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Figs. 1 & 2.
Holotype . CU 85157 (original specimen no. 2161), male, 43.5 mm SL. Gabon , “ Bialé Creek”: a small tributary of the Ivindo River inside the Ipassa Plateau Reserve near Makokou, ( 0° 32.3’ N , 12° 49.6’ E ). Dip net, C.D. Hopkins, 2 September 1975 .
Paratypes . Seventy-two specimens from three localities. The 10 paratypes selected for a complete set of measurements are indicated in Table 1.
Near village of Ébiègn on Rte. N.4, ca. 18 km E. of Makokou, Gabon ( 0° 35.5’ N , 12° 43.0’ E ), 3 Oct 1979 , R. Lewis: CU 85465 (32, without specimen nos.), 20.9-53.3 mm SL. AMNH 235053 (10, without specimen nos.), 34.2-49.8 mm SL. MNHN 2004-1750-1759 (10, without specimen nos.), 38.4-50.7 mm SL. MRAC 2004-32-P-1-10 (10, without specimen nos.), 29.8-48.1 mm SL.
Balé Creek below bridge, ca. 500 m from the IRET field station on Ipassa Plateau, ca. 7 km SE of Makokou, Gabon ( 0° 31.1’ N , 12° 48.0’ E ): CU 75437 (2, specimen nos. 1003, 1004), 37.9, 40.8 mm SL, 27 Oct 93, CD. Hopkins. CU 86244 (2, specimen nos. 2143, 2144), 39.3, 41.1 mm SL, 16 Jan 98, CD. Hopkins et al. CU 86247 (3, specimen nos. 2002, 2010, 2012), 42.3-49.5 mm SL, 11 Jan 98, CD. Hopkins et al.
Creek at Makatabongoy crossing road from Makokou-Okondja (R.15), Gabon ( 0° 08.15’ N , 13° 43.0’ E ): CU 86245 (3, specimen nos. 2223, 2352, 2358), 45.2-48.3 mm SL, 3 Oct 79, CD. Hopkins et al.
Additional (non-type) specimens.
Balé Creek, IRET field station on Ipassa Plateau, SE of Makokou, Gabon ( 0° 31’ N , 12° 48.0’ E ): CU 85385 (3) 30.0-44.8 mm SL, 29 Nov 1976 , CD. Hopkins. CU 85486 (1, specimen no. 2367) 43.0 mm SL, 28 Jul 1976 , CD. Hopkins. CU 85496 (1, specimen no. 85-230) 38.7 mm SL, 9 Dec 1985 , J. Crawford. CU 83094 (1, specimen no. 4879) 39.8 mm SL, 26 Aug 2001 , CD. Hopkins & M.E. Arnegard. CU 75442 (1, specimen no. 1027) 36.8 mm SL, 29 Oct 1993 , CD. Hopkins & MA. Friedman. CU 79703 (1, specimen no. 2074) 45.1 mm SL, 15 Jan 1998 , CD. Hopkins et al. CU 86243 (2, specimen nos. 2078, 2155) 33.6, 46.7 mm SL, 15 Jan 1998 , CD. Hopkins et al.
NyaméPendé Creek inside Ipassa Plateau Reserve ( 0° 30’ N , 12° 47’ E ): CU 85152 (1, specimen no. 2101) 44.3 mm SL, 5 Aug 1975 , CD. Hopkins & R. Askins. CU 86246 (2, specimen nos. 2133, 2136) 38.0, 40.0 mm SL, 17 Jan 1998 , CD. Hopkins et al. CU 89030 (1) 43.0 mm SL, 20 Sep 2002 , Hopkins et al.
Mindouba Creek on route to IRET station ( 0° 31.6’ N , 12° 48.2’ E ): CU 89048 (1, specimen no. 5701) 41.9 mm SL, 18 Sep 2002 , CD. Hopkins et al.
Near mouth of Mié Creek into Ivindo River, across from Ipassa Reserve ( 0° 27.3’ N , 12° 48.5’E ): CU 85123 (6), 21.3-49.6 mm SL, 20 Aug 1975 , C.D. Hopkins & R. Askins. CU 85128 (10), 35.5-48.0 mm SL, 20 Aug 1975 , C.D. Hopkins & R. Askins. CU 85156 (1, specimen no. 2150), 47.4 mm SL, 20 Aug 1975 , C.D. Hopkins & R. Askins.
