Comprehensive DescriptionRead full entry
(Figs. 1-3, Table 1)
Holotype : CU 89005, male, 44.1 mm SL; Gabon , Haut-Ogôoué Province , Ogôoué River at and below the Rapids of Massoukou (Masuku), 1°39’30”S13°32’14”E ; M.E. Arnegard, A. Chow, S. Lavoué , J.F. Liwouwou and J.P. Sullivan, August 15, 1999 .
Paratypes : CU 89006, two specimens, one male, 45.9 mm SL, and one female, 40.4 mm SL; collection data as for holotype . MRAC A4-13-P-1, one male, 40.4 mm SL; collection data as for holotype , SAIAB 74202, one male, 40.5 mm SL; collection data as for holotype . CU 80105, eight specimens, three males, 30.0-31.4 mm SL, three females, 34.5-42.3 mm SL, and two unsexed juveniles, 27.0-28.7 mm SL; Gabon , Ngounié Province , Louétsi River just below falls at Bongolo Hydroelectric Facility, 2°14’2”S11°27’42”E ; M.E. Arnegard, J. Beck, C.D. Hopkins and J.P. Sullivan, July 20, 1999 . AMNH 236128, four specimens, one male, 46.4 mm SL, and three females, 30.2-38.4 mm SL; collection data as for CU 80105 .
Diagnosis: Synodontis acanthoperca ZBK is a relatively small species that can be distinguished from all congeners by a distinctive pigmentation pattern that includes a pair of dark patches on the caudal fin. One patch is present in the middle of each lobe of the fin (Figs. 1 & 3). Furthermore, this species is distinguished by the presence of a welldeveloped opercular spine in sexually mature males (Figs. 1, 2A & 3). In all other mochokid species the opercle lacks such well-developed ornamentation and sexual dimorphism of the opercle is unknown.
Description: Dorsal, lateral and ventral views in Figure 1 illustrate body shape, form and position of fins and barbels. Morphometric and meristic data for holotype and 4 syntopic paratypes are summarized in Table 1.
Small species with a maximum standard length <50 mm. Body compressed. Predorsal profile gently convex; postdorsal body sloping gently ventrally. Preanal profile horizontal. Anus and urogenital opening located at vertical though origin of adipose fin. Skin on body smooth without any enlarged tubercles, lateral line complete and midlateral along side of body.
Head depressed and broad, rounded when viewed laterally with a rounded snout margin when viewed dorsally. Gill opening restricted to lateral aspect of head from level of the base of pectoral spine dorsally to level of the ventral margin of the eye. Gill membranes broadly united to, and attached across the isthmus, supported by 6 branchiostegal rays. Bony elements of skull roof lack any superficial ornamentation. Skin covering skull roof with numerous small unculiferous tubercles. Occipital-nuchal shield large and terminating posteriorly with two rounded processes on each side of dorsal fin.
Each opercle with a single posterolaterally directed spine variously developed depending on gender and size of specimen. In male specimens>30 mm SL, the spine is hypertrophied and ranges from 2.0-2.5 mm long (Fig. 2A). In adult female specimens it measures <1 mm long, but is usually visible protruding from the skin surface (Fig. 2B). In immature specimens of both sexes <30 mm SL the opercular spine may not be distinguishable beneath the overlying skin.
Barbels in three pairs. Maxillary barbel long, slender and unbranched, extending to just beyond base of last pectoral-fin ray. No basal membrane present on maxillary barbel. Mandibular barbels originate immediately posterior to lower lip in a transverse row; with primary branches; secondary branches absent. Inner mandibular barbels origin close to midline, extending to just beyond anterior margin of pectoral girdle; with 2-5 paired branches proximally and 2-4 longer unpaired branches distally. Outer mandibular barbels originate lateral to inner mandibular barbels, extending to posterior margin of pectoral girdle; with 4-6 elongate unpaired branches.
Eyes large and ovoid, horizontal axis longest; approximately two thirds of interorbital distance. Orbit with a free margin. Anterior nostrils slightly closer together than posterior nostrils. Anterior nostrils tubular with a short raised rim. Posterior nostrils with elevated flaps along anterior margin.
