Microsternarchus longicaudatus, new species

Figs. 1–6; Table 1

Holotype. MPEG 14690, 84.31 mm LEA, male, Brazil, Pará, Juruti, Igarapé da Ponte, tributary of Rio Amazonas, 02º10’48.0”S 56º04’37.2”W, A. Hercos, 30 Nov 2007.

Paratypes. All from Brazil, Pará, Juruti, Rio Amazonas basin ,: MPEG 12764, 1, 49.9 mm LEA, undetermined sex, Igarapé Socó Barroso, tributary of Rio Arapiuns, 2°27’30.7”S 56°00’54.3”W, L. Montag, 12 Dec 2006 ; MPEG 11083, 5 (2 females, 2 males, 1 undetermined sex)+ 2 CS (undetermined sex), 42.7–76.2 mm LEA, Igarapé Vitória, A. Hercos, 24 Aug 2006; MPEG 12749, 66.1 mm LEA, male, Igarapé da Ponte, 2º10’48.0”S 56º04’37.2”W, L. Montag, 13 Dec 2006 ; MPEG 14685, 76.2 mm LEA, male, Igarapé Mutum, tributary of Rio Arapiuns, 2°36’45.0” S 56°11’37.4”W, A. Hercos, 27 Nov 2007 ; MPEG 25970, 59.1 mm LEA, undetermined sex, Igarapé São Pedro, 2°13’56.52”S 56°03’18.06”W, M. Mendonça, 2 Dec 2012 ; MPEG 12750, 65.8 mm LEA, undetermined sex, Igarapé Itapiranga, 2°28’19.8”S 56°11’ 51.3”W, L. Montag, 16 Dec 2006 ; MPEG 11084, 4 +1 CS (undetermined sex), 51.8– 67.4 mm LEA, Igarapé Vitória, A. Hercos, 24 Aug 2006 . MPEG 39638, 4, 1 female +3 undetermined sex, 70.0– 84.3 mm LEA; MZUSP 129864, 1, female, 80.0 mm, collected with the holotype .

Diagnosis. Microsternarchus longicaudatus can be distinguished from M. bilineatus and M. brevis by the presence of distinct irregular suborbital blotches, with chromatophores more concentrated near to the rictus and immediately below the orbits (Fig. 1a; vs. absence, Cox-Fernandes & Williston 2017: Fig. 2; Cox-Fernandes et al. 2015: Fig. 1, respectively); and by the caudal filament length in both sexes, 35.1–36.8% TL (vs. 29.3–34.3% TL in M. bilineatus; and 12.8–20.9% TL in M. brevis). The new species can be additionally differentiated from M. bilineatus by the presence of small black spots overlying the mid-dorsal region of the body (Fig. 1b; vs. absence; Cox-Fernandes & Williston 2017: Fig. 2); by the number of displaced haemal spines, three or four (Fig. 2; vs. two); and by the dentary morphology, coronoid process of dentary continues posteriorly beyond dentary hook (Fig. 3; vs. dentary terminates immediately after hook-like process on the posterodorsal portion; Cox-Fernandes & Williston 2017: Fig. 4). Microsternarchus longicaudatus can be additionally differentiated from M. brevis by the number of anal-fin rays (152–173, vs. 124–151).

Description. Morphometric data in Table 1. Largest examined specimen 84.3 mm LEA. Body elongate, distinctly compressed laterally. Greatest body depth between vertical through anal-fin origin and vertical through anal-fin middle. Dorsal profile of body straight. Ventral profile slightly convex. Caudal filament long in both sexes, equal to LEA midlenght.

Head compressed, greatest width in opercular region and greatest depth at nape. Dorsal profile of head convex. Ventral profile of head slightly straight. Snout conical in lateral view. Mouth slightly subterminal with rictus at vertical through anterior margin of posterior nostril. Anterior naris small, just above upper lip; posterior naris rounded, twice in size of anterior naris. Eye small, circular, completely covered by thin membrane, on anterior onethird of HL and laterally oriented. No accessory electric organ over operculum. Gill opening limited to posterior margin of opercle and extending above and below pectoral-fin base. Branchial membranes joined at isthmus. Anus and urogenital papilla adjacent. Position of anus and urogenital papilla shifting through ontogeny from vertical through middle of operculum to vertical through eye.

