Melanorivulus terena Volcan, Severo-Neto & Lanés, 2024

Volcan, Matheus Vieira, Severo-Neto, Francisco & Lanés, Luis Esteban Krause, 2024, A new species of Melanorivulus (Cyprinodontiformes: Rivulidae) from headwaters of the Taquari River basin, Paraguay River basin, central Brazilian Cerrado, Journal of Natural History (J. Nat. Hist.) 58 (37 - 40), pp. 1493-1508 : 1495-1501

Melanorivulus terena Volcan, Severo-Neto & Lanés sp. n.

urn:lsid:zoobank.org:pub:8B0027D9-8346-4ADB-AE9E-8C770823B8D9urn:lsid:zoobank.org:act:9C25B380-D5DA-402B-8E98-C0296777AE0B

Holotype ( Figure 1 ). ZUFMS-PIS08450 , male, 27.2 mm SL, Brazil, Mato Grosso do Sul State, municipality of Camapuã, road MS-422, Ribeirão Cachoeirinha, first-order stream in the headwaters of Rio Coxim , Rio Taquari drainage, Rio Paraguay basin, - 19.34333°S, - 53.89277°W, altitude 518 m a.s.l.; F. Severo-Neto and T. R. F. Sinani, 4 April 2021.

Paratypes. ZUFMS-PIS08471 , 2 males, 26.1–28.1 mm SL (2 C&S), 2 females, 20.9–26.1 mm SL (2 C&S); M . V. Volcan and L.E.K. Lanés, 12 December 2016, same locality as holotype . MZUSP 130257 , 3 males, 19.1–19.8 mm SL, 5 females, 17.8–20.8 mm SL, 19 juveniles, sex undetermined, 11.2–18.7 mm SL, all collected with the holotype . ZUFMS-PIS06575 , 5 males, 21.1–26.1 mm SL, 4 females, 20.9–26.1 mm SL, all collected with the holotype .

Diagnosis. Melanorivulus terena is distinguished from all species of the M. pictus group by the distal margin of the unpaired fins and pelvic-fin orangish-red in males (vs never with the combination of distal margin orangish-red in all unpaired fins and pelvic fin). Melanorivulus terena is similar to M. egens , M. linearis and M. interruptus , and distinguished from all other species of the M. pictus group by the absence of distinctive dark marks on the humeral region (vs presence). Melanorivulus terena is distinguished from M. interruptus , M. egens and M. linearis by having a larger number of scales in the transversal series (10–11 vs 8–9). It is distinguished from M. interruptus by having fewer pectoral fin rays (13 vs 14), smaller pelvic fin (pelvic-fin length 10.3–11.6% SL, vs 12.1– 13.9% SL), larger head in males (head length 27.8–30.5% SL vs 26.2–27.5% SL), and larger caudal fin in females (caudal-fin length 30.6–32.1% SL vs 26.8–29.4% SL). Melanorivulus terena is distinguished from M. egens by the presence of red oblique bars in the caudal fin in males (vs caudal fin without bars), smaller eye diameter in males (eye diameter 28.8– 32.3% HL vs 33.1–36.4% HL); larger body depth (22.0–23.1% SL vs 21.3–21.8% SL), and smaller pre-pelvic length (52.8–55.7% SL vs 56.7–56.8% SL). Melanorivulus terena is distinguished from M. linearis by smaller caudal fin in males (caudal-fin length 29.9– 33.6% SL vs 36.6–36.9% SL), and in females (caudal-fin length 30.6–32.1% SL vs 34.2– 37.4% SL); by smaller pelvic fin in males (pelvic-fin length 10.3–11.6% SL, vs 13.1–15.4% SL); and by larger head width in females (head width 75.0–80.6% HL vs 70.5–74.0% HL).

Description

Morphometric data is presented in Table 1 . Males larger than females. Largest male examined 28.1 mm SL, largest female 26.1 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 slightly pointed in males and rounded in females. Dorsal-fin origin on vertical through between base of 8th and 10th anal-fin ray. Anal-fin rays 14–16. Anal fin slightly pointed in males and females. Caudal fin rounded 28–32 rays. Pectoral-fin rays 13. Pectoral fin rounded, with posterior margin reaching vertical at about 70–80% of length between pectoral-fin and pelvic-fin bases. Pelvic-fin rays 7. Pelvic-fin posterior tip reaching vertical at anterior to the anus to 1st 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 40% 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 31–32; transverse series of scales 10–11; 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 + 11–14 + 1, preorbital 1–2, otic 1, postotic 1, supratemporal 1, median opercular 1, ventral opercular 1, preopercular 2 + 4, mandibular 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 approximately straight. Dorsal portion of preopercle short and pointed. Ventral process of posttemporal absent. Total number of vertebrae 30–31, 13 precaudal vertebrae in males and 14–15 in females, 17 caudal vertebrae in males and 16–17 in females. Dorsal-fin origin on vertical between neural spines of 19th and 20th vertebrae in males and between the 20th and 21st vertebrae in females. Origin of anal fin at vertical through pleural ribs of 13th and 14th vertebrae in males and between the 14th and 15th vertebrae in females. Ventral process of angulo-articular short, pointed. Vomerine teeth 1–3. Dermosphenotic present. Basihyal sub-triangular, greatest width 55% of length; basihyal cartilage 25% of total basihyal length. Second pharyngobranchial teeth absent. Teeth on the fourth ceratobranchial divided into two rows exhibiting a median gap. Interarcual cartilage well developed. Lateral process of ethmoid well developed and free.

