Potamoglanis, Costa Correspondence, 2017

Costa Correspondence, Elisabeth Henschel José Leonardo O. Mattos Axel M. Katz Wilson J. E. M., 2017, Position of enigmatic miniature trichomycterid catfishes inferred from molecular data (Siluriformes), Zoologica Scripta 47 (1), pp. 1-10 : 4-8

3.2.1 | Potamoglanis , gen.n

Etymology

From the greek potamo %river; the gods of rivers in the Greek mythology) and glanis %catfish). Gender masculine.

Type species

Pygidium hasemani Eigenmann, 1914 .

Diagnosis

Potamoglanis is distinguished from all other Trichomycteridae by an angle of 35–40° between the main longitudinal axis of the head and the main axis of the autopalatine %Figure 2a,b) and by the unique thin tubular shape of the second hypobranchial %Figure 2d). Potamoglanis is similar to other Tridentinae genera by the presence of a wide cranial fontanelle %Figure 3a–c); the presence of a short ventral process in the opercular bone %Figure 2f–g) and by the origin of the dorsal fin placed in a vertical through the anal-fin origin or just posterior to it. The new genus differs from other genera of Tridentinae by the presence of six or seven anal-fin rays %vs. 15 or more); eyes dorsally placed on head, not ventrally visible %vs. laterally placed; ventrally visible); opercular and interopercular odontodes patches not juxtaposed, separated by more than the width of the interopercular patch %vs. odontodes patches juxtaposed, separated by less than the width of the interopercular patch); absence of a distal process on the hyomandibula %vs. presence) and the presence of a long process on the anterior region of the hyomandibula %vs. absence) %Figure 2f).

Included species

Pygidium hasemani Eigenmann, 1914 ; Pygidium johnsoni Fowler, 1932 ; Trichomycterus anhanga Dutra, Wosiacki, & de Pinna, 2012 ; Trichomycterus wapixana Henschel, 2016 .

Distribution and habitat

Potamoglanis hasemani , Potamoglanis anhanga and Potamoglanis wapixana are found throughout the Amazon river basin, whereas Po. johnsoni is endemic to Pantanal wetlands, Paraguay river basin. Potamoglanis hasemani is found in aquatic plants roots and Po. wapixana inhabits marginal vegetation of small streams %E. Henschel, personal observation, November 30, 2016). Potamoglanis anhanga lives in places with mud and roots of aquatic plants of a small stream %mainly composed of sandbanks) %E. Henschel, personal observation, September 20, 2016). Potamoglanis johnsoni is endemic to the Pantanal and inhabits roots of aquatic plants %W.J.E.M. Costa, personal observation, April 30, 1996; E. Henschel personal observation, November 30, 2016).

Remarks

Several populations of small catfishes have been collected throughout the Amazon basin and identified as Po. hasemani %E. Henschel , personal observation, August 7, 2016), but this identification requires confirmation.

4 | DISCUSSION

4.1 | Phylogenetic positioning of Potamoglanis and monophyly of Tridentinae

Our results corroborate Potamoglanis as the sister group of species traditionally placed in the Tridentinae . Tridentinae , until the present study, comprised the genera Miuroglanis , Tridensimilis , Tridentopsis and Tridens . Potamoglanis is herein considered as the fifth Tridentinae genus.

Baskin %1973) enumerated eight synapomorphies for the Tridentinae : 1 – cranial fontanelle expanded; 2 – maxillary bone very small; 3 – eyes exposed ventrally; 4 – opercular and interopercular tooth patches juxtaposed; 5 – opercular bone with a short ventral process; 6 – origin of the dorsal fin just above or posterior to anal-fin origin; 7 – hyomandibular with a distal process and 8 – anal-fin rays 15 or more. When regarding Potamoglanis as a new Tridentinae genus, the characters 1, 5 and 6 are maintained as synapomorphies for the whole subfamily %see below). The other five character states are thus no longer considered as valid synapomorphies for the subfamily but are useful to diagnose the clade comprising Miuroglanis %Eigenmann & Eigenmann, 1889); Tridens %Eigenmann & Eigenmann, 1889); Tridentopsis %Myers, 1925) and Tridensimilis %Schultz, 1944).

