Gymnotus coropinae Hoedeman

William G. R. Crampton & James S. Albert, 2003, Redescription of Gymnotus coropinae (Gymnotiformes, Gymnotidae), an often misidentified species of Neotropical electric fish, with notes on natural history and electric signals., Zootaxa 348, pp. 1-20 : 6-14

[[ Gymnotus coropinae Hoedeman ]]

The differential diagnosis presented here is limited to salient characters that do not necessitate clearing and staining. Mean, median, or modal values are given for diagnostic characters that have slightly overlapping ranges but divergent measures of central tendency. Morphometric and meristic data for diagnoses are from data pooled from three geographical populations of G. coropinae , and from specimens of G. anguillaris from the region of the type locality in Surinam (Tables 2-3). We distinguish G. coropinae from members of the G. carapo and G. pantherinus species-groups endemic to the Amazon and Orinoco Basins and to the Guyana Shield (species-groups and geographical areas sensu Albert and Crampton, 2003). Species in the G. cylindricus species-group are confined to Middle America where G. coropinae does not occur.

Gymnotus coropinae Hoedeman

(Figs. 2-5)

Gymnotus coropinae . - Hoedeman (1962): 55, fig. 1c [type locality: Surinam: Coropina Creek, sta. 17, May 20, 1956] .

Gymnotus anguillaris . - Nijssen & Isbrücker (1968): 162, fig. 1 [in part, places G. coropinae as a synonym of G. anguillaris , Surinam: Marowijne, Suriname, Saramacca, Coppename, Nickerie rivers ]. - Knöppel (1970): 257 [ Brazil: Rio Negro ]. - Mago-Leccia (1994): 56, fig. 84 [in part, regards G. coropinae as synonym of G. anguillaris ]. - Planquette et al. (1996): 410, fig. and distribution map. pp. 411 (map may also contain records for G. anguillaris sensu stricto) [ French Guyana ]. - Crampton (1998a): 817 [ Brazil: Rio Tefé ]. - Crampton (1998b): 314 [ Brazil: Rio Tefé ]. - Crampton (1999): 17 [ Brazil: Rio Tefé ]. - Albert (2001): 112 [in part, regards G. coropinae as synonym of G. anguillaris : Guyana , Surinam , Amazon drainages of Brazil , Bolivia , Peru ]. - Campos-da-Paz (2003): 484 [in part, regards G. coropinae as synonym of G. anguillaris ].

Diagnosis. Gymnotus coropinae is unique among all congeners endemic to the Amazon-Orinoco basins and the Guyana Shield in possessing a dark brown background color marked by 8-18 white or pale yellowish bands (or 4-22 dark inter-bands, see Fig. 2) which in anterior 1/3 to 2/3 of body are absent or largely restricted to the ventral part of the lateral surface (Figs. 2, 4). Gymnotus coropinae and all congeners in the G. pantherinus species-group can be distinguished from species in the G. carapo species-group by the absence of a clear or pale patch near the caudal end of the anal fin and by the presence of one (vs. two) laterosensory canal pores in the preopercular-mandibular series at the dorsoposterior portion of the preopercle. Gymnotus coropinae can be distinguished from congeners of the G. pantherinus species-group endemic to the Amazon-Orinoco basins and the Guyana Shield in possessing the following unique combination of characters: 1, maximum known body size 162 mm (vs. more than 300 mm in G. anguillaris , G. cataniapo and G. pedanopterus ); 2, precaudal vertebrate (PCV) 39-43 [mode 41] (vs. 31-32 in G. pedanopterus , vs. 37-38 in G. anguillaris , G. melanopleura , and G. onca , vs. 36-39 [mode 37] in G. jonasi , and vs. 47-51 in G. cataniapo ); 3, pectoral-fin rays (P1R) 12-14 (vs. 16-18 in G. anguillaris , and vs. 15 in G. coatesi ); 4, scales above lateral line (SAL) 6-8 [mode 7] (vs. 8-13 [mode 9] in G. pedanopterus ); 5, pored lateral-line scales to first ramus (PLR) 44-61 [median 52] (vs. 31-40 in G. jonasi , vs. 60-69 [median 63] in G. cataniapo , and vs. 61-65 [mode 65] in G. coatesi ); 6, total number of pored lateral-line scales (PLL) 72-102 [median 86] (vs. 102-130 [median 124] in G. anguillaris , and vs. 114-120 in G. cataniapo ); 7, anal-fin pterygiophore scales (APS) 5-7 (vs. 9-10 in G. javari ); 8, adult preorbital length (PR) 29.3-38.2 % HL (vs. 26.4-29.2 in G. jonasi ); 9, adult mouth width (MW) 35.2-46.6 % HL (vs. 27.8-35.1 in G. coatesi and G. jonasi ); 10, adult body width (BW) 3.5-5.4 [mean 4.8] % TL (vs. 5.6-6.7 in G. anguillaris and G. coatesi , and vs. 5.4-7.3 [mean 6.4] in G. cataniapo ).