Near villages of Ébiègn and Èdzua Megne on route N.4 ca. 16 km E. of Makokou (approx. 0° 35.5’ N , 12° 43’E ): CU 85131 (11), 29.5-55.7 mm SL, 9 Sep 1976 , C.D. Hopkins. CU 85132 (17, specimen nos. between 3067 and 3093), 21.0-44.7 mm SL, 1 Oct 1979 , R. Lewis. CU 85151 (12, specimen nos. between 3114 and 3125), 36.5-47.1 mm SL, 1 Oct 1979 , R. Lewis. CU 85153 (15, specimen nos. between 3095 and 3113), 27.1- 47.1 mm SL, 1 Oct 1979 , R. Lewis. CU 85154 (19, specimen nos. between 3045 and 3066), 26.2-51.2 mm SL, 1 Oct 1979 , R. Lewis. CU 85446 (71), 20.5-52.0 mm SL, 3 Oct 1979 , R. Lewis. CU 85464 (16), 32.2-50.8 mm SL, 10 Oct 1979 , R. Lewis. CU 85487 (1, specimen no. 5015) 38.9 mm SL, Sep 2002 , C.D. Hopkins. CU 85124 (4) 30.0-48.5 mm SL, 7 Nov 1979 , R. Lewis. CU 85126 (4) 24.6-44.5 mm SL, 24 Oct 1979 , R. Lewis. CU 85503 (16) 32.6-51.0 mm SL, 7 Nov 1979 , R. Lewis. CU 85378 (19) 32.6-52.7 mm SL, 1 Dec 1979 , R. Lewis. AMNH231101 (21) 23.6-51.7mm SL, 6 Nov 1975 , W. Harder. AMNH231102 (51) 22.1-49.8 mm SL, 18 Nov 1975 , W. Harder. AMNH231173 (17) 23.2-51.33 mm SL, 3 Aug 1975 , W. Harder.
Near village of Mbès on route N.4 ca. 18 km E. of Makokou ( 0° 34.8’ N , 12° 41’E ): CU 85125 (6), 37.0-45.7 mm SL, 12 Aug 1976 , C.D. Hopkins. CU 85155 (1, specimen no. 2365), 52.0 mm SL, 12 Aug 1976 , C.D. Hopkins. AMNH231100 (30) 32.7-55.1 mm SL. 20 Sept. 1975, W. Harder. AMNH231104 (1) 45.9 mm SL, 20 Nov 1975 , W. Harder. AMNH231105 (2) 44.5, 48.8 mm SL, 20 Nov 1975 , W. Harder. AMNH231106 (2) 41.3, 46.4 mm SL, 20 Nov 1975 , W. Harder.
8 km downstream of IRET Ipassa Plateau Reserve station ( 0° 27.3’ N , 12° 48.8’ E ): CU 85160 (9), 28.0-40.1 mm SL, 3 Aug 1976 , C.D. Hopkins.
23 km W of Makokou along “route Fang” (N.4) ( 0° 33.5’ N , 12° 37.7’ E ): CU 85159 (2) 40.3, 43.5 mm SL, 1 Dec 1979 , R. Lewis.
7 km W of Makokou along “route Fang” (N.4) ( 0° 34.3’ N , 12° 46.5’ E ): CU 85158 (1, specimen no. 2366) 44.0 mm SL, 1 Dec 1976 , C.D. Hopkins.
Along banks of Ivindo River near Makokou ( 0° 33’ N , 12° 51’E ): CU 85127 (13) 27.2- 46.3 mm SL, 1 Aug 1975 , C.D. Hopkins & R. Askins. CU 85477 (3) 27.9-45.5 mm SL, 1 Dec 1976 , C.D. Hopkins.
Weni Creek, Makokou ( 30° N , 12°50’E ): CU 85130 (8) 28.0-41.8 mm SL. 22 Sep 1976 , C.D. Hopkins. CU 85129 (16) 26.7-47.6 mm SL. 1 Oct 1976 . C.D. Hopkins. CU 85499 (1) 30.6 mm SL, 11 Oct 1976 , C. D. Hopkins.
