Myloplus aylan Pereira, Ota, Machado, Collins, Ândrade, Garcia-Ayala, Jégu, Farias & Hrbek, 2024

Machado, Valéria N., Pereira, Victória D., Ota, Rafaela P., Collins, Rupert A., Ândrade, Marcelo, Garcia-Ayala, James R., Jégu, Michel, Farias, Izeni P. & Hrbek, Tomas, 2024, Integrative taxonomy of the black-barred disk pacus (Characiformes: Serrasalmidae), including the redescription of Myloplus schomburgkii and the description of two new species, Neotropical Ichthyology (e 230095) 22 (2), pp. 1-45 : 8-15

Myloplus aylan Pereira, Ota, Machado, Collins, Ândrade, Garcia-Ayala, Jégu, Farias & Hrbek , new species

urn:lsid:zoobank.org:act:2EF3345A-B36F-4BC7-9507-FFECBEB043FC

( Figs. 2–5 ; Tab. 3 )

Prosomyleus (Myleus) schomburgkii Géry, 1977:266 [listed, brief description of the subgenus; photo in page 269, above; locality: Alto Solimões ( Brazil)].

Myleus schomburgkii . — Machado-Allison, Fink, 1995:62–63 [cover figure; brief description; illustration, fig. 26:63; locality: Orinoco and Casiquiare rivers ( Venezuela)] ―Vari et al., 2009:72 [listed; photo F; locality: Orinoco ( Venezuela)].

Myloplus schomburgkii . — Murrieta-Morey et al., 2019:511–19 [description of a new species of parasite, locality: Rio Nanay basin ( Peru)]. — Murrieta-Morey et al., 2021:110 [mortality and water quality; figs. 1C, D; locality: Rio Nanay basin]. — Kolmann et al., 2020:2 [exon-based phylogeny; fig. 1]. — Mateussi et al., 2020:3 [Listed].

Holotype. INPA 60150 , male, 213.01 mm SL, A3742 (GenBank accession OR366877 ), Brazil, Roraima, Caracaraí municipality, rio Baraúna , lago do Bento , 01°15’34.4”N 60°54’32.4”W, 17 Jan 2021, M. S. Rocha.

Paratypes. Brazil: Amazonas: Apuí: INPA 60151 , 1, 91.32 mm SL, rio Guariba tributary of Aripuanã basin, at Reserva Extrativista do Guariba , 08°42’42”S 60°25’53”W, 7 Aug 2008, W. S. Pedroza, W. Ohara, F. R. Ribeiro & T. F. Teixeira. INPA 60153 , 1, 81.97 mm SL, rio Guariba at Reserva Extrativista do Guariba , 08°42’42”S 60°25’53”W, 14 Nov 2008, W. S. Pedroza, W. Ohara, F. R. Ribeiro & T. F. Teixeira. Carauari: INPA 60673 , 1, 185.0 mm SL, (CTGA 22529), Igarapé Pucá tributary of rio Juruá , 02°50’40”S 66°57’51”W, 1 Jul 2022, T. Hrbek. Careiro: INPA 60702 , 1, 131.2 mm SL, (CTGA 22278), rio Juma , interfluve of Madeira-Purus , 03°42’19”S 59°48’46”W, 26 Sep 2021, V. N. Machado. Jutaí: INPA 60672 , 1, 178.1 mm SL, (CTGA 22524), rio Jutaí , 04°46’12”S 66°37’06”W, 1 Sep 2022, V. N. Machado. INPA 60709 , 1, 155.3 mm SL, (CTGA 23316), rio Jutaí , 02°54’13”S 67°02’35”W, 10 Fev 2023, V. N. Machado. BMNH 2024.2.12.1-2 , 2, 171.0– 187.1 mm SL, (CTGA 22523, 22525) same collection data as INPA 60672. Presidente Figueiredo: INPA 22192 , 2, 275.58– 309.33 mm SL, Balbina, rio Uatumã , 01°55’21”S 59°28’21”W, 9 Nov 1985, M. Jégu. Roraima: Caracaraí: INPA 60152 , 6, 190.0– 207.7 mm SL, rio Baraúna , lago do Bento , 01°15’34.4”N 60°54’32.4”W, 17 Jan 2021, M. S. Rocha. MUBIO 110 , 1, 198.70 mm SL, same collection data as INPA 60152 . Peru: Iquitos: Maynas: Loreto: ANSP 180369 , 4, 67.68–95.01 mm SL (1 x-ray, 71.3 mm SL), ( OR366880 ), rio Nanay , large sandy beach on downstream end of island upstream from Santa Clara , 03°46’56.9”S 73°20’33.3”W, 14 Aug 2003, M. H. Sabaj, N. J. Salcedo & B. Sidlauskas. ANSP 188823 , 2, 53.38– 62.73 mm SL, ( OR366879 ), rio Nanay , large sandy beach on downstream end of island upstream from Santa Clara , 03°46’56.9”S 73°20’33.3”W, 14 Aug 2003, M. H. Sabaj, N. J. Salcedo & B. Sidlauskas. ANSP 199909 , 3, 55.55–61.04 mm SL, rio Nanay , just downstream of sandy beach Las Camelias , 7 km from Iquitos, 03°39’51”S 73°15’01”W, 8 Aug 2010, M. H. Sabaj, B. Sidlauskas, C. A. Phillips, J. Tiemann & E. V. Correa Roldán. CIIAP 401 , 1, 206.1 mm SL, rio Pucacuro , tributary of rio Tigre , 03°19’07”S 74°59’05”W, 15 May 2003, J. Ruiz. CIIAP 402 , 3, 85.2– 8.9 mm SL, rio Nanay , cocha Anguilla , 03°54’34.7”S 73°39’57.7”W, 18 Jul 2018, C. Chavez. CIIAP 403 , 5 (3, 97.0– 113.0 mm SL), rio Nanay , large sandy beach on downstream end of island upstream from Panpachica , 03°45’07”S 73°17’01”W, 20 Jan 2019, M. Ruiz-Tafur.

