Aphanotorulus phrixosoma (Fowler 1940)

(Fig. 13)

Plecostomus phrixosoma Fowler, 1940a: 233, figs. 21–23. Type locality: Ucayali River basin, Contamana, Peru. Holotype: ANSP 68650.

Material examined. Holotype: ANSP 68650, Ucayali River basin, Contamana, Peru.

Diagnosis. Aphanotorulus phrixosoma differs from A. ammophilus and A. unicolor by a single, large buccal papillae (vs. multiple buccal papillae in A. ammophilus and A. unicolor); from all other Aphanotorulus and Isorineloricaria by the presence of hypertrophied odontodes covering all lateral and ventral surfaces except the head in breeding males (vs. covering all dorsal and lateral surfaces including the head in I. spinosissima and vs.

covering portions of dorsal and lateral surfaces in all other Isorineloricaria and Aphanotorulus). This species is known only from its holotype, which is a breeding male.

Description. Morphometric data given in Table 4. Head moderately deep. Interorbital surface flat; nape of unknown shape due to damage.

Pectoral-fin spine reaches past point of insertion on the pelvic fin. Pelvic-fin spine reaches past the point of insertion of the anal fin. Caudal fin highly forked with lower lobe longer than upper lobe.

Lateral line plates 28; dorsal fin base plates seven; folded dorsal plates 12; plates between dorsal and adipose fin seven; adipose fin plates two; anal fin base plates two; plates from anal fin insertion to last plate on caudal peduncle 15; plates in folded pectoral fin six; number of teeth on dentary 21; number of teeth on premaxilla 15.

Sexual Dimorphism. Although small, holotype breeding male. Entire dorsal and lateral surfaces of body covered with hypertrophied odontodes. Dorsal surface of head without hypertrophied odontodes.

Color in alcohol. Body background color light, almost white. Spots on head small and circular; densely packed. Dorsal-fin membranes with two rows of spots. Few spots visible on body due to hypertrophied odontodes.

Distribution. Rio Ucayali, near Contamana, Peru. Known only from type locality (Fig. 10).

Comments. We hypothesize that this species is a hybrid. This specimen was collected in an area where both A. horridus and A. unicolor are sympatric. It should be noted that the specimen does not share the multiple buccal papillae or the unicuspid teeth of nuptial A. unicolor, yet it is the smallest breeding male (with hypertrophied odontodes) collected of any species of Aphanotorulus or Isorineloricaria, other than A. ammophilus and A. unicolor. Given these combinations of characters and the high sampling effort in the drainages around the type locality resulting in no more specimens of the species, hybridization is a credible possibility.