Psilorhynchus, , Ramaswami, 1952

Conway, Kevin W., 2011, Osteology of the South Asian Genus Psilorhynchus McClelland, 1839 (Teleostei: Ostariophysi: Psilorhynchidae), with investigation of its phylogenetic relationships within the order Cypriniformes, Zoological Journal of the Linnean Society 163 (5), pp. 50-154 : 136-138

MONOPHYLY OF PSILORHYNCHUS

The phylogenetic analysis recovered 16 derived characters in support of the monophyly of Psilorhynchus , including: (1) the presence of a large preepiphysial fontanelle, bordered anteriorly by the ethmoid complex, and laterally and posteriorly by the frontals (character 9, 0> 1); (2) the presence of a heavily ossified shelf on the medial face of infraorbital 2 that extends medially and cups the ventral surface of the lateral ethmoid (character 33, 0> 1); (3) a greatly elongate autopalatine that extends far anterior past the ethmoid region, and terminates dorsal to the posterodorsal edge of the maxilla (39, 0> 1); (4) the presence of a connection between the infraorbital sensory canal and preopercular sensory canal (character 49, 0> 1); (5) the presence of an interopercular– preopercular articulation (character 51, 0> 1); (6) the presence of basibranchial 4 (character 74, 0> 1); (7) the absence of hypobranchial 2 (hypobranchial 2 cartilage present only) (character 75, 0> 1); (8) cartilage between upper gill-arch elements confluent (character 85, 0> 1); (9) the absence of the neural spine on the fourth vertebral centrum (character 94, 0> 2; reversed in P. robustus ); (10) lateral process of the second vertebral centrum fused to the outer arm of the os suspensorium (character 95, 0> 1); (11) the presence of a large cup-shaped depression on the anterior face of the fifth rib, close to its point of articulation with parapophysis (character 105, 0> 1); (12) the presence of a post-Weberian process (character 106, 0> 1); (13) the absence of the posttemporal (character 110, 0> 1); (14) the complete absence of postcleithra (character 111, 1> 2); (15) rib associated with pelvic splint greatly thickened and robust (character 117, 0> 1); and (16) the absence of hypural 6 (character 122, 0> 1). Of these 16 derived characters, eight (39, 49, 51, 74, 95, 111, 117, and 122) also provide support for clades of cypriniforms other than Psilorhynchus in the phylogenetic analysis (see above). The remaining eight characters (9, 33, 75, 85, 94, 105, 106, and 110) provide support only for the Psilorhynchus clade. Of these eight, four [the presence of a preepiphysial fontanelle (character 9), the presence of a medial shelf on infraorbital 2 (character 33), the confluence of cartilage between the upper gill-arch elements (character 85), and the presence of the post-Weberian process (character 106)] appear to be unique to Psilorhynchus amongst cypriniform fishes. Although unique to Psilorhynchus amongst the taxa included for the phylogenetic analysis herein, the absence of hypobranchial 2 (character 75), neural spine 4 (character 94), and the posttemporal (character 110), and a cup-shaped depression on the fifth rib of males (character 105), have been reported for other cypriniform fishes ( Conway & Britz, 2007; Britz & Conway, 2009).

