MCP 53145, male, 32.3 mm SL, Brazil, Mato Grosso do Sul State, municipality of Campo Grande, first order stream, in the headwaters of Rio Pardo, 20°28’11”S, 54°29’25”W, altitude 589 m a.s.l.; M.V. Volcan & L.E.K Lanés, 13 Dec 2016.

MCP 53146, 4 males, 21.6–31.0 mm SL (1 C&S), 3 females, 22.5–29.0 mm SL, 1 juvenile, sex undetermined, 16.9 mm, all collected with the holotype. ZUFMS 5409, 7 females, 20.2–26.0 mm SL (3 C&S), same locality as holotype; F. Severo-Neto, T.R.F. Sinani & S. Ichikawa, 5 Aug 2017.

Melanorivulus interruptus is distinguished from all species of the M. pictus group, except M. nigromarginatus, by the presence of oblique chevron-like bars interrupted, mainly on the midline of the flank in males (vs. bars continuous, vestigial, irregular or presence of longitudinal rows of red dots on the side of the body; never bars interrupted on the midline of the body). It is distinguished from all other species in the M. pictus species group, except M. planaltinus, M. leali and M. pictus, by the presence of a rounded dorsal fin (vs. slight pointed or pointed dorsal fin). In addition, M. interruptus differs from M. amambaiensis, M. apiamici, M. egens, M. faucireticulatus, M. ivinhemensis and M. leali by a higher caudal fin ray count (32–33 vs. 28–31); from M. egens, M. faucireticulatus, M. leali, M. litteratus, M. rutilicaudus and M. formosensis by the position of the anal-fin origin between the pleural ribs of 15th and 16th vertebrae (vs. 13th and 15th vertebrae); from M. apiamici, M. egens, M. faucireticulatus, M. pictus, M. polychromus and M. proximus by the dorsal-fin origin at a vertical through the base of the 8th anal-fin ray (vs. vertical through base of 7th or 9th-10th anal-fin ray); from M. amambaiensis, M. egens, M. faucireticulatus, M. ivinhemensis, M. planaltinus, M. polychromus, M. nigropunctatus, M. ofaie and M. formosensis by the dorsal-fin origin between neural spines of 18th and 19th vertebrae (vs. 19th and 22th); from M. egens, M. faucireticulatus, M. leali, M. litteratus, M. scalaris, M. vittatus, M. linearis and M. proximus by the tip of pelvic fin reaching the urogenital papilla in males (vs. tip of pelvic fin reaching 1st to 4th anal fin ray). Another interesting diagnostic character is the presence of 8 rays in the pelvic fin in about 35% of the analysed specimens (vs. 5–7 rays, never 8 rays present in the pelvic fin).

Morphometric data are presented in Table 1. Males larger than females. Largest male examined 32.3 mm SL, largest female 29.0 mm SL. Dorsal profile slightly convex from snout to end of dorsal-fin base. Ventral profile convex from lower jaw to origin of anal-fin. Dorsal and ventral profiles of caudal peduncle nearly straight. Body slender, approximately cylindrical and compressed. Greatest body depth at pelvic-fin base. Snout weakly pointed in lateral view. Jaws short.

Short dorsal and anal fins. Dorsal-fin rays 9–10. Dorsal fin rounded in males and females. Dorsal-fin origin on vertical through base of 8th anal-fin ray, and between neural spines of 18th and 19th vertebrae. Anal-fin rays 12–13. Anal fin slightly pointed in males and females. Origin of anal fin at vertical through pleural ribs of 15th-16th vertebrae. Caudal fin rounded 32–33 rays. Pectoral fin rays 14. Pectoral fins rounded, with posterior margin reaching vertical at about 60–90% of length between pectoral-fin and pelvic-fin bases. Pelvic-fin rays 7–8. Pelvic-fin posterior tip reaching vertical at anus to 3rd anal-fin ray. Pelvic-fin bases in close proximity.