Baskin %1973) described the wide cranial fontanelle of the Tridentinae as being anteriorly delimited by the posterior edge of the mesethmoid, laterally by the frontals and sphenotics and posteriorly by the parietosupraoccipital %Figure 3a–c). de Pinna %1989) considered the presence of a similar structure in Po. hasemani and Po. johnsoni as evidence of relationship between these species and Tridentinae . An expanded cranial fontanelle is also seen in Paravandellia %Miranda-Ribeiro, 1912), but regarded as homoplastic % de Pinna, 1989) since it is the only Vandelliinae genus exhibiting this condition, and this subfamily is a well-corroborated clade, phylogenetically distant of the Tridentinae . In other Trichomycteridae , this cranial fontanelle is restricted to one or two openings in the frontals %Figure 3e). Our results corroborate de Pinna’s %1989) hypothesis that Potamoglanis is more closely related to the other Tridentinae genera and that Paravandellia is not closely related to them %Figure 1).

The presence of a short ventral process in the opercular bone, described by Baskin %1973) being half as long as the width of the tooth patch are also observed in Potamoglanis %Figure 2f). Among other Trichomycteridae , this ventral process is usually as long as the width of the opercular tooth patch.

The origin of the dorsal fin above or just posterior to the anal-fin origin was first proposed by Eigenmann %1914) as a diagnostic character for Tridentinae . Later, Baskin %1973) recovered this character state as a valid synapomorphy for the subfamily. As Potamoglanis also possess this unique positioning of both dorsal and anal fins, this character state is herein regarded as a synapomorphy for the Tridentinae .

The small size of the maxilla, described by Baskin %1973) to be less than one tenth the length of the entire neurocranium, is not seen in Po. anhanga . All the remaining Tridentinae genera and Potamoglanis species have a laminar and small maxillary bone, except Po. anhanga , which has the maxilla approximately equal in size to the premaxilla %Figure 2a). Despite being the smallest species among all the Trichomycteridae , Po. anhanga possesses the proportionally largest maxilla within the Tridentinae . Thus, the small size of the maxilla cannot be considered as a synapomorphy for the Tridentinae .

Costa and Bockmann %1994) delimited the TSVSG clade, comprising the Tridentinae , Sarcoglanidinae , Vandelliinae , Stegophilinae and Glanapteryginae . This hypothesis was based on four characters states: 1 – reduced interopercular patch of odontodes, with 15 or fewer odontodes; 2 – reduced number of pleural ribs %one to eight); 3 – metapterygoid reduced or absent and 4 – tip of parasphenoid not reaching or reaching only the anterior portion of basioccipital. According to the present topologies, the TSVSG clade is recovered as well as the morphological diagnostic character states of the TSVSG clade are found in all species of Potamoglanis . The interopercular patch of odontodes is reduced, with six to 15 odontodes %Figure 2f); Po. hasemani , Po. johnsoni and Po. wapixana possess two pairs of pleural ribs and Po. anhanga , one; the metapterygoid is extremely reduced in all species %Figure 2f), and the parasphenoid does not reach the basioccipital.

The Trichomycterinae , comprising the genera Trichomycterus , Hatcheria %Eigenmann, 1909), Scleronema %Eigenmann, 1917), Bullockia % Arratia, Chang, Menu-Marque & Rojas, 1978) and Ituglanis %Costa & Bockmann, 1993), was recovered as a monophyletic group by Datovo and Bockmann %2010). According to these authors, monophyly of Trichomycterinae is supported by the origin of the muscle levator internus 4 attaching the ventral surface of the pterotic and both ventral and dorsal surfaces of the supracleithrum. In other Trichomycteridae , this muscle only inserts onto the ventral surface of the pterotic and/or ventral surface of the posttemporosupracleithrum % Datovo & Bockmann, 2010). Despite not including Po. hasemani in their data matrix, Datovo and Bockmann %2010) mentioned that the condition of the levator internus 4 in this species is similar to the non-Trichomycterinae trichomycterids. The hypothesis of Trichomycterinae monophyly supported by Datovo and Bockmann %2010) contrasted with previous studies, in which Scleronema and Ituglanis were considered more closely related to members of the TSVSG clade % Costa & Bockmann, 1994; de Pinna, 1989, 1998). Our molecular analysis highly supports the morphological study by Datovo and Bockmann %2010).

