Myloplus taphorni, Andrade & López-Fernández & Liverpool, 2019

Andrade, Marcelo C., López-Fernández, Hernán & Liverpool, Elford A., 2019, New Myloplus from Essequibo River basin, Guyana, with discussion on the taxonomic status of Myleus pacu (Characiformes: Serrasalmidae), Neotropical Ichthyology 17 (4), pp. 1-9 : 2-4

Myloplus taphorni , new species

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Holotype. CSBD F 3611, 143.3 mm SL, Guyana, Mazaruni River at mouth of Kurupung River, 6º12.846’N 60º09.394’W, 21 March 2016, E. Liverpool, D.C. Taphorn, H. López-Fernández, M. Ram, K. Dookram, D. Hemraj, J. Correia.

Paratypes. ROM 101261 , 1, 145.7 mm SL, same data of holotype ; ROM 101262 , 1, 142.4 mm SL, Guyana, Lower Kurupung River , 22 March 2016, E. Liverpool, D.C. Taphorn, H. López-Fernández, M. Ram, K. Dookram, D. Hemraj, J. Correia ; UMMZ 251911 , 1, 154.9 mm SL, Guyana, Eping Creek , tributary of the Mazaruni River, 6°08.151’N 60°04.470’W, 20 March 2016, E. Liverpool, D.C. Taphorn, H. López-Fernández, M. Ram, K. Dookram, D. Hemraj, J. Correia .

Diagnosis. Myloplus taphorni differs from all congeners, except M. asterias , M. levis (Eigenmann, McAtee, 1907) , M. rubripinnis , and M. tumukumak , by combination of a higher number of branched rays in both dorsal and anal fin (23–25 and 33–34, branched dorsal-fin and anal-fin rays, respectively, vs. 20–21 and 27–29 in M. arnoldi , 21–22 and 32–34 in M. lobatus , 18–22 and 31–34 in M. lucienae Andrade, Ota, Bastos, Jégu, 2016 , 18–21 and 32–34 in M. planquettei , 21–22 and 30–33 in M. rhomboidalis , 20–21 and 30–36 in M. schomburgkii , 22–25 and 28–29 in M. ternetzi , 24–25 and 30–33 in M. tiete (Eigenmann, Norris, 1900) , 23–24 and 29–30 in M. torquatus , 20–22 and 32– 34 in M. zorroi Andrade, Jégu, Giarrizzo, 2016 ). Myloplus taphorni differs from M. asterias by having a shorter caudal-peduncle length (8.8–9.7% SL vs. 10.2–12.0% SL), and further by possessing anterior and posterior fontanels elongated and about the same size vs. anterior fontanel large, wide and rounded and posterior fontanel minute, narrow and triangle shaped. Myloplus taphorni differs from M. levis by having a reddish gray (male) or completely hyaline (female) anal fin vs. anal fin with an evident black mark present on the anterodistal portion in both sexes. Myloplus taphorni is distinguished from M. tumukumak , and additionally from M. lucienae , M. planquettei , and M. zorroi by having more series of scales between the lateral line and the pelvic-fin origin (37–39 vs. 24–29 in M. tumukumak , 27–31 in M. lucienae , 32–40 in M. planquettei , and 36–42 in M. zorroi ). Additionally, M. taphorni differs from M. arnoldi , M. asterias , M. lobatus , M. lucienae , M. rhomboidalis , M. rubripinnis , and M. torquatus by having a shorter postorbital distance (24.4–25.7% HL vs. 27.8–32.0% HL in M. arnoldi , 27.9– 34.2% HL in M. asterias , 26.9–32.3% HL in M. lobatus , 30.4–36.8% HL in M. lucienae , 26.9–32.0% HL in M. rhomboidalis , 26.2–32.6% HL in M. rubripinnis , and 38.1–38.8% HL in M. torquatus ).

Description. Morphometric data in Tab. 1 . Body deep, rounded to slightly elongated (and see Sexual dimorphism), highest body depth at dorsal-fin origin ( Figs. 1a–c ). Blunt snout. Big eye. Dorsal profile of head gently concave, predorsal profile and dorsal-fin base slightly convex. Dorsal profile between dorsal-fin base terminus and adipose-fin origin short and straight. Ventral profile of head and body convex. Anal-fin base straight to pronouncedly convex. Caudal peduncle with dorsal and ventral profiles convex.

Upper and lower jaws equal, mouth terminal. Rows of premaxillary teeth separated by an edentulous gap. Five*(4) molariform teeth in outer premaxillary row and 2*(4) in inner row. Dentary with 5*(4) molariform teeth, and pair*(4) of teeth at dentary symphysis. Symphyseal tooth with cutting edge in anterior portion. Maxilla edentulous. First gill arch with 14(1) gill rakers on upper branch, 15(1) rakers on lower branch, and 1(1) raker on intermediate cartilage.

