Rivulus albae , a new species of killifish ( Teleostei : Cyprinodontiformes : Rivulidae ) from northeastern Brazil

The killifi sh genus Rivulus constitutes the most speciose and geographically widespread assemblage of aplocheiloid fi shes. It is widely distributed in both Middle and South America, where it occurs from Mexico and the Caribbean islands to Argentina. According to Costa (2006) the subgenus Melanorivulus, a well supported monophyletic clade endemic to central South America, is diagnosed by an osteological feature (dorsal portion of preopercle short and pointed) and three derived color patterns (melanophores concentrated on opercular region form oblique stripes; females with melanophores concentrated on margins of unpaired and pelvic fi ns and with black spot on upper portion of caudal-fi n base not close to fi n margin). Melanorivulus includes 33 valid species (Costa, 2010). The species known so far occur in the rivers south of the main channel of the Amazon River. The new species (belonging to this subgenus) described here is from Amapá and hence the fi rst one collected north of the Amazon River.


Introduction
The killifi sh genus Rivulus constitutes the most speciose and geographically widespread assemblage of aplocheiloid fi shes.It is widely distributed in both Middle and South America, where it occurs from Mexico and the Caribbean islands to Argentina.According to Costa (2006) the subgenus Melanorivulus, a well supported monophyletic clade endemic to central South America, is diagnosed by an osteological feature (dorsal portion of preopercle short and pointed) and three derived color patterns (melanophores concentrated on opercular region form oblique stripes; females with melanophores concentrated on margins of unpaired and pelvic fi ns and with black spot on upper portion of caudal-fi n base not close to fi n margin).
Melanorivulus includes 33 valid species (Costa, 2010).The species known so far occur in the rivers south of the main channel of the Amazon River.The new species (belonging to this subgenus) described here is from Amapá and hence the fi rst one collected north of the Amazon River.

Materials and Methods
Measurements and counts were taken as described in Amiet (1987), Huber (1992) and Valdesalici (2010).Measurements were made with a digital calli-per under a dissecting microscope and rounded to the nearest 0.1 mm.
Nine measurements are presented as percentages of standard length (SL), except for eye diameter and snout length, which are given as a percentage of head length (HL).Terminology for the cephalic neuromast series follows Costa (2001), for the frontal squamation as described in Hoedeman (1958).Osteological preparations (cleared & stained, C&S) were made according to Taylor & van Dyke (1985).

Diagnosis
Rivulus albae is a member of the subgenus Melanorivulus by having all the subgeneric synapomorphic features: dorsal portion of preopercle short and pointed, melanophores concentrated on opercular region to form oblique stripes, females with melanophores concentrated on margins of unpaired fi ns and with a black spot on upper portion of caudal-fi n base not close to fi n margin in females.
It is similar to Rivulus decoratus and distinguished from all other species of Melanorivulus by brown oblique bars on the entire fl ank, which on the dorsal portion of the fl ank often form chevron-like marks with a posterior vertex (vs.chevron-like pattern with vertex pointing anteriorly when present).It differs from R. decoratus by the possession of 6 branchiostegal rays (versus 5), 13 anal fi n rays (versus 10 -11) and 24 -26 scales on lateral series (versus 25 -28).

