Introduction

Gelanoglanis was proposed by ^{Böhlke (1980)} for a distinctive species of previously undescribed driftwood catfish, G. stroudi. This unusual, soft-bodied, new species was described from samples that originated in the río Metica and río Manacacias, both of which are whitewater tributaries to the río Orinoco in eastern Colombia. In his original description of G. stroudi, ^{Böhlke (1980)} discussed the relationships of the species and concluded that it was a member of the Auchenipterinae (the Auchenipteridae of present usage), but "not closely related to any described auchenipterine". ^{Ferraris (1988)} was the first author to resolve the phylogenetic placement of Gelanoglanis which he assigned to a monophyletic group composed by that genus, Centromochlus Kner, 1857, GlanidiumLütken, 1874, Tatia Miranda Ribeiro, 1911, plus two additional undescribed genera. Subsequently, ^{Soares-Porto (1998)} published a phylogenetic hypothesis of the Centromochlinae in which Gelanoglanis resolved as deeply nested within Centromochlus and hence she proposed a synonymy of Gelanoglanis intoCentromochlus. Soon thereafter, ^{Soares-Porto et al. (1999)} reversed the synonymy proposed by ^{Soares-Porto (1998)} by their recognition of Gelanoglanis as valid in their description of a new species, G. nanonocticolus, from localities in the río Asisa, a blackwater tributary of the upper río Orinoco in southern Venezuela and from the middle rio Negro in the northern reaches of the Amazon basin. In that publication, those authors recognized Gelanoglanis as a member of the subfamily Centromochlinae which they equated with the monophyletic group recognized as the family Auchenipteridae by ^{Ferraris (1988)}. More recently, a third species of Gelanoglanis, G. travieso, was described by ^{Rengifo et al. (2008)}; from the río Marañon, a whitewater tributary to the mainstream Amazon in northeastern Peru.

Analysis of a sample of Gelanoglanis collected in the rio Teles Pires within the upper reaches of the rio Tapajós basin, a southern tributary of the rio Amazonas, revealed it to represent a new species of the genus. This record is the first for the genus from the southern portions of the Amazon basin and also the first reported sample of Gelanoglanis originating in a clear water habitat. Comparative surveys identified additional synapomorphies for the species of Gelanoglanis, but also that one previously proposed synapomorphy for the species of the genus was lacking in the new form. Gelanoglanis has been considered a case of miniaturization within the Auchenipteridae, with the contained species reaching maximum standard lengths of 36 mm SL (^{Soares-Porto et al. 1999}). Various reductive features observed in the new species described in the present study and its congeners corroborate the characterization of the genus as miniaturized.

Material and Methods

Morphometric measurements were obtained to the nearest 0.1 mm with digital calipers under a stereomicroscope, preferably on the left side of specimen, and follow ^{Sarmento-Soares & Martins-Pinheiro (2008)} and ^{Soares-Porto et al. (1998)}. ^{Soares-Porto et al. (1998)}and ^{Rengifo et al. (2008)} served as the source of the comparative information in the diagnosis of the new species. The count of vertebrae includes the terminal element associated with the hypural complex. This count assumes that the fifth vertebra, which lacks articulated ribs, is suturally united to the anterior four elements incorporated into the Weberian Complex and treats the first rib-bearing centrum as the sixth vertebra (^{Ferraris & Vari, 1999}). Osteological features were evaluated on specimens cleared and counterstained (c&s) for bone and cartilage according to the technique of ^{Taylor & Van Dyke (1985)}. The term gonopodium herein refers to the intromittent organ of mature males. Abbreviations used in text are: SL, standard length and HL, head length. Museum abbreviations are: ANSP, The Academy of Natural Sciences, Philadelphia; CAS, California Academy of Sciences, San Francisco; LACM, Los Angeles County Museum, Los Angeles; MCP, Museu de Ciências e Tecnologia da Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre; MZUSP, Museu de Zoologia da Universidade de São Paulo, São Paulo; SU, Stanford University, collections now at CAS, San Francisco; UF, Florida Museum of Natural History, Gainesville; and USNM, National Museum of Natural History, Smithsonian Institution, Washington, D.C.

Results

Gelanoglanis monophyly

Gelanoglanis has been to date diagnosed on its possession of a combination of multiple characters, six of which were proposed by ^{Böhlke (1980)} and subsequently utilized with some modifications by ^{Soares-Porto et al. (1999)}. These are: 1- the pectoral-fin spine, if present, lacking serrations along its anterior margin; 2- the posterior naris being long, narrow, and located immediately anterior to the eye; 3- the possession of a single pair of mental barbels; 4- the laterally situated contralateral premaxillary tooth patches widely separated at the jaw symphysis; 5- the mouth oblique and sinuous, with free fleshy flange around the angle of the mouth opening; and 6- the bases of all fins, except for the caudal fin, being short and the fins including relatively few fin rays. In their analysis ^{Soares-Porto et al. (1999)} noted that several of these features, albeit derived, occur homoplastically elsewhere in the Auchenipteridae. Those authors also advanced five additional characters that they hypothesized were diagnostic for Gelanoglanis: 7 - the absence of the third pectoral-fin radial; 8 - the mandibular ramus of the laterosensory canal being free from the dentary; 9 - the reduction of the mesopterygoid to a small, round ossification; 10- the absence of the vomer; and 11- the absence of an anterior fontanel.

