This species is named for Dr. Glover Allen, who first collected it on July 19, 1926 (Barbour and Loveridge, 1927).
Conraua alleni lives in the vicinity of fast-flowing streams within rainforests, ranging from northern Sierra Leone, through southern Guinea and Liberia to southwestern Côte d'Ivoire. The species exhibits vomerine teeth in two oblique rows, a small and somewhat indistinct tympanum, and a strongly developed fringe on the third finger. No dermal fold is present on the forearm. The dorsum is ashy gray with five slighly darker areas, the limbs are mottled and irregularly cross-barred with dark gray on the light gray field, and the venter is immaculate white.
The IUCN Red List (2010) categorizes this species as Vulnerable because it is dependent on streams in rainforests, and so its area of occupancy is probably less than 2,000 km2, its distribution is severely fragmented, and there is continuing decline in the extent and quality of its habitat in West Africa (Rödel and Schiøtz, 2004).
It occurs in several protected areas, including Pic de Fon Classified Forest (Guinea), Haute Dodo Classified Forest and Mont Sangbe National Park (Côte d'Ivoire), and the Mount Nimba World Heritage Site (Liberia and Guinea; Rödel and Schiøtz, 2004).
It is adversely affected by the loss of forest habitat for agriculture, logging, and human settlements. In certain places its habitat is also being lost due to mining, for instance at Simandou in the Pic de Fon Classified Forest in Guinea, and Mount Nimba in Guinea and Liberia. Mining is probably also leading to the pollution of breeding streams (Rödel and Schiøtz, 2004).
Populations of this species are decreasing (Rödel and Schiøtz, 2004).
Conraua alleni is morphologically quite similar to Hoplobatrachus occipitalis (Barbour and Loveridge, 1930). For example, they both exhibit a conspicuous transverse fold that connects the posterior edges of the upper eyelids, and the toes are completely webbed to the tips (Loveridge, 1941). Barbour and Loveridge (1930) compared a 28 mm specimen of H. occipitalis (then classified as Rana occipitalis) with the holotype 28 mm specimen of C. alleni and made the following observations:
1) The snout of C. alleni is much shorter (4.25 mm) than that of H. occipitalis (5.5 mm), and as a result its overall shape is rounder and less sharply tapered anteriorly.
2) The nostrils of C. alleni are positioned much more closely together (2 mm) than in H. occipitalis (2.65 mm), and they barely affect the overall contour of the head, whereas the more prominent nostrils of H. occipitalis give the head an angular appearance when viewed laterally.
3) The tympanum is small and indistinct in C. alleni but is large and conspicuous in H. occipitalis, with a diameter of 2.25 mm.
4) Conraua alleni and H. occipitalis both exhibit a strongly pronounced transverse dermal fold that meets the posterior borders of the orbits; however, in very juvenile H. occipitalis only a smooth, broad depression is present.
5) The thighs of C. alleni are much stouter, with a transverse measurement of 7.5 mm, as opposed to 5.5 mm in H. occipitalis.
6) Dorsally, C. alleni is covered with subequal warts, giving its back an almost granular appearance, whereas the dorsal warts of H. occipitalis are very unequal and tend to form ridge-like series.
7) The two species are differently colored, and the upper lip of H. occipitalis is blotched, unlike in C. alleni (Barbour and Loveridge, 1930).
Loveridge (1941) also compared C. alleni with H. occipitalis, and added the following distinctions:
8) The vomerine teeth in C. alleni form two oblique rows; anteriorly, they touch the inner posterior edge of the choanae, and they converge posteriorly. Meanwhile, vomerine teeth are absent in young H. occipitalis, but form two round groups located between but posterior to an imaginary line connecting the hind edges of the choanae.
9) The tips of the toes of C. alleni are dilated into small, distinct disks, whereas the tips of the toes of H. occipitalis are not dilated (Loveridge, 1941).
The species can be distinguished from Conraua crassipes by its indistinct tympanum, a strongly developed fringe on the third finger, no dermal fold on the forearm, and relatively shorter hindlimbs. C. crassipes, meanwhile, has a distinct tympanum, no finger fringes, a fold on the outer side of the arm near the olecranon, and a relatively longer leg (Parker, 1936).
The head is rather short and broad and the eyes are positioned rather dorsolaterally (Barbour and Loveridge, 1927). The snout is rounded and is about 1.5 times longer than the diameter of the orbit. The tympanum is small and indistinct, and its diameter is about one third that of the orbit. The vomerine teeth in Conraua alleni form two oblique rows; anteriorly, they touch the inner posterior edge of the choanae, and they converge posteriorly (Loveridge, 1941). The species exhibits a strongly developed fringe on the third finger and no dermal fold on the forearm (Parker, 1936). A conspicuous groove runs from the posterior border of the eye to the insertion of hind limb. The body is strongly depressed and rather Xenopus-like. The fingers, four in number and of equal length, are long, slender and poorly developed. The fingers have sharp apices that are probably tactile, another Xenopus-like feature. The thighs and tibia are expanded and depressed. The feet are large and completely webbed. The toes are long and slender and the tibio-tarsal articulation of the adpressed hind limb reaches the posterior border of the eye (Barbour and Loveridge, 1927). The tips of the toes are dilated into distinct, though small, disks (Loveridge, 1941). The skin is covered with subequal warts on all dorsal surfaces. The warts are best developed over the dorsal region, but are present, although smaller, on the head and limbs. The belly is smooth, with a faintly outlined discoidal fold, and the lower side of the limbs is smooth (Barbour and Loveridge, 1927).
Coloration: Dorsally, C. alleni is ashy gray, with five dark dorsal areas: the first on the nasal region; the second a band connecting the orbits; the third a band across intertympanic region; the fourth a fainter band across the back just posterior to the insertion of the forelimbs; and fifth, a curved marking across the sacral region. The fore and hind limbs are mottled and irregularly cross-barred with dark gray on the light gray field. All ventral surfaces are immaculate white (Barbour and Loveridge, 1927).
The holotype specimen measured 28 mm snout-vent length (SVL), but the specimen was later determined to be a juvenile (Barbour and Loveridge, 1930). Goldsmith (2010, unpubl.) examined two specimens measuring 32 mm and 43 mm SVL. A sexually mature adult male examined by Loveridge (1941) measured 65 mm SVL.
This species ranges from northern Sierra Leone, through southern Guinea and Liberia to southwestern Côte d'Ivoire. Records from eastern Ghana and Togo refer to Conraua derooi. A record from eastern Côte d'Ivoire refers to a juvenile Hoplobatrachus occipitalis (Rödel and Schiøtz, 2004).
It lives in the vicinity of fast-flowing permanent streams in rainforests in hilly country. It is forest-dependent, and is not found in open areas. It occurs from low altitudes to more than 1,000m asl (Rödel and Schiøtz, 2004). It is commonly found on stones and root structures close to the water's edge or out of the water (Schiøtz, 1964).
It is extremely localized, but abundant wherever it occurs (Rödel and Schiøtz, 2004).
It breeds in streams, and the tadpoles are usually found in the slow-flowing or nearly stagnant sections of streams (Rödel and Schiøtz, 2004). However, Schiøtz (1964; A preliminary list of amphibians collected in Ghana) reports finding both larval and adult specimens in fast-flowing rainforest streams.