Scales cycloid. Body and head entirely scaled, except anterior ventral surface of head. Body squamation extending over anterior 15–25% of caudal-fin base. No scales on dorsal and anal-fin bases. Frontal squamation E-patterned; E-scales not overlapping medially; scales arranged in regular circular pattern around A-scale without exposed margins; transverse row of scales anterior to H-scale. Longitudinal series of scales 30–32; transverse series of scales 8–9; scale rows around caudal peduncle 16. No contact organs on flank and fins.

Cephalic neuromasts: supraorbital 3+3, parietal 1, anterior rostral 1, posterior rostral 1, infraorbital 1+9-11+1, preorbital 2, otic 1, postotic 1-2, supratemporal 1, median opercular 1, ventral opercular 1, preopercular 2+4, mandibular 2-3+1, lateral mandibular 1-2, paramandibular 1. Two neuromasts on caudal-fin base.

Six branchiostegal rays. Gill rakers on first branchial arch 1+7. First epibranchial slightly curved. Total number of vertebrae 29–30, 13–14 precaudal vertebrae, 16–17 caudal vertebrae. Ventral process of angulo-articular short, pointed. Vomerine teeth 1–3. Dermosphenotic present. Basihyal sub-triangular, greatest width 45–50% of length; basihyal cartilage 20–25% of total basihyal length. Second pharyngobranchial teeth absent.

Males (Figs 1 and 2). Flank light grey or light metallic blue; sometimes purple-blue close to anal fin; 7–9 oblique narrow red bars anteriorly directed, often forming chevron-like marks irregularly arranged and usually interrupted in the midline of the body; commonly, the chevron-like bars are vestigial in the anterior region and begin at the end of the pectoral fin; well-defined, but interrupted, bars begin behind insertion of the pelvic fins. Inconspicuous dark pigmentation between postorbital region and area above midlength of pectoral fin, forming an irregular mid-lateral dark stripe; vertical rows of reddish dots on antero-ventral part of flank, between pectoral and pelvic fins. Dorsum yellowish-grey, venter whitish-grey. Dorsal portion of the head yellowish-grey; ventral portion yellowish-grey or pale golden without dots, marks or any pigmentation pattern; pale golden-greenish iridescence on opercular region. Jaws dark grey or brown. Iris pale yellow to pale brown, sometimes with dark brown bars on anterior and posterior portions. Dorsal fin yellow with four or five narrow red bars. Anal fin yellow, basal portion light blue-whitish, posterior portion pale blue with two or three faint red marks; distal region becoming gradually dark red-brown. Caudal fin yellow with five to seven red oblique parallel bars, covered generally 1/3–1/2 of caudal fin on mid-dorsal portion, sometimes red bars inconspicuous; ventral portion light yellow without bars, distal ventral region becoming gradually dark red-brown with a not well delineated margin of orange-brown colour. Pectoral fin yellowish-hyaline. Pelvic fin yellow with posterior margin orange-brown.

Figure 1. 

Melanorivulus interruptus sp. n., MCP 53145, holotype, male, 32.3 mm SL, Campo Grande, Mato Grosso do Sul, Brazil.

Females (Fig. 2). Generally flank similar to males, but with paler colours. Venter white. Dorsum and dorsal portion of head yellowish-grey; ventral portion of head yellowish-grey without spots and without any colouration pattern. Jaws yellowish-grey or brown; opercular region pale greenish-gold. Dorsal fin yellow, with three or four faint grey bars on posterior region forming a reticulated pattern. Anal fin yellow; basal portion light blue; distal region becoming dark red-brown gradually; distal margin with high concentration of melanophores. Caudal fin pale yellow, with four or six grey bars usually interrupted in the inferior region (lower third of fin); a small irregularly shaped black spot on dorso-basal portion of fin, sometimes overlapping the first bar; faint grey margin. Pectoral fin hyaline or yellowish hyaline. Pelvic fin yellowish hyaline.