Diagnosis. Myloplus aylan can be distinguished from all congeners, except M. schomburgkii and M. sauron n. sp. (described below), by the presence of a vertical black bar on the flank commonly extending from the dorsal-fin base to the pelvic-fin distal end (vs. absence of any conspicuous mark on the flank in the rest of the congeners). The new species can be diagnosed from M. schomburgkii and M. sauron by the following characters: parietal bone with dorsal surface markedly concave in lateral view (vs. straight to slightly concave), 82–95 (mode 84) total perforated scales on lateral line [vs. 68–87 (mode 79) in M. schomburgkii and 70–82 (mode 78) in M. sauron ]; 40–41 total vertebrae (vs. 36–38), serrae with narrow and long spines (vs. short with wide base); 30–39 prepelvic spines (vs. 17–29 in M. schomburgkii and 20–28 in M. sauron ); 38–55 (mode 49) total spines [vs. 27–41 (mode 33) in M. schomburgkii and 29–40 (mode 35) in M. sauron ). Also, M. aylan differs from M. schomburgkii and M. sauron by presenting, in juveniles and females, anteroposterior decreasing of anal-fin rays length almost uniform, forming broad lobe, occupying half of the anal-fin extension (vs. abrupt anteroposterior decreasing of anal-fin rays length, forming narrow falcated anal-fin lobe in juveniles and females, restricted to the anterior half of the fin, not reaching the middle portion of anal-fin base length, with conspicuous dark-red to black pigmentation on the entire anal fin (vs. orange to reddish-orange pigmentation along its length, mostly concentrated on anterior rays). Additionally, the new species can be diagnosed from M. schomburgkii by presenting anterior ventral-keel spine at the vertical through pectoral-fin origin or anterior to it (vs. anterior spine of ventral-keel always inserted posterior to the vertical through pectoral-fin origin); and from M. sauron by having greater number of branched dorsal-fin rays (21–25 vs. 17–19), wider dorsal-fin base (31.1–37.1% SL vs. 25.6–29.1%), shorter distance between last branched dorsal-fin ray and the last branched anal-fin ray [23.4–28.6% SL (mean 25.3%) vs. 27.8–32.0% (mean 29.7%)], shorter adipose-fin base [5.0–7.7% SL (mean 6.3%) vs. 7.1–9.7% (mean 8.4%)] and greater adipose-fin height (height 0.7–1.1 times in its base vs. 0.4–0.6).

