Hypopygus nijsseni, De Santana & Crampton, 2011

De Santana, Carlos D. & Crampton, William G. R., 2011, Phylogenetic interrelationships, taxonomy, and reductive evolution in the Neotropical electric fish genus Hypopygus (Teleostei, Ostariophysi, Gymnotiformes), Zoological Journal of the Linnean Society 163 (4), pp. 1096-1156 : 1146-1148

HYPOPYGUS NIJSSENI SP. NOV.

(FIGS 23B, 29; TABLE 5)

Diagnosis: Hypopygus nijsseni is diagnosed from congeners by the following combination of characters: the total number of anal-fin rays (145–152 versus 102–135 in H. hoedemani , H. lepturus , and H. minissimus ; 155–174 in H. cryptogenes ), the presence of the sixth infraorbital bone (versus absence in H. hoedemani , H. lepturus , and H. minissimus ), the presence of oblique bands (versus absence in H. minissimus ), the presence of scales at midbody (versus absence in H. minissimus ), the dorsal rami of intermittent branch of anterior lateral line nerve visible as two black parallel lines (versus not visible in H. ortegai ), the head length (13.2–15.3% of LEA versus 12.6–13.9 in H. ortegai ), the distance at the nape (12.5–13.8% of LEA versus 11.0–12.5 in H. ortegai ), the total number of pectoral fin rays (ten to 12 versus 12–16 in H. isbruckeri ), the number of scales above the lateral line (five to six versus seven to eight in H. isbruckeri ), and the distance from the anus to the anal fin (7.9–9.4% of LEA versus 10.0– 12.0 in H. neblinae ).

distance to dorsal edge of pterygiophores. Pectoral-fin ray and interradial membrane hyaline. Anal-fin ray hyaline and interradial membrane hyaline.

Dorsal rami of intermittent branch of anterior lateral line nerve visible as two black parallel lines, on each side of upper back from approximately one to one-half pectoral fin length behind the occiput, running posteriorly to mid-dorsal portion of body.

Lateral line nerve visible as dark line, extending from above pectoral fin to approximately two-thirds into caudal filament.

Description: Head and body shape, and pigmentation illustrated in Figures 23B and 29. Morphometric data for examined specimens are presented in Table 5. Body size moderate, maximum examined TL 95.0 mm (N = 15). Maximum examined length from snout to end of BC 18.7 mm. Sexual dimorphism in body or head shape not possible to document. Snout slightly convex. Upper jaw slightly longer than lower jaw. Pectoral-fin rays ten to 12 [12] (N = 7). Postpectoral EO with two columns and five to six rows of electrocytes. EO groove extending approximately two to two and half orbital diameters behind posterior border of eye. Scales present on mid-dorsal region of body. Scales above lateral line at midbody four to five [five] (N = 7). Scales below lateral line five to six [six] (N = 2). Total anal-fin rays 145–154 [154] (N = 7). Caudal filament moderate. Precaudal vertebrae 15 (11 anterior; four transitional; N = 6).

Pigmentation ( Fig. 29 and Fig. 23B for colour of live specimen): Body background dark brown. Light tan vertical bands extending from across dorsal surface to margin of anal fin, from nape to end of caudal filament (15-18 crossing dorsal region of body counted from nape to last anal-fin ray). Light bands thinner in mid-body. Head dark brown, sometimes with a depigmented area ventroposterior to eye. Prominent series of more or less regularly spaced inverted U-shaped dark spots over pterygiophores, at margin of anal fin, separated by light margins, extending about one half Electric organ discharge: Data available only from four specimens at the type locality, in the vicinity of Tefé , Amazonas, Brazil. Hypopygus nijsseni generates a tetraphasic EOD ( Fig. 17 ) ranging in duration from 1.01 to 1.69 ms (mean 1.36, N = 4), and with a PPF ranging from 1.991 to 2.175 kHz (mean 2.069, N = 4). Mean pulse rate during the day ranges from 44.0 to 51.8 Hz (mean 48.3, CV 1.06 , N = 4), and during the night from 49.3 to 57.8 Hz (mean 55.4, CV 2.80 , N = 4) .

Distribution: Hypopygus nijsseni is known only from small streams in the vicinity of Tefé ( Fig. 28 ).

