Skeletal morphology of the holotype of Gymnallabes nops Roberts & Stewart, 1976, using micro CT-scanning by Stijn DEVAERE (1), Dominique ADRIAENS (1), Guy G. TEUGELS† (2), Nora M. DE CLERCK (3) & Andrei A. POSTNOV (3, 4) AbSTRACT. - One of the major problems in the anguilliform Clariidae is the taxonomical validity and the systematic position of Gymnallabes nops. This vagueness is largely due to the fact that the description of this species is based on the holotype only, and no additional specimens are known. This also means that past studies on this holotype were limited to non-invasive research, such as external morphology and X-rays. These methods, however, yield only a limited number of valuable characters. In this study a high-resolution desktop X-ray microtomography instrument (CT-scan) was used. This enables us to confirm the results of the radiographies but also to perform a detailed osteological study, without damaging the holotype. The osteological survey showed similarities with other anguilliform clariids, in particular with Platyallabes tihoni and Gymnallabes typus. Besides these traits, G. nops shows a set of characters, revealed in this study, unique among the clariids, such as a reduction of the infraorbital bones in number and size, a large anterior outgrowth of the opercle, a clearly visible epiphyseal bridge and the typical positioning of entopterygoid and metapterygoid bones. These extra results may help to clarify the validity and the phylogenetic position of G. nops. RéSumé. - Morphologie squelettique de l’holotype de Gymnallabes nops Roberts & Stewart, 1976, étudiée par tomoden- sitométrie à haute résolution. Un des plus grands problèmes chez les Clariidae anguilliformes est la validité taxinomique et la position systématique de Gymnallabes nops. Cette situation est largement due au fait que la description de cette espèce n’est fondée que sur l’ho- lotype. Cela signifie également que les études réalisées sur celui-ci sont limitées aux études non invasives, comme la mor- phologie extérieure et l’usage des radiographies. Ces méthodes ne produisent que des données valables mais limitées. Dans cette étude, un scanner rayons X à haute résolution (CT-scan) a été utilisé, permettant de confirmer les résultats des radio- graphies, mais aussi d’accomplir une étude ostéologique détaillée, sans abîmer l’holotype. Cette étude ostéologique montre les similarités existant avec d’autres Clariidae anguilliformes, en particulier avec Platyallabes tihoni et Gymnallabes typus. En plus de ces caractères, Gymnallabes nops montre un ensemble de caractères uniques, comme par exemple une réduction de la taille et du nombre des osselets infraorbitaires, un operculaire avec une grande expansion antérieure, un pont épiphy- saire non couvert par les plaques frontales et la mise en place typique des osselets entoptérygoïde et métaptérygoïde. Ces résultats supplémentaires peuvent aider à clarifier la validité et la position phylogénétiques de cette espèce. Key words. - Siluriformes - Clariidae - Gymnallabes nops - Holotype - skeleton - X-ray scanning. The freshwater Clariidae is one of the 37 catfish families bels, and especially by the unique presence of a supra- known within the Siluriformes order (Sabaj et al., 2004). branchial organ, formed by arborescent structures from the Their diversity is the largest on the African continent. second and fourth gill arches (Greenwood, 1961; Teugels Besides in Africa, they can also be found in Syria, southern and Adriaens, 2003). Turkey and some parts of Southeast Asia (Teugels, 2003). The anatomy of the anguilliform clariids is poorly Although, some of the more generalized, fusiform species, known. Only a few studies have superficially described the in particular Clarias gariepinus (Burchell, 1922), show a cranial morphology into detail (Poll, 1957, 1977; Cabuy et large distribution, the occupation of the anguilliform species al., 1999; Devaere et al., 2001). This is certainly the case for is a more specialized, burrowing niche (Adriaens et al., Gymnallabes nops, on which, to this moment, no morpho- 2001). They occur only in Equatorial Central and West Afri- logical research has been done. The fact that the holotype is ca (Boulenger, 1911; Poll, 1957; Teugels et al., 1990; Teu- the only found representative of G. nops can be considered gels, 2003). as the main reason for this deficiency, as this allows only a Clariid catfishes are characterized by an elongate body, narrow field of non-invasive studies that can be performed long dorsal and anal fins, the presence of four pairs of bar- on such unique material. Thanks to the use of computerized (1) Ghent University, Evolutionary Morphology of Vertebrates, K.L. Ledeganckstraat 35, B-9000 Ghent, BELGIUM. [[email protected]] (2) Africa Museum, Ichthyology Department, B-3080 Tervuren, BELGIUM and KULeuven, Section for Ecology and Systematics, B-3000 Leuven, BELGIUM. (3) University of Antwerp (RUCA), Microtomography, Department of Biomedical Sciences, Groenerborgerlaan 171, B-2018 Antwerpen, BELGIUM. (4) University of Antwerp (RUCA), Department of Physics, Groenerborgerlaan 171, B-2018 Antwerpen, BELGIUM. Cybium 2005, 29(3): 281-293. Skeletal morphology of the holotype of Gymnallabes nops Devaere et al. microtomography (mCT), we obtained 3D information of Falls, BMNH 1889.11.20.5, 1 ex.; New Antwerp, Upper Congo, the complete skeleton in a non-destructive manner. The BMNH 1899.2.20.16, 1 ex.; Siala-Ntoto Swamps, BMNH 99.11.27.92, 1 ex.; Bangyville, Ubangi, BMNH 1907.12.26.34, 1 objectives of this paper are therefore: (1) to give a detailed ex.; Kashi, Lulua, MHNG 1248.3, 1 ex.; Banana, NMW 47240- description of the cranium and the most relevant post-cranial 42; Mollunda, NMW 47245, 4 ex., NMW 47246, 1 ex. Congo: structures; (2) to use this new information for the compari- Yangala Youbi, MNHN 1967-0146, 1 ex.; Djembo, Kouilou, son with other anguilliform clariids; (3) to describe G. nops MNHN 1967-0147, 1 ex.; Cayo, MNHN 1989-0527, 1 ex.; Riv. based on characters, found in this study and (4) to provide Nanga, between Boukou-Zassi and Kouilou swamp area, MRAC diagnostic characters for this species. 