Monotocheirodon drilos, A.Menezes & Weitzman & Quagio-Grassiotto, 2013

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 : 135-139

Monotocheirodon drilos , new species

Figs. 9-11 , Table 2

Monotocheirodon sp. – Weitzman et al., 2005: 357, Burns & Weitzman, 2006: 529-530 ( MUSM 11082, ANSP 143791, 143792).

Specimens examined: All specimens from Peru.

Holotype: MUSM 41541 , male, SL 33.3 mm, Sandia , Zona Reservada Tambopata-Candamo , stream Ebebahuaeji (empties into Río Candamo), 13°14’56.4”S, 70°00’34.5”W, 31 March 1997, Fonchii Chang.

Paratypes: MUSM 11082 , 2 (SL 28 and 32 mm) , USNM 405296 , 2 (31.8 and 34.8 mm) collected with holotype . ANSP 143790 , 1 (SL 37.2 mm), Río Shintuya at Shintuya (exact coordinates not found) . ANSP 143792 , 6 (SL 300.8-37.8), border between Departamento of Cuzco and Departamento of Madre de Dios, Río Carbón (empties into Río Madre de Dios), 12°53’S, 71°20’W .

Diagnosis: Males and females of Monotocheirodon drilos have an externally visible urogenital papilla which is absent in M. pearsoni . The urogenital papilla

136 MENEZES, N.A. ET AL.: REVIEW OF MONOTOCHEIRODON ( CHARACIFORMES : CHARACIDAE )

of sexually active males of M. drilos is shorter (half length of anal-fin base versus about equal to length of anal fin-base in M. kontos ). Females and juveniles of M. drilos and M. kontos can be differentiated in the height of the dorsal fin (16.1-17.8% SL in M. drilos versus 13.4-15.8% SL in M. kontos ). The number of premaxillary tooth cusps (5 in M. drilos vs 7 in M. kontos ) is also useful to distinguish adult males and females of both species ( Figs. 10 and 14 ).

Description: Morphometric data of holotype and paratypes presented in Table 2. Stevardiine characid reaching at least 37.8 mm SL. Body cylindrical in cross section; greatest body depth between verticals through middle and tip of pectoral fin. Dorsal profile of head anterior to nape strongly convex to snout region in males, less so in females. Snout bluntly convex; tip of snout at about horizontal through mid-point of orbit. Lower jaw convex in profile and somewhat included below upper jaw. Ventral profile of head gently convex, and continuous with strongly convex abdominal region as far as 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 slightly convex. 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 posterior terminus of dorsal fin and caudal-fin base slightly convex in males and almost straight in females.

Unbranched dorsal-fin rays 2 in all specimens, branched rays 7-8, 7.1, (7) n = 12, SD = 0.4); 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 in all specimens; branched rays 9-12, 10, (9), n = 12, SD = 0.9; posterior ray split to its base and counted as one ray. No hooks on anal fin of males. Pectoral-fin rays i, 9-10, 9.1, (9), n = 12, SD = 0.6. Pectoral fin longer in mature males, with tip almost reaching pelvic-fin origin; shorter in immatures and females, with tip distant from pelvic-fin origin. Pectoral-fin rays without hooks. Pelvic fin with one anterior and one posterior unbranched ray, and branched rays 4-5, 4.7, (7) n = 12, SD = 0.4. Sexually active males lacking pelvic-fin hooks. Pelvic-fin length of sexually mature specimens sexually dimorphic (see discussion under sexual dimorphism). Principal caudal-fin rays 10/ 9 in all specimens.

Scales cycloid: Lateral line complete, perforated scales 33-39, 36.8, (33), n = 12, SD = 1.9. Predoral scales 13-15, 14.5, (14), n = 12, SD = 0.7. Scale rows between dorsal-fin origin and lateral line 4-5, 4.5, (5), n = 12, SD = 0.5. Scale rows from pelvic-fin origin to lateral line 3, n = 12. Scale rows around caudal peduncle 10 in all specimens, n = 12. Row of enlarged scales present along anal-fin base.

Premaxilla with single row of 4 multicuspid teeth ( Fig. 10 ) in all specimens. All teeth compressed, pedunculate with distal parts spatulate with 5 cusps; three middle cusps largest, and marginal cusps reduced. Maxillary teeth ( Fig. 10 ) identical in form to premaxillary teeth, also with 5 cusps, but with three middle cusps slightly smaller than those of premaxillary teeth. Total number of maxillary teeth 6-9, 7.4, (6), n = 10, SD = 1.2. Dentary teeth ( Fig. 10 ) identical to premaxillary and maxillary teeth, with 3 large middle cusps and reduced marginal cusps. Total number of dentary teeth 8-11, 9.3, (9), n = 12, SD = 0.4.

