Parancistrus aurantiacus, photo by K.S. Cummings
Parancistrus is diagnosed by two unique characteristics: the posterior of the hyomandibula greatly deflected mesially such that the opercle sits at nearly a right angle to the main axis of the body and the presence of fleshy folds on the naked area around the dorsal fin and at the pectoral fin insertion in at least the breeding males. Other characteristics considered synapomorphic for Parancistrus are: an ossified second basibranchial, a medially placed interohyal , a deep posterior pouch on the posterohyal, loss of the mesial contact of the hyomandibula and quadrate , loss of the raised triangular section at the intersection of the levator arcus palatini crest and the preoperculyomandibular ridge, a mesial ridge at the anterodorsal corner of the metapterygoid facing anteriorly, a taller mesial than lateral wall of the pterygoid channel, loss of contact of the metapterygoid facet and the lateral wall of the pterygoid channel, a wide, long anterior process of the metapterygoid, presence of a cartilage at the anterior corner of the mandible, an unexposed opercle, a short, wide parasphenoid, a wide ventral process of the sphenotic, a greatly expanded lower lobe of the hypural plate, thick dorsal processes of the tripus, an expanded ventral portion of the dorsal fin membrane that contacts the preadipose plate, the transverse process of the first dorsal fin pterygiophore complete to the anteroventral margin, a straight, tall posterior margin of the posterior process of the cleithrum, an angled contact of the posterior processes of the pelvic basipterygium, large gill openings, elongated odontodes on the sides of breeding males, and small tentacules on the pectoral fin spines.
Parancistrus is a stout loricariid with the dorsal fin membrane connected to the adipose fin spine. Color is typically slate gray to black, occasionally with white streaks. Abdomen completely plated in adults. Caudal fin is almost straight and angled posteroventrally. Three predorsal plates. Five rows of plates on the caudal peduncle. The naked area between the lateral plates and the dorsal fin is large and has numerous fleshy, fimbriate folds (see below). The gill openings are large.
Breeding males have elongated odontodes on the pectoral fin spines and the posterolateral plates. The fleshy, fimbriate folds at the dorsal and pectoral fin insertions may be a breeding male trait.
Flesh around dorsal fin showing fimbriate folds in at least breeding male Parancistrus,anterior is left. Photo by J.W. Armbruster
Parancistrus differs from all other loricariids by the presence of fleshy, fimbriate folds on the naked area surrounding the dorsal fin (may be a breeding male character, see above) and from all but some Pogonopoma
and Rhinelepis by the large gill openings. Parancistrus differs from all loricariids except some Baryancistrus, Delturus, and
by having the dorsal fin membrane attached to the preadipose plate.
Additionally Parancistrus can be separated from from Delturus by 7 (vs. 8-10) dorsal-fin rays, one (vs. three or more) preadipose plate, a caudal peduncle that is round (vs. triangular) in cross-section, and evertible cheek odontodes, from Spectracanthicus by a wide, rounded head (vs. slender and pointed), premaxillas nearly forming a straight line at their union (vs. forming an acute angle), and thin teeth (vs. stout).
In her review of the genus, Rapp Py-Daniel (1989) stated that the only
time P. aurantiacus was collected was when the river had dried to
isolated pools. She suggested that the large gill openings may be
an adaptation to low oxygen levels. Rhinelepis similarly has
large gill openings but also has a diverticulum hypothesized to act as
an accessory respiratory system (Armbruster, 1998). It is possible
that the expanded gill openings are not associated with respiration and
may be due to some other factor such as feeding.
The Rio Tocantins of Brazil. The range may be greater as the type
locality for P. aurantiacus is given as L'Ucayale; however, no specimens
are known from outside of the Tocantins.
Armbruster, J.W. 1997. Phylogenetic relationships of the sucker-mouth armored catfishes (Loricariidae) with particular emphasis on the Ancistrinae, Hypostominae, and Neoplecostominae. Unpubl. Ph.D. dissertation. University of Illinois, Urbana-Champaign. 409 pp.
Rapp Py-Daniel, L.H. 1989. Redescription of Parancistrus aurantiacus
1855) and a preliminary description of two new genera:
n.g. and Oligancistrus n.g. (Siluroidei; Loricariidae). Cybium