Paramormyrops undetermined, Rich et al

Rich, Madeline, Sullivan, John P. & Hopkins, Carl D., 2017, Rediscovery and description of Paramormyrops sphekodes (Sauvage, 1879) and a new cryptic Paramormyrops (Mormyridae: Osteoglossiformes) from the Ogooué River of Gabon using morphometrics, DNA sequencing and electrophysiology, Zoological Journal of the Linnean Society 180, pp. 613-646 : 628-638

PARAMORMYROPS NTOTOM SP. NOV.

( FIG. 12 , TABLE 2)

urn:lsid:zoobank.org:act:7BD331A4-1E58-4420-91D9-DA197125340B

Holotype: CUMV 98138, 175 mm SL, male, tag number JPS-1189, Gabon, Ogooué-Lolo, Ogooué River at Doumé , GPS coordinates: 0.84137°S, 12.96548°E, J.P. Sullivan & B. Sidlauskas, 17 September 2014.

Paratypes (22): Ogooué River at Doumé (10) (0.843°S, 12.96°E): J.P. Sullivan , 29 May 2011: CUMV 96811 , JPS- 1117, 120 mm SL ; AMNH 264795 , formerly CUMV 96811 , JPS- 1119, 110 mm SL ; MNHN 2016-0016 , formerly CUMV 96811 , JPS- 1120, 152 mm SL ; MRAC 2016- 010 -P-00001, formerly CUMV 96811 , JPS- 1122, 162 mm SL. (–0.841, 12.965): J.P. Sullivan & B. Sidlauskas , 17 September 2014: CUMV 98129 , JPS- 1179, 121.5 mm SL ; AMNH 264796 , formerly CUMV 98132 , JPS- 1183, 126 mm SL ; MNHN 2016-0017 , formerly CUMV 98133 , JPS- 1184, 125.5 mm SL ; MRAC 2016-010 -P-00002, formerly CUMV 98141 , JPS- 1198, 137 mm SL ; CUMV 98142 , JPS- 1199, 137.5 mm SL ; AMNH 264797 , formerly CUMV 98144 , JPS- 1202,133 mm SL .

Upper Ogooué River (12) Under bridge near Franceville (1.637°S, 13.530°E) C.D. Hopkins et al., 7 August 1999. (9): AMNH 264798 , formerly CUMV 80463 , tag no. 3465, 7 August 1999 ; MNHN 2016-0018 , formerly CUMV 80463 , tag no. 3466, 132 mm SL ; MRAC 2016-010 -P-00003, formerly CUMV 80463 , tag no. 3467, 157 mm ; CUMV 96850 (6, specimen nos. JPS- 1095, JPS-1099 = AMNH 264799 JPS-1102, JPS-1103, JPS-1104, JPS-1105) 116–137 mm SL, J.P. Sullivan, 27 May 2011 .

Ogooué River, 30 minutes downriver from bridge near Franceville (1.603°S, 13.529°E): 14 August 1999. (3); MNHN 2016-0020 , formerly CUMV 80507 , tag no. 3667, 157 mm SL .; CUMV 80507 , tag no. 3671, 187 mm SL ; CUMV 80507 , tag no. 3720, 139 mm SL .

Non-type specimens used for phylogenetic analysis: Okano River near Mitzig (0.80983°N, 11.64633°E): AMNH 231500 (specimen no. 4149, referred to as B. cf. curvifrons in Sullivan et al., 2004), J.P. Sullivan et al., 2001-08-17 .

Non-type specimens used for analysis of EODs: Doumé rapids, left bank of Ogooué River (0.841° S, 12.965° E): J.P. Sullivan, 16 September 2014: CUMV 98092 (1, specimen no. JPS-1176) 154 mm

SL, EOD no. 197457. J.P. Sullivan, 17 September 2014: CUMV 98127 (1, specimen no. JPS-1177) 162 mm SL, EOD no. 197458. CUMV 98128 (1, specimen no. JPS-1178) 147 mm SL, EOD no. 197459. CUMV CUMV 98143 (1, specimen no. JPS-1200) 150 mm SL, EOD no. 197481 .