Description. Morphometric data presented in Tab. 3 . Body compressed, overall rounded to oval, with highest body depth at dorsal-fin origin. Predorsal and postdorsal length almost equivalent. Head rounded, snout length slightly shorter than postorbital distance. Dorsal profile of head convex from mouth to anterior margin of parietal bone, and straight to slightly concave from this point to base of supraoocipital. Dorsal-fin base straight to slightly convex. Last dorsal-fin ray distal end not reaching adipose-fin origin, when adpressed. Distance between dorsal-fin insertion and adipose-fin origin about two times the adipose-fin base. Adipose fin as long as deep, with straight base. Ventral profile of head and body convex from lower lip to anal-fin origin. Anal-fin base slightly convex. Dorsal and ventral profile of caudal peduncle concave.

Mouth terminal. Premaxillary teeth in two rows, outer row with 5*(24) molariform teeth, teeth 1–4 almost equal-sized, tooth 5 smaller, all with sharp, convex edges; inner row with 2*(24) equal-sized teeth with sharp, concave edges; in ventral view, contralateral outer rows forming a V-shaped arch with apex anteriorly pointed; contralateral inner rows forming straight line between the 3 rd teeth of outer series, space between rows forming triangular gap. Dentary with 5*(24) molariform teeth, teeth 1–3 substantially bigger than 4–5. Conical symphyseal tooth immediately behind tooth 1 of labial row. Maxilla edentulous.

Scale cycloid, small. Total of perforated scales on lateral line 82(1), 83(3), 84*(5), 85(3), 86(3), 87(1), 89(4), 90(1), 91(1), 94(1), or 95(1). Dorsal-fin base covered by skin flap bearing one or two scale rows. Scale rows between dorsal-fin origin and lateral-line 41(2), 44(2), 45(2), 46(2), 47(2), 49(1), 50(1), 51(1), 55(1), 56(1), 57*(4), 58(2), 60(1), or 61(1). Scale rows between lateral-line and pelvic-fin origin 43(1), 44(1), 45(2), 46(4), 47(3), 48(1), 49(2), 50(3), 51(1), 52*(3), 53(1), 54(1), or 57(1). Adipose-fin base covered by three or four scale rows. Scale rows between the adipose-fin origin and lateral-line 19(2), 20(5), 21(7), 22(8), 23(1), 24(1), or 25*(1). Anal-fin base covered by four or five scales rows. Circumpeduncular scales 34(4), 35(2), 36(5), 37(2), 38*(4), 40(3), 41(2), 42(1), or 44(1).

Dorsal-fin origin slightly anterior to vertical through pelvic-fin origin. Dorsal-fin rays ii, iii*, or iv, 21(1), 22(6), 23*(7), 24(10), or 25(1). Adipose-fin square, length, and depth almost equivalent. Pectoral fin feather-shaped, anterior rays longest. Pectoral-fin rays i*, 13(3), 14(4), 15(6), 16*(12), or 17(2). Anterior pelvic-fin rays longest, not reaching vertical through last spines of serrae. Pelvic-fin rays i,7*(25). Last unbranched anal-fin ray most developed (longest and thicker). Anal-fin rays iii or iv*, 29(1), 30(6), 31*(11), 32(4), 33(2), or 34(1). Caudal-fin forked with almost equal-sized lobes.

Total gill rakers on first branchial arch 27(2), 28(1), 29*(13), 30(5), or 31(2). Upper branch with 12(2), 13*(13), 14(6), or 15(3) rakers; lower branch with 13(1), 14(5), or 15*(17); 1*(23) at cartilage between cerato- and epibranchial.