Description. Figs. 2-4 illustrate body shape and pigment patterns. Fig. 3 illustrates a preserved specimen from the region of the type locality in Surinam. Fig. 4 illustrates a live specimen from the vicinity of Tefé, Brazil. Nijssen and Isbrücker (1968, fig. 1) also provide a photograph of a specimen from the region of the type locality in Surinam (ZMA 105.904, 125 mm). Morphometric and meristic data presented in Tables 2 and 3. Cephalic sensory canal pore configurations illustrated in Fig. 5. Size up to 162 mm in males and 140 mm in females. Maturity at about 90-120 mm in males and females. Sexually monomorphic other than size differences. Scales cycloid, ovoid, present on entire post-cranial portion of body from nape to tip of caudal appendage. Scales on dorsal surface relatively large at midbody; 6-8 rows from lateral line to dorsal midline. In adult specimen, 150 mm, lateral-line scales approximately 1.5 mm high by 1.3 mm long at midbody, 1 mm high by 1.3 mm long at one head length anterior to distal end of caudal appendage. Gape size in mature specimens moderate, extending three quarters distance from anterior tip of snout to posterior nares. Mouth position superior, lower jaw longer than upper, rictus decurved. Chin fleshy and bulbous with thick pad of electroreceptor organs and support tissues overlying tip of snout and oral jaws. Anterior narial pore partially or entirely included within gape in large narial fold. Anterior nares large, half diameter of eye. Branchial opening moderate, 24.0-30.4% HL. Circumorbital series ovoid. Anterior margin of ethmoid region rounded. Eye position lateral, lower margin of eye dorsal to rictus.

Premaxilla triangular in ventral view, with 11-12 teeth (mode 12, n = 2) teeth disposed in single row along outer edge, and 4-5 (mode 5, n = 2) in inner row. Outer row teeth large and needle shaped. Inner teeth short and needle shaped. Dentary with 19-21 (mode 21, n = 2) teeth disposed in single row along outer edge, and 10-12 (mode 11, n = 2) teeth in inner row. Outer row with 1-10 slender recurved needle-shaped teeth anteriorly, those posteriorly larger and less slender. Inner tooth row extends about two-thirds along the tooth-bearing portion of dentary bone, teeth long, needle shaped and closely spaced anteriorly; smaller, less slender and more distantly spaced posteriorly. Rib 5 triangular in lateral view, narrow proximally and distally, broad in middle, more than 3 times width of rib 6, with a large medial triangular shelf. Hemal spines present. Displaced hemal spines absent. Multiple anal-fin ray branching posterior to ray 18. Variable number (8-27) of asymmetrically arranged lateral-line rami extending posteroventrally at posterior end of lateral line. Dorsal lateral-line rami absent in all specimens. Anal-fin pterygiophores at posterior end of body cavity equal to or longer than first hemal spine. Caudal appendage short, less than half pectoral-fin length in undamaged and unregenerated specimens. Single hypaxial electric organ, extending along entire ventral margin of body. Two or 3 (mode 3) rows of electroplates at one head length from distal end of caudal appendage.

