Costus loangensis (Fig. 1) is a short-stemmed (about 0.5 m tall) plant with few (6–7) leaves and with completely yellow flowers. Stems and leaves are covered with a dense indument of erect to half-appressed hairs. Costus loangensis differs from Costus spectabilis (Fenzl) K.Schum., another short-stemmed yellow-flowered species, by having a well developed aerial stem. Costus spectabilis inhabits savannas and has only 4 leaves that remain strongly pressed to the ground. Costus loangensis differs from Costus ligularis Baker, also a short-stemmed and indumented plant, by the colour of its flowers (Costus ligularis has a pale pink flower) and the length of the calyx (5-7mm in Costus ligularis v. 11–12mm in Costus loangensis) (Table 1).

P.J.M. Maas, F.J. Breteler, C.D. Specht, H. Maas-van de Kamer, R. Niangadouma 10184 (holotype WAG; isotypes K, LBV, MO, UC), Gabon, prov. Ogooué-Maritime: Parc Nacional de Loango, between Lodge and Staff building, 1°54'43.3"S, 9°19'33.6"E, wet forest on white sand, along forest trail, at about sea level, 9 November 2011.

Terrestrial herb, 0.5–0.6 m tall, stems dark brownish red. Leaves: dark olive-green, several (6–7) concentrated at the apex of the stem; sheaths dark red, 0.6–0.8 cm diam.; ligule green, 2-lobed, 15–18 mm long, membranous; petiole 5–6 mm long; sheaths sparsely to rather densely covered with erect to half-appressed hairs ca. 2 mm long, ligule and petiole densely to rather densely so; lamina narrowly elliptic to elliptic, 14–16 by 5–6 cm, densely to rather densely covered with erect to half-appressed hairs 1.5–2 mm long on both sides, zone along midrib sometimes reddish, base attenuate, apex acute. Inflorescence: 3–5-flowered, ovoid, 2 by 1–1.5 cm, terminating the leafy stem; outer side of bracts, bracteoles and calyx densely covered with appressed to half-appressed hairs ca. 0.2 mm long, ovary sparsely so. Flower: 1 per bract; bracts brown to reddish brown, chartaceous, narrowly ovate-triangular to ovate-triangular, 1.7–2 by 0.5–1 cm, callus 2.5–3 mm long; appendages absent; bracteole reddish, boat-shaped, 15–18 mm long, callus 1.5–2 mm long; calyx reddish to greenish, 11–12 mm long, lobes deltate, ca. 2 mm long, callus ca. 1 mm long; corolla yellow, 50–55 mm long, tube 20–25 mm long, lobes narrowly elliptic, 30–35 mm long, outer side rather densely covered with half-appressed hairs ca. 1 mm long particularly near the apex, together forming a hood over the throat opposite the labellum, apex with a callus-like thickening; labellum yellow, horizontally flattened with funnel-shaped base, broadly obovate when spread out, 30–40 by 40–50 mm, margin fimbriate (fimbriae 2–3 mm long); stamen yellow, 25–30 by 7–10 mm, apex reflexed, anther 5–7 mm long; ovary narrowly obovoid, ca. 6 mm long, stigma bilamellate, dorsal appendage 2-lobed. Fruit and seeds not seen.

Gabon (Fig. 2). Only known from the type location.

In wet forest, on white sand soil under rain forest canopy (Fig. 3). Elevation just above sea level. Flowering: November; fruiting: unknown.

Based on a combined 4 molecular marker (CAM, ITS, ETS, rpb2) phylogenetic analysis with taxon sampling that included African species in the genus Costus, Costus loangensis H. Maas & Maas is found to be well supported as sister to a clade of Costus ligularis Baker specimens (Fig. 4). Costus loangensis fits within the general distribution range of Costus ligularis, however no populations of Costus ligularis were found in sympatry. Costus loangensis is restricted in elevation to just above sea level, while Costus ligularis is commonly found 0–600m above sea level throughout low-elevation rain forests in Cameroon and Gabon.

Not recorded. This plant seems to go unnoticed by the local population and has no known uses.

Costus loangensis is named after the place where it was photographed and now has been collected and documented for the first time: near the Loango Lodge, in “Parc Nacional de Loango” in Gabon.