Osteology. Dorsal profile of neurocranium convex from premaxilla to the posterior margin of frontal bone, markedly concave at parietal, convex from the base to tip of supraoccipital process. Lateral view of supraoccipital triangular. Supraneurals 5(6) or 6(3). Dorsal-fin pterygiophores 23(1), 24(4), or 25(4). First dorsal-fin pterygiophore inserted between neural spines of 9 th and 10 th (8) or 10 th and 11 th (1) vertebrae, more developed than remaining pterygiophores, with expanded anterior lamella, and bearing forward-oriented predorsal spine. Predorsal spine somewhat similar to scythe, its dorsal surface smooth; almost completely covered by skin. Anal-fin pterygiophores 31(2), 32(4), or 33(4) ( Fig. S1 ).

Total vertebrae 40(8) or 41(1); Weberian apparatus, 4(10); abdominal 18(6) or 19(3) [Predorsal, 5(7) or 6(2); under dorsal-fin 13(8) or 14(1)]; caudal 17(3) or 18(6) [under dorsal-fin 3(4) or 4(5), posterior to dorsal-fin 14(9)]. Anterior spine of ventral keel never reaching vertical through pectoral-fin origin. Long spines, with an almost uniform width throughout its length, with piercing tips. First prepelvic spines covered by skin. Postpelvic spines more developed than prepelvic spines. Total ventral keel spines 38(1), 40(1), 41(1), 46(2), 47(5), 48(3), 49(7), 51(2), 53(1), or 55*(1). Prepelvic spines 30(2), 31(1), 32(4), 33(4), 35(5), or 39*(1); unpaired post-pelvic spines 10(4), 11*(11), or 12(7); and paired spines around anus 4(16), 5*(7), or 6(1).

Coloration in alcohol. Ground coloration yellow to pale brown. Brown pigmentation on dorsal portion of head and body. Middle portion of flanks and belly light yellow. Sclera yellow. Bright yellow pigmentation skirting ventral keel extension. High concentration of melanophores form a broad, vertical brown bar extending from the midpoint of dorsal-fin base to midpoint of pelvic-fin length, wider at lateral line. Pectoral, pelvic and caudal fins yellowish hyaline. Dorsal and anal fins yellow, with conspicuous dark-brown pigmentation most concentrated along base and in extension of anterior rays. Adipose-fin yellow, with dark black outlining on its distal margins ( Fig. 2 ).

Coloration in life. Ground coloration grayish silver. Purplish-silver scales along dorsal region of body, and silvery white scales covering middle portion of flank and belly. Scattered orange-red pigmentation on body, more concentrated on head and pectoral region.Dark vertical bar similar to color in alcohol.Dark olive-yellow pigmentation mostly concentrated on antero-dorsal portion of head. Scattered orangish-red pigmentation on postero-ventral portion of head and pectoral girdle. Sclera white. Paired fins yellowishhyaline. Basal half of dorsal and caudal fins yellow, followed by white area, and distal ends with dark pigmentation. Adipose fin grayish yellow, with subtle dark margins. Anal fin hyaline with black or dark-red pigmentation throughout ( Fig. 3 ).

Sexual dimorphism. Mature males with two anal-fin lobes; first lobe at anterior rays, less developed; second lobe centered on 13 th or 14 th branched ray, about twice as long as first lobe. Females and juveniles with single, broad lobe, formed by remarkable prolongation of anterior rays, and occupying more than half of fin (i.e., rays decreasing gradually in length from anteriormost rays to middle rays). Breeding males present black pigmentation throughout body, more conspicuous on head, predorsal region and fins, and dark-red pigmentation mostly concentrated on middle portion of flank. Females present grayish-brown pigmentation on antero-dorsal region of head and predorsal region, all fins grayish-hyaline, except for anal fin (which is dark red to black), red to dark-red pigmentation on middle portion of flank and well-marked vertical bar surrounded by light area. Males also differ from females by presenting long filaments extending dorsal-fin branched rays, and stiff hooks on distal-most lepidotrichia segments of anal-fin branched rays.

Geographical distribution. Myloplus aylan is widespread through Nanay, Tigre (tributary of Marañon), Branco, Juruá, Jutaí, Purus, Madeira, and Uatumã basin, in Peru and Brazil, mostly restricted to the western portion of the Amazon basin. In white water rivers the species is only captured in tributaries with black or clear water ( Fig. 4 ).