Scales cycloid. Perforated lateral line scales from supracleithrum to edge of hypural plate 63(2) or 65*(2); total perforated scales on lateral line from supracleithrum to base of median caudal-fin rays 67(1), 68(1), 69*(1) or 70(1). Longitudinal scale rows between line and dorsalfin origin 36(3) or 37*(1); longitudinal scale rows between lateral line and pelvic-fin origin 37(1), 38(1) or 39*(2). Circumpeduncular scale rows 32*(3) or 33(1).

Forward directed spine anterior to dorsal fin. Dorsal-fin base relatively long compared to the short space between dorsal-fin base terminus and adipose-fin origin. Dorsal-fin rays ii*(3) or iii(1), and 23*(2), 24(1) or 25(1). Anal-fin rays iii*(4), and 33*(2) or 34(2). Pectoral-fin rays i*(4), and 12(1), 14(1) or 15*(2). Pelvic-fin rays i,6*(4). Adipose fin tear drop shaped. Caudal fin forked, dorsal and ventral lobes approximately equal in size.

Ventral keel scutes prominent, with large spines. Anterior most spine reaching or surpassing vertical through pectoralfin origin ( Fig. 2 ). Prepelvic serra with 21(1), 22*(1), 23(1) or 25(1) spines. Postpelvic serra with 5(1), 6*(2) or 7(1) simple spines, and 5*(2) or 6(2) pairs of spines around anus. Total serra with 32(1) 33*(1) or 36(2) spines.

Neurocranium triangular, as long as high ( Fig. 2 ). Fontanels elongated and equal sized. Ascending premaxilla process elongated, massive, and well fused to neurocranium. Dentary short with ventral edge straight. First dorsal-fin pterygiophore inserted between neural spines of vertebrae 10th and 11th(1), or between 9th and 10th(1), 5(1) or 6(1) supraneurals. Nine (1) or 10(1) predorsal vertebrae, 5(2) vertebrae between first anal-fin pterygiophore and last dorsal-fin pterygiophore, 12(2) vertebrae posterior to last dorsal-fin pterygiophore. First dorsal-fin pterygiophore posterior to neural spine of 9th(1) or 10th(1) centrum. First anal-fin pterygiophore posterior to haemal spine of 22nd(2) centrum. Thirty-seven(1) to 38(1) total vertebrae, 21(2) precaudal and 17(1) or 18(1) caudal vertebra ( Fig. 2 ).

Coloration in alcohol. Background coloration yellowish silver ( Figs. 1a–c ). Flank homogeneously yellowish silver, countershaded (females, Fig. 1c ) or with black coloration pattern (males, and see coloration under Sexual dimorphism for details), mainly concentrated on dorsal half of flank ( Figs. 1a–b ). Eye with well-marked vertical black bar, anterior and posterior portions of sclerotic clear to light yellow. Infraorbital series lacking pigmentation or with very few dark chromatophores on 1st, 2nd and/or 3rd infraorbitals. Opercle silver, anteromedial portion with very few dark chromatophores. Belly silver or light brown. Fins hyaline to light brown with few chromatophores scattered on rays and hyaline membrane between rays. Dorsal fin generally light brown with discrete black pigmentation distally on anterior rays. Adipose light brown to clear or with a very thin brown edge. Anal rays with dark chromatophores along base and extending to middle portion of rays or uniformly light brown. Caudal fin yellow to light brown, distally whitish gray.

Coloration in life. Background coloration silver to purplish silver, especially in males ( Fig. 3a ); females silver to bluish silver ( Fig. 3b ). Dorsal portion of head brownish black, ventrally and posterior to eye light silver. Upper portion of 3rd and 4th infraorbital and operculum light olive green. Eye with conspicuous vertical black bar, anterior and posterior portions of sclerotic bright yellow. Flank homogeneous silvery or with evident scattered pattern of black scales, mainly on upper flank. Belly silver or light black and metallic purple. Pectoral, pelvic and adipose hyaline to light yellow. Dorsal, anal and caudal fins yellow-orange to bright or brownish red.

Sexual dimorphism. Males and females are distinguished mainly by differences in coloration. Background coloration of dorsal region of males distinctly brownish red ( Figs. 1a–b , 3 ), ventrally purple to purplish silver with a dark purple, almost black, diffuse band on the ventral-most quarter of the flank. Females are primarily silver with a distinct bluish veneer in life ( Figs. 1c , 3 ). Male flank with a whitish silver medial region above and below the lateral line, bordered by deep black scales, most of which are overlaid with a metallic green sheen; perforated scales and lateral line pigmented black at center. Males also recognized by the presence of elongated middle anal fin rays, forming a lobe with apex centered on 16th branched ray. Females with first anal fin ray clear, longest, decreasing in size towards posterior portion of fin. Males lack stiff hooks on distalmost lepidotrichia of anal-fin rays and filamentous extensions on dorsal-fin rays.

Geographical distribution. Myloplus taphorni is currently known only from two tributaries of the middle reaches of the Mazaruni River, Essequibo River basin, Guyana ( Fig. 4 ).

