Colossimystax Armbruster & Lujan, 2024

Armbruster, Jonathan W. & Lujan, Nathan K., 2024, New tribe-level classification of Hypostominae (Loricariidae) based on optimization of morphological states on DNA-based relationships, with descriptions of three new tribes and two new genera, Neotropical Ichthyology 22 (4), pp. e 240108-e 240108 : e240108-240108

Colossimystax Armbruster & Lujan , new genus

urn:lsid:zoobank.org:act:3F6C7DC3-24AC-4BE2-8D7F-BE77FC4CAAD5

( Figs. 1A , 2C )

Type-species. Pseudancistrus pectegenitor Lujan, Armbruster & Sabaj Pérez, 2007

Included species.

Pseudancistrus pectegenitor Lujan, Armbruster, Sabaj Pérez, 2007:164 , figs. 1b, 2-3.

Río Casiquiare , 73 kilometers northeast of San Carlos de Río Negro, 02.3525°, -066.5753°, Amazonas, Venezuela.

Phylogenetic diagnosis. Spindle-shaped hypohyal (21: 0>1), upper pharyngeal jaw with invagination in shelf (29: 0>1), reversal to horizontally oriented preopercle (61: 1>0), wide, blunt articulating condyle of quadrate (67: 0>1), tall ridge on lateral ethmoid for contact with metapterygoid (97:2), reduction to three to eight vertebrae from first normal neural spine posterior to dorsal fin to spine under preadipose plate (120: 1>0; this is concomitant with a unique shortening of the body posterior to the dorsal fin), flared distal margin of rib of sixth vertebral centrum (128: 0>1), ten dorsal-fin rays (142: 0>1), reduction to five or six dorsal-fin radial elements with transverse processes (145: 1>0), reversal to wide posterior process of coracoid (158: 2>1), presence of dentary papillae (180: 0>1), presence of tentacules on hypertrophied snout and pectoral-fin spine odontodes, tentacules shorter than associated odontodes (208: 0>1, 209: 0>1). Further, Colossimystax has a larger supraoccipital crest than Stellantia or other species currently or formerly in Pseudancistrus , with crest most similar to Hemiancistrus medians but still taller and with a greater ventromedial lamina ( Fig. 6 ).

Comparative diagnosis. Colossimystax is readily distinguished from all other Hypostominae except Acanthicini , Chaetostomatini , and Pterygoplichthys by having 10 dorsal-fin rays (vs. 7); from all other Hypostominae except Stellantia by having the dorsal lamina of each ventral plate of the caudal peduncle concave, accentuating the ventrolateral keel of the caudal peduncle (vs. ventral plates on caudal peduncle lacking strongly concave dorsal lamina or plates rounded and keel absent); from other species with more than seven dorsal-fin rays except Dolichancistrus by nuptial males having hypertrophied cheek odontodes reaching to at least the edge of the third plate of the midventral series (vs. maximally reaching the first plate); from Dolichancistrus by nuptial males having many hypertrophied cheek odontodes (vs. 1 or 2); from Hypostominae except Chaetostoma and Lithoxancistrus by having a single or small cluster of enlarged papillae located medially on dentaries interior to tooth rows (vs. all papillae small).

Description. See Tab. S3 and Lujan et al. (2007).

Geographical distribution. Found in main channels and major tributaries of the Orinoco River upstream of San Fernando de Atabapo and in the Casiquiare River above its confluence with the Negro River, including the lower Ventuari River. Currently known exclusively from Venezuela but may also occur in neighboring basins of Colombia and Brazil ( Fig. 5 ).

Etymology. Colossimystax is from the Latin colossicon for gigantic and mystax for moustache in reference to the very long cheek odontodes that look like a moustache, a masculine noun.

Conservation status. Colossimystax pectegenitor was evaluated as Least Concern (LC) by the IUCN ( Echevarría, 2019). Only three disjunct localities are known, but all are main channel habitats, suggesting a likely contiguous distribution throughout a remote area of southern Venezuela in which the most threatening impact is gold mining.

Material examined. Amazonas, Venezuela. Holotype: MCNG 54797 [ex AUM 42130], 241.6 mm SL, Río Casiquiare . Paratypes: ANSP 182801 [ex AUM 42181], 1, 225.1 mm SL, Río Orinoco ; AUM 42202 , 1, 227.0 mm SL, Río Casiquiare ; AUM 43192 , 1 c&s, 173.6 mm SL, Río Orinoco .

Figures

FIGURE 1 | A. Colossimystax pectegenitor, Holotype, MCNG 54797, 241.6 mm SL. Photo by NKL. B. Stellantia siderea, AUM 43443, 147.1 mm SL. The two genera are described below. Photo by M. H. Sabaj.

FIGURE 2 | Caudal peduncles of A. Pseudancistrus nigrescens, AUM 35742, 137.2 mm SL; B. Stellantia siderea, AUM 42185, 180.0 mm SL; and C. Colossimystax pectegenitor, paratype, AUM 42202, 227.0 mm SL. DL is the dorsal lamina of the indicated plate, and the keel lies just ventral. In A, there is only a slight keel (~3 plates) with the dorsal laminae straight to very slightly concave while the keel extends in front of the adipose fin in B and C and the dorsal laminae are concave.

FIGURE 5 | Distribution of Colossimystax pectegenitor (black dots) and Stellantia siderea (white dots).

FIGURE 6 | µCT renderings of neurocrania (left, lateral view) of A. Hemiancistrus medians, ANSP 187122, B. Pseudancistrus barbatus, ROM 86176, C. Colossimystax pectegenitor, AUM 42130, D. Stellantia siderea, AUM 54310, E. Lithoxancistrus coquenani, AMNH 91025, F. Lithoxancistrus yekuana, AUM 39473. Scale bars = 1 cm.