Figure 2. 

Melanorivulus interruptus sp. n., MCP 53146, paratypes, living male (above) and female (below), Campo Grande city, Mato Grosso do Sul state, Brazil.

Melanorivulus interruptus is only known so far from a small stream, in the headwaters of the Rio Pardo, a tributary of the right margin of Rio Paraná, state of Mato Grosso do Sul, Brazil (Fig. 3).

Figure 3. 

Distribution of species of the Melanorivulus pictus group in the Rio Paraná (grey) and Rio Araguaia and Tocantins (light grey) basins. Stars indicate the type localities of the new species.

The name interruptus, from the Latin adjective interrupted, referring to the presence of conspicuous oblique red bars forming chevron-like marks usually disrupted in the midline of the body in males.

The species was recorded in a typical Cerrado area, inserted into the urban area of Campo Grande town at an altitude of 594 m a.s.l.. It was recorded in a small first order creek, but was only captured in border areas with dense aquatic vegetation, mainly grasses (Fig. 4). The biotope, which is locally called “veredas”, a freshwater ecosystem comprising streams bordered by the buriti-palm, typically found in the Cerrado Biome, has moderate current speed. But, the place where the species was recorded is formed by small backwaters, with a clayey substrate and with a maximum depth of 10 cm. The species is encountered in areas exposed to the sun as well as in areas shaded by trees, mainly buritis-palm (Mauritia flexuosa). The region has a slightly undulating terrain and according to the classification of Koppen-Geiger has rainy summers and dry winters.

There are imminent threats to the population of M. interruptus: the stream is located in ​​the Campo Grande urban area, which is developing rapidly around the area of ​​the new species; there is a large real estate development alongside the stream that can cause significant impacts on the environment quality; the stream is crossed by a high traffic road and it is subject to accidents with potentially dangerous loads; besides, there are agricultural activities over its complete extension. Although larger collection efforts are likely to be undertaken in the area to better understand the species distribution pattern, M. interruptus appears to have a very restricted distribution. In the adjacent watersheds distinct species occur (e.g. M. apiamici in the mouth of the Rio Pardo, M. egens in the upper Rio São Domingos and M. nigropunctatus and M. ofaie in the Rio Verde – see Figure 4 with the distribution map of the M. pictus species group). Thus, according to the previously mentioned threats, a high degree of endemism and the observations made in the field, M. interruptus was considered “Vulnerable”, following the criteria of IUCN (2012). The species has an area of ​​occupation of less than 2000 km², it occurs in only one locality and it suffers from a reduction in its occupation area and in its habitat quality (VU B2abii,iii).

Figure 4. 

First order stream (typical “veredas” habitat), tributary of headwaters of Rio Pardo, M. interruptus type locality.

MCP 53147, male, 30.1 mm SL, Brazil, Mato Grosso do Sul State, municipality of Nova Andradina, first order stream of Rio Ivinhema, 22°11’53”S, 53°20’12”W, altitude 300 m a.s.l.; M.V. Volcan & L.E.K Lanés, 09 Dec 2016.

MCP 53153, 6 males, 20.3–27.7–31.0 mm SL, 7 females, 18.5–29.7 mm SL (1 C&S), 2 juvenile, sex undetermined, 14.0–15.4 mm, all collected with the holotype. MCP 53148, 2 juvenile, sex undetermined, 14.3–16.9 mm, wetland in the boarder of BR 376 highway, 22°21’22”S, 53°30’32”W, altitude 252 m a.s.l.; M.V. Volcan & L.E.K Lanés, 08 Dec 2016. MCP 53149, 1 male, 18.6 mm SL, 2 females, 25.7–30.4 mm SL, first order stream of Rio Ivinhema, 22°21’51”S, 53°39’42”W, altitude 310 m a.s.l.; M.V. Volcan & L.E.K Lanés, 08 Dec 2016. ZUFMS 5365, 5 males, 21.8–29.5 mm SL (3 C&S), 3 females, 18.1–20.4 mm SL, first order stream of Rio Ivinhema, 22°12’17”S, 53°27’17”W, altitude 317 m a.s.l.; T.R.F. Sinani & M.O. Bordignon, 17 Sep 2016.

