Micralestes schelly, Stiassny, Melanie L. J. & Mamonekene, Victor, 2007

Stiassny, Melanie L. J. & Mamonekene, Victor, 2007, Micralestes (Characiformes, Alestidae) of the lower Congo River, with a description of a new species endemic to the lower Congo River rapids in the Democratic Republic of Congo, Zootaxa 1614, pp. 17-29 : 21-25

Micralestes schelly , new species

( Figs 1 F, 2D, 3–6)

Holotype ( Fig. 5 B) AMNH 240662, male, 40.1 mm SL; Democratic Republic of Congo, Bas Congo Province, Congo River main channel near Inga at point 50 (5° 31.69 S 13° 36.47 E), R.C. Schelly et al., 26 Sept. 2002.

Paratypes ( Fig. 1 F, 5B), with same data as holotype. AMNH 239518, 33.9 – 48.9 mm SL (24 alcoholic, 5 cleared and stained), MRAC 2007-26 -P-1-2, 36.5 – 39.0 mm SL (2 alcoholic), MNHN 2007-1629, 35.3 – 40.1 mm SL (2 alcoholic), ZSM 33982, 39.0 – 44.2 mm SL (2 alcoholic), MCZ 166773, 34.6 – 36.5 mm SL (2 alcoholic), CU 93431, 36.9 – 40.0 mm SL (2 alcoholic).

Diagnosis. A member of a clade of small alestid fishes characterized by the combination of the absence of a supraorbital bone, and the presence of a deep-lying midlateral stripe extending along the body onto the caudal peduncle, a band of chromatophores above the anal fin, and a small pair of inner row symphyseal teeth on the dentary. Micralestes schelly is distinguished from all putative congeners in having an elevated vertebral count of 38 – 40 (mean 39) versus 34 – 36 (mean 35) vertebrae. Additionally the species is characterized by the presence of 4–6 outer row premaxillary teeth with a majority of specimens (23 of 40) with only four outer row teeth implanted in an alternating pattern with respect to the anterior inner row premaxillary teeth. Mature males are further diagnosed by live coloration, which is dominated by a broad, blue-green iridescent midlateral band extending from behind the opercle to the base of the caudal fin, and by a marked expansion of the band of black chromatophores above the anal fin.

Description. Based on the holotype and 39 paratypes. See Figs. 5 and 6 for general appearance, and Table 1 for summary of morphometric and meristic data. Diminutive species, maximum observed size 48.9 mm SL. Relatively deep bodied, somewhat laterally compressed with greatest body depth at, or a little in front of, dorsal-fin origin. Dorsal body profile more-or-less smoothly convex to caudal peduncle, less strongly so in mature males; ventral profile markedly convex to anal-fin insertion. Caudal peduncle slightly longer than deep. Mouth terminal, gape not reaching beyond level of anterior margin of eye. Eye moderately large, flanked by adipose membrane extending over snout to level of nostril and posterodorsally over postorbital region. Nostril large and prominent; nasal and antorbital bones greatly reduced in size ( Fig. 2 D).

Teeth. Stout tri- or quadri-cuspid teeth in outer row on premaxillae variable in number; majority of individual have two teeth on either premaxilla (23 of 40 specimens), 13 specimens have two teeth on one side and three teeth contralaterally, and four specimens have three teeth on both premaxillae. When only two teeth are present each is positioned opposite interspaces between and alternating with, anterior inner row teeth ( Fig. 3 A). Four inner row teeth on each premaxilla, most with one large central cusp and two or three minor cusps on either side, symphyseal teeth often with single minor cusp on medial face. Four outer row teeth on contralateral dentaries, each with large central cusp and two or three minor cusps on either side. A single pair of small, conically recurved, symphyseal inner row teeth on lower jaw invariably present ( Fig. 3 B).

Fins. Dorsal-fin rays ii, 6–7 (total rays, 8–9), anal-fin rays iii, 16–17 (total rays 19–20). Origin of dorsal fin at, or slightly behind, vertical through pelvic-fin origin. Marked sexual dimorphism in morphology of anal fin (see Figs. 4 , 5 ). However, unlike condition in Rhabdalestes , Hemigrammopetersius and Virilia ( Stiassny and Schaefer, 2005; Zanata and Vari, 2005) there is no posterior curvature or hypertrophy of third unbranched anterior ray of mature males ( Fig. 4 C).

Squamation. Scales in longitudinal series 26–28 (mean 27) to caudal fin flexure, 1–3 smaller scales continuing over caudal fin base, 4.5 transverse scales between lateral line and dorsal-fin origin, 2.5 transverse scale rows between lateral line and pelvic-fin insertion. Lateral line complete, with 25–29 canal-bearing scales to point of hypural flexure, 2–4 smaller pored scales continuing over caudal-fin base.

Total number of gill rakers on first gill arch 18–22 (mean 19). Total number of vertebrae 38–40 (mean 39).

