Three new species of the killifish genus Melanorivulus from the Rio Paraná Basin, central Brazilian Cerrado (Cyprinodontiformes, Aplocheilidae)

Wilson J.E.M. Costa

doi:10.3897/zse.94.21321

Abstract

Three new species of Melanorivulus are described from the upper and middle Rio Paraná Basin, central Brazilian Cerrado. These species are members of the M. pictus species group, endemic to central Brazilian plateaus and adjacent areas, and are easily diagnosed by colour pattern characters, but their relationships with other congeners of the group are still uncertain. Melanorivulus proximus sp. n., from the middle Rio Aporé drainage, and M. nigromarginatus sp. n., from the Rio Corrente drainage, are possibly more closely related to other species endemic to streams draining the slopes of the Caiapó range, whereas M. linearis sp. n., from the upper Rio Pardo drainage, middle Rio Paraná Basin, is considered more closely related to M. egens, a species also endemic to this part of the Basin. This study corroborates the high diversity of species of Melanorivulus in the central Brazilian Cerrado plateaus repeatedly reported in previous studies, indicating once more that different species are often found restricted to short segments of the same river drainage. The intense habitat loss recorded in recent years combined to the high species diversity limited to specific Cerrado freshwater ecosystems, the veredas, indicates that species of Melanorivulus endemic to this part of the Brazilian Cerrado are highly threatened with extinction.

Key Words

Biodiversity hotspot, colour patterns, conservation, systematics, taxonomy

Introduction

The Cerrado savannas of central Brazil, with an area of about 2,000,000 km², is among the 25 most important biodiversity hotspots of the world (Myers et al. 2000). The present study is directed to a group of small killifishes of the genus Melanorivulus Costa, 2006, with species reaching 45 mm of standard length (SL) or less, inhabiting the shallowest parts of the veredas, a typical Cerrado ecosystem comprising streams bordered by the buriti-palm (Mauritia flexuosa). About 50 valid species are presently placed in Melanorivulus, formerly considered as a subgenus of Rivulus Poey, 1860 (Costa 2011). Melanorivulus is similar to other South American aplocheiloid killifishes living in similar biotopes, such as Anablepsoides Huber, 1992, Atlantirivulus Costa, 2008, Cynodonichthys Meek, 1904, Laimosemion Huber, 1999, Rivulus Poey, 1860, which have slender body, short fins and long neural prezygapophyses on caudal vertebrae (Costa 1990). Species of Melanorivulus are distinguishable from species of those genera by the presence of black pigmentation concentrated on the whole margin of the caudal fin and on the distal margin of the dorsal and anal fins in females, ventral process of the angulo-articular short or rudimentary, and preopercular canal absent (Costa 2011).

A great diversity of species of Melanorivulus has been reported for the Cerrado region drained by the Rio Paraná Basin, the second largest river basin in South America. A total of 14 endemic species have been described for this region (Costa 1989, 2005, 2007a, b, 2008; Nielsen et al. 2016; Volcan and Lanés 2017). These species belong to different and not closely related species groups (Costa et al. 2016): the M. punctatus group, containing slender small species, mainly diagnosed by the presence of oblique rows of red dots on the flank in males (e.g., Costa 2005); the M. pinima group, easily diagnosed by the reduction of black pigmentation on the head and humeral region in males, presence of longitudinal rows of red dots on the flank, and a longitudinally elongated white to light yellow mark above caudal spot in females (Costa 2007a), and the M. pictus group, diagnosed by a deeper body (i.e., body depth reaching about 25 % SL) and oblique red bars on the flank (Costa 2017). Eleven of the 14 endemic species are members of the last group, which are mostly concentrated in the rivers draining the Caiapó range and in the south adjacent areas of the middle Rio Paraná Basin (Costa 2005, 2012).