Melanorivulus ivinhemensis is distinguished from all other species of the M. pictus species group by having a yellow caudal fin with thin red bars arranged only in the median region of the fin in males (vs. red bars absent or red bars extending from the ventral or median region to the dorsal region of the caudal fin), except in M. rutilicaudus. Besides, it is distinguished from all M. pictus species by the origin of the anal fin at a vertical through pleural ribs of 15th-18th vertebrae (vs. 13–15 vertebrae), except in M. amambaiensis, M. planaltinus, M. scalaris, M. polychromus, M. ofaie, M. interruptus and M. nigropunctatus; and by the greater snout length in males (16.1–21.3 vs. 11.4–16.3), except in M. amambaiensis, M. apiamici, M. interruptus, M. vittatus, M. polychromus and M. nigropunctatus. Distinguished from M. planaltinus, M. rutilicaudus, M. interruptus by a lower caudal fin ray count (29–31 vs. 32–34) and from M. amambaiensis by a higher caudal fin ray count (29–31 vs. 25–28). Distinguished from M. faucireticulatus, M. giarettai, M. interruptus, M. leali, M. vittatus and M. formosensis by a short basihyal cartilage 10–15% of total basihyal length (vs. 20–25%). From M. interruptus, M. vittatus, M. ofaie, M. nigropunctatus, M. proximus, M. linearis and M. nigromarginatus it is distinguished by a smaller dorsal fin base length in females (8.9–10.4% vs. 10.5–14.8%). Distinguished from M. giarettai, M. proximus, M. linearis, M. nigromarginatus and M. planaltinus by lower number of gill rakers on the first branchial arch (1+7 vs 1-2+8), and from the M. pictus by the dorsal-fin origin on a vertical through base of 8th or 9th anal-fin ray (vs. on vertical through base of 7th anal-fin ray).

Morphometric data are presented in Table 2. Females usually larger than males, largest female 30.4 mm SL, largest male examined 30.1 mm SL. Dorsal profile slightly convex from snout to end of dorsal-fin base, straight on caudal peduncle. Ventral profile convex from lower jaw to origin of anal-fin, approximately straight to end of caudal peduncle. Body slender, approximately cylindrical and compressed, greatest body depth anterior of pelvic fin base, in the pelvic fin base or at the anus. Snout slightly pointed. Jaws short.

Short dorsal and anal fins. Dorsal-fin rays 7–9. Dorsal fin rounded or slightly pointed in males, rounded in females. Dorsal-fin origin on a vertical through base of 8th or 9th anal-fin ray, and between neural spines of 20th and 22th vertebrae. Anal-fin rays 12–14. Anal fin slightly pointed in males and females. Origin of anal fin at a vertical through pleural ribs of 15th-18th vertebrae. Caudal fin oval shaped, deeper than long, 29–31 rays. Pectoral fin rays 11–13. Pectoral fins rounded, with posterior margin reaching vertical at about 60–90% of the length between pectoral-fin and pelvic-fin bases. Pelvic-fin rays 6–7, one individual with one single pelvic fin. Posterior tip of pelvic fin reaching vertical between anus to 2nd anal-fin ray. Position of pelvic-fin bases variable, in close proximity, or with bases separated by a great distance, similar to the size of the base of the pelvic fin.