DoNascimiento %2015) provided a morphological phylogeny focusing on internal relationships of the Stegophilinae , but also including representatives of every Trichomycteridae subfamily. This study was the first to assign a species of Potamoglanis in a phylogenetic framework. Potamoglanis hasemani was positioned within the TSVSG clade, as the sister group of the clade Tridentinae + Vandelliinae + Stegophil inae. As the data matrix was not included in DoNascimiento’s %2015) paper, and it is not available elsewhere, their characters cannot be evaluated.

4.2 | Monophyly of Potamoglanis

The synapomorphic character states herein proposed for Potamoglanis are as follows: an angle of 35–40° between the main longitudinal axis of the head and the main axis of the autopalatine %Figure 2a,b); the tubular thin shape of the second hypobranchial %Figure 2d); the fewer number of anal fin rays %six or seven); smaller eyes %not ventrally visible); interopercular and opercular patches of odontodes not juxtaposed; the presence of a long process on the anterior margin of the hyomandibula and the absence of a distal process in the hyomandibula %Figure 2f).

Costa and Bockmann %1994) proposed the presence of an anteriorly directed process on the hyomandibula as a synapomorphy for a clade composed by Glanapteryginae + Sarco glanidinae % Costa & Bockmann, 1994; fig. 6). This process is also found in Potamoglanis species %Figure 2f), which is herein treated as a distinct character from the posterodorsal process on the hyomandibula proposed by Baskin %1973) as a synapomorphy for the Tridentinae %Figure 2g). The posteriorly directed process present in Tridentinae is positioned in a different region of the hyomandibula and is thus treated as a synapomorphy for the clade comprising Miuroglanis , Tridens and Tridensimilis . The anterior process of the hyomandibula found in Potalmoglanis is unique among the Tridentinae .

Dutra et al. %2012) proposed the following character states to diagnose the T. hasemani group: 1 – presence of a wide cranial fontanelle; 2 – a “medially bent” autopalatine; 3 – first pectoral-fin ray longer than other rays; 4 – absence of the anterior portion of the infraorbital canal corresponding to pores i1 and i3; 5 – presence of one or none branchiostegal rays in the posterior ceratohyal. The presence of an elongate first pectoral-fin as synapomoprhic ray is subjective and Dutra et al. %2012) proposed no measurement or proportion. A long first pectoral-fin ray is also present in Tridentopsis pearsoni %Myers, 1925 ) % Baskin, 1973), making this character state invalid to diagnose Potamoglanis . The infraorbital canal could not be examined by us in other Tridentinae genera, but Arratia and Huaquin %1995) reported the absence of pores i1 and i 3 in T. pearsoni %Myers, 1925 ). Therefore, absence of the anterior portion of the intra-orbital canal system cannot be considered as diagnostic for Potamoglanis . The presence of one or none branchiostegal rays in the posterior ceratohyal cannot be regarded as a valid character state to diagnose the Potamoglanis due to intrapopulational polymorphism: in some specimens four or more branchiostegal rays are found in the posterior ceratohyal %Figure 2h,i), whereas in others, no branchiostegal rays are found in this region of the hyoid arch.

The “medially bent” autopalatine is herein observed to constitute an angle of 35–40° between the main longitudinal axis of the head and the main axis of the autopalatine in species of Potamoglanis and considered to be an exclusive character state among trichomycterids %Figure 2a,b). Furthermore, the posterior edge of the autopalatine in Po. hasemani , Po. johnsoni and Po. wapixana is directed to the lateral ethmoid and frontals and in Po. anhanga , it is extremely reduced, uniquely among trichomycterids. In the remaining Trichomycteridae , the autopalatine is directed to the suspensorium %Figure 2c).

The unique thin tubular shape of the second hypobranchial %Figure 3a) is herein proposed as a diagnostic character state for Potamoglanis . Usually among Trichomycteridae , the second hypobranchial is triangular, with its posterior margin equal in size or longer than the lateral margins %Figure 3b). In Potamoglanis , the lateral facets are elongated when compared to the posterior margin, thus forming a tubular shape.