The M. pictus group was first studied by Costa (1989), based on fish collections deposited in the Museu de Zoologia, Universidade de São Paulo, when M. apiamici (Costa, 1989), M. pictus (Costa, 1989) and M. vittatus (Costa, 1989) were described. In September 1994, efforts were directed to firstly sample typical Melanorivulus habitats along the Paraná River Basin, but based on the great morphological variability found among populations inhabiting close neighbouring areas, Costa (1995) concluded that all populations of this vast geographical area belong to a single polymorphic species, M. pictus. However, after more accurate field studies since 2004, numerous new species have been continuously described for the Paraná and other adjacent river basins (e.g., Costa 2005, 2006, 2012, 2017; Deprá et al. 2017). These studies have indicated that each species is limited to small areas and that different species may inhabit the same river drainage at different altitudes (Costa 2007b, 2017; Volcan et al. 2017). However, some new species collected in recent years are still await formal descriptions. In this paper, three new species of the M. pictus group from the Paraná River Basin are described.

Materials and methods

Specimens were captured with small dip nets (40 × 30 cm) and fixed in formalin for a period of 10 days, and then transferred to 70 % ethanol. Collections were made with permits provided by ICMBio (Instituto Chico Mendes de Conservação da Biodiversidade) and field methods have been approved by CEUA-CCS-UFRJ (Ethics Committee for Animal Use of Federal University of Rio de Janeiro; permit number: 01200.001568/2013-87). Material is deposited in Instituto de Biologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro (UFRJ) and Coleção Ictiológica do Centro de Ciências Agrárias e Ambientais, Universidade Federal do Maranhão, Chapadinha (CICCAA). Descriptions of colouration of living fish were based on observations just after collections, in small transparent plastic bottles. Type specimens were photographed live about 24 hours after collection. 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. Osteological preparations followed Taylor and Van Dyke (1985); the abbreviation C&S in lists of material indicates those specimens that were cleared and stained for osteological examination. Terminology for osteological structures followed Costa (2006), for frontal squamation Hoedeman (1958), and for cephalic neuromast series Costa (2001). In lists of material, geographical features are written according to Brazilian Portuguese local use (e.g., córrego, ribeirão, rio), allowing more accurate identifications of localities in the field and avoiding common mistakes when tentatively translating them to English.

Results

Melanorivulus proximus sp. n.

http://zoobank.org/79A16259-1DEF-4F9E-8AE1-047802792F3F

Figs 1, 2, Table 1

Rivulus pictus (non Rivulus pictus Costa, 1989); Costa 1995: 216 (misidentification).

Rivulus scalaris (non Rivulus scalaris Costa, 2005); Costa 2005: 79 (misidentification).

Holotype

UFRJ 11681, male, 27.7 mm SL; Brazil: Mato Grosso do Sul state: Cassilândia municipality: stream crossing the road MS-306, Rio Aporé drainage, Rio Paraná Basin, 19°03’54”S, 51°49’56”W, altitude about 515 m asl; W.J.E.M. Costa et al., 20 Sep. 2011.

Figure 1.

Melanorivulus proximus sp. n., holotype, UFRJ 11681, male, 27.7 mm SL. Photograph by W.J.E.M. Costa.

Paratypes

All from Brazil: Rio Aporé drainage, upper Rio Paraná Basin. Mato Grosso do Sul state: UFRJ 10792, 4 males, 25.6–30.0 mm SL, 1 female, 25.6 mm SL; UFRJ 10793, 1 male, 23.6 mm SL, 1 female, 21.1 mm SL (C&S); collected with holotype. – UFRJ 10788, 6 males, 15.7–27.7 mm SL, 4 females, 24.0–27.4 mm SL; UFRJ 10789, 3 males, 20.9–21.9 mm SL, 2 females, 18.5–20.0 mm SL (C&S); CICCAA 00692, 5 males, 21.9–25.8 mm SL, 5 females, 21.7–25.6 mm SL; Cassilândia municipality: road MS-306, 19°02’15”S, 52°01’57”W, altitude about 540 m asl; W.J.E.M. Costa et al., 20 Sep. 2011.– UFRJ 2207, 8 males, 20.2–27.1 mm SL, 12 females, 17.8–35.4 mm SL.; UFRJ 2280, 1 male, 25.9 mm SL, 2 females, 27.9–28.7 mm SL (C&S); Cassilândia municipality: swamp close to Ribeirão Grande, road MS-306, about 30 km SE from the town of Cassilândia, 19°15’40”S, 51°30’03”W, altitude about 495 m asl; W.J.E.M. Costa et al., 17 Sep. 1994. Goiás state: UFRJ 10821, 1 male, 27.7 mm SL, 1 female, 22.7 mm SL; UFRJ 10822, 4 males, 15.6–18.4 mm SL, 12 females, 16.2–22.1 mm SL; Itajá municipality: Ribeirão Bagageiro, road GO-302, 19°06’17”S, 51°42’15”W, altitude about 455 m asl; W.J.E.M. Costa, B.B. Costa & C.P. Bove, 15 Jan. 2007.