Scales small, cycloid. Body and head entirely scaled, except anterior ventral surface of head. Body squamation extending over anterior 15–25% of caudal-fin base. No scales on dorsal and anal-fin bases. Frontal squamation E and F-patterned or with two patterns, one on each side in one specimen. E-scales generally not overlapping medially. In one specimen E-scales marginally overlapped. Transverse row of scales anterior to H-scale; scales arranged in regular circular pattern around A-scale. A-scale usually without exposed margins, four specimens with posterior margin exposed, overlapping the B-scale. Longitudinal series of scales 29–31; transverse series of scales 8–9; scale rows around caudal peduncle 16. No contact organs on flank and fins.

Cephalic neuromasts: supraorbital 3+3, parietal 1, anterior rostral 1, posterior rostral 1, infraorbital 1+11–12+1, preorbital 2, otic 1, postotic 1–2, supratemporal 1, median opercular 1, ventral opercular 1–2, preopercular 2+4–6, mandibular 3–4+1, lateral mandibular 1–2, paramandibular 1. Two neuromasts on caudal-fin base.

Six branchiostegal rays. Gill rakers on first branchial arch 1+7. First epibranchial slightly curved. Total number of vertebrae 29–31, 14 precaudal vertebrae, 15–17 caudal vertebrae. Ventral process of angulo-articular short, pointed. Vomerine teeth 2–5. Dermosphenotic present. Basihyal sub-triangular, greatest width 45–50% of length; basihyal cartilage 10–15% of total basihyal length. Second pharyngobranchial teeth absent.

Males (Fig. 5). Metallic greenish-gray flank, purple-blue close to anal fin; red dots on anterior flank area, sometimes few red dots on the anteroventral region; one to three oblique narrow bars between the medial pectoral fin portion and close to the pelvic fin base, sometimes one or two of these bars forming chevron-like marks with an angle on the ventral region of the flank; 7–9 oblique narrow red bars anteriorly directed, forming chevron-like marks with an angle on midline of flank, and generally positioned posteriorly to the pelvic fin base on the caudal peduncle. Two parallel and oblique dark brown bars often interconnected by a brown-dark horizontal line between postorbital and posterior opercular region, sometimes forming a broad and well-defined mid-lateral dark stripe between postorbital region and area above the pectoral fin at mid length. Yellowish-grey dorsum with small black dots, venter white. Dorsal portion of the head yellowish-grey sometimes with small black dots; ventral portion white; golden iridescence on opercular region. Dark grey jaws varying to pale yellowish-grey. Iris pale yellow. Dorsal fin yellow with three or five oblique narrow red bars on posterior portion of fin. Anal fin yellow or orange-yellow, sometimes with a distal dark margin, basal portion light metallic blue-whitish, posterior portion pale blue with two or three faint reddish oblique bars. Caudal fin yellow or hyaline-yellowish, presenting three to six vertical or slightly oblique parallel red or red brownish bars in the central region. Pectoral fin yellowish hyaline. Pelvic fin yellowish hyaline or orange and hyaline, sometimes with a distal dark margin.

Figure 5. 

Colour pattern variation of male paratypes of M. ivinhemensis sp. n., MCP 53153, Nova Andradina municipality, Rio Ivinhema, Mato Grosso do Sul state.

Females (Fig. 6). Flank similar to males. Dorsum and dorsal portion of head greenish-gray with small black dots; ventral portion of head white, with black marks, sometimes inconspicuous; pale golden iridescence on opercular region. Jaws pale yellowish-grey. Iris pale yellow, sometimes with dark brown bars on anterior and posterior portions. Dorsal fin yellowish-orange, with horizontally elongated dark brown to black spots or bars in the medial region; dark grey narrow margin delineating entire dorsal fin. Anal fin yellowish-orange, basal portion pale blue with two interrupted reddish bars, posterior portion pale blue with two or three reddish oblique bars, distal region becoming gradually dark reddish brown on marginal border, distal margin with high concentration of melanophores. Caudal fin orange, sometimes orange and hyaline in central portion, with three to six dark brown to black bars, sometimes bars formed by dense, vertically elongated spots, the rays and inter-spaces between them pigmented; dark brown to black circular or sub triangular spot on dorso-basal portion of the caudal fin; dark grey margin. Pectoral fins hyaline. Pelvic fins orange-yellow, with a reddish-brown anterior margin.

