Monotocheirodon pearsoni Eigenmann, 1924

A. Menezes, Naércio, Weitzman, Stanley H. & Quagio-Grassiotto, Irani, 2013, Two New Species And A Review Of The Inseminating Freshwater Fish Genus Monotocheirodon (Characiformes: Characidae) From Peru And Bolivia, Papéis Avulsos de Zoologia 53 (10), pp. 129-144 : 131-135

Monotocheirodon pearsoni Eigenmann, 1924

Figs. 1 , 2 , 5 , 6 , Table 1

Monotocheirodon pearsoni Eigenmann , in Pearson, 1924: 34, pl. 11, fig. 1 (original description, type locality: Bolivia, Espia, Beni river basin). Malabarba, 1998: 200 (structure of pseudotympanum). Malabarba & Weitzman, 2000: 269-283 (insemination). Lima et al., 2003: 150 (maximum length; distribution; remarks and references). Weitzman et al., 2005: 357 (listed in comparative specimens examined).

Specimens examined: All specimens from Bolivia.

Lectotype: CAS 59792 (SL 35.5 mm), Río Beni basin, confluence of Ríos La Paz and Miquilla where they form Río Bopi near Espia , 16°33’S, 67°51’W.

Paralectotypes: CAS 233970 (6, SL 19.5-29.1) , UMMZ 66484 (4, SL 23.5-29.2), collected with lectotype ; UMMZ 66485 (5, SL 23-27.8 mm), Río Iniqui (exact coordinates not found) .

Diagnosis: Monocheirodon pearsoni can be easily distinguished lacking the externally visible urogenital papilla present in males and females of M. drilos and M. kontos (see Figs. 1 , 9 , and 12). Aditionally it has a smaller horizontal eye diameter (13.5-16.6% of HL) than its congeners (18.3-22.6% in M. drilos and 18.0-21.6% in M. kontos ).

Description: Morphometric data of lectotype and paralectotypes presented in Table 1. Stevardiine characid reaching at least 35.5 mm SL. Body cylindrical in cross section; greatest depth situated between verticals through tip of pectoral fin and dorsal-fin origin. Dorsal profile of head anterior to nape slightly convex to snout region dorsal to nostril. Snout bluntly convex with tip at about level of horizontal through mid-point of orbit. Lower jaw margin convex with jaw somewhat included below upper jaw. Ventral profile of head gently convex, continuous with gently convex abdominal region extending to anal-fin origin. Body profile along anal-fin base approximately straight to slightly convex to posterior termination of anal fin. Ventral profile of caudal peduncle almost straight. Dorsal profile of body between nape and dorsal-fin origin gently convex. Base of dorsal fin slightly convex and somewhat inclined posteroventrally. Body profile between termination of base of dorsal fin and caudal-fin base slightly concave.

Two unbranched dorsal-fin rays in all specimens, branched rays 7-8, 7.5, (7), n = 17, SD = 0.5; posterior ray not split to its base. Dorsal-fin height apparently sexually dimorphic (see discussion under sexual dimorphism). Adipose fin absent. Unbranched anal-fin rays ii or iii, usually ii; branched rays 8-10, 9.5, (10), n = 17, SD = 0.6; posterior ray split to its base and counted as one ray. No hooks on anal fin of males ( Fig. 5 ). Pectoral-fin rays i, 8-10, 9, (9), n = 17, SD = 0.4. Tip of pectoral fin falling short of pelvic-fin origin. Pectoral-fin rays lacking hooks. Pelvic-fin rays i, 5, i, n = 17. Sexually active males without pelvic-fin hooks ( Fig. 6 ). Pelvic-fin length of sexually mature specimens apparently sexually dimorphic (see discussion under sexual dimorphism). Principal caudal-fin rays 10/ 9 in all specimens.

Scales cycloid: Lateral line complete; perforated scales 32-38, 35.1, (36), n = 14, SD = 1.5. Predorsal scales 13-17, 16, (13), n = 16, SD = 1.0. Scale rows between dorsal-fin origin and lateral line 4-5, 4.6, (5), n = 16, SD = 0.5. Scale rows from pelvic-fin origin to lateral line 2-3, 2.8, (3), n = 16, SD = 0.3. Scale rows around caudal peduncle 10 in all specimens, n = 16. Row of enlarged scales present along anal-fin base.