Sébé River, left bank rocks below bridge (0.93442°S, 13.35777°E): J.P. Sullivan, 20 September 2014: CUMV 98261 (1, specimen no. JPS-1217) 111 mm SL, EOD no. 197494. CUMV 98086 (1, specimen no. JPS-1222) 122 mm SL, EOD no. 197499. CUMV 98087 (1, specimen no. JPS-1224) 135 mm SL, EOD no. 197501. CUMV 98088 (1, specimen no. JPS-1225) 147 mm SL, EOD no. 197502. CUMV 98089 (1, specimen no. JPS-1228) 152 mm SL, EOD no. 197505. CUMV 98090 (1, specimen no. JPS-1229) 177 mm SL, EOD no. 197506. J.P. Sullivan, 22 September 2014: CUMV 98077 (1, specimen no. JPS-1241) 136 mm SL, EOD no. 197518. CUMV 98080 (1, specimen no. JPS-1242) 133 mm SL, EOD no. 197519. CUMV 98081 (1, specimen no. JPS- 1243) 146 mm SL, EOD no. 197520. CUMV 98078 (1, specimen no. JPS-1244) 141 mm SL, EOD no. 197521. CUMV 98079 (1, specimen no. JPS-1245) 146 mm SL, EOD no. 197522 .

98130 (1, specimen no. JPS-1182) 132 mm SL, EOD no. 197462. CUMV 98131 (1, specimen no. JPS-1182) 163 mm SL, EOD no. 197463. CUMV 98134 (1, specimen no. JPS-1185) 176 mm SL, EOD no. 197466. CUMV 98135 (1, specimen no. JPS-1186) 162 mm SL, EOD no. 197467. CUMV 98136 (1, specimen no. JPS-1187) 165 mm SL, EOD no. 197468. CUMV 98137 (1, specimen no. JPS-1188) 144 mm SL, EOD no. 197469. CUMV 98139 (1, specimen no. JPS-1191) 116 mm SL, EOD no. 197472. CUMV 98140 (1, specimen no. JPS-1197) 145 mm SL, EOD no. 197478.

Diagnosis: Paramormyrops ntotom sp. nov. is distinguished from all other Lower-Guinea Paramormyrops by a combination of morphological and electrical characteristics: 5 teeth in upper-jaw, 6 in lower; 12 circumpeduncular scales; ‘V’-shaped snout profile viewed from above, snout angle 38–50° ( Fig. 5D ), corresponding to an interorbital width 0.8–1.05 times the snout length; BD 14.9–18.1% SL, 81–91.6% of BD at urogenital pore ( Fig. 5E ); eye diameter 12.4–14.7% HL measured to end of opercular bone ( Fig. 5F ); snout length 23.8–28.3% HL; ratio of HL to depth (HLx/ HDx measured from radiographs) 1.25–1.40 ( Fig. 4A ); HL 20.4–24.6% SL; EOD waveform with two phases, head positive then negative, duration 4.78 ± 1.10 ms with a corresponding peak power spectral frequency, 265 ± 67 Hz; electric organ composed of type NPp electrocytes ( Sullivan et al., 2002).

Comparison with other Paramormyrops: With its 5/6 teeth, 12 circumpeduncular scales and sharp snouts, P. ntotom sp. nov. differs from all other described Paramormyrops except P. curvifrons and P. sphekodes . We treat both of these in turn.

Four morphological characters distinguish P. ntotom sp. nov. from P. curvifrons . (1) ratio of HL to SL shorter in P. ntotom sp. nov. compared with P. curvifrons ( Fig. 5A ); (2) slight concave depression in the forehead in advance of the orbit and a relatively compact snout compared to P. curvifrons which has a longer snout, downward sloping, often with a slightly protruding chin and upper lip; (3) shorter prepectoral distance relative to predorsal distance ( Fig. 5B ); (3) snout angle greater in P. ntotom sp. nov. than P. curvifrons ( Fig. 5D, G ) and (4) the HD relative to HL greater in P. ntotom sp. nov. than P. curvifrons ( Fig. 5C ).

EOD duration slightly longer in P. ntotom sp. nov. compared with P. curvifrons , with extensive overlap ( Fig. 15 ). Both species have type NPp electrocytes in the electric organ and both exhibit sex differences in EOD duration ( Table 3, Fig. 15 ). Although these two species have overlapping EOD types, our collections indicate they are not anywhere sympatric. Paramormyrops curvifrons is known only from the Ivindo River in Gabon, while P. ntotom sp. nov. is known only from the Ogooué ( Fig. 10 ).