Ecological notes. Myloplus aylan appears to be more abundant in black water rivers such as the Nanay in Peru, Pitinga, and Jutaí rivers in Brazil. In white water rivers such as the Juruá and Madeira, the species was captured in black water lakes. However, it also occurs in clear water rivers such as Água Boa do Univini and Baraúna, tributaries of the Branco River. White water rivers of Andean origin seem to constitute a chemical barrier for this species, since in the Branco, Juruá and Madeira rivers, the species was captured only in black water lakes of these basins. The same distribution pattern was registered to its congener M. nigrolineatus ( Ota et al., 2020) . Ríos-Villamizar et al. (2020) classify the black waters of the várzea environments as Intermediate type B, since they present intermediate levels of suspended solids originating from ancient sediments and those recently eroded from the Andes. These characteristics allow the presence of M. aylan in Amazonian floodplain environments.

Conservation status. This new taxon is threatened by exploitation from commercial fishing ( Fig. 5 ) (upper Solimões River), by pollution, primarily from mining (Branco River) in the environments where it occurs ( Nyholt et al., 2022; Vasconcellos et al., 2022), and by the proposed construction of hydroelectric projects in the region of the upper Solimões River ( Winemiller et al., 2016; Castello, Macedo, 2016). This species is also exploited to a limited extent by fishing for the ornamental trade, mainly in the upper Solimões region in Peru ( García-Dávila et al., 2022), where attempts have already been made to reproduce this species in captivity (Murrieta et al., 2021), without success. Fish farming of M. aylan aims to meet the market demand for ornamental fish and for food consumption, but induced reproduction has not yet been successful. In addition, since 2009 Myloplus aylan is considered a species prohibited from extracting and commercializing, unless it comes from management programs in Peru ( PERU, 2009). Although many threats are identified in the range of occurrence of M. aylan , this species has a wide distribution in western Amazonia. Following the IUCN categories and criteria the species can be categorized as Least Concern (LC) (IUCN Standards and Petitions Subcommittee, 2022).

Etymology. The specific name honors the late Aylan Moraes Andrade, Carine Moraes and Marcelo Andrade’s son, born on December 23, 2022, who passed away prematurely on July 6, 2023. Marcelo is one of the authors of this manuscript and this is a tribute to record all the love and dedication of his parents who will never forget him. A noun in apposition.

Figures

FIGURE 1 | Schematic drawing of Myloplus schomburgkii with the landmarks used in the measurements. 1, anteriormost point of the snout. 2, distal margin of the hypurals. 3, dorsal-fin insertion. 4, pelvic-fin insertion. 5, posteriormost point of the opercle, not including opercular membrane. 6, supraocciptal’s base. 7, supraocciptal’s distal tip. 8, pectoral-fin insertion. 9, anal-fin insertion. 10, distal tip of the longest dorsal-fin unbranched ray. 11, base of the last dorsal-fin ray. 12, anterior end of the adipose-fin base. 13, pectoral-fin distal tip. 14, pelvic-fin distal tip. 15, distal tip of the longest anal-fin unbranched ray. 16, base of the longest ray of the second anal-fin lobe. 17, distal tip of the longest branched ray of the second anal-fin lobe. 18, distal end of the anterodorsal margin of the adipose fin. 19, posterior end of the adipose-fin base. 20, posterior end of the anal-fin base. 21, point of the dorsal profile of the caudal peduncle in which it is closest to ventral profile. 22, point of the ventral profile of the caudal peduncle in which it is closest to dorsal profile. 23, posteriormost point of the eye. 24, middle of posterior margin of the fourth infraorbital. 25, middle of anterodorsal margin of third infraorbital. 26, middle of posteroventral margin of third infraorbital. 27, internal angle of pre-opercle. 28, dorsalmost point of the eye. 29, ventralmost point of the eye. 30, anteriormost point of the eye. 31, distal end of the premaxilla. 32, point in which the head is wider. 33, point in which the caudal peduncle is wider. The ventral keel count comprises prepelvic spines (prPvSp), postpelvic spines (poPvSp), and the paired spines around anus (bISp).

FIGURE 2 | Myloplus aylan, INPA 60150, holotype, male, 213.01 mm SL, Brazil, Roraima, Caracaraí municipality, rio Baraúna, lago do Bento.