Creek crossing road from Makokou to Okondja (R.15), ca. 60 km SE of Makokou ( 0° 18’ N , 13° 13’E ): CU 85420 (1, specimen no. 5014), 40.2 mm SL, C.D. Hopkins & R. Lewis.
Near Makokou, Gabon ; precise locality not specified: AMNH 231103 (1) 42.8 mm SL, 20 Nov 1975 , W. Harder. AMNH 231151 (2) 30.0, 49.2 mm SL, 1978, W. Harder. AMNH 231166 (1) 25.5 mm SL, 29 July 1975 , W. Harder. AMNH 231215 (1) 50.4 mm SL, no date, W. Harder.
Diagnosis: Stomatorhinus ivindoensis ZBK is recognized by the combination of the following characteristics: (1) very small adult size (specimens larger than 56 mm SL never observed) with most adults between 35 and 48 mm SL; (2) caudal peduncle of moderate width (caudal peduncle depth 37-45% of caudal penduncle length); (3) a small eye, 6.7- 10% of HL; (4) 8 to 9 pectoral rays and 14 to 17 dorsal fin rays; (5) first 22 to 28 scales along lateral line bearing pores, last pore-bearing scale lying posterior to a vertical through origin of anal fin, but positioned well in advance of caudal peduncle; (6) lobes of the caudal fin broad and rounded, without distinct points at apices.
Description: A very small Stomatorhinus ZBK not known to surpass 56 mm SL. Morphometric ratios and meristics presented in Table 1. Body depth and width as proportions of SL are in middle range for species of this genus. In all but gravid females, depth at origin of anal fin equal to or slightly less than depth at mid-body. Greatest width at opercles. Dorsal and ventral profiles gently convex. The head profile is slightly convex above the eye, the snout is rounded to somewhat truncate and projects slightly beyond the mouth. Mouth small. No submental swelling. Eye small, covered by epidermis. Inter-orbital distance is 72-90% of head width. Nostrils well separated with posterior nostril located near rictus of mouth. Teeth 7-8/8-9, usually 7/8, bicuspid. Head (including eye), upper back and belly covered by epidermis that is translucent in life, semi-opaque in preservation. When pressed to side, tip of pectoral fin extends well past origin of pelvic fin. Pectoral fin has 8 or 9 rays. Dorsal and anal fins roughly symmetrical above and below body axis, the anal fin (of 19-21 rays) slightly longer, with the dorsal fin (of 14-17 rays) originating at vertical through the origin of the fourth or fifth anal fin ray; the origins of the last dorsal and last anal fin rays at roughly the same vertical. Lobes of caudal fin short, very rounded and blunt. Caudal peduncle relatively short (14-16.5% of SL), of moderate depth (37-45% of length), with 12 circumpeduncular scales. First 22-28 scales along lateral line bearing pores, last pore-bearing scale lying past vertical through origin of anal fin. Total scales along longitudinal line 44-50. Scale rows between origin of pelvic fin and lateral line scale 11 to 13. Total vertebrae 36 (excluding urostyle).
Color: In life, specimens are uniformly dark chocolate brown, with little patterning in pigment along the body. Fins are translucent with dark brown pigmented rays. In alcohol, thick epidermis overlying head (including eye), upper back and belly appears opaque.
Size and sexual maturity: Review of 504 specimens in 54 lots from several localities reveals no specimens larger than 56 mm SL. Mature males are identifiable by a pronounced anal fin notch from about 35 mm SL. Gravid females are often enormously distended with eggs (e.g. female paratype shown in Fig 2D).
Distribution and ecology: All known collections of S. ivindoensis ZBK come from the upper Ivindo River (Fig. 3). Most were made in the vicinity of Makokou and the nearby field station located inside the Ipassa Plateau Reserve on the right bank of the Ivindo River.
The species is usually taken along the margin of small forest streams (Fig. 4) in root tangle where it occurs with other species of mormyrids, but has also been taken along the margin of the Ivindo River proper in deep water. Individuals are often found spaced apart at intervals of 50 cm to one meter. Diet of this species has not been studied, but is probably similar to that reported for S. polli Matthes 1964 ZBK in which stomach contents were dominated by small crustaceans and insect larvae.
