Knodus angustus, Menezes & Ferreira & Netto-Ferreira, 2020

Menezes, Naércio A., Ferreira, Katiane M. & Netto-Ferreira, André L., 2020, A new species of Knodus (Characiformes: Characidae: Stevardiinae) from the rio Aripuanã, rio Madeira basin, Brazil, Neotropical Ichthyology (e 190139) 18 (2), pp. 1-14 : 4-8

Knodus angustus , new species urn:lsid:zoobank.org:act:53520CEE-9439-4508-91E8-F7FD24A4B8B3

( Figs. 1–5 , Tab. 1)

Holotype. MZUSP 122222 , female 36.0 mm SL, Brazil, Amazonas , Apuí , Prainha Nova beach in rio Aripuanã at end of road BR-174, 7°13’49.15”S 60°39’3.56”W, 05 Oct 2016, O. T. Oyakawa, W. Ohara, T. Teixeira & M. Pastana.

Paratypes. MZUSP 124932 , 70 (19.0–36.0 mm SL, 5, 31.0–36.0 mm SL C&S) ,

UFRGS 28150 , 5 (24.0– 30.5 mm SL) collected with holotype .

Diagnosis. Knodus angustus is distinguished from most congeners, except K. figueredoi Esguícero, Castro, 2014 , K. heteresthes (Eigenmann, 1908) , K. longus Zarske, Géry, 2006 , K. meridae Eigenmann, 1911 , K. mizquae (Fowler, 1943) , K. moenkhausii (Eigenmann, Kennedy, 1903) , and K. victoriae (Steindachner, 1907) by its very low body depth (20.0–25.2% SL vs. above 30% SL). The new species can be further distinguished from K. caquetae Fowler, 1945 , K. cinarucoense (data from Román-Valencia et al., 2008), K. hypopterus (Fowler, 1943) , K. smithi (Fowler, 1913) , K. gamma Géry, 1972 , and K. jacunda (Fowler, 1913) (data from Fowler, 1913) by having fewer anal-fin branched rays (15–19 vs. 20–27 branched rays); from K. shinahota Ferreira, Carvajal, 2007 by the presence of 3 scales below the lateral line (vs. 5–6 scales); from K. alpha , K. breviceps (Eigenmann, 1908) , K. chapadae (Fowler, 1906) , K. orteguasae (Fowler, 1943) , K. pasco Zarske, 2007 , and K. septentrionalis Géry, 1972 by having 5 longitudinal scale series from dorsal-fin origin to lateral line (vs. 4 in K. orteguasae and K. septentrionalis (data from Géry, 1972); and 6 in K. breviceps , K. chapadae , and K. alpha (data from Eigenmann et al., 1914, and Ferreira, 2007); and 7 in K. pasco (data from Zarske, 2007); from K. delta Géry, 1972 and K. borki by having a complete lateral line with 36–38 perforated scales (vs. incomplete lateral line with 8–12 and 6–14 perforated scales (data from Géry, 1972 and Zarske, 2008); from K. dorsomaculatus Ferreira, Netto-Ferreira, 2010 by the lack of a dark dorsal-fin blotch (vs. dark blotch present); from K. geryi Lima, Britski, Machado, 2004 by the lack of dark pigmentation on the caudal-fin lobes (vs. caudal-fin lobes distinctly pigmented). Knodus angustus further differs from K. deuterodonoides by having 3 scale series between the pelvic-fin origin and the lateral line series (vs. 4 series); from K. tiquiensis Ferreira , Lima, 2006 by the lack of expansion of anal-fin rays (vs. anteriormost rays expanded with thickened integument until the sixth branched ray); and from K. tanaothoros (Weitzman, Menezes, Evers, Burns, 2005) and K. weitzmani (Menezes, Netto-Ferreira, Ferreira, 2009) by the lack of modified glandular tissue on the anal-fin lobe (vs. glandular tissue present). Finally, Knodus angustus can be distinguished from K. figueredoi , K. heteresthes , K. meridae , K. mizquae , K. moenkhausii , and K. victoriae by having 11–12 circumpeduncular scales (vs. 13–14); and from K. longus by the presence of 11–13 predorsal scales and the presence of a faint longitudinal stripe (vs. 17–18 predorsal scales and the presence of a conspicuous dark stripe (data from Zarske, Géry, 2006).

Description. Morphometric data of holotype and paratypes in Tab. 1. Body comparatively small (largest examined specimen 73 mm SL). Head and body elongate and laterally compressed; greatest body depth at dorsal-fin origin. Profile distinctly convex from upper jaw to posterior nostril, slightly convex from latter point to dorsal-fin origin, straight along dorsal-fin base, slightly concave from latter point to adiposefin origin, and slightly concave to nearly straight from latter point to anterior most dorsal procurrent caudal-fin ray. Ventral body profile convex from tip of lower jaw to isthmus, nearly straight from that point to vertical through pectoral-fin origin, convex from latter point to pelvic-fin origin, and straight from that point to anal-fin origin, and straight along anal-fin base and concave along caudal peduncle.