Colouration in life – males ( Figure 2 ). Metallic greenish-grey flank, becoming purplish-blue close to the base of the anal fin; red dots scattered on anterior flank area, above the pectoral-fin insertion, forming 3–4 small oblique bars; 1 or 2 narrow continuous or interrupted red oblique bars, between the medial pectoral-fin portion and the pelvic-fin base; continuous bars forming chevron-like marks with an angle on the dorsal region of the flank; 7–9 continuous or interrupted oblique narrow red bars irregularly arranged between the pelvic-fin base and caudal peduncle, anteriorly directed, forming chevron-like marks, with an angle that goes from dorsal to midline in the anterior-posterior direction, no bar interruption pattern; vestigial red bars in the lateral-dorsal region of flank, from the medial pectoral fin portion to the dorsal origin of caudal fin. Dark red pigmentation in the postorbital region of the head, forming 2–3 oblique bars, sometimes contiguous with the red dots in the humeral region. Dorsum brownish-green, venter whitish-grey. Dorsal portion of the head brownish-green; ventral portion yellowish-grey; pale greenish-golden iridescence on opercular region. Dark grey jaws. Iridescent yellow iris. Dorsal fin yellow with 4–5 oblique red bars, orangish-red distal margin. Anal fin yellow, proximal portion light blue, medial posterior proximal portion with faint red marks, sometimes forming a U-shape; distal region with an orangish-red margin. Caudal fin yellow with 4–5 red oblique parallel bars, covering generally 1/3–1/2 of caudal fin on mid-dorsal portion; ventral portion light yellow without bars; distal margin of fin with an orangish-red colour, well delineated. Pectoral fin yellowish-hyaline. Pelvic fin yellow with orangish-red posterior margin.

Females ( Figure 2 ). Same bar pattern as males, with a greyish-ochre flank, becoming lighter towards the base of the caudal fin. Light ochre venter. Dorsum and dorsal portion of head brownish-ochre; ventral portion of head yellowish. Jaws brownishorange; opercular region pale ochre, with a few chromatophores scattered randomly and a conspicuous greenish-gold iridescent spot. Dorsal fin yellow, with 2 or 3 faint brown bars; distal margin faint dark brown. Anal fin yellow; basal portion light yellow becoming dark brown gradually towards distal margin; distal region with a dark brown margin. Caudal fin pale yellow, with 5 dark brown bars in the mid upper portion of the fin; faint dark brown distal margin, with mixed orange dots in the dorsal margin; conspicuous black spot on the dorso-basal portion of the fin, generally contiguous with the first bar. Pectoral fin hyaline with narrow orange margins. Pelvic fin yellowish hyaline with dark brown posterior margin with orange dots.

Distribution ( Figure 3 ). Melanorivulus terena is only known from one location in Ribeirão Cachoeirinha, a small stream in the headwaters of the Coxim River, a tributary of the Taquari River, in the Paraguay River basin, municipality of Camapuã, state of Mato Grosso do Sul, Brazil.

Etymology. The specific epithet ‘terena’ is in allusion to the Terena ethnic group. The Terena people belong to the Aruak linguistic family and are the last remaining members of the Guará nation. The Terena are currently the most visible Indigenous people in the upper Paraguay portion in the state of Mato Grosso do Sul.

Habitat. The new species was found in a small first-order stream (locally called ‘veredas’), with a depth of about 20 cm, at the headwaters of the Taquari River ( Figure 4 ). Melanorivulus terena was recorded in the stream bank and adjacent shallow pools where the water was clear, still or gently flowing, with muddy and clay substrates, within the savanna-like Cerrado domain. The species is found in areas exposed to sun, as well as in areas shaded by trees, mainly buritis palm ( Mauritia flexuosa ).

Conservation status. The new species is endemic to the Coxim River, a tributary of Taquari River, and was recorded in only one location, but there are potential habitats for its occurrence along the stream and in the surrounding area. It has been recorded in an area with major impacts from intense agricultural activity. Another main activity in the region is cattle farming, which has drastically altered the original vegetation. The main threats are erosion and excessive sedimentation in the drainages of Ribeirão Cachoeirinha and its tributaries, as well as trampling by cattle in shallow wetlands (veredas) located along the stream banks. In addition, the area of occurrence of M. terena is crossed by the MS-422 highway. Consequently, the species is also subject to transportation-based accidents with toxic loads, which pose a threat to the M. terena population. Because of the previously mentioned threats, the high degree of endemism, and the field observations, M. terena is considered ‘Vulnerable’, following the criteria of the International Union for Conservation of Nature ( IUCN 2012). The species has an area of occupation of less than 2000 km 2, it occurs in only one locality, and it suffers from a reduction in its present area of occurrence and in its habitat quality (VU B2abii,iii).

Figures

Figure 1. Melanorivulus terena, ZUFMS-PIS08450, male, holotype, 27.2 mm SL.

Figure 2. Male ZUFMS-PIS08471 (above), and female ZUFMS-PIS06575 (below) paratypes of Melanorivulus terena, sp.n. Camapuã municipality, Mato Grosso do Sul, Brazil. Photos: Matheus Vieira Volcan (male) and Francisco Severo-Neto (female).

Figure 3. Occurrence area of Melanorivulus terena (type locality) in the upper Rio Coxim basin, Rio Paraguay basin and closest related species of M. pictus group in the Rio Paraná basin.

Figure 4. Type locality of Melanorivulus terena sp. n. in Ribeirão Cachoeirinha, tributary of headwaters of Rio Coxim, Paraguay River Basin, Camapuã municipality, Mato Grosso do Sul state, Brazil (A). Photo: Matheus Volcan. Detail of the type of environment where the species was recorded (B). Photo: Francisco Severo-Neto.