90-57-P2315, 1 ex.; Sintou, Riv. Kibombo, Kouilou, MNHN 1967-0144, 1 ex.; Riv. Loadjili, Songolo, Pointe Noire, MNHN 1967-0145, 6 ex.; Angola: Caungula, Mabete, Riv. Uamba, MRAC 162088, 1 ex.; Riv. Camuconda, Tchimenji, MRAC mATERIALS AND mETHODS 162089, 162090-094, 5 ex., 162095-100, 6 ex.; Riv Ganga-Lud- chimo, MRAC162083-086, 4 ex. material examined Dolichallabes microphthalmus. - Dem. Rep. Congo: Kunungu, MRAC 44655, adult male, 229 mm SL (holotype), MRAC 44656- The holotype of Gymnallabes nops Roberts & Stewart, 659, 3 ex. (196-210 mm SL) and 62407, 1 ex., 188 mm SL (para- 1976 (MCZ 50.298) (Fig. 1) was obtained from the Museum types), MRAC 57662, 1 ex., 196 mm SL, MRAC 18850, 1 ex., 90 of comparative Zoology, Harvard University (MCZ). The mm SL; Boende swamps, MRAC 101843, 1 ex., 149 mm SL, specimen was collected in Tadi, the lower Congo-River 176123-124, 1 ex., 68 mm SL; Bokuma, MRAC 79093, 1 ex., 134 basin (5°14’S, 13°56’E). The sample site is a 2 m deep, mm SL, 93774, 1 ex., 66 mm SL; Bokuma - Tchuapa, MRAC 79258-260, 3 ex. (85-126 mm SL); Ndwa (Boloko), MRAC 78808- backwater with boulders, with a silty or sandy bottom (Rob- 810, 3 ex. (99-110 mm SL); Inonge, MRAC 96672, 1 ex., 110 mm erts and Stewart, 1976). SL; Maylimbe, Tshela, MRAC 66721, 1 ex., 97 mm SL. Gymnallabes alvarezi. - Rep. Congo: Zanaga, Lésala, MRAC Comparative material examined 8-22-P-1047-050, 1 ex., 141 mm SL. Museum abbreviations are listed in Leviton et al. (1985). Gymnallabes typus. - Nigeria: Old Calabar, BMNH 1866.12.4, Channallabes apus. - Angola: Ambriz, BMNH 1873.7.28.16 2 ex. (Syntypes); Umu-Eze Amambra, MRAC 84-16-P-1-2, 1 ex.; (holotype). Dem. Rep. Congo: Bokalakala, MRAC 175247-270, Riv. Sombreiro, East of Erema, MRAC 91-067-P0134, 1 ex.; Niger 10 ex.; Kinshasa, MRAC 97-056-P-0001-0003, 2 ex.; Bumba, Delta, MRAC 97-030-P-0001-0010, 10 ex.; lake Odediginni, Agu- MRAC 88-25-P-2192-227, 36 ex.; Boma, MRAC 939, 1 ex.; Riv. dama, Yenagoa, MRAC 92-083-P-0035-0036, 1 ex.; Okaka, Epie Lula, Bushimaie, MRAC 1535051 ex.; Kelé, MRAC 1491, 1 ex.; Creek, Between Nun an Rashi Riv, MRAC 97-085-P-0001-0004, 4 Stanleyville, MRAC 30893-30900, 8 ex., MRAC 88-01-P-1976- ex.; Riv. Sombreiro, Odiemerenyi, Ahoada, MRAC 91-067-P- 1992, 17 ex.; Riv. Ruki, Eala, MRAC 14747-49, 3 ex.; Lake 0135-0136, 1 ex.; New Calabar, Choba, MRAC 91-105-P-1, 1 ex.; Tumba swamp area, MRAC 46299, 1 ex.; Katanga, MRAC 39480, Rumuji Swamps, MRAC 86-10-P-72, 1 ex.; Oshika, MRAC 84-28- 1 ex.; Riv. Botota, Keseki, MRAC 67763-77, 15 ex.; Mwilam- P-28, 1 ex., MRAC 84-28-P-25, 1 ex.; Riv. Cron, Itu, MRAC bongo, MRAC 72886-887, 2 ex.; Dekese, Riv. Lofu, Anga, 88-36-P-10, 1 ex.; Between Sapele and War, Niger Delta, MRAC MRAC 153352, 1 ex.; Yangambi, MRAC 68700,1 ex.; Riv. 74-29-P-600, 1 ex.; Muoha, New Calabar, MRAC 91-10-P-478, 1 Oubangui, Imfondo, MNHN 1922-0029,1 ex.; Loango, MNHN ex.; Biseni, Taylor Creek, MRAC 91-01-P278, 1 ex.; Ossomari, 1924-0079, 1 ex.; MNHN 1924-0080, 1 ex.; Sangha, MNHN BMNH 1902.11.10.119, 1 ex. Cameroun: Riv. Kom, Ntem, Abou- 1925-0137, 1 ex.; Mogende, MNHN 1926-0155-59, 5 ex.; Riv lou, MRAC 73-18-P-3307-309, 1 ex. Congo, MNHN, 1937-0124-25, 1 ex.; Stanleypool, Bamu, MNHN Platyclarias machadoi. - Angola: Cuango, Cafunfo, Riv. Borio, 1958-0111, 1 ex.; Boloko, Riv. Likouala, MNHN 1962-0401, 7 MRAC 78-6-P-1345, adult female, 181 mm SL (holotype), 78-6-P- ex.; Mossaka, Riv. Likouala, MNHN 1963-0402, 2 ex.; Riv. Load- 1346-1367, 21 ex., (76.0-146 mm SL). jili, Songolo, MNHN 1967-0143, 6 ex.; Mangala, BMNH Platyallabes tihoni. - Dem. Rep. Congo: Kingabwa, Stanley 1896.3.9.17, 1 ex.; Riv. Lebuzi, Kaka Muno, BMNH 1912.4.1411- pool, MRAC 13307 (Holotype); Kinsuka, MRAC 73-68-P-143, 1 12, 2 ex.; Lower Congo, BMNH 1887.1.13.8-9, 2 ex.; Stanley ex., 138698-699, 2 ex., 125345-349, 4 ex., 73-22-P-3127, 3 ex.; Figure 1. - A: Holotype of Gymnallabes nops (56.57 mm SL) (MCZ 50298); b: Lateral side; C: Dorsal side; D: Ventral side of head. (Scale bars = 5 mm). (Photos: S. Devaere). [A : Holotype de Gymnallabes nops ; B : Vue latérale ; C : Vue dorsale ; D : Vue ventrale de la tête. (Échelles = 5 mm). (Photos : S. Devaere).] 282 Cybium 2005, 29(3) Devaere et al. Skeletal morphology of the holotype of Gymnallabes nops Bulu, Luozi, BMNH 1976.5.21.30-39, 9 ex., MCZ 50239, 13 ex.; nostril interdistance (ANID); posterior nostril interdistance Inga, 88947, 50537, 15 ex.; Tadi, Kibunzi, MCZ 50297, 5 ex. (PNID); rostral skull width (RSkW); orbital skull width (OskW); skull height (SkH); eye diameter (ED); snout height measurements (SnH); prehyoid length (PhL); internal mandibular interdis- All measurements were made on the holotype using a tance (ImnID); external mandibular interdistance (EmnID); high-resolution desktop X-ray microtomography instrument mouth width (MW) and skull roof width (SkR). The follow- [Skyscan-1072, Belgium (www.skyscan.be)]. A Tungsten ing meristic counts were made, using the radiographies made air-cooled micro-focus X-rays tube with 9-µm spot size and with a Bennett X-ray (25-50kV, 25-150 Ma, 1/30-20 sec, a maximum voltage of 80 kV was used as a source. A 12-bit