FIGURE 3 | Myloplus aylan, coloration immediately after capture. Mature female, lago Uauaçu, Purus River basin. Photographed by Lucia Rapp Py-Daniel.

FIGURE 4 | Geographic distribution of Myloplus schomburgkii (blue circles; the pink star at rio Negro basin represents the type-locality), M. sauron (red squares; the yellow star at rio Xingu basin represents the type-locality) and M. aylan (purple triangles; the black star at rio Branco basin represents the type-locality).

FIGURE 5 | Sympatric occurrence of Myloplus schomburgkii (left) and M. aylan (right) in rio Água Boa do Univini, tributary of rio Branco. Both specimens are mature males, the specimens has not being preserved.

Tables

TABLE 3 | Morphometric data ofMyloplus aylan. Range including the holotype.N = Number of specimens; SD = Standard deviation.

	Holotype	N	Range	Mean	SD
Standard length (mm)	213.0	27	53.4–309.3	–	–
Percentages of standard length
Body depth	72.4	27	59.2–79.9	73.1	4.7
Head length	29.0	27	26.2–33.7	29.7	2.0
Supraoccipital process	15.8	27	12.1–18.0	14.8	1.4
Predorsal length	61.2	27	57.2–64.5	60.5	1.7
Postdorsal length	55.1	27	51.1–66.5	57.3	3.5
Prepectoral length	29.2	27	28.0–33.8	30.4	1.5
Prepelvic length	60.8	27	57.5–64.8	60.6	1.8
Preanal length	80.1	27	76.3–84.0	79.7	1.8
Dorsal-fin length	23.6	20	22.4–47.7	29.1	8.4
Interdorsal length	9.0	27	7.0–11.8	9.8	1.2
Pectoral-fin length	21.3	27	18.0–23.6	21.3	1.3
Pelvic-fin length	13.8	27	12.8–17.2	16.1	1.5
First anal-fin lobe length	19.2	25	18.6–37.0	25.1	5.0
Second anal-fin lobe length	30.1	3	13.2–14.2	13.2	0.5
Dorsal-fin base length	34.3	27	31.1–37.1	34.5	1.6
Adipose-fin base length	6.4	27	5.0–7.7	6.3	0.7
Anal-fin base length	33.8	27	32.6–40.9	37.0	2.5
Caudal-peduncle depth	10.4	27	8.9–11.5	10.3	0.7
Width of peduncle	4.2	27	2.3–5.8	4.1	1.0
Supraoccipital to dorsal-fin	48.5	27	32.9–48.6	44.5	3.0
Snout to Supraoccipital	29.9	27	27.9–35.6	31.0	2.1
Snout to base of supraocciptal	13.4	21	13.4–18.8	16.4	1.6
Pelvic-anal distance	23.5	27	19.9–24.6	21.9	1.3
Pectoral-pelvic distance	29.8	27	28.0–33.3	30.5	1.3
Dorsal-fin origin to anal-fin origin	74.5	27	64.4–82.2	75.5	4.1
Dorsal-fin end to anal-fin origin	50.9	27	44.6–57.7	53.4	3.1
Dorsal-fin end to anal-fin end	24.5	27	23.4–28.6	25.3	1.3
Percentages of head length
Head width	17.9	27	13.6–19.4	17.1	1.4
Postorbital distance	41.4	27	25.0–41.4	31.0	4.2
Fourth infraorbital width	20.4	27	7.4–20.4	14.4	3.0
Third infraorbital width	14.9	27	6.9–15.3	11.9	2.6
Cheek gap width	14.6	27	10.8–23.2	15.4	2.3
Interorbital width	58.0	27	38.4–63.6	50.6	6.2
Eye vertical diameter	26.3	27	26.3–40.9	29.6	3.8
Snout length	37.9	27	22.0–38.9	28.7	6.3
Mouth length	43.3	27	19.2–47.9	32.5	8.9
Mouth width	35.3	27	29.2–45.3	34.5	3.8
Percentages of adipose-fin base length
Adipose-fin length	0.8	21	0.7–1.1	0.8	0.1