Figure 6. 

Female paratypes of M. ivinhemensis sp. n., MCP 53153, Nova Andradina municipality, Rio Ivinhema, Mato Grosso do Sul state.

Melanorivulus ivinhemensis is so far only known from first order streams and small wetlands associated with both margins of the lower course of the Rio Ivinhema, Rio Paraná basin (Figure 3).

The name ivinhemensis is a reference to the occurrence of the new species in the Rio Ivinhema basin.

Melanorivulus ivinhemensis was recorded in marginal areas of small first order streams and in wetlands completely exposed to the sun on a slightly undulating terrain at altitudes ranging from 267 to 315 m a.s.l. (Fig. 7). One wetland was deep (about 70 cm of maximum depth), with muddy substratum and turbid water (Fig. 7b), while in the other localities the species was recorded in shallow areas, not exceeding 20 cm depth, in crystalline waters and with a clayey substrate (Fig. 7a, c). Eight fish species were recorded co-occurring with M. ivinhemensis: Aphyocharax dentatus Eigenmann & Kennedy, 1903, Pyrrhulina australis Eigenmann & Kennedy, 1903, Hyphessobrycon anisitsi (Eigenmann, 1907), Hyphessobrycon eques (Steindachner, 1882), Serrapinnus kriegi (Schindler, 1937), Cichlasoma dimerus (Heckel, 1840), Hoplerythrinus unitaeniatus (Spix & Agassiz, 1829) and Moenkhausia sanctaefilomenae (Steindachner, 1907).

Figure 7. 

Sampling sites of Melanorivulus ivinhemensis sp. n. (a) Wetland associated with a small drainage, direct tributary of Rio Ivinhema, type locality, (b) wetland along BR 376 highway, (c) small drainage, direct tributary of Rio Ivinhema.

The new species is endemic of Rio Ivinhema and was recorded in four different areas, in a range of about 25 km along both sides of the river and there is a large density of potential environments for its occurrence along this stretch. It has been recorded in areas with no major impacts – although under intense agricultural activity – and therefore there is no evidence that the species is threatened with extinction.

MCP 53150, male, 27.9 mm SL, Brazil, Mato Grosso do Sul State, near of municipality of Naviraí, first order stream of Rio Amambai, margin of BR 487 highway, 22°57’11”S, 54°26’52”W, altitude 332 m a.s.l.; M.V. Volcan & L.E.K Lanés, 14 Dec 2016.

MCP 53151, 4 males, 20.9–25.9.0 mm SL (1 C&S), 10 females, 22.8–28.7 mm SL (3 C&S), all collected with the holotype. ZUFMS 5497, 1 male, 21.6 mm SL, 4 females, 22.2–22.9 mm SL, all collected with the holotype.

The presence of an orange or orange-red anal fin with grey or dark grey distal margin (vs. no similar colour pattern) and chevron-like red bars in inverted Y-shape in the flank of males (vs. no similar pattern) distinguishes the M. amambaiensis from all other species of the M. pictus species group. Additionally, the new species is distinguished by the lower caudal fin ray count (25–28 vs. 29–34), except M. faucireticulatus; by the lower body depth in males (29.4–21.6 mm SL vs. 21.8–26.5), except from M. vittatus, M. polychromus, M. nigropunctatus, M. interruptus and M. ivinhemensis; by a lower caudal peduncle depth in males (12.4–13.5 mm SL vs. 13.5–16.8 mm SL), except in M. polychromus, M. nigropunctatus, M. interruptus and M. ivinhemensis. Females are distinguished from M. apiamici, M. faucireticulatus, M. giarettai, M. planaltinus, M. rutilicaudus, M. scalaris, M. nigropunctatus, M. proximus, M. linearis, M. nigromarginatus and M. formosensis by the lower body depth (19.1–21.5 mm SL vs. 21.6–26.0 mm SL); and from M. apiamici, M. egens, M. faucireticulatus, M. giarettai, M. planaltinus, M. nigropunctatus, M. linearis, M. nigromarginatus and M. ofaie by a shorter pre-dorsal length (74.5–76.6 mm SL vs. 76.4–83.3 mm SL).