Premaxilla with single row of 4 multicuspid teeth ( Fig. 2 ) in all specimens. All teeth compressed, pedunculate with wider distal parts spatulate with 6 or 7 cusps and two or three middle cusps largest. Small cusps sometimes appearing only as small rounded process. Maxillary teeth ( Fig. 2 ) shaped like premaxillary teeth with larger anterior teeth bearing 6 or 7 cusps and smaller posterior teeth with 3 or 4 cusps. Total number of maxillary teeth 5-6, 5.7, (6), n = 16, SD = 0.5. Dentary teeth ( Fig. 2 ) shaped like premaxillary and maxillary teeth, progressively decreasing in size posteriorly. Most dentary teeth with 5 cusps, with middle cusp usually largest. Total number of dentary teeth 7-9, 8.1, (8), n = 17, SD = 0.5.

Vertebrae 36-38, 37.1, n = 12, SD = 0.7. Dorsal limb gill rakers 8-10, 8.8, (8) n = 15, SD = 0.8; ventral limb gill rakers 11-15, 12.4, (12) n = 17, SD = 1.1. Branchiostegal rays 4 in one cleared and stained specimen, 3 rays originating on anterior and one on posterior ceratohyal.

Color in alcohol: Pigmentation comparable in both sexes. Overall body color pale to yellowish-brown. Dark chromatophores widespread over all of body, more condensed dorsally and slightly darker in that region than on ventral part of body. Dark roundish humeral blotch situated above anterior portion of pectoral fin and about two scales distant from posterodorsal part of opercle. Dark longitudinal stripe extending on

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body from posterior border of blotch to base of median caudal-fin rays. Stripe anteriorly narrower from posterior border of humeral blotch to point ventral to middle of dorsal-fin base and wider from this point to caudal-fin base. Terminal portion of stripe darker on caudal peduncle, forming inconspicuous dark spot.

Head darker dorsally and on anterior portion of snout; lighter and with scattered dark chromatophores on region anterior to eye, infraorbitals, central portion of opercular area and lower jaw. Enalrged scales on base of each caudal-fin lobe with scattered dark chromatophores. All fins hyaline with scattered dark chromatophores.

Sexual dimorphism: Means corresponding to pelvic-fin length and dorsal-fin height differ considerably between males and females ( Table 1), but tests to evaluate if such differences would be statistically significant are meaningless since only two males are available.

Reproductive mode and gonad anatomy: Histological analysis revealed the presence of spermatozoa with spherical nuclei in ovaries from one mature female of Monotocheirodon pearsoni ( Burns & Weitzman, 2006, fig. 1, Table 1). This indicates that the species can be classified as “aquasperm”, characteristic of externally fertilizing characids and is inseminating.

Distribution: Monotocheirodon pearsoni is known from headwaters of the Río Bopi, Río Beni basin and Río Iniqui (not exactly located), Bolivia, at about 5,000 m of elevation ( Fig. 7 ).

Figures

FIGURE 1: Monotocheirodon pearsoni, CAS 59792, adult male above, 29.4 mm SL and adult female below, 35.5 mm SL.

FIGURE 2: Monotocheirodon pearsoni, UMMZ 66485 (upper jaw), C&S, female, 26.3 mm SL, CAS 5792 (lower jaw), male, 29.4 mm SL, dentition, lateral view, left side, anterior at left.

FIGURE 5: Monotocheirodon pearsoni, CAS 59792, C&S, male, 29.4 mm SL; anal-fin rays, lateral view, left side.

FIGURE 6: Monotocheirodon pearsoni, CAS 59792, C&S, male, 29.4 mm SL; pelvic-fin rays, ventral view, left side.

FIGURE 7: Map of western South America showing collecting sites and type localities (black symbols) of species of Monotocheirodon. Squares: Monotocheirodon kontos; circles: M. drilos; triangle: M. pearsoni. Some symbols may represent more than one locality.

FIGURE 9: Monotocheirodon drilos, MUSM 11082, adult male above, 33.6 mm SL and adult female, below, 33.0 mm SL.