Analysis revealed that Gelanoglanis pan, the species described herein, shares all the aforementioned synapomorphies except for 11- the absence of an anterior fontanel. Gelanoglanis pan conversely has a single, rounded to longitudinally ovoid fontanel bordered solely by the frontal bones. An undivided anterior fontanel is generally present among the remaining members of Centromochlinae except for Centromochlus existimatus Mees, 1974, and Tatia intermedia (Steindachner, 1877) in which the anterior fontanel is subdivided in two openings. When present the fontanel is generally margined by both the mesethmoid and frontal bones other than in Centromochlus altae Fowler, 1945, C. existimatus, Tatia jacaratia Pavanelli & Bifi, 2009, T. meesi ^{Sarmento-Soares & Martins-Pinheiro, 2008}, and T. neivai (Ihering, 1930), in which the anterior fontanel is margined solely by the frontals. In so far as an anterior fontanel is present in almost all taxa in the Auchenipteridae, the presence of that aperture in Gelanoglanis pan is potentially indicative of the basal position of that species within the genus (e.g., the absence of the fontanel being a synapomorphy for all congeneric species); however, a more encompassing phylogenetic analysis is necessary to evaluate this hypothesis.

Our analyses revealed four additional diagnostic and potentially synapomorphic features for Gelanoglanis above and beyond those put forward by ^{Böhlke (1980)} and ^{Soares-Porto et al. (1999)}. The first of these is the anterior portion of the head with a very fleshy region anterior to the mesethmoid. As mentioned above, species of Gelanoglanis have a highly modified morphology of the cranium as exemplified by the arched mesethmoid and the lateral displacement of the premaxillae. This restructuring of the bones at the anterior region of the head is complemented by a thick layer of soft tissues over the anterior portion of the snout. Other members of Centromochlinae alternatively have the anterior portion of the snout formed by the skin-covered mesethmoid.

A second additional synapomorphy for Gelanoglanis is the mesethmoid separate from the anteromedial portion of the premaxillae. In the remaining species of the Centromochlinae, the mesethmoid is in contact with the premaxillae along its length; with the contact in some instances being via a distinct suture. The thin arched mesethmoid of Gelanoglanis is instead anteromedially separate from the laterally displaced premaxillae.

The third diagnostic feature for Gelanoglanis is the lack of ossification of all infraorbital canals other than for the laterosensory canal segment and laminar projections along both the posterior and ventral margins of the canal within the first infraorbital. Remaining infraorbitals (2 to 4) are present as transparent, unossified, elongate, tubular structures.

The fourth and final additional synapomorphy is the incomplete lateral line. Although ^{Soares-Porto et al. (1999)}and ^{Rengifo et al. (2008)}assumed the lateral line to be complete across Gelanoglanis, our observations indicate that the lateral line is incomplete and terminates approximately at a vertical through the posterior terminus of the anal-fin base and well anterior to the caudal fin in all species of Gelanoglanis. This reduction resolves as synapomorphic for the species of Gelanoglanis, since all other genera of the Auchenipteridae have a lateral line extending posteriorly onto the caudal fin for about one-half the length of the caudal-fin rays, or when the lateral line is reduced, it at least reaches the caudal-fin base.

Gelanoglanis pan, new species

Figs. 1-2; 4-5

Gelanoglanis cf. stroudi.-Birindelli, 2014: 535, 544 (comparative material).

Fig. 1 Gelanoglanis pan, MZUSP 114669, holotype, male, 24.7 mm SL; Brazil, Mato Grosso State, Itaúba, rio Teles Pires, tributary to upper rio Tapajós basin; lateral views. Scale bar = 0.5 cm. 

Etymology. The species name, pan, from the name of the Greek God of fertility and male sexuality, refers to the large gonopodium of the males of the species.

Fig. 2 Gonopodium and osteology of the anal fin of an adult male of Gelanoglanis pan, holotype, 24.7 mm SL, MZUSP 114669, showing fusion of the proximal radials. DR = distal radials, G = gonopodium, PR = fused proximal radials, i-iii = anterior unbranched and unsegmented anal-fin rays. Scale bar = 1 mm. 

Fig. 3 Map of southeastern Amazon basin showing location of type locality of Gelanoglanis pan, 1 = rio Tapajós, 2 = rio Teles Pires, 3 = rio Juruena. 

Miniaturization

Gelanoglanis pan is a miniature species under the definition proposed by ^{Weitzman & Vari (1988)} in their overview of miniaturization within South American freshwater fishes. According to that definition, miniature fishes either mature at less than 20 mm SL or do not exceed a maximum body length of 26 mm SL. Although standard length is a readily observed indicator of miniaturization, ^{Weitzman & Vari (1988)} noted that the miniaturization process generally involves a series of other less obvious reductive (paedomorphic) morphological features. Prominent among these are reductions in the degree of development of the laterosensory system on the head and body, in the number of fin rays, and of the sculpturing on the surface bones of the head.