focus distance: 0.3-0.6 m): total number of vertebrae (TV), 1 mega-pixel low-noise CCD camera was applied as a detec- number of ribs (RB). tor. During acquisition both the X-ray source and the camera We compared the holotype with a large sample of four remained motionless while the object was rotated around its other anguilliform species; Platyallabes tihoni (Poll, 1944), vertical axis. The system allows achieving 9-micron resolu- Gymnallabes typus Günther, 1867, Dolichallabes tion but measured resolution depends on the size of the microphthalmus Poll, 1942, Channallabes apus (Günther, object and on the contrast. Biggest field of view possible for 1873) and Platyclarias machadoi Poll, 1977. this device was 20 mm. The investigated specimen was too big for one acquisition so it was scanned 5 times and then Analyses reconstructed separately and assembled in one model after- The morphometric data and the meristic counts were wards. As our fish was fixated for a time in a decalcifying submitted to a Principle Component Analysis (Bookstein et medium, the observed contrast was much lower than for al., 1985). Morphometric data were log-transformed, so that ordinary fresh fish bones. That constrained us to use lower the non-normality effects could be minimized before the energies to improve contrast for soft tissues. The sample was PCA was run on the covariance matrix. The first principle scanned with 40 kV X-ray tube voltage, 0.9-degree rotation component was not used since it is considered as a size fac- step and approximately 10 sec exposure time per individual tor, the other components as shape factors, independent of shadow projection. After the shadow images were collected size (Teugels et al., 1999). Different combinations of com- they were reconstructed into virtual cross-section using ponents were used to give the plot that expressed the most Feldkamp cone-beam algorithm. These cross-sections are variation. the analogues of histological slices. The advantage is that the specimen remained intact although the resolution is lower than in classical histology. Another important advantage is that micro-tomography provides isotropic resolution thus RESuLTS allowing to build 3D-models (see following illustrations). Neurocranium 3D models presented in this paper were created with ANT Ethmoid region. - The nasal bone (Figs 2, 3), covering software supplied by Skyscan with the instrument. (ANT the nasal sac and enclosing the anterior-most part of the software, Skyscan, Belgium) (Postnov et al., 2002; De Cler- supraorbital canal, is reduced to a partially open, elongated ck et al., 2003). bone, with little to no lateral plate extensions present. The Furthermore, a set of 33 metric measurements were taken point-to-point using digital callipers to 0.1 mm (Digital ruler, supraorbital canal appears to split in this bone, as a lateral Mauser), interfaced directly with a computer. Measurements pore can be distinguished. terminology follows that of Devaere et al. (2004): total The mesethmoid (Figs 2, 3A) shows a substantial con- length (TL); standard length (SL); preanal length (PaL); anal striction caudally from the two rostral wings. It is sutured to fin length (AFL); dorsal fin length (DFL); prepelvic length the frontals, by means of large interdigitations. The main (PPvL); prepectoral length (PPcL); predorsal length (PdL); part of the lateral ethmoid is covered by the frontal. Ventral- distance between the occipital process and the dorsal fin ly, the mesethmoid supports the premaxillary bones. (SPDFL); pelvic fin length (PvFL), pectoral fin length Although very restricted, the mesethmoid makes contact (PcFL); pectoral spine length (PcSL); caudal peduncle depth with the anterior fontanel. The lateral ethmoid shows a long, (CPD); body depth at anus (ABD); maxillary barbel length pointed lateral process (Figs 2, 3). Anteriorly, the supraor- (MxB); external mandibular barbel length (EmnB); internal bital canal leaves the neurocranium where the mesethmoid mandibular barbel length (ImnB); nasal barbel length (NB); and lateral ethmoid suture. At this point the supraorbital interpelvic distance (IpvD); interpectoral distance (IpcD); canal is covered by the rostral extension of the frontal. Since skull length (SkL); postorbital length (PoL); skull width there is no second infraorbital bone (see below), the lateral (SkW); supraoccipital process length (SpL); supraoccipital ethmoid shows only an articulation surface for the autopala- process width (SpW); interorbital distance (IoD); anterior tine. The arrow-shaped prevomer interconnects with the Cybium 2005, 29(3) 283 Skeletal morphology of the holotype of Gymnallabes nops Devaere et al. Figure 2. - Dorsal view of the skull of Gymnallabes nops (56.57mm SL) (MCZ 50298). ant: os antorbitale; apal: os au topalatinum; cl: os cleithrum; eoc: os exoccipitale; fr: os frontale; io-IV: os infraorbitale IV; leth: os latero-ethmoi- deum; meth: os mesethmoideum; mnd: mandibula; mx: os maxillare; ns: os nasale; op: os operculare; par-soc: os parieto-supraoccipitale; pp-v4: parapo- physis of vertebra 4; prmx: os premax- illare; pt: os pteroticum; p-trc: transs- capular process; pt-scl: os posttemporo- supracleithrum; pvm: os prae vomerale; sph: os sphenoticum; spop: os suprapraeoperculare. [Vue dorsale du crâne de Gymnallabes nops. ant : antorbital ; apal : autopalatin ; cl : cleithrum ; eoc : exoccipital ; fr : fron- tal ; io-IV : infraorbital IV ; leth : eth- moide latéral ; meth : méseth moi de ; mnd : mandibulaire ; mx : ma xi l laire ; ns : nasal ; op : operculaire ; par-soc : pariéto-supraoccipital ; pp-v4 : parapo physe de la quatrième