A new species of Serrapinnus Malabarba, 1998 (Characidae: Cheirodontinae) from Rio Grande do Norte State, northeastern Brazil

Serrapinnus potiguar, new species, is described from the rio Ceará-Mirim, a coastal drainage in the Rio Grande do Norte State, northeastern Brazil. The new species is distinguished from the other species of the genus by the shape and arrangement of the ventral procurrent caudal-fin rays of the sexually dimorphic males; where the hypertrophied elements present the shape of a series of scimitars arranged radially, forming a semi-circle on the ventral margin of the caudal peduncle. Furthermore, the new species is diagnosed from S. heterodon and S. piaba, sympatric congeners from the northeastern Brazilian drainages, respectively by the presence of incomplete lateral line and teeth bearing at most five cusps.


Introduction
The Neotropical freshwater genus Serrapinnus includes ten species distributed on the main Cis-Andean basins of South America (Malabarba, 2003;Zarske, 2012).The genus, erected to host a monophyletic assemblage of cheirodontines previously assigned to the genera Cheirodon Girard, Odontostilbe Cope, and Holoshesthes Eigenmann, was diagnosed by Malabarba (1998) along with the cladistic definition of the subfamily Cheirodontinae.In that work, Serrapinnus, a member of the tribe Cheirodontini, was supported by a series of synapomorphies related to the sexual dimorphism presented by mature males, such as the presence of a ventrally arched caudal peduncle and main axis of the ventral procurrent caudal-fin rays not supported by the parhypural, perpendicular to the longitudinal axis of the body, while the most anterior elements are anteriorly directed (Malabarba, 1998).Moreover, the species of the genus present a reduction of the number of pored scales on the lateral line, with a single exception found in S. heterodon (Eigenmann).A different hypothesis of relationship for the members of the Cheirodontinae was proposed by Mariguela et al. (2013) based on molecular inferences, where the monophyly of Serrapinnus and some major genera of the subfamily was questioned and not supported.
Two species of Serrapinnus commonly inhabit the northeastern coastal drainages of Brazil: S. heterodon and S. piaba.A third species presenting the morphological synapomorphies of the genus had been previously identified by LRM for the rio Ceará-Mirim in the Rio Grande do Norte State (Dias & Fialho, 2009), and it is herein described.