Color in life. Ground color dark brown in adults and juveniles. 8-18 (median 14, n = 15) pale yellow or white bands on lateral surface which in anterior 1/3 to 2/3 of body of adults are either absent or largely restricted to the ventral part of the lateral surface (Figs. 2, 3) such that the dark inter-bands (ground color between pale bands) in the anterior portion of the body fuse into a uniform dark coloration. Occasionally some bands in the anterior portion of the body extend onto the dorsal part of the lateral surface but never connect with bands from the other side of the body at the dorsal midline. Pale bands are oriented vertically or obliquely in anterior-inferior to posterior-superior orientation. Pale band appearance irregular in shape, width, and arrangement, both on and among individuals. Pale bands progressively more narrow and short anteriorly, only rarely extending dorsal to midline on anterior half of body. Pale band margins highly contrasted with dark ground color. Pale bands never completely divided, although posterior-most 3-6 bands have less intense dark coloration in their middle. Dark area between pale bands sometimes marked by small white or pale yellow pigment patch near ventral margin of anal-fin pterygiophores, especially in posterior portion of body. Pale bands extend to mid-dorsum along posterior 1/4 of body. Pale bands of juveniles resemble adults but sometimes extend further dorsally in anterior portion of body. No pale bands from either side meet on ventral midline, between the anus and anal-fin origin. Two to 3 pale bands lie posterior to last anal-fin ray.

Head never banded, spotted or blotched, dark brown dorsally grading to a slightly lighter brown ventrally, with numerous minute chromatophore speckled over branchiostegal membranes and ventral surface of head. In life the operculum is dark. Pectoral-fin rays dark brown, inter-radial membranes hyaline. Anal-fin membrane grading from charcoal gray anteriorly to black posteriorly with no unpigmented patch at the caudal end. Fin colors similar in juveniles and adults. Specimens fixed in 10% formalin and preserved for 1-5 years in 70% ethanol maintain approximate colors of life, although the darker pigments tend to pale with time. Color variation is not known to be correlated with sex or EOD structure.

Electric organ discharges. EOD waveform with total duration of approximately 0.85- 1.30 ms (with beginnings and ends of EOD, i.e. zero crossovers taken at threshold 1% of peak-to-peak amplitude). EOD comprises four phases with low-voltage pre- and post potentials preceding (P0) and following (P3) a dominant, approximately symmetrical biphasic component (P1, P2) (Fig. 6-A). A very low-amplitude initial positive phase (P-1) precedes P0 in some specimens (e.g. in Fig. 6-A). The Peak Power Frequency (PPF) (Fig. 6-B) of the Fourier Transform of EODs of G. coropinae ranges from 2.1 to 2.7 kHz (mean 2.4, n = 20). The EOD pulse repetition rate of G. coropinae is relatively low and less variable during the day when this species lodges itself into submerged roots or leaf litter (approx. 45-55 Hz). The EOD pulse repetition rate is usually higher and more variable at night, when G. coropinae is active (approx. 50-65 Hz).

Remarks and comparisons. Hoedeman (1962, p. 55, fig. 1c) described G. coropinae from Surinam on the basis of a single 49.3 mm-specimen with a broken caudal appendage. On the basis of a distinct pigmentation pattern comprising only “10 faint, narrow yellowish streaks on the posterior part of the tail, otherwise the fish is dark tan (in alcohol)” and a low anal-fin ray count (142), Hoedeman opted to “provisionally describe it as a new species ”. Nijssen and Isbrücker (1968, p. 164) examined the holotype of G. coropinae and noted that it had the “same appearance as young damaged, dried out specimens of G. anguillaris .” They also observed that the low anal-fin ray count was a consequence of its broken caudal appendage and concluded that G. coropinae is a synonym of G. anguillaris .

We examined the holotype of G. coropinae , the single paratype of G. anguillaris , and other collections of Gymnotus from Surinam and concluded that G. coropinae is a valid species.

Specimens of G. coropinae can be unambiguously distinguished from G. anguillaris on the basis of several characters: 1, pale white or yellow bands usually complete only in posterior 1/3 to 2/3 of body (vs. usually complete along entire body length although more pronounced in posterior part of body in adult G. anguillaris ); 2, body length small, maximum 162 mm (vs. maximum 302 mm in G. anguillaris ); 3, pectoral-fin rays 12-14 [mode 14] (vs. 16-18 [mode 16] in G. anguillaris ); 4, body cavity relatively long, with 39-43 [mode 41] precaudal vertebrae (vs. 37-38 [mode 38] in G. anguillaris ); total number of pored lateral-line scales 72-102 [median 86] (vs. 102-130 [median 124] in G. anguillaris ). Juvenile G. anguillaris have a similar pigmentation pattern to juveniles and adults of G. coropinae , with the pale inter-bands being very faint to absent in the anterior portion of the body. Nonetheless, pectoral-fin ray and precaudal vertebrae counts allow unambiguous recognition. The holotype of G. coropinae has 12 pectoral-fin rays and 39 precaudal vertebrae, well within the range of other specimens from the region of the type locality.