Electric organ discharge: Stomatorhinus ivindoensis ZBK has a 110 to 720 microsecond duration EOD waveform with three to four peaks in the discharge (Fig. 5A-F). The EOD begins with a small head-negative phase or peak, P0, followed by a larger head-positive phase, P1, followed by the largest head-negative peak, P2. In some fish, the discharge ends with a final head-positive phase, P3. The peak spectral frequency of the Fourier Transform of the EOD is between 4000 Hz and 16,000 Hz. Average peak heights, total durations, and spectral peak frequencies for this species, are listed in Table 2. The EOD is sexually dimorphic among sexually mature individuals. Juvenile male EODs are similar to female EODs. Male EODs are longer in duration and have lower FFT peaks than females. They also differ in the heights of peaks P0 and P1 relative to the full peak to peak height compared to females (Table 2). The EOD of a juvenile of this species was reported by Heymer & Harder (1975).
Etymology: Named for the Ivindo River of Gabon.
As pointed out by Poll (1945), S. puncticulatus Boulenger 1899 ZBK and S. patrizii ZBK can be separated from other Stomatorhinus ZBK , including S. ivindoensis ZBK , by their deep bodies (depth more than 33% SL) and long, narrow caudal penduncles (caudal peduncle depth 25%- 31% of caudal peduncle length). In all other species of Stomatorhinus ZBK body depth is 33% of SL or less and caudal peduncle depth is 32% of caudal peduncle length or more. In S. ivindoensis ZBK , body depth ranges from 24-27% of SL and caudal peduncle depth ranges from 37-45% of caudal peduncle length.
Eye diameter distinguishes two Stomatorhinus ZBK species from all others: S. schoutedeni Poll 1945 ZBK has a larger eye than other Stomatorhinus ZBK (~20% of HL, ~66% of interorbital distance), while S. microps Boulenger 1898 ZBK , as its name implies, has a smaller eye than all other species (~6% of HL, ~17% of interorbital distance) that is partially obscured by overlying skin. In S. ivindoensis ZBK the eye diameter is 6.7-10% of HL and 35-44% of interorbital distance.
Members of the type series of three Stomatorhinus ZBK species - S. polylepis Boulenger 1899 ZBK , S. corneti Boulenger 1899 ZBK , and S. ater Pellegrin 1924 ZBK - are far larger than the 56 mm maximum observed adult SL of S. ivindoensis ZBK and additionally differ from the new species in counts and measures. For S. polylepis ZBK , one of the syntypes exceeds 100 mm SL and two others are above 80 mm SL. Further distinguishing this species from S. ivindoensis ZBK are its high scales counts: S. polylepis ZBK has 53 or more scales in longitudinal series and 18-22 around the caudal peduncle vs. 44-50 longitudinal scales and 12 circumpeduncular scales in S. ivindoensis ZBK . Lateral line scales bearing tubes extend onto the caudal peduncle in S. polylepis ZBK while they do not in S. ivindoensis ZBK . The holotype of S. corneti ZBK has a SL of 93.3 mm and a distinctive rectilinear, gently sloping head profile above the eye that makes an abrubt, nearly vertical downward turn above the anterior nostril; the snout is truncate and does not extend beyond the mouth and the tooth count is 9/10. In S. ivindoensis ZBK , the head profile is slightly convex above the eye, the snout is rounded and projects slightly beyond the mouth and the tooth count is modally 7/8. The S. ater ZBK type series ranges in size from 70-94 mm SL and the four examined syntypes (of six total) examined exhibit deeper heads (head depth 74.4-79% of HL) than do specimens of S. ivindoensis ZBK (head depth 59- 69% of HL). Additionally, the eye in S. ater ZBK is significantly larger than that of S. ivindoensis ZBK (12-15% of HL, vs. 6.7-10% in S. ivindoensis ZBK ) and S. ater ZBK has 10 pectoral rays compared to the 8-9 in S. ivindoensis ZBK .
Two described species of Stomatorhinus ZBK - S. humilior Boulenger 1899 ZBK and S. kununguensis Poll 1945 ZBK - are considerably more elongate than S. ivindoensis ZBK . In S. humilior ZBK the body depth above the anal fin origin is 19-21.5% of SL, in S. kununguensis ZBK it is 21-22%, while in S. ivindoensis ZBK it is 23-27 %. Members of the type series of both species exceed 70 mm SL, a significantly larger size than any known S. ivindoensis ZBK . Furthermore, S. kununguensis ZBK has larger eye than S. ivindoensis ZBK (10.5-13% of HL vs. 6.7-10% of HL).