Ecology: In the Tefé region H. nijsseni was found rarely in small rainforest streams (approximately 1–4 m across and up to 1.5 m deep), where it was found in marginal roots and submerged structures, near conspecifics. A single specimen was found in shallow flooded forest in the floodplain of the Rio Tefé, in the lower reaches of a terra firme stream. Stomach contents (N = 5) contained only autochthonous aquatic invertebrates – mostly Chironomidae larvae, but also the larvae of Ephemeroptera, Coleoptera, and Trichoptera. Hypopygus nijsseni co-occurs syntopically with the much more abundant H. lepturus without any obvious partitioning of microhabitat or diet.

Etymology: The specific epithet, nijsseni , is a patronym in honour of Hans Nijssen for his contribution to Neotropical ichthyology.

Remarks: Hypopygus nijsseni possesses two osteological autapomorphies: both the fourth and fifth basibranchials are ossified.

Material examined: (15 specimens.)

Holotype. Brazil, Amazonas , MCP 44650, 1 , 91 mm, WC04.240201, female; Igarapé Repartimento , 1.5 km downstream from Estrada Agrovila , c. 7 km and 230° from Tefé , municipality of Tefé , tributary of Lago Tefé, Rio Tefé drainage, 03°24′30″S, 064°44′12″W; W. Crampton, 24.ii.2001.

species), the total number of anal-fin rays (137–147 versus 102–135 in H. hoedemani , H. lepturus , and H. minissimus ; 155–174 in H. cryptogenes ), the presence of the sixth infraorbital bone (versus absence in H. hoedemani , H. lepturus , and H. minissimus ), the presence of oblique bands (versus absence in H. minissimus ), the presence of scales at midbody (versus absence in H. minissimus ), the head length (12.6– 13.9% of LEA versus 13.2–15.3 in H. nijsseni ), the distance at the nape (11.0–12.5% of LEA versus 12.5– 13.8 in H. nijsseni ), the distance from the anus to the anal-fin ray (6.5–9.5% of LEA versus 10.0–12.0 in H. neblinae ), and the head length (12.6–13.9% of LEA versus 13.2–22.9 in H. isbruckeri ).

Paratypes. Brazil, Amazonas , MCP 44737, 4 , 73–96 mm, all immature (2 CS); same locality as holotype; W. Crampton, 29.xi.2000. MCP 44738, 3 , 82–94 mm, 1 immature (82 mm, CS), 2 male (83– 94 mm); same locality as holotype; W. Crampton, 24.i.2001. MCP 44739, 2 , 72–79 mm, 1 male (72 mm), 1 female (79 mm); same locality as holotype; W. Crampton, 02.iii.2001. MCP 44740, 1 , 70 mm ( WC01.070701 , immature); near Ilha do Martelo , Rio Tefé , 57 km and 214° from Tefé , municipality of Tefé, Rio Tefé drainage, c. 03°46′S, 064°59′W, W. Crampton, 7.vii.2001. MCP 44651, 4 , 74–92 mm (2 CS) (2 recorded: WC01.240201, immature, 76 mm; WC05.240201, male, 74 mm); same collecting data as holotype .

Figures

Figure 17. Electric organ discharges (EODs) of Hypopygus as time-voltage waveforms recorded in the far-field. The EODs were in all cases taken from immature specimens or females with adult morphology, and are representative of most individuals of each species (small juveniles and sexually mature males present EODs that in some cases differ from those of larger juveniles, immature adults, and females). A, Hypopygus cryptogenes, INPA uncatalogued, 80 mm (poor signal recording quality). B, Hypopygus isbruckeri, UF 148537 (WC38.150304), immature, 76 mm. C, Hypopygus lepturus, UF 176883 (WC21.090307), female, 92 mm. D, Hypopygus minissimus, UF 148533 (WC41.120304), female, 42 mm. E, Hypopygus neblinae, UF 148540 (WC36.150304), female, 77 mm. F, Hypopygus nijsseni MCP 44740 (WC01.070703), immature, 70 mm. G, Hypopygus ortegai, MUSM 35305, holotype (WC02.160104), female, 107 mm. Scale bars = 1 ms. Dashed horizontal line = 0 volts. Note that all species generate EODs with a similar four- or five-phase structure, with minimal interspecific variation in duration (except in H. neblinae where the EOD is clearly longer).

Figure 28. Map of northern South America showing collection records of Hypopygus neblinae (circles), Hypopygus nijsseni (squares), and Hypopygus ortegai (triangles). Some symbols represent more than one nearby collecting locality.

Figure 29. Hypopygus nijsseni, holotype, head, and lateral and dorsal views of body, 96 mm, MCP 44650, immature; Brazil, Amazonas, Rio Tefé, Lago Tefé, Igarapé Repartimento on road from Tefé to Agrovila, 03°24′28″S, 64°44′10″W. Scale bars = 5 mm.