Gymnotus coropinae can be readily distinguished from all other species of Gymnotus on the basis of the unique color pattern described above. It can be further distinguished from congeners of the G. pantherinus species-group endemic to the Amazon and Orinoco basins to the Guyana Shield on the basis of the unique combination of characters described in the diagnosis.

Geographical variation. Populations of G. coropinae from the upper Madeira and upper Amazon (see Tables 2 and 3) and also from other areas of the Amazon-Orinoco Basins and the Guyana Shield cannot be unambiguously distinguished from G. coropinae specimens from the region of the type locality in Surinam on the basis of color, morphometric, meristic or osteological characters. The averages of certain morphometric and meristic characters do differ slightly between populations of these regions, although the ranges of all these characters overlap considerably (Table 2 and 3). EOD data is presently unavailable for specimens outside the Tefé region.

Distribution. Gymnotus coropinae is widely distributed throughout the Amazon-Orinoco Basins and the Guyana Shield (Fig. 7).

Ecology. Detailed ecological notes on Gymnotus coropinae were compiled from studies in the Tefé region of Amazonas, Brazil (see Fig. 1 for notes on aquatic habitats). Here, this species is found in small streams ( igarapés) that drain the rainforests of the terra firme peneplain above the Amazonís floodplain (Fig. 8), and also in the lower, seasonally inundated reaches of these streams where they drain into floodplain habitats. Gymnotus coropinae occurs sympatrically in this habitat with three other species of Gymnotus : G. arapaima , G. coatesi and, rarely, G. n. sp. “cur” . Igarapés of the lowland Amazon Basin flow with cool (23-26 °C), low-conductivity (5-25 µScm-1) water. Where igarapés drain sandy soils, leached tannic and folic acids impart a dark tea-like coloration to the water. Where igarapés drain clay-dominated soils these humic substances are sequestered by the soil and the water tends to be clearer. In either case suspended sediment loads are usually low (visibility with Sechi disk 1.5-2.5 m) except following heavy rainfall or disturbance. pH values in igarapés of the Tefé region vary from 3 to 5.

Terra firme streams of the Central Amazon basin are usually devoid of macrophytes, except in forest gaps or clearings, and are choked with leaf litter and submerged branches. Gymnotus coropinae shelters during the day in curtains of dense rootmats that festoon the banks of terra firme streams or within the interstices of submerged banks of leaf litter. The diminutive size and eel-like shape of G. coropinae are well adapted for foraging in these microhabitats. Here G. coropinae prey on aquatic invertebrates of primarily autochthonous origin including small freshwater crustaceans and the nymphs or larvae of a variety of aquatic insects. Chironomidae larvae are often the dominant food. Breeding occurs primarily during the months of most rainfall (December-April) when streams are often swollen with flood waters and flood adjacent areas of low-lying forest. Gymnotus coropinae also colonizes shallow, often ephemeral, swamp-pools in terra firme forest which are temporarily connected to streams following heavy rainfall and flash flooding. Gymnotus n. sp. “cur” is also found in these swamp pools.

Gymnotus coropinae has been documented in ecological studies of small forest streams in other areas of the lowland Amazon Basin, including the Rio Negro drainage (Henderson & Walker, 1990; Knöppel, 1970; pers. obs.) where it occurs sympatrically with G. arapaima , G. pedanopterus , and G. stenoleucus . Gymnotus coropinae is also found in forest streams in the Guyanas where it occurs sympatrically with G. anguillaris and G. carapo : Surinam (Hoedeman, 1962; Hopkins & Heiligenberg, 1978; Nijssen & Isbrücker, 1968; Hopkins pers. comm.), French Guyana (Planquette et al. 1996), and Guyana (G. Watkins, R. Lowe-McConnell pers. comm.). Nijssen and Isbrücker (1968:164) describe G. coropinae ’s habitat in Surinam as “limpid, dark brown -coca-cola colored -, acid water.”

Etymology. Named for the collecting locality of the holotype, at Coropina Creek, Surinam.