Interestingly, two of the characters in support of Psilorhynchus monophyly [presence of the cup-shaped depression on the fifth rib (character 105) and the post-Weberian process (character 106)] relate to sexual dimorphism of the axial skeleton, and are present only in male specimens. Skeletal sexual dimorphisms, which affect structures other than fin rays, are rare amongst cypriniform fishes, and as yet have been reported only in a small number of miniature cyprinids, including Danionella (Britz, 2002; Britz et al., 2009), Paedocypris ( Kottelat et al., 2006; Britz & Conway, 2009), and Sundadanio ( Conway & Britz, 2007) . All three of these miniature cyprinids, which are hypothesized to form a monophyletic group (Britz & Conway, 2009), exhibit unique sexual dimorphisms of the Weberian apparatus that involve hypertrophy of the lateral process of the second vertebral centrum and the outer arm of the os suspensorium. Additionally, these miniature taxa also exhibit sexual dimorphism of the pectoral girdle ( Sundadanio ) or both paired-fin girdles ( Danionella and Paedocypris ). The sexual dimorphisms of Psilorhynchus , which involve hypertrophy of the bony swim-bladder capsule and a cup-like depression on the fifth rib in males, are similar (though not homologous) to the sexual dimorphisms of the axial skeleton exhibited by male Sundadanio . In Sundadanio , the fifth rib is hypertrophied and exhibits a cup-like depression on its anterior edge that serves as the insertion point for a large bulbous muscle that originates on the similarly hypertrophied outer arm of the os suspensorium ( Conway & Britz, 2007). Conway & Britz (2007) hypothesized that this large bulbous muscle, a modification of the epaxial musculature, represented a drumming muscle and was probably involved in sound production in males. Dissections of alcohol specimens of Psilorhynchus revealed the presence of a similar bulbous muscle between the outer arm of the os suspensorium and the fifth rib in males only (a detailed description of which is reserved for a future publication), suggesting that the sexual dimorphisms present in Psilorhynchus may also be involved in sound production.

Of the 12 species of Psilorhynchus examined during this study, cleared and stained specimens of both sexes were only available for six ( P. breviminor , P. gracilis , P. rahmani , P. robustus , P. sucatio , and P. brachyrhynchus ). Of the remaining species ( P. balitora , P. melissa , P. nepalensis , P. pavimentatus , P. pseudecheneis , P. rahmani , and P. tenura ), sexual dimorphism was not directly observed in the cleared and stained specimens. Based on the presence of a post-Weberian process and a cup-like depression on the anterior face of the fifth rib in all cleared and stained specimens of P. balitora , and the single specimens of P. melissa , P. nepalensis , and P. rahmani examined, these specimens were interpreted as males, and although females were not observed, characters 105 and 106 were coded as present for these three taxa in the phylogenetic analysis. The single cleared and stained specimens of P. pavimentatus and P. tenura examined lacked both the post-Weberian process and the cup-shaped depression on the anterior edge of the fifth rib, as did all six of the cleared and stained specimens of P. pseudecheneis examined. It is unclear (based on the available material) whether all cleared and stained specimens of these three latter taxa are female (unfortunately gonadal examination was not conducted prior to clearing and staining), or whether the sexual dimorphisms of the axial skeleton are actually absent in these species. Characters 105 and 106 were thus coded as missing (?) for these three taxa in the phylogenetic analysis. Despite this absence of information, however, these two characters were recovered at the level of the Psilorhynchus clade, providing support for the monophyly of Psilorhynchus as outlined above. Additional specimens of these three taxa should be examined in order to confirm whether the unique sexual dimorphisms of the axial skeleton present in other species of Psilorhynchus are also present in these species.

In addition to the unusual sexual dimorphisms, the skeleton of Psilorhynchus is also characterized by a large number of reductions. The following bones, present in most cypriniforms, are absent in all species of Psilorhynchus examined: (1) intercalar; (2) hypobranchial 2 (only cartilage present); (3) posttemporal; (4) postcleithrum; (5) supraneurals 5–9 (cartilages also absent); (10) second uroneural; and (11) hypural 6; (12) all species, excluding P. balitora , P. breviminor , and P. brachyrhynchus , also lack gill rakers; (13) all species, excluding P. gracilis , P. robustus , and P. melissa , also lack the mandibular sensory canal; (14) all species, excluding P. robustus , lack the neural spine on the fourth vertebral centrum; (15) in addition, P. sucatio is unique amongst species of Psilorhynchus in lacking hypobranchial 3 (only cartilage present); and (16) the supraorbital [Chen (1981) also noted the absence of the supraorbital in P. homaloptera , but this could not be confirmed during the course of this study]. Such reductions, not frequently encountered in fishes within the size range of Psilorhynchus (the adults of which exhibit maxima of 35–110 mm in standard length), are more common in fishes considered to be miniature (maximum standard length 26 mm or less; sensu Weitzman & Vari, 1988), and provide further support for the msonophyly of Psilorhynchus .