Figure 1, Table 1
Holotype. MCP 38304 (WGRC 50.231099, female, 163 mm), Brazil, Amazonas, Rio Tefé, Toco Preto, trawl sample, Municipality of Tefé, 03°47.31’S, 64°59.91’W, collected by W. Crampton, 23 Oct 1999.
Paratypes. INPA 25256, (1 female c&s, 211 mm), same locality as holotype, collected by W. Crampton, 24 Oct 1999. MCP 38305 (2: female 181 mm, male 183 mm), same locality as holotype, collected by W. Crampton, 24- 25 Oct 1999.
Diagnosis. Sternarchorhynchus curumimZBK is diagnosed as a member of the genus SternarchorhynchusZBK by anguloarticular, and endopterygoid bones elongate, five to 10 times longer than deep in specimens larger than 100 mm TL; anterior process of maxilla unossified; orbitosphenoid narrow, with unossified anterior margin; five branchiostegal rays; and Meckel’s cartilage lost or greatly reduced (Campos-da-Paz, 2000). Sternarchorhynchus curumimZBK is diagnosed from congeners by the following combination of characters: head and mid-dorsum as dark as dorsolateral areas (four specimens examined) (Fig. 1) (vs. white or yellow pigments along head and mid-dorsum forming long pale stripe in S. roseniZBK; this character appears to be polymorphic in S. britskiiZBK, S. curvirostris, and S. mesensisZBK (Campos-da-Paz, 2000: 527)); anal and pectoral fins translucent, without conspicuous distal dark margins (vs. dark margins in S. mormyrus, S. oxyrhynchus, and S. roseniZBK); 10-11 scales above lateral line at mid-body (vs. 11-17 in S. britskiiZBK, S. curvirostris, S. mesensisZBK, and 5-8 S. oxyrhynchus); 23-24 anterior unbranched anal-fin rays (vs. 18-22 in S. britskiiZBK); 179-189 total anal-fin rays (vs. 213-227 in S. mormyrus, 209-221 S. oxyrhynchus, and 187-215 S. roseniZBK); greatest body depth 9.0- 10.0% of length to end of anal fin (LEA) (vs. 10.0-14.5% in S. britskiiZBK and S. mesensisZBK); mouth length 3.5-5.6% of head length (HL) (vs. 5.9-9.6% in S. britskiiZBK and S. mormyrus); internarinal distance 2.4-2.8 % of HL (vs. 3.1-5.8 in S. curvirostris, S. mesensisZBK, and S. mormyrus); branchial opening 11.7-12.9% of HL (vs. 13.1-17.9% in S. britskiiZBK); postocular distance 33.6-39.2% of HL (vs. 40.0-50.5% in S. britskiiZBK and S. mormyrus); prepectoral distance 18.3-20.6% of LEA (vs. 21.4-23.8% in S. mesensisZBK); tail length (CL) 19.5-25.2% of LEA (vs. 5.9-14.3% in S. britskiiZBK and S. mesensisZBK); tail depth 10.6-11.9% of CL (vs. 18.9-25.0% in S. britskiiZBK); and 15 precaudal vertebrae (vs. 16-17 in S. britskiiZBK, S. mesensisZBK, and S. mormyrus). Additional characters used to diagnose all valid described species of SternarchorhynchusZBK are summarized in Table 2.