Description
For morphometric data see Table 1.Largest male exam ined 15.4 mm SL; largest female examined 21.2 mm SL.Dorsal profi le slightly convex from snout to posterior end of dorsal-fi n base, about straight to slightly concave on caudal peduncle.Ventral profi le weakly convex from lower jaw to anal-fi n origin, approximately straight or slightly concave to end of caudal peduncle.Body slender, cylindrical, greatest body depth at midlength between pectoral-fi n base and anal-  107 Vertebrate Zoology ■ 61 (1) 2011  Colouration.Males (Fig. 1).Body side light metallic greenish to light greenish blue, yellow greenish on dorsolateral portion of body between postorbital region and anterior portion of fl ank; wide brown to grey oblique bars on entire fl ank, often forming chevronlike marks; black melanophores irregularly extending between postorbital and humeral regions and forming irregular oblique stripes.Dorsum dark yellow.Venter white.Ventral part of head white.Upper jaw yellow greenish, lower jaw dark brown to black.Iris pale yellow to light brown.Dorsal fi n pale yellow with 3 oblique reddish to dark brown stripes, its margin dark grey to black.Anal fi n pale yellow, base whitish to light blue, with 4 oblique brownish bars.Caudal fi n pale yellow to greenish with 5 to 6 narrow brownish to red bars crossing fi n except on dorsal and ventral portion, margin dark grey to black.Pectoral fi n yellowish to hyaline.Females: Body side light metallic greenish, pale brownish on dorsolateral portion of body between postorbital region and anterior portion of fl ank; wide brown oblique bars on entire fl ank, which often form chevron-like marks; black melanophores forming oblique stripes irregularly extending between postorbital and humeral regions.Dorsum brownish.Venter white.Opercular region greenish.Ventral part of head white.Upper jaw light brown, lower jaw dark brown.Iris pale yellow to light brown.Dorsal fi n pale yellow with 3 oblique reddish gray stripes, its margin dark gray to black distal Anal fi n pale yellow, base whitish, with short oblique brownish bars on posterior portion, distal margin dark gray to black.Caudal fi n pale yellow to greenish with 5 narrow brownish bars; margin dark grey to black; on dorsal portion of caudal-fi n base a small, vertically elongate, dark grey spot, dorsally margined by short yellow zone.Pectoral fi n yellowish to hyaline.Distribution (Fig. 3).Rivulus albae is known only from few localities belonging to Comprido Lake and Tartaruga Grande River (Amapa state, northeastern Brazil) at an altitude up to about 50 meter above see level.
Habitat notes (Fig. 4).Rivulus albae was collected in the transition areas between savannah and forests near to the banks of clear and transparent large water bodies, lakes and lagoons with a dense underwater vegetation.At the type locality the water temperature was about 38 °C at 12:00 a.m.The waters was acidic (between pH 6 and 6.5), with a very low conductivity (about 30 microsiemens/cm).A Fluviphylax species was caught together with R. albae at all collecting sites.Additonally, in some localities Copella arnoldi, Hoplias malabaricus, and Nannostomus species were caught.Etymology.Named in honor of Alba García, the daughter of the second author.

Discussion
South America is composed of several well defi ned geological units.The oldest are the Brazilian Shield and the Guyana Shield, which both own different rivu lids faunas (Costa, 1998;Hrbek & Larson, 1999).All the Melanorivulus species described so far are limited to the river basins draining the Brazilian Shield.Rivulus albae, however, was collected on the Guiana Shield, i. e. north of the Amazon River, where a member of Melanorivulus has not been expected.
Another group of minute rivulids, the members of the subgenus Anablespoides (Costa, 2006;Huber, 1992), inhabits the Amazon basin.Rivulus albae has a similar banding colouration as known for the spcecies of Anablespoides.But it is readily distinguished from this assemblage by the possession of the following characters: rostral cartilagine elongated (vs.rounded), absence of external medial teeth laterally displaced of premaxilla and dentary (vs.presence), elliptical caudal fi n (vs.acuminate), H-scale with two margins covered (vs.H-scale with all margins free), males without transverse stripe through chin (vs.present), females with black spot on dorsal portion of caudal-fi n (vs.without).
Field observation in the habitats of Amazonian species of Rivulus revealed that different lineages have distinct habitat preferences, thus suggesting divergent specializations during the evolution of the groups.Species of Melanorivulus, including R. albae, are found in sunny creeks in savanna-like environments (Costa, 2006;Schindler & Etzel, 2008).
n origin.Snout short, rounded.Dorsal and anal fi ns rounded, without fi laments in both sexes.Pectoral fi n rounded, its posterior margin reaching about 65% of distance between pectoral-fi n base and anal-fi n origin.Pelvic fi n small, elliptical tip reaching the anus.Caudal fi n elliptical.Dorsal-fi n origin in vertical through base of 7 th or 8 th anal-fi n ray, between neural spines of vertebrae 18 and 19.Anal-fi n origin between pleural ribs of vertebrae 12 and 13.Dorsal-fi n rays 6 -7; analfi n rays 13; caudal-fi n rays 24 -26; pectoral-fi n rays 13; pelvic-fi n rays 6. Scales large, cycloid.Body and head entirely scaled.No scales on dorsal-and anal-fi n bases.Frontal squamation F-patterned.Longitudinal series of scales 24 -26.No contact organs on fi n rays.Cephalic neuromasts: supraorbital 3 + 3. Lateral line interrupted.Six branchiostegal rays.Second pharyngobranchial with one tooth.Gill-rakers on fi rst branchial arch 1 + 6. Vomerine teeth 2 -3.Total vertebrae 30.

Table 1 .
Morphometric data of Rivulus albae.All measurements are presented as percentages of standard length, except eye diameter and snout length as percentages of head length, standard length in mm.Excluded C&S specimens.