Mouth inferior and crescent shaped; lips plicate. All teeth unicuspid. Premaxillae tooth plate wide with broad ventral shelf. Primary, secondary and tertiary premaxillary teeth discrete. Primary teeth 15-17 in a single row. Secondary teeth 37-49 in irregular rows. Tertiary teeth 16-18 in a single row. Mandibular teeth 14-16, in a single row; concentrated at midline; strongly recurved.
Dorsal fin located at anterior third of body. Dorsal fin with spinelet, spine and 7 rays; fin membrane not adnate with body. Dorsal-fin spine long and slightly curved; smooth along anterior margin; small serrations on posterior margin. Adipose fin well developed; margin convex. Caudal fin forked, count i,7,8,i. Procurrent caudal-fin rays symmetrical and extend only slightly anterior to fin base. Anal-fin base located ventral to adipose fin; margin convex. Anal-fin count iii, 8. Pelvic-fin origin at vertical ventral to posterior end of dorsal fin base. Pelvic-fin margins convex, tip of appressed fin just reaches anal-fin origin. Pelvic-fin count i,5-6. Pectoral-fin count I,7-8. Pectoral fin with slightly curved, stout spine bearing large serrations. Anterior spine margin with 8-17 distally directed serrations along entire length of spine. Posterior spine margin with 8-13 proximally directed serrations along the entire length.
Cleithral process elongated, narrow with a distinct lateral ridge, terminating in a sharp point. Surface concave dorsal to its lateral ridge, without bony ornamentation on its surface. Slit-like axillary pore present just ventral to cleithral process.
Coloration: The preserved holotype and syntopic paratypes were kept in a darkened aquarium prior to fixation and display a darker coloration than was observed when they were collected or is seen in other specimens fixed immediately after collection. Thus it is more difficult to see the contrast between light and dark areas on the head and body of these specimens (Figs. 1-2). A composite illustration is presented that depicts the bold pigmentation pattern typically observed in this species (Fig. 3).
In dorsal view, specimens appear banded with six dark patches over a lighter colored background. Area between the nares dark brown. Maxillary barbels dark brown to black. Mandibular barbels cream colored. Head with a light band between eyes and a light transverse collar at the level of the opercular spine, band lighter ventrally. Large dark spot posterior to lighter collar continuous with pigmentation at base of dorsal fin and dorsal spine; extending posteriorly to level of pelvic fins. A small dark dorsal spot posterior to dorsal fin. A small dark ventral spot posterior to pelvic fins. A large dark patch from the base of the adipose fin extends ventrally to the base of the anal-fin rays. Large dark spot covering most of the caudal peduncle.
Ventral surface cream colored anteriorly with a series of thick, somewhat indistinct, dark bands separated by cream colored or lighter brown bands posteriorly. Darker bands are found: just anterior to the pelvic fins in v-shape (chevron); from slightly beyond posterior margin of pelvic fins to anus; along entire base of anal fin; and from narrowest point along caudal peduncle to base of caudal fin.
Dorsal and pectoral spine very dark; rays blackish against translucent, milky membranes; faint blackish band near margin in dorsal fin. Pelvic and anal fins milky with a diffuse blackish band approximately midway along the rays. Adipose fin black basally with wide cream-colored band at distal margin. Caudal fin milky colored with a pair of prominent black spots in the dorsal and ventral lobes just anterior to the fork of the fin.
Etymology: The specific name is a Latinized combination of the Greek acantha, meaning a thorn and the Latin opercul, meaning a cover or lid, alluding to the distinctive opercular spines developed by mature males in this species. Gender: feminine.
Distribution: This species is known from two sites within the Ogôoué River basin of Gabon. The type locality is the Rapids of Massoukou (Masuku) on the Ogôoué River near Franceville and a second population has been collected in rapids on the Louétsi River near Bongolo (Fig. 4). Given the significant river distance between these sites (> 500 km), we speculate that S. acanthoperca ZBK is widely distributed in the upper Ogôoué basin wherever similar rapid habitats exist.
