Modifications of the digestive tract for holding air in loricariid and scoloplacid catfishes

Loricariid catfishes have evolved several modifications of the digestive tract that appear to function as accessory respiratory organs or hydrostatic organs. Adaptations include an enlarged stomach in Pterygoplichthys, Liposarcus, Glyptoperichthys, Hemiancistrus annectens, Hemiancistrus maracaiboensis, Hypostomus panamensis, and Lithoxus; a U-shaped diverticulum in Rhinelepis, Pseudorinelepis, Pogonopoma, and Pogonopomoides; and a ringlike diverticulum in Otocinclus. Scoloplacids, closely related to loricariids, have enlarged, clear, air-filled stomachs similar to that of Lithoxus. The ability to breathe air in Otocinclus was confirmed; the ability of Lithoxus and Scoloplax to breathe air is inferred from morphology. The diverticula of Pogonopomoides and Pogonopoma are similar to swim bladders and may be used as hydrostatic organs. The various modifications of the stomach probably represent characters that define monophyletic clades. The ovaries of Lithoxus were also examined and were shown to have very few (15-17) mature eggs that were large (1.6-2.2 mm) for the small size of the fish (38.6-41.4 mm SL.).