ver tèbre ; prmx : prémaxillaire ; pt : ptérotique ; p-trc : processus transscapulaire ; pt- scl : posttemporo-supracleithrum ; pvm : prévomer ; sph : sphénotique ; spop : suprapréoperculaire.] parasphenoid through one short interdigitating spine. It car- orbitosphenoid and pterosphenoid (Figs 3A, 3B, 4), which ries one tooth plate, which shows a constriction in the middle are ventrally connected to the parasphenoid (Fig. 3B). This (Fig. 10). latter bone runs from the rostral end of the orbital region up Orbital region. - The circumorbital series is highly to the occipital region; it bears one elongated process. reduced, both in shape and the number of bones. Of the Temporal region. - The skull roof in this region is formed whole set of five bones present in most other clariids (Cabuy by the frontals, the sphenotic and the pterotic (Fig. 2). The et al., 1999; Devaere et al., 2001), only the infraorbital IV sphenotic (Figs 2, 3A) interdigitates rostro-medially with the and the antorbital are present in Gymnallabes nops (Figs 2, frontal and caudally with the pterotic. The pterotic (Figs 2, 3A) [for discussion on the nomenclature of the circumorbital 3A), in turn, contacts the parieto-supraoccipital on the medi- bones in clariids, we refer to Adriaens et al. (1997)]. The al side and the posttemporo-supracleithrum caudally. Both antorbital bone is very small, lying dorso-posteriorly of the the sphenotic and pterotic lack lateral plates. They form a base of the maxillary barbel, close to the maxillary bone and firm connection between the neurocranium and the suspen- dorsal to the rostral end of the autopalatine. The infraorbital sorium through a set of interdigitating processes. On the IV is reduced to a more or less completely open, canal bone. anterior side of the sphenotic one large and one small pro- This makes that the infraorbital canal runs almost complete- cess are present. On the posterior side of the pterotic, two ly unprotected along the lateral side of the head. This infraor- additional processes are present. In the pterotic, however, the bital bone has the most dorsal position of the neurocranial largest process is situated most externally to the hyomandib- bones, elevated by the hypertrophied jaw adductor muscles. ula, which is opposite for the sphenotic spines. Ventrally, the The frontals (Figs 2, 3A) form the largest bones of the neurocranium is formed by the paired prootics (Fig. 3B). skull roof. The two contralateral parts are lying closely Occipital region. - The neurocranium is caudally bor- against each other, leaving a clear joint. The frontals enclose dered by the parieto-supraoccipital (Figs 2, 3A), bearing a the anterior fontanel entirely, except for the anterior most medial, long, caudally pointed spine. This bony complex border contacting the mesethmoid. The anterior fontanel encloses the posterior fontanel, which is very small and lies separates the two frontals along the rostral half. Within this in the caudal half of the parieto-supraoccipital. The posttem- fontanel, the epiphyseal bridge is clearly visible; with the poro-supracleithrum (Figs 2, 3A) rostrally makes contact anterior part of the postpineal fontanel being exposed. The with the pterotic. Further, it is connected to the pectoral gir- infraorbital canal exits the skull roof at the level of the fron- dle and to the parapophysis of the fourth vertebra, through a tals, close to the connection between the latter bone and the solid transscapular process. Ventrally, this region of the skull lateral ethmoid. The lateral side of the skull is formed by the consists of the exoccipitals and the basioccipitals (Fig. 3B). 284 Cybium 2005, 29(3) Devaere et al. Skeletal morphology of the holotype of Gymnallabes nops maxillaries bears several rows of slightly curved teeth, while the posterior, some- what smaller, part remains teethless. The maxilla is a rod-like bone (Figs 2, 3A, 4). It has a little indentation, in which the base of the barbel is enclosed. The proxi- mal part of the maxilla is broader and bears two articulatory facets for the artic- ulation with the autopalatine. Splanchnocranium Lower jaw. - The lower jaw consists of two parts: the dentary and the angular complexes (Figs 3A, 4). The coronoid process on the lower jaw is distinct and firm. The anterior part of the lower jaw is covered with a large tooth battery, with slightly curved teeth, which run close up to the top of the coronoid process. The lower jaw shows five pores for branches of the preopercular-mandibular canal. The well-developed retroarticular process lies mostly ventral from the articulation facet of the quadrate. The lower jaw embeds the rostral end of the preoperculo-mandibular canal of the lateral line system. The first three pores (PM1-3) lie in the dento- splenio-mentomeckelium complex, the fourth (PM4) is situated on the border of both bone complexes, while the fifth one (PM5) is located on the caudal end of the lower jaw. Figure 3. - Lateral view of the skull of Gymnallabes nops (56.57 mm SL) (MCZ 50298). A: Left lateral view; b: Sagittal view. ang-c: angular complex; ant: os antorbitale; apal: Suspensorium. - The suspensorium os autopalatinum; boc: os basioccipitale ; ch-a: os ceratohyale anterior; ch-p: os cera- consists of the hyomandibula, quadrate, tohyale posterior; cl: os cleithrum; cor: os scapulo-coracoideum; den-c: dentary complex; entopterygoid, metapterygoid and the pre- enp: os entopterygoideum; eoc: os exoccipitale; fr: os frontale; fr-n-II: foramen of the optic nerve; hh-v: os hypohyale ventrale; hm: os hyomandibulare; iop: os interopercu- opercle (Figs 3A, 5A). This latter