Figure 2.

Melanorivulus proximus sp. n., paratype, UFRJ 10792, female, 26.6 mm SL. Photograph by W.J.E.M. Costa.

Additional material (non-types)

UFRJ 10819, 12; Brazil: Goiás state: Itajá municipality: road GO-302, 19°05’09”S, 51°36’27”W, altitude about 440 m asl; W.J.E.M. Costa et al., 20 Sep. 2011.

Diagnosis. Melanorivulus proximus is distinguished from all other congeners of the M. pictus group except M. scalaris by the presence of irregularly arranged, interconnected oblique red bars on flank, forming Y- and X-shaped marks. Melanorivulus proximus is distinguished from M. scalaris by: caudal fin base colour pale orangish pink in females (vs. pale yellow); dorsal and anal fin sharply pointed in males (vs. rounded to moderately pointed), dorsal-fin origin at vertical between base of 9th and 10th (vs. between base of 7th and 8th); longitudinal series of scales 29–30 (vs. 31–34); pre-dorsal length longer in males (75.9–78.4 % SL vs. 73.0–75.0 % SL); longer anal-fin base (21.1–24.7 % SL in males and 18.8–21.4 % SL in females vs. 18.1–21.0 % SL in males and 16.2–18.5 % SL in females); and fewer infraorbital neuromasts around orbit (9–10 vs. 11–12).

Description

Morphometric data appear in Table 1. Body relatively deep, subcylindrical anteriorly, slightly deeper than wide, compressed posteriorly. Greatest body depth at vertical just anterior to pelvic-fin base. Dorsal and ventral profiles of trunk slightly convex in lateral view, approximately straight on caudal peduncle. Head moderately wide, sub-triangular in lateral view, dorsal profile nearly straight, ventral profile convex. Snout blunt. Jaws short; teeth numerous, conical, irregularly arranged; outer teeth hypertrophied, inner teeth small and numerous. Vomerine teeth 3–5. Gill-rakers on first branchial arch 2 + 7–8.

Dorsal and anal fins short, sharply pointed in males, rounded to slightly pointed in females. Caudal fin rounded, slightly longer than deep. Pectoral fin rounded, posterior margin reaching vertical at about 90 % of length between pectoral-fin and pelvic-fin bases. Pelvic fin small, longer in males, tip reaching between base of 2nd and 3rd anal-fin rays in males, reaching anus in females; pelvic-fin bases medially in close proximity. Dorsal-fin origin at vertical between base of 9th and 10th anal-fin rays. Dorsal-fin rays 9–11; anal-fin rays 13–15; caudal-fin rays 30–32; pectoral-fin rays 13–14; pelvic-fin rays 7. No contact organs on fins. Second proximal radial of dorsal fin between neural spines of 18^th and 20^th vertebrae; first proximal radial of anal fin between pleural ribs of 13th and 14th vertebrae; total vertebrae 30–31.

Scales small, cycloid. Body and head entirely scaled, except anterior ventral surface of head. Body squamation extending over anterior 25 % of caudal-fin base; no scales on dorsal, anal and pectoral-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. Five supraorbital scales. Longitudinal series of scales 29–30; transverse series of scales 8; scale rows around caudal peduncle 16. No contact organs on scales. Cephalic neuromasts: supraorbital 3 + 3, parietal 1, anterior rostral 1, posterior rostral 1, infraorbital 1 + 9–10 + 1, preorbital 2, otic 1, post-otic 2, supratemporal 1, median opercular 1, ventral opercular 1, preopercular 2 + 4, mandibular 3 + 1, lateral mandibular 2–3, paramandibular 1. Lateral line interrupted, alternating sets of 3–4 scales with one neuromast and without neuromasts. Two neuromasts on caudal-fin base.