Morphometric data are presented in Table 3. Females larger than males, largest female examined 28.7 mm SL, largest male 27.9 mm SL. Dorsal profile slightly convex from snout to end of dorsal-fin base, straight on caudal peduncle. Ventral profile weakly convex from lower jaw to operculum. Straight from operculum to origin of pelvic-fin and in the caudal peduncle. Body slender, cylindrical and compressed, greatest body depth at origin of pelvic-fin base. Snout blunt. Jaws short.

Short dorsal and anal fins. Dorsal-fin rays 8–9. Dorsal fin slightly pointed in males, rounded in females. Dorsal-fin origin on a vertical through base of 8th or 9th anal-fin ray, and between neural spines of the 20th and 21th vertebrae. Anal-fin rays 12–14. Anal fin slightly pointed in males and females. Origin of anal fin at a vertical through pleural ribs of 15th–16th vertebrae. Caudal fin oval shaped, longer than deep, 25–28 rays. Pectoral fin rays 11–13. Pectoral fins rounded, with posterior margin reaching vertical at about 70–90% of length between pectoral-fin and pelvic-fin bases. Pelvic-fin rays 6–7. Posterior tip of pelvic fin reaching a vertical at slightly anterior to the anus to 1st anal-fin ray. Pelvic-fin bases in close proximity.

Scales small, cycloid. Body and head entirely scaled, except anterior ventral surface of the head. Body squamation extending over anterior 15–20% of caudal-fin base. No scales on dorsal and anal-fin bases. Frontal squamation E and F-patterned or with two patterns, one on each side (present in one specimen). E-scales not overlapping medially. Transverse row of scales anterior to H-scale; scales arranged in regular circular pattern around A-scale. A-scale without exposed margins. Longitudinal series of scales 27–31; transverse series of scales 8–9; scale rows around caudal peduncle 16. No contact organs on flank and fins.

Cephalic neuromasts: supraorbital 3+3, parietal 1, anterior rostral 1, posterior rostral 1, infraorbital 1+9–11+1–2, preorbital 2, otic 1, postotic 1–2, supratemporal 1, median opercular 1, ventral opercular 1–2, preopercular 2+4–5, mandibular 2–3+1, lateral mandibular 1–2, paramandibular 1. Two neuromasts on caudal-fin base.

Six branchiostegal rays. Gill rakers on first branchial arch 1+7–8. First epibranchial slightly curved. Total number of vertebrae 30–31, 13–14 precaudal vertebrae, 17 caudal vertebrae. Ventral process of angulo-articular short, pointed. Vomerine teeth 2–3. Dermosphenotic present. Second pharyngobranchial teeth absent.

