Comprehensive DescriptionRead full entry
A West African species of Acanthixalus, morphologically similar to the only other known member of the genus, the central African A. spinosus. The only substantial difference seems to be the wider head of male A. sonjae (Schiøtz 2007, pers. comm.). The head has a long, pointed snout, with canthi and slightly concave lores. Eyes are protuberant and bear rhomboid-shaped pupils. The tympanum is absent. Paired, oval subgular glands are present,extending longitudinally. The body is stout and flattened with quite warty skin dorsally and on the extremities. In addition, a transverse row of bigger warts in the neck area gives the appearance of a "crown"; four particularly large warts are present in the shoulder area; and two particularly large warts are visible on the sacral region. Ventrally the skin is granular. Both hands and feet are webbed, and fingers and toes bear enlarged discs. Males bear a pair of subgular glands and numerous hook-shaped tarsal spines, and have larger digital discs, but possess no vocal sacs (Rödel et al. 2003).
The dorsum and extremities are yellow-green and bear numerous white warts with black spines. Three black crossbands as well as black spots and lines are present on the dorsum, as well as a transverse black band on the sacral region. A single black triangular mark is located between the eyes, and paired black triangles are visible on the shoulders, with each triangle apex pointing towards the posterior. Black transverse bands are also present on the upper lip, femur and tibula fibula. Flanks are green to white. Ventral surfaces are gray brown with black markings and white warts lacking black spines. Gular glands (present on males) are beige-colored. The anal region is bright yellow. Finger and toe discs are white with a black spot above and beige below; heels are orange to yellow. This frog is capable of metachrosis, with colors darkening almost to black in the sunlight, within a few minutes. The iris is black with white stripes radiating outward (Rödel et al. 2003).
Juveniles are more brightly colored, with the dorsal ground color often being yellow to orange, and the ventral side orange as well. Juveniles also have four enlarged yellow warts on the neck (Rödel et al. 2003).
Tadpoles are similar to those of A. spinosus but have longer tails. The body is compact, rounded dorsally and slightly depressed laterally. Eyes are very small and lateral. The oral disc is subterminal, with a tooth formula 1: 2 + 2/ 3, wide serrated, curved jaw sheaths, a single row of lateral papillae on either side and two rows of caudal papillae (medially uniserial). The spiracle is sinistral and the vent is medial. The tail tip is broadly rounded (Rödel et al. 2003).
Tadpoles taken directly from treeholes were a fleshy violet color, which quickly changed to black in the sunlight. In some tadpoles, the tail tip is almost transparent (Rödel et al. 2003).
Genetic analysis (using mitochondrial 16S RNA) points to differences between A. sonjae and A. spinosus at a level comparable to species differences in other amphibian groups, and A. sonjae appears to be separated from A. spinosus by a gap of 1700 km (Schiøtz 2007, pers. comm.).