Color in alcohol. Base body coloration yellowish brown dorsally and laterally, yellow-orange ventrally. A darkly pigmented, deep-lying midlateral stripe extends from posterior margin of opercle to base of caudal fin. Well-marked band of dark melanophores present above anal fin. Pectoral and pelvic fins pale yellow. Adipose fin tipped in black. Dorsal and caudal fins somewhat dusky. Anal fin of females dusky proximally, in males pigmentation is somewhat expanded distally in posterior portion of fin.

Color in life. Recently two male specimens have been collected from the type locality and live coloration recorded ( Fig. 6 ). In life a broad, blue-green iridescent midlateral band covers the lateral flanks and tail from behind the opercle to the base of the caudal fin. This iridescent band overlies and obscures the deep lying midlateral stripe evident in preserved specimens ( Fig. 5 ). Above and below the midlateral band the body is silvery white. Numerous melanophores are present on the head and opercle, and scattered over the body with a concentration around lateral scale margins. An expanded band of black melanophores reaching to the lateral line is present above anal fin. Dorsal fin and pectoral fins are dusky gray; adipose fin is pale gray with a dark distal margin. Caudal fin is dusky gray at its base with a median dark band extending to mid-fork, distally the fin is dusky gray and medially each fin fork is deep crimson red. Anterior and distal margins of the anal fin are white, while the remainder of the fin is jet black. Pelvic fins are white. Iris is silvery white with a small red patch dorsally. Unfortunately no female specimens were collected during a recent site visit and female life coloration remains unknown.

Geographical distribution. Known only from the type locality in Bas Congo Province, at Point 50 on the Congo River main channel near Inga, Democratic Republic of Congo (5°31.69’S 13° 36.47E). Specimens were collected with cast nets in a complex, partially protected riffle habitat adjacent to rapids (Fig. 7). A smallunnamed stream enters the Congo River at this point and supports a patch of forest in the draw between two steep, mostly-bare hills. Micralestes schelly was collected over a sand and gravel substrate in the main channel amongst large rock slabs of up to 4 m high thrusting upward at angles of 60–70 degrees, in pools up to 2 m in depth. At this site water depth and flow is highly variable due to water surges every 5–10 minutes resulting in depth fluctuations in excess of half a meter.

Etymology. Named for our colleague and the intrepid collector of the type series, Robert C. Schelly .

Discussion. Recent collections in the region allow us to document the presence of six Micralestes in the stretch of the lower Congo River from Pool Malebo to Boma near the river’s mouth, and as an aid for field identification we provide here an illustrated key to those species.

1A Teeth with numerous small cusps, first inner row premaxillary tooth with 10–12 cusps (Key Fig. 1 A). Inner row tooth pair on dentary usually multicuspid (Key Fig. 3 A). Dorsal fin with distinctive black apical patch (Key Fig. 2 A) .................................................................................................................. M. acutidens

1B Teeth with fewer cusps, first inner row premaxillary tooth with 6–8 cusps (Key Fig. 1 B). Inner row tooth pair on dentary invariably unicuspid (Key Fig. 3 B). Dorsal fin without black apical patch (Key Fig. 2 B)................................................................................................................................................................2

2A 23–28 scales in longitudinal series from opercle to point of caudal flexure (Key Fig. 2 A)........................ 3

Figures

FIGURE 1. Micralestes of the lower Congo River. A) Micralestes acutidens, AMNH 238378, female, 51.1 mm SL, B) Micralestes holargyreus, AMNH 239479, male, 60.5 mm SL, C) Micralestes humilis, AMNH 238406, female, 56.8 mm SL, D) Micralestes lualabae, AMNH 238386, female, 65.4 mm SL, E) Micralestes stormsi, AMNH 238440, male, 51.0 mm SL, F) Micralestes schelly sp. nov., AMNH 239518, male, 37.7 mm SL. Scale bars = 1 cm.

FIGURE 2. Isolated orbital series in A) Bryconaethiops yseuxi, B) Brachypetersius altus, C) Micralestes humilis, D) Micralestes schelly sp. nov., E) pectoral girdle in medial view of Micralestes humilis, F) caudal skeleton and adjacent vertebrae in Micralestes humilis.

FIGURE 3. Micralestes schelly sp. nov. A) buccal jaws in lateral view, B) dentary in medial view.

FIGURE 4. Micralestes schelly sp. nov. A) schematic of dominant color pattering in preservation in female, B) male. C) Skeletal elements of anterior region of anal fin of mature male.

FIGURE 5. Micralestes schelly sp. nov. A) AMNH 239518, paratype, female, 44.5 mm SL, B) AMNH 240662, holo-

FIGURE 6. Micralestes schelly sp. nov. AMNH 240671, photograph of mature male taken immediately post mortem. Specimen collected from the type locality in July 2007, photo by Jake Lowenstein.

Tables

TABLE 1. Morphometric and meristic data for holotype and paratypes of Micralestes, new species.

	holotype	holotype + paratypes
Morphometrics		min max	n	mean
Standard length (mm)	40.1	33.9 48.9	40	38.1
% head length
Eye diameter	35.9	32.1 43.7	37	37.8
Interorbital width	38.4	27.5 45.6	37	36.2