Mouth sub-terminal; lower jaw short, included in upper jaw when mouth closed. Posterior tip of maxilla reaching vertical through anterior border of orbit. Outer premaxillary series with 4(29), 5*(39), or 6(12) tri- to pentacispid teeth; inner series with 4(80) pentacuspid teeth, larger than those of outer series ( Fig. 2 ). Maxillary with 2(25), 3*(50), or 4(5) teeth; anterior teeth largest, pentacuspid, posterior tri- to pentacuspid ( Fig. 2 ). Dentary with 4(80) large pentacuspid teeth anteriorly, followed by 3(7), 4(32), 5*(37), or 6(3) smaller tri- to pentacuspid teeth abruptly decreasing in size posteriorly ( Fig. 2 ). First gill arch with external and internal series of gill-rakers; external series with 14(4), 15(23), 16*(44), 17(8), or 18(1) gill-rakers. Branchiostegal rays 4(5), 3 originating on anterior and 1 on posterior ceratohyal.

Scales cycloid, two radii present, originating at focus and vertically opposed to each other. Lateral line complete with 36(35), 37(39), or 38*(6) perforated scales. Predorsal scales 11(10), 12(65), or 13*(5). Scale series between lateral line and dorsal-fin origin 5(80); series between lateral line and pelvic-fin origin 3(80); circumpeduncular scales 11(2) or 12*(78). Single series of scales with sinuous posterior borders forming sheath along base of all anal-fin rays.

Pectoral-fin rays i,7,i(1), i,8,iii(6), i,9,i(2), i,9,ii(42), i,9,iii(2), i,10,i*(9), i,10,ii(12), or i,11,i(1). Distal tip of longest pectoral-fin ray not reaching vertical through pelvic-fin origin when adpressed to body. Pelvic-fin rays i,6,i*(80), tip of fin falling short of anal-fin origin. Supraneurals 5(1), or 6(4), located anterior to spines of 5 th to 9 th (3) or

10 th (2) vertebral centra; rod-shaped, or with discrete anteroposterior lamellar expansions of dorsal portion. Dorsal-fin rays ii,7,i,*(80). First dorsal-fin pterygiophore inserting behind neural spine of 11 th (5) vertebral centrum. Distal margin of extended dorsal fin straight to slightly convex. Dorsal-fin origin closer to caudal-fin base than to snout tip. Dorsal-fin ray terminus at vertical through base of third unbranched anal-fin ray. Anal-fin rays iii,15(6), 16(19), 17(39), 18(*15), or 19(1), posterior-most ray adnate. Anal fin with short, inconspicuous lobe anteriorly, including last unbranched ray plus first 5–6 branched rays. Distal margin of anal fin concave. First anal-fin pterygiophore inserting behind haemal arch of centra 16 th (2) or 17 th (3). Adipose fin present. Principal caudal-fin rays i,9,8,i(31). Dorsal and ventral procurrent caudal-fin rays 10(2), 11(1) or 12(2), respectively. Total vertebrae 36(5).

Coloration in alcohol. Ground color pale yellowish to brown. Color pattern poorly counter shaded. Upper portion of head from tip of snout to end of supraoccipital spine and predorsal scales only slightly more pigmented, and somewhat darker than lateral and ventral portions of body; minute dark chromatophores scattered around eye extending laterally over maxilla, first, second, fourth, fifth and sixth infraorbitals,

upper half of third infraorbital, upper half of opercle and interopercle; anterior part of lower jaw with scattered dark chromatophores. Postorbital portion of the head with disperse chromatophores forming a discrete darker pigmentation from the infraorbitals 5 and 6, and opercle to the pectoral girdle. Dark chromatophores onto distal margins of scales, distinctly lighter at focus; scattered dark chromatophores on lower part of body below lateral line series, darker above anal-fin base. A vertically elongate humeral blotch present onto scales of 3 to 4 longitudinal series, becoming narrower downward ( Fig. 1 ). Diffuse stripe with scattered chromatophores present on the body, from vertical through pectoral-fin midline to caudal-fin base, becoming more conspicuous near vertical through anal-fin origin, being slightly enlarged over caudal peduncle ( Fig. 1 ). All fins hyaline with scattered dark chromatophores on intervening membranes.

Sexual dimorphism. Mature males of Knodus angustus present bilateral hooks on first unbranched and the following six branched pelvic-fin rays ( Fig. 3 ). Bilateral bony hooks present on last unbranched plus four to six anteriormost branched anal-fin rays ( Fig. 4). Mature males also with a gill gland on the lower portion of the first gill arch involving 8 gill filaments. Hooks and gill gland absent on mature females.

Etymology. The species epithet angustus is from the Latin, meaning “narrow” in allusion to the narrowness of the body of this species. An adjective.

Geographical distribution. Knodus angustus is known so far from the rio Aripuanã, a tributary of the rio Madeira, Amazonas, Brazil ( Fig. 5 ).

Conservation status. Although Knodus angustus is known only from its type locality in the rio Aripuanã, the area where the specimens were sampled is adjacent to the Floresta Nacional do Aripuanã. Despite the recent, on-going deforestation peak in the Brazilian Amazon, it is likely that anthropic impacts on the forest will take several years to generate impacts on the population of K. angustus , therefore, the species is classified herein as Least Concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2019).

Figures

FIGURE 1 | Knodus angustus, MZUSP 122222, female. 36 mm SL, Brazil, Amazonas, rio Aripuanã, tributary of rio Madeira.

FIGURE 2 | Knodus angustus, MZUSP 124932, C&S, jaws and dentition, lateral view, left side, anterior at left.

FIGURE 3 | Knodus angustus, MZUSP 124932, sexually mature male; pelvic-fin rays, ventral view, left side showing pelvic-fin hooks.

FIGURE 5 | Map of northern South America showing collection site of Knodus angustus in the rio Aripuanã, rio Madeira basin.