Ecological notes. The species was collected in black water channels ( Fig. 5 ) between 25 and 75 m wide. Water characteristics at the two sites were similar, with temperature between 26.7–29 ºC, pH of 5.1, extremely low conductivity between 4–12 µS/m and secchi depth between 65–85 cm. Both sites had bottom consisting of a mixture of sand and coarse gravel, likely a semi-natural substrate transformed at least partially by ongoing or past gold mining activities in the area (see Discussion). Specimens were collected near large rocks or submerged woody debris at depths of 1 m or deeper.

Etymology. The species is named in honor of American ichthyologist Dr. Donald C. Taphorn in recognition of over four decades of continuing contributions to Neotropical ichthyology, his expansive role in training South American ichthyologists (including the authors), and for his participation in the expedition to the middle Mazaruni during which the new Myloplus was collected.

Figures

Fig. 1. Myloplus taphorni: Guyana, Essequibo Basin. a. Holotype, CSBD F 3611, male, 143.3 mm SL; Paratypes, b. UMMZ 251911, male, 154.9 mm SL, and c. ROM 101262, female, 142.4 mm SL. Photos: MCA.

Fig. 2. Radiograph of Myloplus taphorni, paratype, UMMZ 251911, male, 154.9 mm SL. Courtesy E. Holm (ROM).

Fig. 3. Myloplus taphorni photographed shortly after capture. a. Male, paratype, UMMZ 251911, male, 154.9 mm SL, Guyana, Eping creek. b. Female, paratype, ROM 101262, 142.4 mm SL, Guyana, Lower Kurupung River. Photos: HLF.

Fig. 4. Distribution map of Myloplus taphorni in middle Mazaruni River indicated by the black star.

Fig. 5. Collection localities for Myloplus taphorni. a. Eping Creek; b. Type locality, Lower Kurupung River near the mouth. Both streams are tributaries of the middle Mazaruni River, Guyana. Photos: HLF.

Tables

Tab. 1. Morphometric data of Myloplus taphorni from Essequibo River basin, Guyana. SD=Standard Deviation.

	Holotype		Paratypes		Mean	SD
CSBD F 3611 ROM 101261 ROM 101262 UMMZ 251911
Standard length (mm)	143.3	145.7	142.4	154.9
Percentages of SL
Body depth	72.7	72.8	78.9	69.1	73.4	4.1
Head length	26.1	25.4	25.8	25.5	25.7	0.3
Distance from snout to supraoccipital spine	33.0	34.5	33.8	34.2	33.9	0.7
Predorsal length	58.3	59.0	60.9	58.5	59.2	1.2
Dorsal-fin base length	36.5	34.6	36.3	36.2	35.9	0.9
Interdorsal length	9.1	10.1	9.0	9.2	9.4	0.5
Adipose-fin base length	4.6	3.7	4.7	4.2	4.3	0.5
Caudal-peduncle length	9.7	9.2	8.9	8.8	9.2	0.4
Caudal-peduncle depth	10.5	10.0	9.9	9.3	9.9	0.5
Caudal peduncle width	3.3	3.4	3.9	3.6	3.6	0.3
Prepectoral length	29.7	28.4	27.9	28.1	28.5	0.8
Pectoral-fin length	21.7	19.9	21.3	20.3	20.8	0.8
Pelvic-fin origin to anal-fin origin	20.4	22.8	21.2	21.8	21.6	1.0
Pectoral-fin origin to pelvic-fin origin	31.6	32.7	37.6	32.6	33.6	2.7
Prepelvic length	60.6	60.8	65.3	60.2	61.7	2.4
Pelvic-fin length	14.7	14.0	14.1	14.1	14.2	0.3
Preanal length	79.0	80.2	75.0	79.3	78.4	2.3
Anal-fin base length	40.4	38.2	41.4	37.7	39.4	1.8
First anal-fin lobe length	20.8	-	24.1	17.7	20.9	3.2
Second anal-fin lobe length	14.3	-	-	16.2	15.3	1.3
Dorsal-fin lobe length	27.1	25.9	28.4	25.3	26.7	1.4
Dorsal-fin origin to anal-fin origin	75.2	74.4	79.5	72.0	75.3	3.1
Dorsal-fin end to anal-fin origin	55.7	54.0	58.8	51.6	55.0	3.0
Dorsal-fin end to anal-fin end	22.2	21.4	21.7	20.5	21.5	0.7
Percentages of HL
Snout length	27.9	28.6	28.3	28.9	28.4	0.4
Mouth length	16.9	16.9	14.7	17.3	16.5	1.2
Mouth width	37.3	35.2	36.7	34.9	36.0	1.2
Interorbital width	55.7	59.0	56.9	53.3	56.2	2.4
Head width	64.8	66.0	67.5	64.7	65.8	1.3
Third infraorbital width	12.5	13.1	12.4	11.8	12.5	0.5
Cheek gap width	8.7	9.0	10.0	10.0	9.4	0.7
Fourth infraorbital width	9.5	10.8	10.6	9.3	10.1	0.8
Eye vertical diameter	47.3	46.6	46.7	45.6	46.6	0.7
Postorbital distance	25.7	25.1	24.4	24.6	25.0	0.6