Description. Morphometric data for holotype and paratypes are presented in Table 1. Body laterally compressed, with greatest body depth at abdominal cavity or a little posterior to this area. Dorsal profile nearly straight. Lateral line extending to tail, but absent on caudal fin. First anterior perforated scale of lateral line above pectoral-fin origin (Fig. 1). Head laterally compressed, widest at opercular region and deepest at nape; snout elongate, compressed and curved downward. Eyes small, located laterally on head, and completely covered by a thin membrane. Anterior nares at end of a small tube, close to tip of snout; posterior ones ellipsoid, without a tube and closer to tip of snout than to eyes. Branchial opening slightly anterior to pectoral-fin origin; branchial membranes joined at isthmus. Anus and urogenital papilla adjacent, ventral, located in a vertical posterior to eyes, does not noticeable move anteriorly with age. Neurocranium partially reticulated. Infraorbital series not ossified. Mesethmoid long and narrow. Lateral ethmoid absent. Premaxillae of small size, somewhat rounded, seven to 10 teeth (n=1). Dentary elongate, a row of seven curved conical teeth (n=1). Meckel’s cartilage greatly reduced. Coronomeckelian bone absent. Anguloarticular elongate. Mouth terminal, rictus not passing a vertical through anterior nares. Endopterygoid thin and long, without teeth or an ascending process connecting orbitosphenoid. Five branchiostegal rays, first, second and third almost filamentous, ventromedial branchiostegals large and broad. Urohyal with trapezoidal head, and well-developed at posterior laminar portion. Basihyal elongate, narrow at base and broad posteriorly, about same size or bigger than first ceratobranchial. First and second basibranchial ossified, posterior ones cartilaginous. Epibranchials one to four ossified, fifth one cartilaginous. Lower pharyngeal tooth plates with six teeth (n=1); upper pharyngeal tooth plates with seven to eight teeth (n=1). Four ossified independent proximal pectoral radials; third postcleithrum not ossified; anterior portion of coracoid reaching anterior edge of cleithrum. Absence of medial ridge on cleithrum, scapula without scapular foramen; supracleithrum and postemporal fused. Pectoral fin elongate, broad and pointed distally, with ii+11-12 rays (n=4). Anal-fin origin anterior to opercular region; 23-24 (n= 2) anterior unbranched anal-fin rays and 179-189 (n= 3) total anal-fin rays. 10-11 (n=4) scales above lateral line at mid-body. Dorsal mid-sagittal electroreceptive filament origin on posterior half of body, and inserted into a narrow middorsal groove, almost extending to, or slightly passing end of anal fin. Tail compressed and short, ending in a small and elongate caudal fin, 13-15 rays (n=3). Number of precaudal vertebrae 15 (12 anterior, three transitional), (n= 4).
Secondary sexual dimorphism. Single male specimen with wider snout than in all three female specimens (see Fig. 1). External teeth resembling those known from S. roseniZBK from the Orinoco basin (Mago Leccia, 1994) and S. cf. roseniZBK from the Amazon (PyDaniel& Cox-Fernandes, 2005: 105, fig. 5) were not present in the male specimen. No sexual differences in size or post-cranial morphology were evident.
Coloration in alcohol. Body dark brown; pectoral-fins hyaline to translucent; anal-fin hyaline to translucent; caudal-fin black. Coloration in life is similar, with brown pigments less faded and with a purplish hue.
Electric organ discharge. Known from a single sexually mature female (the holotype). Continuous periodic wave-type EOD with biphasic waveform and stable repetition rate of 1168 Hz. Downward inflection in rising voltage above zero line. The EOD of S. curumimZBK has a harmonic spectral content, with energy concentrated into the fundamental frequency (F0) and higher harmonics (at integer multiples of F0) (Fig. 2). No spontaneous amplitude modulations (‘chirps’) or other EOD modulations (Zakon et al., 2002) were observed during the recording.
Ecology. Sternarchorhynchus curumimZBK is known only from the type series captured at depths of 5-14 m in the Rio Tefé towards the end of the low-water period. The Rio Tefé is a typical nutrient poor blackwater river with low electrical conductivity (6-20 µScm-1). All four specimens were in reproductive condition. The females possessed swollen ovaries containing large yellow globular eggs typical of spawning fish. The male specimen had enlarged white testes typical of a spawning specimen. The stomachs of all specimens contained only autochthonous invertebrates, mainly the larvae of Chironomidae, Coleoptera and Plecoptera.
Distribution. Known only from the type series collected in the lower Rio Tefé, Amazonas, Brazil.
Etymology. The specific epithet is from curumim, the Brazilian Portuguese derivate of the Tupi-Guarani for child, curumi, or kurumí; referring to the small size of this species. A noun in apposition.