Material and Methods
Counts and measurements follow Fink & Weitzman (1974) except by the number of scale series below the lateral line, which was counted from the series just below the lateral line to the scale bordering anteriorly the pelvic-fin origin.Measurements New Serrapinnus from Northeastern Brazil 302 were made with a caliper 0.01 mm precision under a microscope, and are presented as percentages of standard length (SL), or head length (HL) for subunits of the head.Teeth, tooth cusps, gill rakers, supraneurals, procurrent caudal-fin rays, and vertebrae were counted on specimens cleared and stained (c&s) according to Taylor & Van Dyke (1985) procedure.The four vertebrae of the Weberian apparatus and the terminal centrum were counted as a single element (Weitzman & Malabarba, 1999).On the description counts are followed by the number of occurrence in parentheses, and the value observed on the holotype is marked with asterisk.Images of jaws and teeth were taken on a Scanning Electron Microscope (SEM).Pictures of the ventral procurrent caudal-fin rays and anal fin were taken from cleared and stained specimens with a camera connected to a microscope.Nomenclature follows Weitzman (1962) and Zanata & Vari (2005)
Color in alcohol.Overall body coloration yellowish to dun.Head darker over dorsal region of neurocranium, snout and maxilla, due to higher concentration of dark chromatophores.Dark brown chromatophores sparsely scattered over infraorbitals and opercular apparatus.Opercular apparatus, some infraorbitals and branchiostegal rays silver in some individuals.Ventral region of head and abdomen light.Humeral region with triangular darkened area due to pseudotympanum muscular hiatus.Longitudinal line of dorsum darker than lateral of body.Body scales with higher concentration of black chromatophores on distal margin, resulting in reticulated coloration pattern more conspicuous on upper portion of body.Longitudinal black subcutaneous gradually decreasing in size posteriorly.Adipose-fin origin slightly posterior to vertical through base of last anal fin.Anal-fin origin posterior to vertical through base of last dorsalfin ray.Anal-fin rays ii(5), iii*(17), or iv(7), 17(1), 18(10), 19*(16), or 20(4).Anal-fin distal profile rounded on anterior lobe and concave posteriorly on females; pointed on anterior lobe and deeply concave posteriorly on males.Last unbranched and first two branched anal-fin rays longer, remaining rays Ecological notes.The type locality of Serrapinnus potiguar in the rio Ceará-Mirim (05°37'47''S 35°37'09''W) was described by Dias & Fialho (2009) as clear and transparent water, sandy bottom high depth 1.2 m.The water current varied from slow to line on lateral of body extending from vertical through dorsalfin insertion to caudal peduncle spot.Black chromatophores aligned and following line of miosepta on body above anal fin.Caudal spot black, slightly rounded, occupying most of caudal peduncle termination, not reaching upper or lower margins of caudal peduncle.Dorsal, anal, pectoral, pelvic, and caudal fins mostly hyaline, with dark chromatophores scattered on interradial membrane.Unbranched dorsal-fin rays darker than branched rays due to higher concentration of black chromatophores.Adipose fin mostly hyaline, with few black chromatophores distributed near dorsal margin.Caudal fin with clear yellowish areas on base of caudal-fin lobes, bordering caudal-fin spot posteriorly.
Color in life.Dark pigmentation as described on alcohol preserved specimens.Body light olive brown slightly translucent.Pectoral fin slightly orange or red; dorsal, anal, pelvic and caudal fins intense orange or red (Fig. 5).Distal tip of longest anal-and pelvic-fin rays white or hyaline.Middle caudal-fin rays hyaline.
Sexual dimorphism.Bony hooks were only observed on pelvic and anal-fin rays of sexually dimorphic males, as well as hypertrophied and ventrally exposed procurrent caudalfin rays (Fig. 2a).Sexually dimorphic males also present those anal-fin rays that bear bony hooks hypertrophied and expanded in the sagittal plane (Fig. 4).On fixed and preserved specimens the caudal peduncle is generally found arched ventrally only on adult males (Fig. 1a).Furthermore, sexually mature males have gill glands, located on the anteriormost portion of the lower branch of the first gill arch, extending posteriorly through six to eight of gill filaments.piaba) (S. calliurus (S. calliurus + S. notomelas)))).Species of Serrapinnus present a unique set of morphological characters related to the number and shape of the ventral procurrent caudal-fin rays, sexually dimorphic hypertrophy of these structures and ventral curvature of the caudal peduncle on males, which are not found in any other genera of the Cheirodontinae (Figs. 2a;Malabarba, 1998: figs. 4, 13 a,b).
Those characters, as well as the hypertrophy of the anal-fin rays of mature males and the remaining synapomorphies shared by Serrapinnus and other genera of the tribe Cheirodontini sensu Malabarba, 1998(Bührnheim et al., 2008) are absent in all members of Compsura and Odontostilbe.Secondary sexual dimorphism in Compsura as well as in other members of the Compsurini is expressed in other body parts, mostly by the presence of hooks, hypertrophied scales and hypertrophied tissues on caudal fin (Eigenmann, 1915;López, 1972;Fink & Weitzman, 1974;Malabarba, 1998;Malabarba & Weitzman, 1999;Malabarba et al., 2004;Jerep & Malabarba, 2011), that are absent in Serrapinnus.Conversely, Odontostilbe species lack most secondary sexual characters observed in the representatives of the Cheirodontini and Compsurini, being otherwise characterized by the elongation of unbranched rays of the dorsal and pelvic fins (Bührnheim & Malabarba, 2006, 2007).The acceptance of Mariguela et al. (2013) hypothesis would require the recognition that the whole set of secondary sexual characters of both the Cheirodontini, the Compsurini and of Odontostilbe had multiple origins in the Cheirodontinae.Mariguela et al. (2013: 32) also argues that the expansion of the three posteriormost ventral procurrent caudal-fin rays forming a small caudal keel in Serrapinnus calliurus, S. kriegi, S. notomelas, and S. piaba "reinforc(es) the hypothesis of their possible close relationship".This assumption lacks phylogenetic support and fails as a premise to justify the relationship of those species in the cladogram found in their molecular study for two reasons: 1) S. kriegi is sister group of the remaining species of that clade, hence it is as related to the species of Serrapinnus as to Compsura heterura and to the species of Odontostilbe; 2) according to their cladogram, the independent acquisition of such character in those four species of Serrapinnus is more parsimonious than a hypothetic synapomorphy for the clade with reversion in Compsura heterura and species of Odontostilbe.In this context, this character does not support their hypothesis of relationships among those species as proposed.
Serrapinnus potiguar presents all the synapomorphies proposed by Malabarba (1998: 207-211) to the subfamily Cheirodontinae, to the tribe Cheirodontini, as well as the synapomorphies proposed for the genus (Malabarba, 1998: 216).Serrapinnus potiguar is the only known species of the Cheirodontinae whose mature males present scimitarshaped ventral procurrent caudal-fin rays (Fig. 2), a condition moderate and in that stretch the river presented a large amount of floating and submerse vegetation.In that locality S. potiguar is sympatric and syntopic with the cheirodontines Compsura heterura, S. heterodon and S. piaba; and its diet was mainly composed by vegetal matter and algae, but also by insects and microcrustaceans (Dias & Fialho, 2009).