In Tables 2-5 and in Figures 6-9, S. ivindoensis ZBK is compared with the two species it resembles most closely, S. fuliginosus ZBK and S. polli ZBK , as well as with S. walkeri , the sole Stomatorhinus ZBK species previously described from the Ogooué River basin.
S. fuliginosus ZBK , from the Itimbiri River in the Congo basin (Fig. 6 C, D), is the only described Stomatorhinus ZBK for which the entire type series is within the observed size range of S. ivindoensis ZBK and many of its proportions and counts are similar to those observed for S. ivindoensis ZBK . Harder (2000) assigned the Stomatorhinus ZBK specimens from the Ivindo basin to this species. However, S. fuliginosus ZBK differs from S. ivindoensis ZBK by having a deeper caudal peduncle (caudal peduncle depth is 50-69% of caudal peduncle length vs. 37-45% in S. ivindoensis ZBK ), a wider head (head width is 50-56% of head length vs. 43-50% in S. ivindoensis ZBK ), and a smaller inter-orbital width (57-69% of head width vs. 72-90% in S. ivindoensis ZBK ). Differences between these two species in two morphometric ratios are illustrated in Fig. 7A. Furthermore, S. fuliginosus ZBK has fewer lateral line scales with exposed tubes and pores relative to S. ivindoensis ZBK (12-21 in S. fuliginosus ZBK vs. 22-34 in S. ivindoensis ZBK ). In all eight syntypes of S. fuliginosus ZBK , the lateral line series of tubes and pores terminates midway along the side, well in advance of a vertical through the origin of the anal fin. In S. ivindoensis ZBK , the lateral line series of tubes and pores continues at least a short distance beyond a vertical through the origin of the anal fin, but always terminates well in advance of the caudal peduncle. The caudal fins of all eight S. fuliginosus ZBK syntypes are damaged to one degree or another. Poll (1941) describes the lobes of the caudal fin of S. fuliginosus ZBK as “obtuse,” however, examination of the S. fuliginosus ZBK syntypes and the illustration of the holotype indicates that the lobes of the caudal fin are narrower than those observed in S. ivindoensis ZBK specimens. No EOD information is available for S. fuliginosus ZBK . Taken together, we view the differences between the Ivindo basin Stomatorhinus ZBK specimens and the type series of S. fuliginosus ZBK as sufficient to recognize the two as distinct species.
It remains unclear whether S. fuliginosus ZBK is actually as small a species as S. ivindoensis ZBK . Four of the eight S. fuliginosus ZBK syntypes are within the size range of sexually mature individuals of S. ivindoensis ZBK , yet none show the characteristic anal fin notch of males, nor abdominal distension from eggs (gonads were not examined). It is thus possible that all the syntypes of S. fuliginosus ZBK are juveniles and considerably smaller than adult size. (No nontype specimens from near the type locality are available for to ascertain the size range of adults.) If, as it seems likely, adults of S. fuliginosus ZBK achieve larger sizes than adults of S. ivindoensis ZBK , S. ivindoensis ZBK is the smallest known mormyrid species.
S. polli ZBK from the Tshuapa River in the Middle Congo basin (Fig. 6E, F) is somewhat larger than S. ivindoensis ZBK : the largest in the type series measures 63.5 mm SL and six of the eight others exceed the greatest size observed for S. ivindoensis ZBK . The depth to width ratio of this species is approximately the same as that for S. ivindoensis ZBK , as are scale counts and the number of scales bearing lateral line tubes and pores. As in S. ivindoensis ZBK , the lobes of the caudal fin are broad and rounded, lacking distinct points at the apices. However, this species differs from S. ivindoensis ZBK in having a shorter snout relative to body length (7- 7.6% of SL vs. 7.8-8.4% in S ivindoensis ZBK ), a larger eye (11-13.4% of HL vs. 6.7-10% in S. ivindoensis ZBK ) and a smaller inter-orbital width as a proportion of head width (53-61.5% vs. 72-90.4% in S. ivindoensis ZBK ). Additionally, S. polli ZBK has a narrower head (head width 59- 64% of HL to end of opercle) than S. ivindoensis ZBK (head width 66-78% of HL to end of opercle). Differences between these two species in two morphometric ratios are illustrated in Fig. 7B.