bone is lare; io-IV: os infraorbitale IV; leth: os latero-ethmoideum; meth: os mesethmoideum; mentioned here since it is fused to the sus- mnd-sym: mandibular symphysis; mp: os metapterygoideum; mx: os maxillare; ns: os pensorium and forms one functional unit. nasale; op: os operculare; osph: os orbitosphenoideum; para: os parasphenoideum; par- soc: os parieto-supraoccipitale; pop: os praeoperculare; prmx: os praemaxillare; prot: os The hyomandibula connects to the sphe- prooticum; psph: os pterosphenoideum; pt: os pteroticum; pt-scl: os posttemporo-supra- notic and the pterotic through a set of pro- cleithrum; pvm: os praevomerale; q: os quadratum; r-br-IX: radius branchiostegus IX; cesses. At the level of the sphenotic, a sph: os sphenoticum; spl: os spleniale; spop: os suprapraeoperculare. [Crâne de Gymnal- labes nops. A : Vue latérale ; B : Vue sagittale. ang-c : complexe angulaire ; ant : antor- plate-like process is present anteriorly, bital ; apal : autopalatin ; boc : basioccipital ; ch-a : cératohyal antérieur ; ch-p : céra- followed by a smaller, pointed process. tohyal postérieur ; cl : cleithrum ; cor : scapulo-coracoïde ; den-c : complexe dentaire ; On the caudal side of the articulation enp : entoptérygoïde ; eoc : exoccipital ; fr : frontal ; fr-n-II : foramen du nerf optique ; hh-v : hypohyal ventral ; hm : hyomandibulaire ; iop : interoperculaire ; io-IV : infraor- ridge, three processes are present, rostral- bital IV ; leth : ethmoide latéral ; meth : mésethmoide ; mnd-sym : symphyse mandibu- ly to caudally, in size increasing for the laire ; mp : métaptérygoïde ; mx : maxillaire ; ns : nasal ; op : operculaire ; osph : orbit- interdigitation with the pterotic. Between osphénoïde ; para : parasphenoïde ; par-soc : pariéto-supraoccipital ; pop : pré- operculaire ; prmx : prémaxillaire ; prot : prootique ; psph : pterosphénoïde ; pt : ptéro- these two sets of processes an articulation tique ; pt-scl : posttemporo-supracleithrum ; pvm : prévomer ; q : carré ; r-br-IX : rayon ridge is present, making contact only with branchiostège IX ; sph : sphénotique ; spl : splénial ; spop : suprapréoperculaire.] the sphenotic bone. On the lateral side, a Maxillary bones. - The premaxillaries (Figs 2, 3) are clear ridge is present, inclosing a foramen. On the rostral large, toothed plate-like bones. The anterior part of the pre- side of the hyomandibula no plate-like outgrowth is present. Cybium 2005, 29(3) 285 Skeletal morphology of the holotype of Gymnallabes nops Devaere et al. Figure 4. - Ventral view of the skull of Gymnallabes nops (56.57 mm SL) (MCZ 50298). ang-c: angulare com- plex; apal: os autopalatinum; boc: os basioccipitale; ch-a: os ceratohyale anterior; ch-p: os ceratohyale posterior; cl: os cleithrum; cor: os scapulo-cora- coideum; den-c: dentary complex; iop: os interoperculare; hh-v: os hypohyale ventrale; meth: os mesethmoideum; mx: os maxillare; op: os operculare; osph: os orbitosphenoideum; pect-sp: pectoral spine; pop: os praeoperculare; pvm: os praevomerale; puh: os paru- rohyale; r-br-IX: radius branchiostegus IX; spl: os spleniale. [Vue ventrale du crâne de Gymnallabes nops. ang-c : com plexe angulaire ; apal : auto pa- latin ; boc : basioccipital ; ch-a : céra- tohyal antérieur ; ch-p : cératohyal pos térieur ; cl : cleithrum ; cor : scapu- lo-coracoïde ; den-c : complexe den- taire ; iop : interoperculaire ; hh-v : hypohyal ventral ; meth : méseth- moide ; mx : maxillaire ; op : opercu- laire ; osph : orbitosphénoïde ; pect- sp : épine pectorale ; pop : préopercu- laire ; pvm : prévomer ; puh : paru- rohyal ; r-br-IX : rayon branchiostège IX ; spl : splénial.] The opercular process is ventro-caudally oriented. The quad- lates with nine branchiostegal rays. The first six branchioste- rate ventro-laterally contacts with the lower jaw through a gal rays articulate with the ventral rim of the anterior cera- well-developed articulation head. It makes a clear contact tohyal, the following ray is placed at the small cartilaginous with the complete caudal side of the metapterygoid bone; region between the anterior and posterior ceratohyals. The dorsally this occurs through an interdigitation zone, ventrally last two rays are situated on the posterior ceratohyal. The through a synchondrosis. On the other hand no contact parurohyal lies in between the two hyoid arches and bears between the quadrate and the entopterygoid can be observed, two caudo-lateral processes and one very small, spiny, cau- as they are completely separated by the metapterygoid. The dal process (Fig. 4). last bone is the preopercle (see below), which is completely Branchial arches. - The “Bauplan” of the branchial bas- incorporated in the suspensorium. This bone surrounds a part ket corresponds to the general clariid situation. Only the size of the preoperculo-mandibular canal, and shows two pores and the number form somewhat an exception to this general (PM5-6). situation. The gill rakers in Gymnallabes nops are not only The autopalatine bone (Figs 2, 3A, 4) shows a clear con- small, but also occur in small numbers. For a detailed cave medial side, with a rostral and caudal cartilaginous tip. description of this general configuration in clariids, we refer The autopalatine lies ventrolateral to the lateral ethmoid and to Adriaens and Verraes (1998). shows a clear and well-pronounced articulation facet at the Opercular series. - The opercular bone is a triangular, articulation site with the lateral ethmoid. On the rostral side caudally, pointed structure (Fig. 3A). On the rostro-dorsal it articulates with the maxillary bone, thus being part of the side it bears