Discussion
The new species is herein assigned to the genus Serrapinnus as diagnosed by Malabarba (1998).Mariguela et al. (2013), in a phylogenetic analysis based on molecular data, proposed Serrapinnus as polyphyletic, with its species distributed among different clades in the Cheirodontinae, including genera of both tribes Cheirodontini and Compsurini, as well as incertae sedis cheirodontine genera sensu Malabarba (1998).Based on their results, Mariguela et al. (2013) discussed species relationships and tentatively assigned a few selected morphological characters that agreed with the topology of their cladogram to support some of their hypotheses of relationships, but failed to find characters defining several of their clades, as well as their newly defined tribes Cheirodontini, Compsurini and for their new tribe Pseudocheirodontini.Our main criticism to Mariguela et al. (2013) is that they ignored and did not discuss the large amount of evidences, including those associated to the primary and secondary sexual systems, that supports the recognition of the tribes Cheirodontini and Compsurini sensu Malabarba (1998), as well as several cheirodontine genera.We will exemplify those problems with Serrapinnus that is directly related to the taxonomic decisions of this paper.Mariguela et al. (2013) included six species of Serrapinnus in their analysis, S. calliurus, S. heterodon, S. kriegi, S. microdon, S. notomelas, and S. piaba.Those species were not recovered as a natural lineage.Instead, S. microdon was found to be sister group of (Odontostilbe pequira (Odontostilbe sp.2 + Serrapinnus heterodon)); and in another distant clade S. kriegi was found as sister group of (Odontostilbe fugitiva (O.ecuadorensis + O. paraguayensis) + ((C.heterura + S. considered herein to be an autapomorphy of that species.On the remaining species of Serrapinnus, the hypertrophied ventral procurrent caudal-fin rays on mature males are rod-shaped with an acute distal tip (Zarske, 2012: fig. 3), or spatulate, with a rounded or square-shaped distal tip (Malabarba, 1998: fig. 13ab).On both conditions mentioned above each ventral element present few variation in width along its length; nevertheless in S. potiguar, those elements are noticeable expanded on the longitudinal axis, reaching maximum width in the muscular ventral line of the caudal peduncle, than narrowing distally as they project outside the body (Fig. 2).
On northwestern Brazil, Serrapinnus potiguar is sympatric and syntopic with S. heterodon and S. piaba in the rio Ceará-Mirim.According to Dias & Fialho (2009) these three species present an overlap in their diet all year long.Serrapinnus heterodon and S. potiguar are omnivores species with a very similar diet based on algae, vegetal matter and invertebrates, while S. piaba, also classified as omnivore, has a tendency to herbivory.Moreover, Compsura heterura also occurs in the same locality and shares with S. piaba the same feeding habit.The coexistence of these species partitioning the same alimentary source can be explained by two possible hypotheses: the abundance of food resources and/or the existence of a spatial or temporal segregation, which may avoid interspecific competition (Dias & Fialho, 2009).
body depth at vertical through dorsal-fin origin.Snout slightly rounded.Dorsal profile of head convex from snout to vertical through posterior border of nares, slightly convex from that point to distal tip of supraoccipital bone.Predorsal profile convex from posterior end of supraoccipital to dorsal-fin origin.Dorsal-fin origin located at vertical at midlength of standard length.Dorsal profile from last dorsal-fin ray to adipose-fin base slightly convex in females and strongly ventrally arched on preserved mature males.Caudal-peduncle dorsal profile slightly concave from adipose-fin origin to caudal-fin origin.Caudal-peduncle ventral profile straight to slightly concave in females, pronounced convex with exposed bony spines due to hypertrophy of ventral procurrent caudal-fin rays on males.Caudal peduncle slightly longer than deep.Ventral profile of head slightly convex from mouth to pelvic-fin origin; straight from that point to anal-fin origin in females and concave in mature males.Anal-fin base straight to slightly concave in females, convex anteriorly and deeply concave posteriorly on mature males.Head pointed on lateral view.Mouth terminal, at horizontal line through middle of pupil.Maxilla angled posteroventrally, posterior tip reaching vertical through anterior border of eye and surpassing ventrally the horizontal through ventral border of eye.Teeth pedunculated, distally expanded, with similar shape and cusp number.Four (1), 5(7), or 6(2) premaxillary teeth aligned in one single row, bearing 5 cusps.Midcentral cusp longer and wider than lateral ones.Three (3) or 4(7) maxillary teeth bearing 3 to 5 cusps.Six (8) to 7(2) large dentary teeth bearing 5 cusps, followed posteriorly by one smaller tooth with 3 to 4 cusps and one or two conical teeth.Central cusp larger than lateral cusps.All dentary tooth cusps slightly directed lingually (Fig.3).Dorsal-fin origin on midlength of standard length.Dorsal-fin rays ii,9*(31).First unbranched dorsal-fin ray about half length of second unbranched dorsal-fin ray, following branched rays

Fig. 4 .
Fig. 4. Anal fin of a mature male of the Serrapinnus potiguar, paratype, UFRGS 9216, 27.4 mm SL, showing hypertrophied anal-fin rays and hooks shape and distribution.Lateral view, anterior to the left.Scale bar = 1 mm.
for bones.Museums abbreviations are: CAS -California Academy of Sciences, San Francisco, USA; FMNH -Field Museum of Natural History, Chicago, USA; MCP -Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, 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.Measured and counted specimens are indicated between parentheses in the list of paratypes or comparative material.

Table 1 .
Morphometric data for Serrapinnus potiguar.N = number of specimens; SD = Standard deviation.Male range includes the holotype.