Table 1.

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Morphometric data of Melanorivulus proximus.

Measurements	Holotype	Paratypes
Measurements	male	males (7)	females (5)
Standard length (mm)	27.7	25.6–30.0	24.0–27.4
Percent of standard length
Body depth	25.6	22.6–25.0	22.8–23.5
Caudal peduncle depth	15.1	14.1–14.9	13.1–14.2
Pre-dorsal length	78.4	75.9–78.3	76.1–79.7
Pre-pelvic length	57.1	54.7–56.5	55.0–57.9
Length of dorsal-fin base	11.2	10.4–12.8	10.7–12.9
Length of anal-fin base	22.2	21.1–24.7	18.8–21.4
Caudal-fin length	36.9	33.7–39.8	31.6–36.8
Pectoral-fin length	22.8	19.9–22.3	18.9–22.8
Pelvic-fin length	12.4	12.0–13.4	9.1–9.6
Head length	29.3	27.0–29.2	26.9–29.3
Percent of head length
Head depth	72.0	68.6–72.8	69.3–73.0
Head width	71.1	70.5–73.4	72.7–76.0
Snout length	13.5	13.6–15.2	14.5–16.8
Lower jaw length	18.4	19.9–22.0	19.7–21.8
Eye diameter	33.2	30.3–34.6	30.2–35.7

Colouration in life

Males. Flank metallic greenish blue to bright blue, with narrow oblique red bars between humeral region and posterior portion of caudal peduncle; bars irregularly arranged, forming chevron-like marks with angle varying in position on flank, often connected to short adjacent bars, forming Y- and X-shaped marks; bars with minute vertical extensions on each scale margin; dorsal portion of flank with oblique rows of red dots; anteroventral portion of flank with rows of red dots, often coalesced to form zigzag red marks. Dorsolateral portion of body, between posterior part of head and anterior part of flank, above humeral region, pale golden. Humeral region with horizontally elongated black spot. Dorsum light brown, venter white. Opercular region greenish golden with dark red reticulation on scale margins; suborbital region yellowish white; lower jaw dark grey. Iris pale yellow, with dark brown bar on anterior and posterior portions. Dorsal fin bluish white, sometimes yellowish on distal portion, with 4–5 transverse, narrow faint red or red stripes. Anal fin pale yellow, base and posterior portion bluish white with row of light red dots or short stripes. Caudal fin pale yellow to bluish white, with 5–6 narrow red or reddish orange stripes. Pectoral fin yellowish hyaline. Pelvic fin orangish pale yellow.

Females. Similar to males, except flank base colour pale greenish golden; dorsal and caudal fin bars dark grey; caudal fin base colour pale orangish pink; and presence of black spot on dorsal portion of caudal-fin base and dark grey pigmentation concentrated on distal margins of dorsal and anal fins, anterior margin of pelvic fin and entire caudal-fin margin.

Colouration in alcohol

Head and trunk pale brown, fins whitish hyaline; dark marks recorded for live specimens varying from dark brown to black.

Distribution

Middle section of the Rio Aporé drainage, upper Rio Paraná Basin, central Brazil (Fig. 3).

Figure 3.

Geographical distribution of killifishes of the Melanorivulus pictus species group in the rivers draining the south-eastern slope of the Caiapó range and the adjacent middle Rio Paraná Basin. Blue triangle: M. vittatus; light green dot: M. scalaris; light green triangle: M. linearis; pink lozenges: M. faucireticulatus; pink triangle: M. egens; purple dot: M. nigropunctatus; purple stars: M. proximus; red square: M. rutilicaudus; red triangle: M. ofaie; yellow dots: M. nigromarginatus; yellow stars: M. formosensis.

Etymology

From the Latin proximus (near, neighbour), referring to its distribution area at the same drainage as M. scalaris.