Despite in some instances exceeding the 26 mm SL maximum originally proposed by ^{Weitzman & Vari (1988)}, all species of Gelanoglanis share a number of reductive features and, thus, are considered miniatures. Reductions general across Gelanoglanis include the decrease to one ossified infraorbital, the first infraorbital; a mandibular ramus of the lateral sensory canal separate from the dentary and instead extending as a separate ossification lateral of the dentary and anguloarticular (see Fig. 4); the incomplete lateral line terminating posteriorly anterior to the caudal-fin base; the reduction of the mesethmoid to an elongate narrow arch; the very small premaxilla in the form of a thin plate bearing relatively few teeth anteriorly; and the reduction to 8 total anal-fin rays (vs. 9-11 in Tatia and Centromochlus; and 12-14 in Glanidium). Hence, Gelanoglanis pan fits the definition of a miniature both in sharing this series of developmentally truncated features with its congeners, and also in its maximum length of 24.7 mm SL.

Fig. 4 Lateral view of the mandibular arch of Gelanoglanis pan, MZUSP 96032, paratype, 20.2 mm SL. AA = anguloarticular, D = dentary, PM = premaxilla, LC = subdivided autogenous laterosensory canal. Scale bar = 0.5 mm. 

Discussion

Gelanoglanis pan is characterized by a rugose dorsal surface of the entire length of the maxillary barbel resultant from the presence of soft rounded papillae in a single series along the barbel (Fig. 5; see description for details). No comparable structures have been reported to date elsewhere in the genus. Our analysis revealed that Gelanoglanis pan shares this feature of the maxillary barbel with its congeners. The rounded papillae on the maxillary barbel were, however, encountered only in a few of the examined specimens of G. stroudi, and G. travieso. Gelanoglanis pan, conversely, possesses such structures in juveniles and adults across all examined specimens. Further studies are necessary to throughly investigate the ontogeny of this character. Papillae-laden tissues elaborated into various structures are unusual among siluriforms although reported in several genera elsewhere in the Auchenipteridae. Ferraris &Vari (1998; Fig. 1) documented various forms of such elaborations on the surface of the ossified maxillary barbel in mature males of Auchenipterus Valenciennes, 1840, and described a much different form of the barbel elaboration in Trachelyopterus insignis (Steindachner, 1878). Those same authors reported that the medial surface of the ossified portion of the maxillary barbel in mature males of the two then known species of Tetranematichthys Bleeker, 1858, was covered with a series of epidermal ridges (^{Vari & Ferraris, 2006}: 175); a modification also reported in a third species of the genus by ^{Peixoto & Wosiacki (2010)}. Fleshy elaborations of the barbel were also identified in two specimens of Pseudepapterus Steindachner, 1915, by ^{Ferraris & Vari (1999)}. The rounded fleshy elaborations of the barbel in G. pan clearly differ both in form and location from those in Auchenipterus,Pseudepapterus, Tetranematichthys, and Trachelyopterus. Another notable difference between G. pan versus Auchenipterus, Pseudepapterus, and Tetranematichthys is that the elaborations of the maxillary barbel in the three latter genera have been observed solely in nuptial males in which the transitory enlarged, ossified barbels are present solely during the breeding season (^{Ferraris & Vari, 1999}, ²⁰⁰⁰; ^{Vari & Ferraris, 2006}) used to hold females during copulation. Gelanoglanis pan conversely has these elaborations well developed in both males and females, with juveniles also bearing small rounded epidermal papillae. These differences and present concepts of higher level relationships within the Auchenipteridae support the hypothesis of the independent acquisition of these modifications of the maxillary barbel in G. pan versus the other cited taxa of the Auchenipteridae.

^{Soares-Porto et al. (1999)}reported a long adipose fin in the form a low fleshy ridge with its posterior margin attached to the body in adults of Gelanoglanis nanonocticolus. Alternatively, adults of G. pan, G. stroudi, and G. travieso lack such a ridge; having instead a distinct adipose fin with a posterior margin free from the middorsal surface of the body. A long middorsal ridge anterior to the adipose-fin base is common in juveniles of the genus, and its persistence in adults of G. nanonocticolus likely represents a paedomorphic feature.

The dorsal-fin spine was cited as absent in Gelanoglanis travieso by ^{Rengifo et al. (2008)}, who rather reported a "second dorsal-fin lepidotrichium filamentous, simple, not a spine, not serrate". Our examination of a cleared and stained paratype of Gelanoglanis travieso (ANSP 182808) reveals a strong dorsal-fin spine with a distal unsegmented filament; a feature shared with G. pan. Interestingly G. pan and G. travieso also have in common similar coloration in preservative; albeit being readily distinguished by the characters detailed in the Diagnosis.

Fig. 5 Maxillary barbel of Gelanoglanis pan, MZUSP 96032, paratype, 12.6 mm SL. Left side, anterior to left. Scale bar = 0.5 mm. 

Key for the species of Gelanoglanis