Lithoxus Eigenmann, 1910


Lithoxus lithoides, photo above by J.W. Armbruster and below by M.H. Sabaj

TAXA LIST


Lithoxus is very dorsoventrally flattened.  Lithoxus (along with its sister, Exastilithoxus) are fairly unique among loricariids in that they subsist entirely on a diet of insect larvae.  Armbruster (1998) describes a unique, air-holding stomach in Lithoxus and suggests, based on the small number of very large eggs, that Lithoxus has well-developed parental care.  The following is an excerpt from Armbruster (1997).

GENUS SYNONYMY

Paralithoxus Boeseman, 1982.  Originally described as a subgenus, but elevated to generic rank by Isbrücker and Nijssen (1985), and subsequently returned to the synonymy of Lithoxus and is no longer recognized as a valid subgenus by Nijssen and Isbrücker (1990).


SPECIES
DIAGNOSIS

Lithoxus is diagnosed by three unique characteristics: a process on the preoperculo-hyomandibular ridge, a ventrally oriented, spoon-shaped anterior process of the metapterygoid, and an enlarged, thin-walled stomach from which the intestine exits dorsally.  Other characteristics considered synapomorphic for Lithoxus are: a reduction in the width of the fifth ceratobranchial such that it is bar-shaped, loss of the accessory process of the first epibranchial, loss of the shelf of the parasphenoid, an expanded distal end of the expanded rib of the sixth vertebral centrum, an abductor fossa of the pectoral girdle that is almost flat anteromesially, and a reversal to thin anterolateral processes of the pelvic basipterygium.


DESCRIPTION

Lithoxus contains some of the most dorsoventrally flattened fishes in the world.  Color is typically slate gray to tan with a few lighter markings on the body and occasionally bands in the pectoral and caudal fins.  Ventral surface ranges from white to slightly lighter than the sides.  Abdomen naked.  Caudal fin slightly forked.  Four or more predorsal plates.  Three rows of plates on the caudal peduncle.  The lower lip is large and round as in Exastilithoxus with the edge sometimes frilled, but not with the barbels seen in Exastilithoxus.  The jaws are reduced with very few teeth (less than six per premaxilla).  The premaxillary teeth are longer than the dentary teeth.

Breeding males develop extremely long odontodes on the leading edge of the pectoral fin spine.  The expanded stomach is slightly larger in males.


COMPARISONS

Lithoxus can be separated from all other Ancistrini except Leporacanthicus and Exastilithoxusby having a round (vs. oval) lower lip. Lithoxus can be separated from Leporacanthicus by lacking tentacles on the upper lip and by having 3 (vs. 5) rows of plates on the caudal peduncle, and from Exastilithoxus by lacking short fimbriae along the lower lip (the posterior rim of the lower lip is smooth in Lithoxus).


ECOLOGY

Lithoxus is said to inhabit both rivulets (Boeseman, 1982) and medium-sized creeks (Eigenmann, 1912). We collected some of Lithoxus from riffle on the main-stem Essequibo River in Guyana.  These peripheral habitats are among the first parts of the river to dry and the respiratory stomach may have evolved to handle this periodic drying.  The large size and small number of eggs suggest that parental care is well developed, but nothing is known of the breeding habits of Lithoxus.  The flattened morphology suggests that Lithoxus lives under rocks.


DISTRIBUTION

Known from throughout the Guyanas and also in the Rio Negro of Brazil. We recently discovered a new species in the upper Rio Orinoco.


LITERATURE CITED

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.

Armbruster, J.W. 1998. Modifications of the digestive tract for holding air in loricariid and scoloplacid catfishes. Copeia 1998:663-675.

Boeseman, M. 1982. The South American mailed catfish genus Lithoxus Eigenmann, 1910, with the description of three new species from Surinam and French Guyana and records of related species (Siluriformes, Loricariidae). Proc. Konink. Nederl. Akad. Wetens., ser. C 85:41?58.

Eigenmann, C.H. 1912. The freshwater fishes of British Guiana, including a study of the ecological grouping of species and the relation of the fauna of the plateau to that of the lowlands. Mem. Carnegie Mus., 5:1?578.

Isbrücker, I.J.H. and H. Nijssen. 1985. Exastilithoxus hoedemani, a new species of mailed catfish from rio Marauiá, est. Amazonas, Brazil (Pisces, Siluriformes, Loricariidae). Spixiana 8:221?229.

Nijssen, H. and I.J.H. Isbrücker. 1990. Lithoxus stocki, a species new to science of ancistrin loricariid catfish from the Maroni River drainage, with a comparison of the primary type specimens of six species of Lithoxus (syn. Paralithoxus) (Pisces, Siluriformes, Loricariidae). Bijd. tot Dierk. 60:327-333.


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