Acestridium Haseman, 1911

Delapieve, Maria Laura S., A., Pablo Lehmann & Reis, Roberto E., 2017, An appraisal of the phylogenetic relationships of Hypoptopomatini cascudinhos with description of two new genera and three new species (Siluriformes: Loricariidae), Neotropical Ichthyology 15 (4), No. e 170079, pp. 1-38 : 21

Genus Acestridium Haseman, 1911

Acestridium Haseman, 1911: 319 . Type-species: Acestridium discus Haseman, 1911 . Type by monotypy. Gender neuter.

Included species. Acestridium colombiensis Retzer, 2005 ; Acestridium dichromum Retzer, Nico, Provenzano, 1999 ; Acestridium discus Haseman, 1911 ; Acestridium gymnogaster Reis, Lehmann, 2009 ; Acestridium martini Retzer, Nico, Provenzano, 1999 ; Acestridium scutatum Reis, Lehmann, 2009 ; Acestridium triplax Rodriguez, Reis, 2007 .

Diagnosis. Acestridium is diagnosed as monophyletic based on ten exclusive synapomorphies: large exposure of mesethmoid on dorsal surface of snout (char. 5.2); anterolateral margin of anterohyal concave (char. 32.1); lack of articulation of neural spine of sixth vertebral centrum to parieto-supraoccipital (char. 39.1); dorsal fin attached to neural spine of vertebral centrum 11-13 (char. 45.2); anterior margin of cleithrum convex, with hollow in mesial lamina (char. 58.1); lateropterygium absent (char. 61.1); ventral margin of infraorbital 4 expanded anteroventrally, its deepest point surpassing anterior margin of bone (char. 72.2); mid-ventral series of lateral plates absent (char. 83.3); and spatulate projections of snout are present (char. 90.1). In addition, there are 14 non-exclusive synapomorphies: ventral condyle of mesethmoid oval (char. 3.3); nasal capsule completely encapsulated (char. 7.0); posterolateral portion of lateral ethmoid narrower or as wide as anterior margin (char. 10.0); dorsal wall of swimbladder formed by compound pterotic and supraoccipital (char. 13.1); condyle of hyomandibula articulates to neurocranium only by prootic (char. 15.3); crest for insertion of levator arcus palatini muscle robust (char. 18.0); one lateral foramen in hyomandibula (char. 20.1); metapterygoyde-hyomandibula suture large (char. 21.0); interhyal absent (char. 33.1); second and third radials of pectoral fin joined along longest axis (char. 54.1); two postrostral plates (char. 66.2); odontodes on dorsal surface of pectoral-fin spine randomly distributed (char. 95.0); iris operculum present (char. 100.0); and lack of skin flap on dorsal surface of first pelvic-fin ray in males (char. 103.1).

Comparisons. The species of Acestridium are further distinguished from other hypoptopomatins by a highly distinctive slender and elongated body, usually with green color in life. Acestridium is further distinguished by a snout that is produced into an anterior spatulate projection that bears hypertrofied odontodes (vs. snout not elongated) and by lacking the mid-ventral series of lateral plates (vs. midventral series present). They also have very delicate and poorly developed fins and the odontodes of the trunk are small and aligned forming conspicuous rows (vs. normally developed fins and odontodes not clearly aligned). It differs from all other genera, except for some species of Otocinclus , by possessing an iris operculum (vs. iris operculum absent). Acestridium is additionally distinguished from Hypoptopoma , Nannoptopoma , Otocinclus , Leptotocinclus , and Nannoxyropsis by the caudal peduncle, which is strongly depressed, elongated, and narrow (vs. caudal peduncle oval in cross-section). Acestridium is further distinguished from Niobichthys , its sister genus, by having a spatulate snout projection (vs. spatulate snout projection absent), by ten or 12 branched rays in the caudal-fin (vs. 14 branched rays); by the lateral-line canal that shifts to the ventral series of lateral plates after truncation of the median plate series (vs. shifting to the dorsal series); and by odontodes on the dorsal surface of the pectoral-fin spine that are randomly arranged (vs. arranged in two or three rows).