Paramormyrops ntotom sp. nov. differs from P. sphekodes in the six following morphological characters: (1) larger overall size at sexual maturity ( Fig. 2 ); (2) head more elongate and less rounded ( Fig. 4 ); (3) snout reduced ( Fig. 3 , Fig. 5G ); (4) larger caudal peduncle depth to length ratio ( Fig. 3 , Table 2); (5) the smaller eye diameter relative to HL, HLBO ( Fig. 5F , Table 2); and (6) the greater EOD duration ( Figs 2 , 14 ).

When alive, P. ntotom sp. nov. is most easily distinguished from P. sphekodes by its longer EOD duration and a correspondingly lower peak spectral frequency ( Table 3, Figs 8 , 14 , 15 ). The EODs of P. ntotom sp. nov. and P. sphekodes also differ in shape, with the width of first and second phase being equal for P. ntotom sp. nov., while W1 is longer than W 2 in P. sphekodes ( Fig. 15 ). Living specimens of these two species also differ in coloration. Dorsal and anal fin pigmentation tends to be darker in P. ntotom sp. nov. than P. sphekodes , and the ground color of the skin tends to be darker chocolate brown compared to yellow brown in P. sphekodes ( Fig. 7 ).

Description: Photos and radiograph of the holotype are shown in Fig. 12 . Table 2 summarizes morphometric ratios and meristics for it and 22 paratypes. Figure 13 shows five live specimens photographed in the field.

A large-bodied Paramormyrops , largest female, 137.5 mm SL (CUMV 98142, #JPS-1199), the largest male, 183 mm SL (CUMV 80507, #3671). Body laterally compressed with maximum width at opercular bones, 8.43–10.42% SL. Viewed laterally, BD increases gradually from base of the pectoral fin to its maximum at origin of the anal fin. BD at pectoral fin, 14.9–18.1% SL, BD at urogenital pore (BDUGP) 16.7–21.5% SL: the ratio of these two depths, 0.819 –0.916, reflects the slight increase posteriorly compared to P. sphekodes ( Table 2). BD decreases from origin of dorsal and anal fins to caudal peduncle. Caudal peduncle length 15.8– 19.4% SL, slightly deeper at origin than centre, CPD 26.1–33.7% CPL. Lobes of caudal fin rounded.

Lateral head profile straight and downward sloping from a point half way between opercular opening and tip of the snout, slightly concave above the eye in some specimens. Head and snout ‘V’-shaped when viewed dorsally ( Figs 9D and 15 ), snout angle, 38–50° intermediate between that of P. curvifrons and P. sphekodes ( Fig. 5D, G , Table 2). HL, 19.6–24.6% SL, similar to P. sphekodes , but shorter than P. curvifrons ( Fig. 5A ). HD measured from external landmarks 64.8–79.8% HL, or HL/HD = 1.13–1.35. When measured from radiographs, HLx/HDx = 1.25–1.38, non-overlapping with P. sphekodes ( Fig. 4 ). HW 38.3–46.9% HL. In lateral view, snout tip lies along mid-horizontal line. Teeth bicuspid, 5 (rarely 4) in upper and 6 in lower jaws.

Mouth small, rictus directly beneath nares. Chin slightly swollen below gular region, not extending beyond snout. Eye small, ED 10.6–15.0% HL ( Fig. 5F ). Eye socket forms light ring around dark eyeball, with gold iris and dark centre. IOW 22.4–31.4% HL. Anterior naris at about 1/3 distance from snout tip to eye, slightly below line drawn through centre of eye, posterior naris halfway between anterior naris and eye, level with lower margin of eye. Opercular opening begins anterior to base of pectoral fin. POL 62.8–68.1% HL.

Pectoral-fin origin beneath posterior terminus of opercular opening, slightly below mid-horizontal line, pectoral length 14.6–16.8% SL, 10–12 rays. Pelvic-fin origin at 34.6–38.8% SL, length 8.2–11.1% SL, positioned ventrally, 6 rays. Pre-dorsal distance 61.4–66.1% SL; anterior margin of dorsal fin gently convex, trailing margin concave in first third, remainder levels off at 1/2 DFH. Maximum DFH 59.0–83.2% DFL, 19–21 total rays. Anal-fin origin slightly anterior to dorsal-fin origin: dorsal-fin origin above seventh anal ray (fifth branched ray).