a large articulatory facet for the articulation with palatine-maxillary mechanism. A single, small, tubular, sple- the hyomandibula. Caudally to this facet, a large process is nial bone (Adriaens et al., 1997) lies laterally to the articula- present. On the medial side of the caudal part, a ridge is pres- tion between the lower jaw and the quadrate (Figs 3A, 4). ent, presumably for the attachment of the levator operculi Hyoid arches. - The hyoid arch consists of two anterior muscle. On the rostro-ventral side of the articulation facet, a and posterior ceratohyals and two ventral hypohyals (Figs large, more plate-like, extension is present, ending on a bor- 3B, 4). On the scans there was no evidence of the dorsal der at the contact zone with the interopercular bone. This lat- hypohyals, this could be due to decalcification, since in other ter bone is situated between the opercular bone and the lower clariids the dorsal hypohyals are present (Cabuy et al., 1999; jaw (Fig. 3A). On the medial side, the interopercle bears a Devaere et al., 2001, 2004). Ventrally, the hyoid arch articu- marked concavity enclosing the caudal tip of the posterior 286 Cybium 2005, 29(3) Devaere et al. Skeletal morphology of the holotype of Gymnallabes nops ing the preopercular canal. As mentioned above, the praeo- percular bone is incorporated in the suspensorium (Fig. 3A). Postcranial skeleton Vertebrae. - The total number of vertebrae is 62 (Fig. 6) (including those comprised by the Weberian apparatus). There are 18 precaudal vertebrae, of which five carry ribs. The dorsal fin comprises a total of 75 fin rays; 59 fin rays are present in the anal fin. A little foramen can be found at the bases of the parapophyses of the first precaudal vertebrae. Pectoral girdle. - The pectoral girdle comprises a scapu- lo-coracoid and a cleithral bone (Figs 3B, 4), which are strongly sutured to each other. Ventrally, the two contralater- al parts are strongly connected to each other. In the caudal part, this occurs through several large processes. A clear fenestra is present between the right cleithral and scapulo- coracoid bone, but completely absent in the left part (Fig. 4). The cleithral bone shows a clear anterior process, as well as a ridge on its rostro-ventral side. On the scapulo-coracoid, a distinct insertion ridge is present for the attachment of the pectoral muscles. The pectoral fin has a non-serrated spine and eight fin rays that articulate with the two radials present. Pelvic girdle. - Although the pelvic girdle is highly decal- cified, the basipterygium shows two clear processes (internal and external anterior process: see Arratia 2003). No more than four pelvic fin rays can be discerned. Caudal skeleton. - The caudal skeleton (Fig. 7) consists out of the parhypural, five hypurals, urostyl and uroneural, which are all fused in a single bony plate. Dorsally to this fusion lies the broad tipped, epural. The neural spine of the second preural vertebra is elongated, broadly tipped and sup- ports the caudal fin rays; the neural spine of the third preural vertebra is also elongated but ends with a spiny tip and does not support the dorsal fin rays. This supporting function is Figure 5. - Lateral view of the suspensorium. A: Gymnallabes nops performed by a pterygiophore. Both haemal spines of the (56.57 mm SL) (MCZ 50298); b: Channallabes apus (28 mm SL) second and third preural vertebrae are elongated, broad and (MRAC 175247-270); C: Gymnallabes typus (192 mm SL) (MRAC support the anal fin rays. Anteriorly, the dorsal and anal fin 97-030-P-0010). af-a: articulatory facet of the quadrate with the angulo-splenio-articulo-retroarticular; af-sph-pt: articulatory facet rays are supported by pterygiophores. of the hyomandibula with the sphenotic and the pterotic; enp: os entopterygoideum; hm: os hyomandibulare; mp: os meta- pterygoideum; pop: os praeoperculare; prc-op: opercle process; prc-pg: pterygoid process; prc-pt: pterotic process; prc-sph: sphe- DISCuSSION notic process; q: os quadratum; spl: os spleniale. [Vue latérale du suspensorium de A : Gymnallabes nops; B : Channallabes apus; C : Gymnallabes typus. af-a : face articulaire du carré avec l’angulo-splénial-articulo-rétroarticulaire ; enp : entoptérygoïde ; Original description hm : hyomandibulaire ; mp : métapterygoïde ; pop : préoperculai- In the original description of Roberts and Stewart (1976), re ; prc-op : processus operculaire ; prc-pg : processus ptérygoïdi- en ; prc-pt : processus ptérotique ; prc-sph : processus sphénotique containing some metric and meristic data, the holotype of ; q : carré ; spl : splénial.] Gymnallabes nops was included in the genus Gymnallabes Günther, 1867. At that time this genus included Gymnallabes ceratohyal. Laterally, on top of the jaw muscles, two suprap- typus Günther, 1867; G. alvarezi Roman, 1970 and G. tihoni reopercular (Figs 2, 3A) bones are present. The larger, dor- (Poll, 1944) of which the latter was later transferred to Platy- sally situated suprapreopercular bone shows a triangular allabes Poll, 1977. Although the largest similarity was shape. The ventral one is reduced to a tubular bone, enclos- observed between G. nops and Platyallabes tihoni, the Cybium 2005, 29(3) 287 Skeletal morphology of the holotype of Gymnallabes nops Devaere et al. Figure 6. - Overall lateral view of Gymnallabes nops (56.57 mm SL) (MCZ 50298). [Vue latérale générale de Gymnallabes nops.] tihoni shows a lot of variation and is not always extremely reduced, as stated in the original description. Further, it was stated that G. nops has a higher abdominal body depth and caudal peduncle depth, shorter