostei species bearing adipose fin (Stewart et al., 2014(Stewart et al., , 2019, Characiformes is the only order in which its developmental pattern differs. In the Characiformes, the adipose fin develops de novo, i.e., the fin appears after the reduction of the median larval finfold, whereas in the other orders, it develops by the retention of the larval finfold between the dorsal and caudal fin (Fuiman, 1983;Bender, Moritz, 2013;Marinho, 2017;Stewart et al., 2014Stewart et al., , 2019. Absence of adipose fin in Characiformes is more frequent among miniature to smallsized species and has long been related to miniaturization (Weitzman, Malabarba, 1999;Bührnheim et al., 2008), although large species may also lack it (e.g., Erythrinidae, Mattox et al., 2020). Its absence in miniature to small species is probable a consequence of truncation in their development during the evolution of small-body size, in which late-forming structures, such as the "de novo" formation of the adipose fin, are the first to be lost (Marinho, 2017). Besides, morphological variability of characters formed in late developmental stages is also associated with body-size reduction, resulting in intrapopulational variation of that structure (Hanken, Wake, 1993;Marinho et al., 2021). Intraspecific variation regarding presence/absence of adipose fin has been documented for miniature to small characids (Tab. 2) and are herein interpreted as a consequence of developmental truncation. Polymorphisms are not equivalent though. The frequency of the presence of adipose fin varies among species (Fig. 5), evidencing this is a very labile character for some taxa.
Polymorphism regarding presence of adipose fin directly affect decisions on the systematic of characiforms, especially the family Characidae. This is because its absence or presence is still widely used to diagnose genera and/or species in the family. For example, Hasemania Ellis, 1911, was originally defined as "like a Hyphessobrycon, but without an adipose" (Ellis, 1911), despite species of both genera present intraespecific variation in this character (Tab. 2; Fig. 5), evidencing the fragility of such definition. Therefore, the use of such labile character in systematics needs to be made with caution. Polymorphism in the presence of adipose fin in Hyphessobrycon comodoro, along with the still poorly known interspecific relationships of large polyphyletic genera within Stethaprioninae (see Mirande, 2019) raise questions on the allocation of the new species in Hyphessobrycon. The monophyletic nature of the genus has long been disputed (Weitzman, Fink, 1983;Weitzman, Palmer, 1997;Mirande, 2010Mirande, , 2019 and today, Hyphessobrycon is largely accepted as polyphyletic. However, some groups are likely monophyletic, such as the rosy-tetra clade (see Mirande, 2019). The evidence of monophyly of the rosy tetras has long been proposed, based in the unique coloration pattern shared by its species (Weitzman, Palmer, 1997), and was confirmed in recent cladistic works (e.g., Mirande, 2019). Unfortunately, only a restricted sample of Hyphessobrycon could be included, not representing the whole diversity of the genus. Therefore, still needing efforts to advance the knowledge of the phylogenetic relationships of the species now attributed to Hyphessobrycon. As consequence, the composition, and limits of Hyphessobrycon remain open to question. Despite generic allocation to be tentative, the new species can be distinguished from all remaining characids by the combination of a well-defined and relatively narrow dark midlateral stripe on body extending from the upper half of the posterior margin of the eye to the middle caudal-fin ray, orange fins, a total of 33-35 scales on the longitudinal series, in which only few of them are perforated.
It is premature to infer a close evolutionary relationship between the new species with other characids, but morphological features may indicate it is more closely related to species nowadays allocated in Hyphessobrycon.  half of the posterior margin of the eye, with a green to bluish stripe above it and base of anal fin sexually dimorphic, convex in males. Besides very similar morphologically, H. comodoro and H. melanostichos occur very close to each other in neighboring tributaries of the rio Camararé, Juruena river basin. They also share the fact of having a very restricted known distribution, with H. melanostichos so far known only from the rio Doze de Outubro and H. comodoro from the rio Mutum. Populations of H. melanostichos from shield tributaries of the rio Madeira basin (e.g., rio Cabixi, rio Machado) are probably closely related undescribed species that are being studied (FCPD, pers. obs.).
The description of an additional new species already known worldwide in the aquarium trade reveals how scarce is the knowledge on the diversity of Neotropical freshwater fishes (Reis et al., 2016;Albert et al., 2020). Despite being widely sampled in the last decade, the Chapada dos Parecis still provides new and endemic taxa that reinforces the status of being a biogeographic region distinct from the rest of the Amazon basin (Dagosta, de Pinna, 2019;Dagosta et al., 2020).