Anal fin mirrors general shape of dorsal fin, maximum height 50.4–71.8% AFL. In males, anterior AFR thickened and stiff, noticeable notch in body spanning anterior half of anal-fin base. Anal-fin base terminus directly beneath that of dorsal fin, rays 24–26. Lobes of caudal fin rounded, equal, slightly wider than caudal peduncle, deeply cleft, scaled at the bases.

Scales fine, cycloid, absent from head. Pierced lateral line scales 57–72, 9–12 scale rows between anterior base of dorsal fin and lateral line, 10–14 scale rows from pelvic fin to lateral line. Circumpeduncular scales 12.

Vertebrae: 41–44 total, 18 pre-caudal and 22–26 caudal.

The EODs of P. ntotom sp. nov. differ between males and females ( Figs 2 and 14C ) with a corresponding difference in EOD power spectrum ( Figs 2 and 14E ). EOD duration of males, significantly longer than that for females (Student’s t = 2.107, P <0.05, Table 3). Widths of the first and second peaks nearly equal in P. ntotom sp. nov. both for males and females ( Table 3) contrasting with P. sphekodes where the first peak is longer than the second, especially for males. Variation in EOD duration amongst male P. ntotom sp. nov. results from EOD elongation during male sexual maturation – only appearing in the larger males during the breeding season. Adult males and females recorded during the dry season have similar EODs, a common pattern among mormyrids studied in the laboratory ( Kirschbaum, 1984, 1987) and in the field ( Bass et al., 1983).

Coloration. All fins with lightly to heavily pigmented rays, membranes hyaline. No dark band bases of dorsal and anal fins. Body darker dorsally, lighter ventrally. When alive, tan to light chocolate brown body with olive accents on top of head, back and belly. Mouth, chin and sometimes gular region unpigmented, white to grey. Many small white pores (mormyromast and ampullary electroreceptors) visible on top of head and back, with fewer, large white pores (Knollenorgans) on head. Preserved specimens are uniform greyish-brown.

Electric organ discharge. EOD composed of biphasic pulses, head positive then negative, total duration, 4.79 ± 1.1 ms ( Fig. 14A, C ; Table 3). Width, W1, of first peak 2.25 ± 0.429 ms, approximately equal to width of second peak ( Table 3). Power spectrum peak, 265 ± 67 Hz. Other quantitative measures in Table 3 and Fig. 14 . Marked inflection point on first rising phase of EOD in advance of peak P1; first derivative of EOD with two positive peaks before P1 ( Fig. 14B , arrows 1 and 2), contrasting with P. sphekodes , which lacks an inflection point and has but a single peak in the first derivative ( Fig. 8B ). No head-negative peak in advance of P1 (see 20× expanded trace in Fig. 14A ) as occurs in all mormyrids with Type Pa electrocytes (P enetrating stalks with a nterior innervation), indicating that electrocytes are type NPp (i.e. have N on- P enetrating stalks with p osterior innervation), the same as P. sphekodes (not shown).

Distribution: Paramormyrops ntotom sp. nov. is found in Gabon in the Ogooué River and some of its tributaries ( Fig. 10 ). We have confirmed its presence in the lower Ogooué near Lambaréné, the Okano River near Mitzig, the middle Ogooué near Lopé and the upper Ogooué from Doumé to Franceville. This species is notably missing from the Ivindo River, a major tributary of the Ogooué to the North East of Gabon, connected to the Ogooué through a series of major waterfalls.

Etymology: The species name, ntotom is the word for mormyrid fish in the language of the Fang people from northern Gabon, Equatorial Guinea and southern Cameroon.

Figures

Figure 2. Electric organ discharge (EOD) waveforms recorded from 40 specimens of P. sphekodes-like mormyrids from the Ogooué River Basin of Gabon suggest the possibility of two species with distinct EOD waveforms. (A) For each specimen, EOD duration is plotted against standard length (SL). (B) Histogram of EOD durations reveals two modal peaks: one for short EODs, <2 ms duration, and one for longer EODs,> 2 ms. (C) EOD waveforms of longer (above) and shorter duration (below) are superimposed after each EOD’s amplitude is normalized to the same peak-to-peak height and centred on the zero-crossing between positive and negative peaks. Blue lines are males and red lines are females. Head positivity is upward. EOD duration is measured between T1 and T2 (in E), first and last points of the waveform that deviate above or below the baseline by more than 2% of the peak-to-peak height. In previous publications, the longer EOD type was referred to by the code name ‘SN4’. The fish with the ‘short EOD’ waveform is new to this study. (D) Histograms of SLs of all 40 specimens separated by EOD-type and by sex/age class show that within each EOD type there are males recognized by their dimorphic anal fins.Within each group, males tend to have the longest duration waveforms.This sex difference is especially pronounced for SN4 males recorded during the breeding season. Fish of both EOD types co-occur at two sites in Gabon: the main channel of the Ogooué River at Doumé and the Sébé River nearby (see map). (E) EOD waveform of specimen CUMV 98177 tag JPS-1238 showing how EOD duration is measured.