Scales small, cycloid; squamation incomplete with naked head, nape and anterior first third of trunk above lateral line. Lateral line canal complete, with large pores posterior to head.

PLLN visible in live and preserved specimens, followed by dorsal rami of posterior lateral line nerves, running from sides to near dorsal midline as two parallel lines. PLLN configuration variable with only VSLLR visible; or both DSLLR and VSLLR clearly visible in preserved specimens.

Total pectoral-fin rays 10(6), 11*(10), or 12(4). Distal pectoral-fin margin rounded. Total anal-fin rays 152(1), 156(2), 164(1), 166(2), 167*(3), 168(5), 169(2), 170(2), 171(3), 172(1), or 173(1). Anal-fin origin posterior to pectoral-fin base. Distal margin of anal fin straight. First unbranched anal-fin rays tiny, progressively increasing in size to first branched rays. Branched anal-fin rays all nearly equal length, except for posterior most rays with progressively decreasing sizes. One (7), or two*(13) bilateral horizontal columns of electrocytes at anal-fin terminus.

Relevant osteological features of Microsternarchus . Teeth absent in both jaws in adults and juveniles. Edentulous maxilla with descending process broad throughout ontogeny. Dorsoposterior portion of dentary with hook-like process; coronoid process of dentary continues posteriorly dentary hook (Fig. 3). Lateral ethmoid absent. Four (3) branchiostegal rays. First branchiostegal ray approximately half length of second one. Seventeen (1) or 18(3) precaudal vertebrae. Two (4) transitional vertebrae. Eleven (1) or 12(3) pleural ribs, Three (3) or four (1) displaced hemal spines (Fig. 2).

Color in alcohol. Background color pale. Dorsal region of head and body dark brown, gradually lightening below lateral line. Lips and suborbital region light brown. Eye with distinct irregular suborbital blotches composed of chromatophores and subcutaneous pigmentation. All scales on flanks homogeneously dark brown, except scales overlying anal-fin pterygiophores, with clear pigmentation. Mid-dorsal region darkened with small black spots, without prominent depigmented pale stripe extending along midline or dark brown patches. Vertical stripes or lines on flanks absent. Dorsal ramus of posterior lateral line nerve and segmental lateral line rami visible as conspicuous dark lines. Caudal filament uniformly dark brown, darker than body. Pectoral- and anal-fin interradial membranes hyaline. All anal-fin rays with black scattered tiny chromatophores. Juvenile specimens background color light brown; irregular suborbital blotches and small black spots on dorsal region of trunk poorly evident (Fig. 4).

Color in life. Based on photographs taken in the field (Figs. 5 and 6). Similar to color pattern found in preserved specimens. Overall body translucent, darker in mid-dorsal region of trunk; dorsal region of body dark brown. Mottling on middorsum towards posterior portion of caudal filament. Dorsal region of dark brown, ventral region lighter, with diffuse chromatophores. Pectoral and anal fins hyaline with scattered dark brown chromatophores overlying fin rays.

Sexual dimorphism. No sexual dimorphism was observed.

Etymology. The specific epithet, longicaudatus, is from the Latin “ longus ” for long and “ cauda ” for tail, in allusion to the larger size of the caudal filament length.

Distribution and habitat. Microsternarchus longicaudatus is currently known only from clearwater streams directly emptying in the Amazon River, as well as streams tributaries of the Rio Arapiuns (itself a tributary of the rio Tapajós basin), in the lower Amazon basin, Pará, Brazil (Fig. 7). The type-locality is a clearwater stream, 1–5 m wide and 1–2.5 m deep, with preserved riparian vegetation, swift current, and dead leaves on the bottom (Fig. 8). Other species sampled syntopically were Carnegiella strigata (Günther 1864), Copella nattereri (Steindachner 1876), Apistogramma gr. agassizii, and Sternopygus macrurus (Bloch & Schneider 1801) .