Vertebrae 37-38, 37.7, n = 8, SD = 0.4. Dorsal limb gill rakers 9-10, 9.3, (9), n = 11, SD = 0.4; ventral limb gill rakers 11-15, 13.6, (12), n = 11, SD = 0.4. Branchiostegal rays 4 in one cleared and stained specimen; 3 rays originating on anterior and one on posterior ceratohyal.

Color in alcohol: Background body color pale to yellowish brown, darker dorsally due to presence of dark chromatophores largely concentrated towards posterior border of scales. Dark chromatophores fewer on posterior border of scales of midlateral and ventral parts of body. Dark longitudinal dark stripe extends from posterodorsal part of opercle to caudal-fin base. Stripe anteriorly inconspicuous and slightly arched dorsally from upper part of opercle to point below dorsal-fin origin; bordered ventrally by lateral line. Stripe more conspicuous and wider from point above anal-fin origin to caudal-fin base. Dark vertically elongate humeral blotch, located about two scales posterior of posterodorsal portion of opercle, and extending one scale ventral of lateral line.

Head darker on upper part of snout and area dorsal to eye with scattered dark chromatophores on ventral portion of infraorbital bones and opercular region. Urogenital papilla and all fins with scattered dark chromatophores. Large scales on basal portion of each caudal-fin lobe with dark chromatophores mostly concentrated on their basal and median portions.

Sexual dimorphism: The p value in Table 2 suggests that the caudal peduncle depth, pectoral-fin length, pelvic-fin length, dorsal-fin height and the distance from dorsal-fin origin to caudal-fin base are sexually dimorphic. Regression data to test the differences more accurately were not used due to the limited number available mature males.

Reproductive mode and gonad anatomy: Males of Monotocheirodon drilos ( MUSM 11082, ANSP 143791 and 143792) identified as Monotocheirodon sp. were used by Burns & Weitzman (2006) for histological sections of the urogenital papilla, which was characterized as a large intromittent organ used for insemination of the females. The sperm was found to have elongate nuclei 1.8-2.1 µm in length, usually characteristic of inseminating and internally fertilizing fishes.

Examination of sperm cell ultrastructure using TEM confirmed that the nucleus is elongate in the direction of the flagellar axis ( Fig. 11 , A-G) and is approximately 2.05 µm in length (SD ± 0.2 µm). It contains highly condensed granular chromatin and in cross section shows a concave outline ( Fig. 11 , A-F). In the centriolar complex, the centrioles are perpendicular to one another ( Fig. 11 , A-B). The mitochondria and a well-developed vesicular system surround the cytoplasmatic canal for its entire dimension ( Fig. 11 , E-L). The flagellum originates at about the middle of the nuclear length ( Fig. 11 , A-B). The midpiece is conical, strongly asymmetric ( Fig. 11 , A-B) and 0.6 µm in length (SD ± 0.1 µm). A single flagellum emerges from the midpiece ( Fig. 1 I-L).

Etymology: The name drilos is Greek masculine meaning penis. The word is used here in reference to the prominent male inseminating organ. A noun in apposition.

Distribution: This species is known from headwaters of Ríos Tambopata and Madre de Dios, Río Madre de Dios basin, Peru ( 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 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.

FIGURE 10: Monotocheirodon drilos, MUSM 11082, C&S, adult male, 43.5 mm SL; jaws and dentition, lateral view, left side, anterior at left.

FIGURE 11: Spermatozoa of Monotocheirodon drilos (ANSP 143791, 33.9 mm SL). A, B: Longitudinal section of spermatozoa. Note lateral position of nucleus (n) in relation to flagellar axis and nuclear fossa (double arrow) near tip of nucleus; short midpiece contains cytoplasmic canal (asterisks) that houses initial segment of flagellum (f), mitochondria (m) that increase in number from tip of base of nucleus and several vesicles (v). Proximal centriole (p) inside first concavity of fossa and distal centriole (d) outside. C-H: Cross sections at different levels of nucleus showing branched format of first concavity of nuclear fossa (arrow), cytoplasmic canal (c) and initial segment of flagellum (f) running along nuclear outline. I-L: Cross sections at different levels of midpiece displaying mitochondria (m) concentrated mainly at base of nucleus and decreasing progressively along midpiece. Cytoplasm of midpiece filled by vesicular system (v). Electron dense dots (arrowhead) indicate connections of vesicles.

FIGURE 14: Monotocheirodon kontos, MUSM11250, C&S, female, 37.1 mm SL; jaws and dentition, lateral view, left side, anterior at left.