Figure 3. The Paramormyrops specimens with short EODs, and those called SN4 have overlapping meristics but differ in a number of morphometric ratios. Here, the short EOD specimens are shown as blue circles, while ‘*’ indicates the lectotype of P. sphekodes and ‘+’ indicates the paralectotype. The red circles show those with longer EODs referred to as SN4 specimens. The short EOD forms have elevated ratios of interorbital width to snout length and correspondingly blunter snout angles than the SN4 specimens. They also have slightly reduced caudal peduncle depth to length ratios. There is overlap in each ratio taken separately, but combined they provide a convenient morphological basis for diagnosis between these two EOD types. EOD traces are 10 ms long.

Figure 4. The short EOD and SN4 Paramormyrops differ in the ratio of head length (HL) to head depth (HD) when measurements are taken from radiographs. (A) HLx/HDx is plotted against standard length for 41 specimens including short EOD specimens (n = 9, blue circles), SN4 specimens (n = 30, red squares) and the two existing types (* = lectotype of P. sphekodes and ‘x’ = the paralectotype). Solid lines show linear regression lines showing that head shape changes little with overall size. The measurements of the lectotype (LT) of P. sphekodes (MNHN-A893) and paralectotype (PLT) (MNHN 1998-1050) identify the short EOD individuals as P. sphekodes. The specimens with SN4-type EODs belong to a new species (red * indicates the new species holotype). Specimen 1185 is shown in x-ray in C. (B) Non-overlapping histograms of HLx/ HDx allow for good diagnosis of the two EOD types even if no EOD is available, as with the two types of P. sphekodes. (C) Radiographs of two specimens (Specimen CUMV 98134 tag number JPS-1185, an SN4 fish and MNHN-A893) illustrate landmarks used for measuring HLx and HDx (see Material and Methods). Scale bars = 1 cm.

Figure 5. Morphometrics and ratios from the three species of Paramormyrops included in this study. (A–C) and (E–G) show measurement ratios useful in diagnosis of these three species. Snout angle measurements (see Material and Methods) are compared in (D) and (G). (D) plots snout angle against IOW/SNL. Holotypes or lectotypes are indicated by ‘*’ symbols and paratypes are indicated by ‘x’ symbols. (H) compares IOW/SNL for specimens of differing standard lengths. Superimposed on the data points in (A–C) and (E–G) are box plots showing range, 25% quartile, median and 75% quartile. Black bars above box plots span samples where means differ significantly (P ≤ 0.05) using Tukey–Kramer multiple comparison tests for differences in sample means. See Table 1 for abbreviations.

Figure 7. Five specimens of P. sphekodes from Ogooué basin of Gabon. From top to bottom: specimen tag number 1192, female, 113.5 mm; 1201, female, 111 mm from the Ogooué River at Doumé; 1214, male, 112.5 mm; 1230, male, 133 mm and 1238, male, 119 mm from the Sébé River nearby. Scale bars = 1 cm.

Figure 8. Electric organ discharges (EODs) from P. sphekodes. (A) Paramormyrops sphekodes specimen CUMV 98177 (tag JPS-1238), a male from type locality showing normalized voltage as a function of time, that is 1.0 V peak-to-peak. Head positivity is upwards on all traces. There are two peaks to the waveform: P1, which is head-positive, and P2, which is headnegative. EOD duration is measured between points marked by open circles – the first and last points exceed ±0.02 V. W1 and W2 are the widths of the first and second peaks. The thin line is a 20× vertical expansion of the waveform which rises gradually from the baseline with no indication of a head-negative pre-pulse, P0, that is present in some other species. The final overshoot, due to AC-coupling of the amplifier, is absent when making DC-coupled recordings. The dashed line indicates the zero baseline. (B) Time derivative, dV/dt, of EOD waveform in (A) has a single positive peak (arrow) in advance of peak P1 and there is no inflection point on the rising phase before P1. (C) EODs from nine specimens of P. sphekodes showing the stereotypy of the species waveform. After normalization to unity peak-to-peak height, all EODs are centred on the zero-crossing between P1 and P2 [zc in (A)]. Males are plotted in blue and females are plotted in red. (D) Power spectral density of EODs of P. sphekodes in (A) with its maximum power at 1125 Hz. Flow and Fhi are the frequencies where the spectral power drops 3 dB below the peak power of the FFT. The bandwidth of the power spectrum is Fhi–Flow. (E) Superimposed power spectra for the nine EODs shown in (C) with peak frequencies marked with ‘x’. Axes units are as in D.

