A. adonis Isbrücker and Nijssen 1988
A. hystrix Spix 1829

Acanthicus is diagnosed by two unique characteristics: a greatly expanded pterotic-supracleithrum due to an extreme increase in the size of the swim-bladder capsule and connection of the dilatator operculi solely to the hyomandibula.  Other characteristics considered synapomorphic for Acanthicus are: a wide anterohyal, an elongate first basibranchial, a wide hyohyal, elongated first hypobranchials, loss of the posterior pouch on the posterohyal, angle of the levator arcus palatini crest to above the metapterygoid facet for articulation with the lateral ethmoid, a raised and posteriorly deflected triangular section at the junction of the levator arcus palatini crest and the preoperculo-hyomandibular ridge, loss of the ventrolateral hyomandibular ridge, a long anterior process of the metapterygoid, a vertical orientation of the preopercle, loss of the longitudinal quadrate ridge, reversal of the size of both jaws from small to large, three rows of plates between the suprapreopercle and the exposed opercle, reduction in the number of infraorbital plates to five, the lateral line continuing onto the plate beyond the hypurals, reversal to the mesethmoid disk not extending anterior to the main body of the mesethmoid, loss of the anterior process of the pterotic-supracleithrum, an increase in the number of postdorsal vertebrae to, a greatly expanded distal tip of the enlarged rib of the sixth vertebra, loss of the adipose fin, a greatly elongated posterior processes of the coracoid, wide anterolateral processes of the pelvic basipterygium, and reduction of the size of the teeth.

Acanthicus is a very large (at least up to 1 m), very spiny loricariid that lacks an adipose fin.  Color is typically black, sometimes gray, occasionally with white spots with the abdomen the same color as the rest of the body.  Abdomen completely covered in small plates.  Caudal fin deeply forked with the lobes of equal length and each proceeding posteriorly as a thin filament.  Four or more predorsal plates.  Five rows of plates on the caudal peduncle.  Pterotic-supracleithrum greatly enlarged.  Pectoral fin spines extremely long.  The entire dorsal surface of the head is covered in stout, sharp odontodes.  The odontodes form a sharp keel on the lateral plates and, in juveniles, there are few to no odontodes on the plates above and below the keel rows.  Cheek odontodes fairly thin, but numerous (up to 100 or more).  A photograph in Burgess (1989, p. 730, right column, third from top) is clearly of an Acanthicus, but the fish possesses a small adipose fin.  It is unknown whether this fish represents an undescribed species or if it is just aberrant.

Males may have more and longer cheek odontodes and greatly elongated odontodes on the pectoral fin spine.

Acanthicus is readily recognized from all other Ancistrini (except some Chaetostoma and Leptoancistrus) by the absence of an adipose fin.  In addition, the pterotic-supracleithrum is larger in Acanthicus than in any other loricariid.  Most similar to Leporacanthicus, Megalancistrus and Pseudacanthicus, but the jaws are long, nearly forming a straight line at union (vs. short, forming an acute angle at union).  Acanthicus can be separated from all other Ancistrini (except Dekeyseria) and most Pterygoplichthini by the presence of enlarged odontodes forming keels along the lateral plates. Acanthicus can be separated from Dekeyseria by the loss of the adipose fin and presence of plates on the abdomen and Pterygoplichthys with keels by loss of the adipose fin, long, stout odontodes on the head, and fewer than nine dorsal fin rays.

Found in the Amazon and Orinoco rivers and possibly in Guyana (Burgess, 1989).

Known from large rivers.  The enlarged swimbladder may be used for sound production by drumming or strumming the enlarged rib of the sixth vertebral centrum upon it.  The swimbladder may also provide hydrostasis, although nothing is known of the life history of Acanthicus and it is unclear how it could make use of the water column.

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.

Burgess, W.E., 1989. An atlas of freshwater and marine catfishes, a preliminary survey of the Siluriformes. T.F.H. Publications, Neptune City, New Jersey. 784 pp.