Stomatorhinus walkeri (Fig 6A, B) is the type species of the genus and is, apart from the species described here, the only other Stomatorhinus ZBK species described from and known to occur within the Ogooué River basin. It is a considerably larger species than S. ivindoensis ZBK , with individuals exceeding a length of 90 mm SL. While the ranges of many morphometric ratios overlap those for S. ivindoensis ZBK , the pre-pectoral distance is a smaller proportion of SL (26.8-30.6%) than in S. ivindoensis ZBK (30.6-33.4%). The lobes of the caudal fin are more narrow and pointed at the apices than those of S. ivindoensis ZBK . Counts easily separate the two species. Stomatorhinus walkeri have 10 pectoral rays; S. ivindoensis ZBK have no more than 9. Stomatorhinus walkeri have 37 to 39 total vertebrae; S. ivindoensis ZBK have 36. S. walkeri have lateral line scales with exposed tubes and pores that extend onto the base of the caudal peduncle. In S. ivindoensis ZBK lateral line scales with exposed tubes and pores terminate well in advance of the caudal penduncle. Furthermore, the body coloration in life of S. walkeri is light to medium gray with a purplish cast in daylight (see Fig. 6B) whereas S. ivindoensis ZBK are always a dark chocolate brown.
In a principal components analysis of the correlation matrix of 24 log-transformed measures, the type specimens of S. ivindoensis ZBK form a coherent cluster on a plot of the second and third factors that is non-overlapping with the type specimens of S. fuliginosus ZBK , S. polli ZBK , S. walkeri , S. corneti ZBK and S. ater ZBK (Fig. 8). We conclude that the significant morphological differences between Stomatorhinus ZBK specimens from the Ivindo River basin of Gabon and all other described species in this genus warrant its recognition as a new species.
The addition of this Ivindo River-endemic Stomatorhinus ZBK further highlights the distinctiveness of this river’s fish fauna, already notable for species of mormyrids (Sullivan et al. 2002), gonorhynchiforms ( Géry 1964), cyprinodontiforms (Wildekamp 1993), and cichlids (Thys van den Audenaerde 1966) either endemic or shared only with the Ntem River basin to the north.
Comparison of electric signals. The EOD of S. ivindoensis ZBK is similar to that of S. walkeri from the Ogooué River basin of Gabon. The latter species has a shorter duration EOD that measures 70 to 270 microseconds with an average spectral frequency peak at 17,355 Hz (Fig. 9A-D). We found no differences between the sexes or different ageclasses of S. walkeri comparing EODs from eight sexually mature females with six sexually mature males and with seven juveniles of 34-99 mm SL, although all samples were taken during the dry season when most mormyrids are not breeding. The EOD of S. walkeri is characterized by two head-positive peaks of about the same amplitude (23% of peak to peak height), separating a single large head negative peak. A weak head-negative phase preceding P1 occurs in some individuals. Table 2 summarizes descriptive statistics on EODs from 14 adult individuals. Although similar, EODs of Stomatorhinus walkeri differ from those of S. ivindoensis ZBK by having a shorter waveform duration, higher peak spectral frequency, and in the heights of peak P0 (Scheffe’s post-hoc comparison).
EOD recordings have been made for only two other species of Stomatorhinus ZBK that we can confidently assign to described species. These are S. ater ZBK and S. patrizii ZBK . These specimens were collected in Odzala Park in the Republic of Congo in 2002 by Drs. John Sullivan, John Friel, and SébastienLavoué . Five individuals of S. ater ZBK , ranging in size from 57.8 to 83.5 mm SL were recorded (Fig. 9 E, F). The mean EOD duration is 69.3 microseconds± 10.0 (std. dev.) and the power spectrum peak occurred at 26,562 Hz ± 3,945. The EOD waveform has four peaks, beginning with a small, head-negative peak of low amplitude (4.5% of the peak to peak height). Thirteen individuals of S. patrizii ZBK , ranging in size from 35 to 62 mm SL, were recorded (Fig. 9 G, H). The mean EOD duration is 88.4 microseconds ± 17.2 (std. dev.) and the power spectrum peak is 36,425 Hz ± 8,706. The EOD waveform is complex, with as many as six peaks in many of the recordings.