Figure 10. Distribution map of West-Central Africa showing collection localities of specimens of P. sphekodes (blue), P. ntotom sp. nov. (red) and P. curvifrons (green). Stars mark collection locations of holotypes (or lectotype) and circles mark locations of other specimens.

Figure 12. Holotype of P. ntotom sp. nov. CUMV 98138, tag number JPS-1189, male, 178 mm SL, from top to bottom photographed when alive, preserved in alcohol left and right sides, and radiograph. Scale bars = 1 cm.

Figure 13. Variation in P. ntotom sp. nov. illustrated by five specimens from the Ogooué River: CUMV 96811 tag number JPS-1117, female, 117 mm; CUMV 98091 tag number JPS-1175, male 130 mm SL; CUMV 98092 tag number JPS-1176, male, 154 mm SL; CUMV 98134 tag number JPS-1185, male, 176 mm and CUMV 98136 tag number JPS-1187, male 165 mm.

Figure 14. Electric organ discharges (EODs) from P. ntotom sp. nov. EOD waveforms plot voltage versus time. Voltage is normalized by setting the peak-to-peak voltage to 1.0 V.The time base is in (C). (A) EOD of holotype, CUMV 98138 tag number JPS-1189, male, SL 178 mm. The first peak, P1, is head-positive and the second, P2, is head-negative. Overall duration is 5.813 ms, measured between the two red dots. The 20× expanded trace (black) indicates the absence of a head-negative phase preceding P1. Abbreviations are as in Figure 10. (B) Time derivative of EOD shown in (A), indicating two points where dV/dt goes through a local maximum leading to inflection points in the EOD (dashed lines). (C) Superimposed EODs from 18 males (blue) and 13 females and juveniles (red) show a clear sex difference in EOD duration. (D) Power spectrum of the EOD of holotype, with magnitude measured in dB relative to the peak power plotted against frequency in Hz. Frequency at peak power indicated by F max. The frequencies where the magnitude of the power spectrum drops 3 dB below the peak power are indicated by Flow and Fhi. (E) Superimposed power spectra of traces shown in (C) demonstrate that spectral power emphasizes higher frequencies among females compared to males, as expected.

Figure 15. EODs from (A) P. sphekodes from the Ogooué River, (B) P. curvifrons from the Ivindo River and (C) P. nototom sp. nov. from the Ogooué River. Each is plotted on the same time scale, all with head positivity upwards. (D) Scatter plot of EOD duration versus the ratio W1/W2 – the widths of the first and second phases of the EOD illustrated in Figs 10 and 14. EOD total duration and W1/W2 overlap for the allopatric pair, P. curvifrons and P. ntotom sp. nov., but not for the sympatric pair, P. sphekodes and P. ntotom sp. nov.

Figure9. Histologyofpara-sagittalsectionofparalectotype of P. sphekodes specimen MNHN 1998-1050 (Female, 98.7 mm, SL) shows electrocytes of type NPp (Non-Penetrating stalks with posterior innervation). The specimen, collected from Doumé Falls by Alfred Marche in 1876–1877 and preserved in alcohol, was embedded in plastic, sectioned with a tungsten carbide knife at 7 µm and stained with toluidine blue. E = main body of the electrocyte; anterior = anterior face of electrocyte; post = posterior face of same electrocyte; c = collagen layer separating two electrocytes; S = stalk of electrocyte which is innervated by the axons from the electromotor nerve (not shown); s = stalklets, or small branches from a dividing stalk that eventually fuse with posterior face of the electrocyte. Stalks are innervated on the posterior side of the electrocyte and all branches of the stalk system remain posterior to the main body of the electrocyte without crossing to the opposite or anterior side.