Males (Fig. 8). Flank light metallic blue-whitish, sometimes purplish above anal fin; numerous oblique narrow red bars irregularly arranged, forming reticulated chevron-like marks anteriorly directed with a branching on the lower portion in inverted Y-shape, sometimes with a branching on the upper and lower portions in X-shape; between these bars one to four red dots obliquely distributed mostly on dorsal portion of flank; reddish dots on anteroventral part of flank, sometimes forming rows, mostly between bases of the pectoral and the pelvic fin; one to three oblique reddish-brown bars between the postorbital region and the pectoral fin; reddish-brown spots on the humeral region. Dorsum metallic grey or yellowish grey, venter white. Dorsal portion of head yellowish-grey; ventral portion white, without marks or any pigmentation pattern; golden iridescence on opercular region. Jaws grey to pale yellowish-grey. Iris pale yellow. Dorsal fin yellowish-hyaline with three or four oblique narrow brownish-red or red bars; basal portion pale yellow. Anal fin orange or orange-red, basal portion pale blue with reddish dots forming reticulated pattern, posterior portion pale blue sometimes with two or three reddish oblique bars, distal region grey or dark grey on the marginal border. Caudal fin hyaline or yellowish-hyaline, yellowish-orange on ventral portion; four to seven narrow oblique brownish-red or red bars cover 2/3 of caudal fin, absent in ventral portion. Pectoral fin hyaline, or yellowish-hyaline. Pelvic fin orange or yellowish-orange, sometimes with orange-brown anterior margin.

Figure 8. 

Colour pattern variation of male paratypes of M. amambaiensis sp. n., MCP 53151, small stream tributary of Rio Amambaí, Mato Grosso do Sul state.

Females (Fig. 9). Flank similar to males, but more evident reddish-brown or brown spots between pectoral and pelvic fins. Numerous oblique narrow red bars irregularly arranged, forming reticulated chevron-like marks anteriorly directed, sometimes with a branching on the lower portion in inverted Y-shape. Dorsum and dorsal portion of head greenish-gray with small black dots; ventral portion of head white, with dark grey spots; golden iridescence on opercular region. Jaws greyish-brown. Iris pale yellow, sometimes bordered with grey. Dorsal fin yellowish-orange or yellowish hyaline, with three reddish-brown narrow horizontal bars forming a reticulated pattern. Anal fin orange-yellow, basal portion pale blue with reddish dots forming a reticulated pattern, posterior portion pale blue with two or three reddish oblique bars, distal region becoming gradually dark reddish brown on the marginal border. Caudal fin yellowish-orange, with three to seven grey or dark grey bars; small elongated black spot on dorso-basal portion of the fin, sometimes overlapping the most anterior bar. Pectoral fins hyaline. Pelvic fins yellowish-orange, sometimes with reddish-brown anterior margin.

Figure 9. 

Colour pattern variation of females of M. amambaiensis (not preserved), sp. n., small stream tributary of Rio Amambaí, Mato Grosso do Sul state.

The species is only known from its type locality, a small first order drainage and direct tributary of Rio Amambaí, state of Mato Grosso do Sul, Brazil (Fig. 3).

The name amambaiensis is a reference to the occurrence of the new species in the Rio Amambaí basin.

Melanorivulus amambaiensis was recorded in a small natural drainage, direct tributary of the Rio Amambaí, with depth not exceeding 50 cm and with low flow in an area totally exposed to the sun, parallel to BR 487 highway (Fig. 10). The region has a smoothly undulating terrain and the area of occurrence is at an altitude of 327 m a.s.l.. The drainage has a clayey substrate, which gives an orange colour to the water when disturbed. Only Gymnotus aff. carapo Linnaeus, 1758 was registered co-occurring with M. amambaiensis.

Figure 10. 

Water source of a first-order stream on the left bank of the Rio Amambaí, marginally situated along the BR 487 highway, type locality of M. amambaiensis.

The species is known only from its type locality. It seems to be a micro endemic of the Rio Amambaí basin, since other species occur in the adjacent basins. Part of its original area was fragmented by BR 487 and there is high frequency traffic of vehicles on this highway. Consequently, the species is subject to accidents with potentially dangerous loads, which present a threat to the population of M. amambaiensis. In addition, the surrounding region is largely degraded by agricultural activities that also threaten its habitat quality. Thus, M. amambaiensis was accounted as “Vulnerable” according to the IUCN (2012) standards. The species has an area of occurrence of less than 2000 km², occurs in only one locality and suffers from a reduction in its area of occupation and in the quality of its habitat (VU B2abii, iii).