pectoral spine and a smaller skull roof width, but all these lie within the ranges of P. tiho- ni (Tab. I). The distance between the occipital process and the dorsal fin was given as a last metric difference. This was stated to be double in G. nops compared to P. tihoni. Since a small distance is one of the distinctive characters of P. tihoni (Devaere, personal observation), the distance is larger in G. nops but not twice that of P. tihoni (Tab. I). Since metric and meristic data of Gymnallabes nops overlap, mostly lying within the range of Platyallabes tihoni, the question raises if G. nops is still a valid species. Further- more, since Platyallabes has become a different, monotypic genus, the question raises if G. nops is designated to the proper genus. A discussion on shared characters follows, Figure 7. - Caudal skeleton of Gymnallabes nops (56.57 mm SL) based on the obtained morphological characters. (MCZ 50298). cc: composed centre; ep: os epurale; h1-h5 + ph + u + un: complex of os hypural 1 to 5, parhypural, urostyl and uroneu- ral; hpu2, 3: haemal spine of preural vertebrae 2, 3; npu2, 3: neural Diagnosis spine of preural vertebrae 2, 3; pu2, 3: preural centre 2, 3. [Squelette caudal de Gymnallabes nops. cc : centre composé ; ep : épural ; Principal Components Analysis h1-h5+ph+u+un : complexe terminal (hypuraux 1 à 5 + parhy- For the analysis, we examined 110 specimens of the wide pural + urostyle + uroneural) ; hpu2, 3 : épine hémale des vertè- area around the type locality of G. nops. This region includes bres préurales 2, 3 ; npu2, 3 : épine neurale des vertèbres préurales 2, 3 ; pu2, 3 : centres préuraux 2, 3.] the lower Congo stream up to Kinshasa, Southern West Coastal Equatorial, and the Kasai region (Thieme et al., authors pointed out that a comprehensive set of differences 2005). These include specimens of Channallabes apus (n = existed. Most of them, however, proved to be invalid, as this 29), Platyallabes tihoni (n = 39), Platyclarias machadoi (n = study indicates, simply by taking more specimens of P. tiho- 21), Gymnallabes alvarezi (n = 1) with type material of ni into account (see below). Gymnallabes nops, Platyallabes tihoni and Platyclarias The authors gave as a first striking difference the lack of machadoi. We also included the specimens of Gymnallabes eyes in G. nops. This is not unique and also occurs in Platy- typus (n = 19), from the Niger delta, Northern and Central allabes tihoni (MCZ 88947, 50297). A second character used West Coastal Equatorial Freshwater Ecoregion (Thieme et was the observation that the pectoral and pelvic fins did not al., 2005) (includes G. typus types). extend respectively beyond the origin of the dorsal and anal Figure 8 plots the second principle component derived fin, in contrast to the situation in P. tihoni. In our data set from a PCA on the covariance matrix for 25 measurements (Tab. I) a large variation on the length of both pectoral and against the first principle component derived from a correla- pelvic fins in P. tihoni was observed, a variation already tion matrix of 5 meristic characters. The dominant characters shown in several other anguilliform clariids (Adriaens et al., for the second principal component are the distance between 2002). Paired fins not reaching or reaching further than the the occipital process and the dorsal fin, the caudal peduncle origin of the respectively dorsal and anal fin could both be depth and the barbel lengths; while for the first component, found. Also the length of the innermost pelvic fin ray in P. the total number of vertebrae and the number of ribs are the 288 Cybium 2005, 29(3) Devaere et al. Skeletal morphology of the holotype of Gymnallabes nops Table I. - Measurements and meristic data for the holotype of Gymnallabes nops, compared to Platyallabes tihoni, Gymnallabes typus and Gymnallabes alvarezi. Abbreviations used are explained in the text. [Mesures et données méristiques de l’holotype de Gymnallabes nops, comparées à celles de Platyallabes tihoni, Gymnallabes typus et Gymnallabes alvarezi. Les abréviations sont expliquées dans le texte]. G. nops P. tihoni G. typus G. alvarezi n min max mean SD n min max mean SD n min max mean SD TL (mm) 61.0 54 63 380 32 49 284 18 35.5 329 SL (mm) 56.6 54 58 359 32 44 259 18 31 318 Measurements in % standard length PaL 40.3 54 18.6 34.7 26.3 3.5 32 29.2 37.1 32.9 2.2 18 24.1 40.6 34.9 4.2 PPcL 16.4 54 8.3 17.0 12.1 2.0 32 10.0 19.3 13.0 1.9 18 8.2 20.8 15.6 3.3 PPvL 35.0 54 18.3 30.6 23.5 3.1 32 27.2 36.2 30.5 2.3 18 32.0 37.8 34.7 2.3 PdL 24.8 54 12.5 23.4 16.9 2.8 32 18.1 27.8 21.6 2.4 18 16.9 35.9 28.3 4.6 SPDFL 8.7 54 2.2 6.6 3.7 1.0 32 5.2 11.2 8.3 1.5 18 8.3 15.8 11.9 1.7 PcFL 9.4 54 4.6 9.9 6.6 1.3 32 3.8 8.5 5.7 1.0 18 3.6 11.3 8.0 2.4 PcSL 5.1 54 3.3 7.0 5.0 0.8 32 1.8 3.8 2.5 0.5 18 1.6 4.6 3.5 0.9 PvFL 5.6 54 4.0 7.3 5.6 0.8 32 1.7 6.8 4.2 0.9 18 3.8 7.0 5.5 1.0 CPD 3.4 54 1.1 3.6 1.9 0.5 32 1.9 4.5 3.0 0.7 18 1.9 5.9 4.0 1.0 ABD 6.6 54 3.0 6.4 4.2 0.8 32 4.6 7.8 6.3 0.8 18 4.8 11.6 8.4 1.9 IpcD 11.7 54 6.9 13.0 9.3 1.5 32 5.9 11.0 8.0 1.1 18 2.2 4.9 3.3 0.8 IpvD 4.8 54 2.9 6.2 4.2 0.8 32 1.4 4.0 2.5 0.6 18 5.8 15.7 11.3 3.1 SkL 15.1 54 7.4 17.2 12.7 2.1 32 11.1 18.0 13.1 1.6 18 7.5 27.0 18.3 5.6 Measurements in % head length PoL * 54 61.2 85.5 74.7 9.2 32 62.0 74.0 67.7 2.4 18 62.5 86.9 74.5 6.7 SpL 13.6 54 6.9 20.3 13.5 3.0 32 10.2 20.0 15.5 2.6 18 13.0 28.6 20.1 3.1 SkW 68.7 54 62.9 113.9 75.8 7.9 32 52.1 69.6 60.3 4.6 18 45.4 88.5 59.5 10.6 SpW 26.0 54 10.2 22.3 16.0 3.0 32 12.1 27.3 18.8 4.0 18 18.6 33.6 26.8 4.1 IoD * 54 30.4 56.9 38.5 4.5 32 24.6 40.4 33.0 3.5 18 24.9 41.9 30.8 4.0 ANID 