This new species is among the smallest known Synodontis ZBK species and has a distinctive pigmentation pattern. We initially considered that these specimens might be conspecific with S. marmoratus Loennberg 1895 ZBK described from the Mémé River of Cameroon and later illustrated by Poll (1971). This is another small Synodontis ZBK species with a similar body pigmentation consisting of contrasting dark patches on a light background. However, S. marmoratus ZBK does not possess a distinctive pair of spots on the caudal fin, lacks any evidence of opercular spines in similar sized specimens and also differs in head, barbel and body proportions from S. acanthoperca ZBK .
While the well-developed opercular spines in S. acanthoperca ZBK are unique in mochokids, bony spines on structures other than the dorsal and pectoral spines have been reported in several other mochokid taxa including some Synodontis ZBK species. The bestknown example is the spiny ornamentation on the humeral processes in three larger Synodontis ZBK species. S. bugetti Boulenger 1911 and S. omias Guenther 1864 ZBK , both from the Niger basin, have up to three spines developed along the ventral portion of the humeral process. A more extreme condition is seen in S. acanthomias Boulenger 1899 ZBK , from the Congo basin. This species develops multiple spines that can completely cover the posterior portion of the humeral process. In this species the number and size of these spines increases with body size and large specimens (>400 mm SL) may have up to 20 welldeveloped spines (personal observations of the authors).
Another type of spiny ornamentation of the pectoral girdle is seen in Acanthocleithron chapini Nichols & Griscom 1917 ZBK . As the genus name implies, this small mochokid from the Congo basin bears one or more small bony spines just above and anterior to the pectoral spine. Furthermore, while reexamining the types series of this species (AMNH 6548, 6552, 6575 & 6681), we discovered that the skull roof bears numerous minute bony spines that give it a prickly texture. This skull ornamentation has not previously been reported and is distinct from the rough surface texture observed on the skull roofs of some mochokids.
Although not as apparent as in S. acanthoperca ZBK , the opercle is weakly ornamented in two other mochokid taxa, Brachysynodontis batensoda ( Rueppell 1832) found in the Nile, Chad, Niger, Sénégal , and Gambia basins, and Hemisynodontis membranaceus (Geoffroy Saint-Hilaire 1809) found in the Nile, Chad, Niger, Sénégal , Gambia and Volta basins. In both species the margin of the opercle bears a comb-like series of small spines at its posterior apex (Taverne & Aloulou-Triki 1974). In Hemisynodontis ZBK the spines are more numerous, but are so small as not be visible through the skin. Whereas in Brachysynodontis ZBK , the spines are fewer, and large enough to be visible externally, yet never reach beyond the opercular membrane. In neither species are the series of opercular spines known to be sexually dimorphic and we do not consider this ornamentation to be homologous with the opercular ornamentation observed in S. acanthoperca ZBK .
Sexual dimorphism has been reported in about half of all families of catfishes (Rapp Py-Daniel & Cox Fernandes 2005). Among African catfishes, sexual dimorphism is probably most widespread and strikingly apparent in the Mochokidae. Well known examples include the elongate anal and caudal fins found in adult males of many Chiloglanis ZBK species (Roberts 1989; Seegers 1996). In addition, enlarged humeral processes have been reported in males of some Chiloglanis ZBK species (Roberts 1989). In such cases, the humeral process is hypertrophied and may protrude away from the body axis, but is not ornamented with any bony spines or dermal tubercles.
Many mochokid species have rough skins that bear unculiferous tubercles and sexual dimorphism of this feature has been reported in Microsynodontis ZBK (Ng 2004). Males in several Microsynodontis ZBK species have denser aggregations of tubercles on the dorsal and lateral surfaces of their heads than females, especially along the area between the snout and the preopercle.
As previously mentioned the skull roof of Acanthocleithron ZBK is covered with numerous small spines and the cleithral region directly anterodorsal to the pectoral spine bears one or more larger spines. Sexual dimorphism may exist in both of these bony features, as they appear to be best developed in males among the type series examined.
In summary, neither spiny ornamentation nor sexual dimorphism appears to be uncommon in mochokid catfishes. However, an association between these features has not previously been reported in any members of this family. Thus the new species described here represents the first documented case in mochokids in which such ornamentation and sexual dimorphism are clearly linked. A similar relationship may also occur in Acanthocleithron ZBK , and we would not be surprised if additional cases are discovered in the future within the Mochokidae.