A new miniature killifish of the genus Melanorivulus (Cyprinodontiformes: Rivulidae) from the coastal plains of north-eastern Brazil

Melanorivulus atlanticus, new species, is described on the basis of specimens collected in the coastal plains of north-eastern Brazil, constituting the first record of the genus for the Atlantic Forest biogeographical province. The new species together M. decoratus and M. jalapensis form a clade of small species, not surpassing 20 mm SL, mainly diagnosed by the presence of five branchiostegal rays. Melanorivulus atlanticus differs from those two species by a narrow black border along the entire caudal fin in males and the presence of 2 + 1 neuromasts in the mandibular series, besides differing from all congeners by having sexually dimorphic pelvic fin, which is pointed and with seven rays in males, elliptical and with five rays in females. The description of M. atlanticus expands in 670 km to east the known geographic range of the genus.


Introduction
The genus Melanorivulus Costa, 2006 comprises a diversified clade of South American small killifishes inhabiting the main river basins east of the Andes (Costa, 2011).Prior to 2011, all included species were placed in Rivulus Poey, 1860, then considered a broad generic unit with weak or no morphological support (e.g., Parenti, 1981;Costa, 1998Costa, , 2006a)).After 1999, some molecular studies (e.g., Hrbek & Collier, 1999;MurPHy et al., 1999) have congruently indicated that Rivulus in the broad sense was paraphyletic, with most lineages more closely related to other genera than to Rivulus cylindraceus Poey, 1860, the type species of the genus.On the other hand, some intrageneric assemblages supported both by morphology and DNA sequences were proposed as subgenera, including Melanorivulus among others (e.g., Costa, 2006a).More recent studies have found morphological evidence cor-roborating molecular hypotheses (Costa, 2011), reinforcing a phylogenetic context in which most Rivulus-like species were more closely related to other genera than to R. cylindraceus, a member of a lineage confined to the Greater Antilles.As a consequence, the traditional classificatory scheme with numerous species constituting a paraphyletic genus Rivulus has been substituted by a new generic classification, in which the former subgenera are elevated to well corroborated monophyletic genera (Costa, 2011).
Species of Melanorivulus are usually found in shallow aquatic habitats, often less than 20 cm deep, situated in marginal areas of streams (e.g., Costa, 2005Costa, , 2006b)).Recent intensive field studies directed to these habitats have revealed an astonishing diversity of undescribed species, placing Melanorivulus among the most species-rich and geographically widespread genera of aplocheiloid killifishes (Costa, 2003a(Costa, -b, 2005(Costa, , 2006b(Costa, , 2007a(Costa, -c, 2008(Costa, a -d, 2009(Costa, , 2010(Costa, , 2012a(Costa, -b, 2013;;Costa & brasil, 2008;Costa & De luCa, 2011;Costa et al., 2014).Despite the broad area occupied by the genus, the great majority of species are found in savannah regions, whereas a few ones are found in transitional areas between the Cerrado savannah and the Amazon Forest.The new species herein described is the first member of the genus found in an area within the Biogeographical Province of the Atlantic Forest, consisting in a considerable enlargement of the generic geographical distribution.