19.4 54 10.5 26.1 14.1 2.7 32 10.6 19.0 15.0 2.2 18 12.1 20.7 16.4 2.0 PNID 33.3 54 22.6 44.8 31.2 4.5 32 23.1 34.1 27.7 2.8 18 23.3 39.6 28.6 3.9 RSkW 45.3 54 30.3 69.0 40.0 7.4 32 28.9 53.4 42.0 5.5 18 25.6 46.0 35.4 5.3 OSkW 58.6 54 43.7 92.9 54.9 6.8 32 42.5 60.8 51.4 3.4 18 40.0 59.7 47.5 4.6 SkH 45.5 54 22.8 61.7 34.6 6.9 32 26.6 49.8 40.0 5.8 18 33.1 129.7 46.7 21.5 ED * 54 3.7 12.8 6.1 1.6 32 4.1 8.4 6.5 1.1 18 6.7 11.8 8.8 1.4 SnH 20.3 54 9.5 21.6 13.3 4.8 32 10.0 18.5 14.2 2.0 18 7.7 25.9 14.1 3.9 OSkH 23.4 54 16.8 44.0 23.4 4.8 32 17.5 36.0 26.0 4.6 18 17.1 35.6 25.1 5.0 PhL 27.7 54 17.2 44.5 27.6 4.1 32 18.6 31.8 24.5 3.5 18 19.1 48.1 26.2 6.3 IMnID 22.4 54 20.3 42.0 27.5 3.7 32 18.5 37.1 24.4 3.5 18 13.2 33.1 21.4 4.4 EMnID 45.5 54 32.1 67.8 43.2 6.0 32 31.5 44.6 39.3 3.6 18 24.9 44.1 34.0 5.2 MW 26.1 54 19.5 76.1 32.7 8.5 32 21.5 39.3 30.8 4.0 18 19.0 36.4 27.9 5.3 SkR 11.3 54 9.5 26.8 18.9 4.6 32 4.0 44.1 15.7 9.5 18 11.5 57.8 25.7 11.4 Meristic counts RB 5 33 4 7 5 0.6 22 6 12 8.6 1.7 3 10 14 12 2 TV 62 33 63 83 74 5.7 22 78 92 84 2.7 3 71 99 81 11 two most important. This reveals the presence of several clariids by the combination of following characters: a large groups that do not overlap. In this plot Gymnallabes nops rostral outgrowth of the hypertrophied adductor mandibulae takes a separate position, apart from other groups and type complex; a reduced infraorbital series, both in number and material, indicating that G. nops is indeed a valid species. size; the absence of the medial expansions of the frontal plates, leaving the epiphyseal bridge clearly exposed; the Diagnosis entopterygoid situated in a completely rostral position of the Gymnallabes nops differs from all other anguilliform metapterygoid, a short posterior process on the prevomer Cybium 2005, 29(3) 289 Skeletal morphology of the holotype of Gymnallabes nops Devaere et al. and a large rostro-ventral process on the opercle. Although the absence of the eyes is not unique, it has some typical conse- quences; the available orbital space is taken by an outgrowth of the hypertro- phied adductor mandibulae complex, covering the lateral and dorsal side of the skull (Fig. 9). Since no discrimina- tion between soft tissues is possible on radiographies or CT-scan, we have no certainty that nothing of the visual sense organ is developed. It cannot be overruled that the eyes could have developed up to a certain embryonic stage, but remain invisible as they are Figure 8. - Combined plot of the second principal component derived from a PCA of 25 covered by the integument. A compa- log-transformed metric variables against the first principal components of a PCA using 5 meristic variables. ❊ Gymnallabes nops; + Channallabes apus;  Gymnallabes alvarezi; rable situation has been observed in ◆ syntypes of Gymnallabes typus;  other specimens of Gymnallabes typus; ▲ holotype of Caecomastacembelus brichardi (Poll, Platyclarias machadoi;  paratypes of Platyclarias machadoi; ■ holotype of Platyallabes 1973). The only certainty that we have tihoni;  other specimens of Platyallabes tihoni. [Graphique d’une analyse en composantes principales, basée sur 25 variables métriques (log-transformé) (PC2) et 5 variables méris- obtained through the CT-scans is that tiques (PC1). ❊ Gymnallabes nops ; + Channallabes apus ;  Gymnallabes alvarezi ; the foramen for the optic nerve is ◆ syntypes de Gymnallabes typus ;  autres spécimens de Gymnallabes typus ; ▲ holotype clearly present between the orbitos- de Platyclarias machadoi ;  paratypes de Platyclarias machadoi ; ■ holotype de Platyal- labes tihoni ;  autres spécimens de Platyallabes tihoni.] phenoid and the pterosphenoid (Fig. 3B), which could indicate the passage of an optic nerve and thus a certain level of development of the visual sense organ itself. As in other anguilliform species, as well as in Uegitgla- nis and the primitive catfish family Diplomystidae (David, 1936; Arratia, 1987), the plesiomorphic state of a reduced series of circumorbital bones occurs in G. nops. However, this reduction in G. nops not only involves a reduction of the plate-like expansions on each of the infraorbital bones, but is also reflected in a reduction of the number of infraorbital bones. Only the first and last one are still present, namely the antorbital and infraorbital IV (Figs 2, 3A). Those two are the most dorsally situated bones and are also the only circumor- bital bones present in Dolichallabes microphthalmus (Devaere et al., 2004). As was stated in Devaere et al. (2004) this could be due to a heterochronic process, although for G. nops some other reasons for this reduction could be suggest- ed. Since G. nops shows no evidence of eyes, a protecting series of bones around the orbital region is not required. This, however, leaves the infraorbital canal unprotected. Although, a discontinuous infraorbital canal does sometimes occur (Webb, 1989), this seems not to be the case here. There is no evidence of a discontinuous canal in clariids (Adriaens Figure 9. - Dorsal view of the skull of Gymnallabes nops (56.57 mm SL) (MCZ 50298), with indication of the position of the adduc- et al., 1997; Cabuy et al., 1999; Devaere et al., 2001) Fur- tor mandibulae complex (shaded) and the degree it covers the skull thermore, the absence of the infraorbital bones II and III roof. [Vue dorsale du crâne de Gymnallabes nops, avec indication could also be linked to the subsequent outgrowth of the jaw de la position du complexe adducteur mandibulaire (hachuré) et du degré de couverture du toit crânien]. muscle complex (Fig. 9). Another explanation is that the 290 Cybium 2005, 29(3)