Megalancistrus aculeatus, photo by J.W. Armbruster
Megalancistrus barrae Holotype, photo by J.W. Armbruster
Megalancistrus is not diagnosed by any unique characteristic.
Characteristics considered synapomorphic for Megalancistrus are: a mesial thin section of the anterohyal, reduction in the width of the accessory process of the first ceratobranchial, an extremely tall levator arcus palatini crest, mesial ridge paralleling the anterior margin of the metapterygoid and facing posteriorly, loss of a posterior facing ridge posteromesially on the metapterygoid, reduction of the lateral wall of the pterygoid channel of the metapterygoid to a slight ridge, loss of the connection of the metapterygoid facet for articulation with the lateral ethmoid, a potentially second loss of the interoperculo-mandibular ligament that has re-evolved in the Acanthicus group, contact of the canal plate solely with the quadrate, reduction in the size of the ossified Baudelots ligament to a short shelf, the metapterygoid condyle of the lateral ethmoid forming a lateral shelf, a large gap between the anterior process and the main body of the pterotic-supracleithrum, eight to nine postdorsal vertebra, greatly expanded lower hypurals, reduction in the width of the dorsal processes of the tripus to thin, and generally 10 or more dorsal fin rays.
A massive loricariid (the holotype of M. gigas is 530 mm TL - Isbrücker, 1980) that is extremely spiny and generally has 10 (occasionally 9 or 11) dorsal fin rays. The color is dark brown with very large spots on the head, sides, and fins or with light vermicualtions. Color of the abdomen is the same as on the sides. Four or more predorsal plates. Five rows of plates on the caudal peduncle. Abdomen completely plated in adults. Caudal fin forked, but without filaments. Cheek odontodes as in Acanthicus. Plates well keeled with rows of odontodes above and below the keel rows incomplete in specimens up to at least 200 mm. Pterotic-supracleithrum enlarged but not as large as in Acanthicus.
Megalancistrus is best identified from other Ancistrini by the
combination of an increase in the number of dorsal fin rays (usually 10,
occasionally 9 or 11 vs. 7-8, occasionally 9) and strongly keeled lateral
plates. Megalancistrus can be further separated from Pseudacanthicus
by the presence of a moderately enlarged pterotic-supracleithrum (Fig.
89), from Acanthicus by the presence of an adipose fin, and short
jaws forming an acute angle at union (vs. long, nearly forming a straight
line at union), from Leporacanthicus by an oval lower lip (vs. round)
and a lack of tentacles on the upper lip, from Dekeyseria by having
plates on the abdomen (abdominal plates develop fairly late ontogenetically),
nine or more dorsal fin rays (vs. 7), and young specimens can be separated
from Dekeyseria by having only the keel row of odontodes developed
(vs. having rows of odontodes above and below the keel rows), and from
Pterygoplichthys by an enlarged pterotic-supracleithrum and by having the odontodes on the head large and stout (vs. small and slender). Megalancistrus
is from the Paraná and São Francisco systems, and Pseudacanthicus
is from the Amazon and Orinoco systems and the Guyanas.
The genus has been reported from the upper Orinoco, the eastern, north-flowing
Amazon tributaries, and the Rio Tocantins (Stawikowski, 1992).
A large-river dweller that eats freshwater sponges and probably other invertebrates. There is also a lot of wood in the gut, but it appears as if this was consumed accidentally.
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. 1994. Scobinancistrus aureatus, a new species of loricariid catfish from the Rio Xingu (Loricariidae: Ancistrinae). Tropical Fish Hobbyist 43:236?242.
Stawikowski, R. 1992. Die Gattung Leporacanthicus Isbrücker &
Nijssen, 1989. Harnischwelse, DATZ-Sonderheft, September 1992: 58-61.