Material and methods
Material is deposited in UFRJ, Instituto de Biologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro.Specimens were fixed in formalin just after collection, for a period of ten days, and then transferred to 70 % ethanol.Descriptions of colour patterns were based on photographs of both sides of live individuals of each population taken in small aquaria one day after collection; colour patterns derived from distribution of melanophores on fins were also observed in all preserved specimens.Measurements and counts follow Costa (1988).Measurements are presented as percentages of standard length (SL), except for those related to head morphology, which are expressed as percentages of head length.Fin-ray counts include all elements.Number of vertebrae and gill-rakers were recorded only from the cleared and stained specimen; the compound caudal centrum was counted as a single element.The osteological preparation was made according to taylor & Van Dyke (1985); the abbreviation c&s means specimens cleared and stained for bone and cartilage.Terminology for cephalic neuromast series follows Costa (2001) and for cephalic squamation patterns, HoeDeMan (1958).
Diagnosis.Melanorivulus atlanticus is distinguished from all other congeners, except M. decoratus and M. jalap ensis, by having five branchiostegal rays (vs.six); it is also similar to M. decoratus and M. jalapen sis by having a narrow basihyal, its width about 35 % of the longitudinal length, a condition also occurring in M. paresi, thus contrasting with the wider basihyal, with 45 -60 % of that length, in all other congeners.It is distinguished from M. decoratus, M. jalapensis and all other species of the genus by having sexually dimorphic pelvic fin, pointed and with seven rays in males, elliptical and with five rays in females (vs.never sexual dimorphism in pelvicfin shape, always seven pelvic-fin rays in both sexes).Also distinguished from M. decoratus and M. jalapensis by having a narrow black border along the entire caudal fin in males (vs. a broad dark grey stripe on the ventral margin of the caudal fin in M. decoratus and no distinctive colour on caudal fin margin border in M. jalapensis) and 2 + 1 neuromasts in the mandibular series (vs. 3 + 1).Similar to M. decoratus by having irregularly arranged, vertically elongated black spots on the caudal fin in females (vs.caudal fin dark reddish grey with white short bars and spots in M. jalapensis).Also distinguished from M. decoratus by the presence of oblique red bars restricted to the basal and posterior portions of the anal fin in males (vs.bars extending through most fin extension), and the presence of four to six red bars on the caudal fin in males (vs.eight).1. Largest male examined 17.8 mm SL, largest female 14.9 mm SL.Dorsal and ventral profiles between snout and posterior end of dorsal and anal fins, about straight on of caudal peduncle.Body slender, sub-cylindrical anteriorly, slightly deeper than wide, compressed posteriorly.Greatest body depth at vertical just in front to pelvic-fin base.Jaws short, snout blunt in lateral view.Dorsal and anal fi ns short, extremity slightly pointed in males, rounded in females.Caudal fi n oval, longer than deep.Pectoral fi n rounded, posterior margin reaching vertical at about 90 % of length between pectoral-fi n and pelvic-fi n bases.Pelvic fi n pointed in males, tip reaching between urogenital papilla and base 2 nd anal-fi n ray, elliptical in females, reaching between anus and urogenital papilla; pelvic-fi n bases medially in close proximity.Dorsal-fi n origin on vertical through base of 10 th anal-fi n ray; second proximal radial of dorsal fi n between neural spines of 18 th and 20 th vertebrae, fi rst proximal radial of anal fi n between pleural ribs of 13 th and 15 th vertebrae.Dorsal-fin rays 8 -9; anal-fin rays 12 -13; caudal-fin rays 26 -28; pectoral-fi n rays 11 -12; pelvic-fi n rays 7 in males, 5 in females.

Description. Morphometric data appear in Table
Scales small, cycloid.Body and head entirely scaled, except anterior ventral surface of head.Body squamation extending over anterior 25 % of caudal-fi n base; no scales on dorsal and anal-fi n bases.Frontal squamation F-or sometimes E-patterned; E-scales not overlapping medially; scales arranged in regular circular pattern around A-scale without exposed margins.Longitudinal series of scales 27 -28; transverse series of scales 8; scale rows around caudal peduncle 16.No contact organs.

Discussion
The description of M. atlanticus greatly expands the known geographic range of the genus Melanorivulus, consisting of the eastern-most record and the fi rst collecting site within the Atlantic Forest biogeographical province.Species of this genus have been previously recorded from open vegetation savannah areas, in the biogeographical provinces of the Amazon, Caatinga, Cerrado, Pantanal and Chaco (bastos & lourenço, 1983;Costa, 2005Costa, , 2006aCosta, -b, 2007bCosta, -c, 2008b;;sCHinDler & etzel, 2008;bragança et al., 2012), but the new species was also found in an open vegetation habitat inserted in a coastal Restinga area.Until now, the eastern-most record of the genus was the type locality of M. decoratus (Costa, 1989), situated in the northern portion of the São Francisco river basin, near the village of Ibiraba (Costa, 1989).The type locality of M. atlanticus is placed 670 km E from the type locality of M. decoratus, which probably is the closest relative of M. atlanticus.Melanorivulus atlanticus, M. decoratus and M. jalapensis form a wellsupported clade of miniature species not surpassing 20 mm SL, diagnosed by the presence of five branchiostegal rays (vs.six in all other species of the genus).
. Males.Flank light grey with pale blue iridescence; dark grey to black stripe between postorbital region and caudal-fi n base mainly visible when fi sh is exposed to strong light; irregular oblique red bars, more concentrated on posterior half of body side.Dorsum light brown, venter white.Dorsal portion of head side light brown, ventral portion white.Upper jaw light brown, lower jaw black.Iris pale yellow.Dorsal and anal fi ns fi n pale blue with oblique dark red bars on basal and posterior portions of fi ns; dark reddish grey to black stripe on distal margin of anal fi n.Caudal fi n pale yellow, with 5 or 6 narrow red bars, and narrow black border on whole fi n margin.Pectoral fi n hyaline.Pelvic fi n pale blue with dark red anterior margin.Females.Similar to males, except by unpaired and pelvic fi ns white, with distal black margin on dorsal and anal fi ns, entire black margin on caudal fi n, and black anterior margin on pelvic fi n; rounded black spot on dorsal portion of caudal-fi n base, and vertically elongated black spots on middle of caudal fi n.Distribution and habitat.Known only from the type locality in the coastal plains of northeastern Brazil, between the villages of Lagoa Grande and Pau Seco, Estado de Sergipe.Etymology.The name atlanticus refers to the fi rst record of the genus Melanorivulus for the Atlantic Forest region of northeastern Brazil.