Didymoplexis avaratraensis P. J. Cribb, Nusb. & L. Gaut., 2013

Cribb, Phillip, Gautier, Laurent & Nusbaumer, Louis, 2013, How many species are there in the holomycotrophic genus Didymoplexis Griff. (Orchidaceae) in Madagascar?, Candollea 68 (1), pp. 41-49 : 43

Didymoplexis avaratraensis P. J. Cribb, Nusb. & L. Gaut. , spec. nova

( Fig. 1-3 ).

Typus: MADAGASCAR. Prov. Diego-Suarez/ Antsiranana: sous-préfecture de Vohemar, commune rurale de Daraina, forêt de Binara , 13°15’S 49°37’E, 818 m, fl., 12.XII.2005, Nusbaumer & Ranirison 1763 (holo-: G [ G00090146 ]! ; iso-: P, TAN!, research herbarium of Daraina).

Ab aliis speciebus Africanis Madagascariensibusque sepalislateralibus ad medium connatis, petalisac sepalo dorsali in parte infernus 1/5 adnatis, columna brachia apicalia brevia recurvata ad apicem acuta ferenti, labello sine seriebus verrucarum et callo ad apicem bilobato distinguendo.

Holomycotrophic herb up to 15 cm tall, stem beige, brown to blackish when old, growing from a subterranean elongated brownish tuberous rhizome. Leaves scale like, very small. Inflorescence erect, racemose, laxly few-flowered, the flowers opening one at a time and successively; peduncle slender, erect, bearing a small ovate bract below inflorescence, white or pale buff-coloured; rachis terete, slender; pedicel with longitudinal ridges 1.5-2 cm during flowering period; bracts ovate, acuminate, 1-1.5 mm long. Flowers with pink or white sepals and petals, flushed with red-brown in the centre, the lip pink or white with reddish brown sides, the callus spotted with yellow. Dorsal sepal strongly curved forward, lanceolate, rounded, 15 × 3.- 3.5 mm; lateral sepals somewhat recurved, fused in basal half, more or less parallel and slightly falcate, ovate, rounded, 13 × 4 mm. Petals elliptic-lanceolate, acute, 13 × 4.5 mm, decurved at tip, adnate in basal fifth with sepals. Lip narrowly clawed at base, obovate-spatulate, truncate, 5-6 × 7-8 mm, the sides upcurved, bearing three patches on the apical lip lamina; callus at base shortly erect and bilobed at tip. Column 5-6 mm long, slender at base, with short recurved apical arms, triangular, broader than long and acute at the tips, 0.7 mm long. Capsule unknown.

Etymology. – The specific epithet refers to the North (“Avaratra” in Malagasy) because the species occurs in the northern part of Madagascar.

Distribution. – The species is known from Toamasina and Antsiranana provinces in North-East Madagascar. Four populations each of between (4)-15-20 individuals were observed, with a total of about 65-75 individuals observed in all sites.

Habitat and ecology. – Didymoplexis avaratraensis was recorded in evergreen forest on metamorphic rock; up to 820 m. This terrestrial herb occurs in forests with dense canopies reaching 10 to 15 m, with emergent trees reaching 18 to 20 m, with two woody strata in the canopy at 4 to 5 m and at 1 to 1.5 m high, and with an absent, sparse or very sparse ground flora, the soil densely covered with dry fallen leaves.

The most frequent species recorded together with D. avaratraensis (considering the Loky-Manambato population) in vegetation surveys include, in decreasing abundance: Croton sp. , Dypsis nodifera Mart. , Ravenea sambiranensis Jum. & H. Perrier , Dypsis aff. madagascariensis (Becc.) Beentje & J. Dransf. and Noronhia sp.

Flowering time. – November to December.

Conservation Status. – With an extent of occurrence (EOO) of 2,022 km 2 and area of occupancy (AOO) of 300 km 2, and only one subpopulation collected and three subpopulations observed, all four in protected areas (Station Forestière à Usage Multiple de Loky-Manambato, Marojejy National Park and Mananara Nord National Park), Didymoplexisavaratraensis is assigned a preliminary status of “Vulnerable” (VU D2) following the IUCN Red List Categories and Criteria ( IUCN, 2012; calculation following CALLMANDER & al. (2007) and performed with MOAT (2012) tools.

Notes. – Didymoplexis avaratraensis is clearly distinguished from the other African and Malagasy species of Didymoplexis known with flowers ( CRIBB, 1984; CRIBB & al., 2011) by its 2 lateral sepals fused to the mid-point while the remaining two petals and sepal are adnate together in the basal fifth, as well as by its column which has short recurved apical arms that are acute at the tips, and by its lip lacking rows of warts but with a bilobed callus at the apex. One observation of predation of flowering plants by some kind of invertebrate in one population is also interesting to note (Mark Clements, pers. comm.).

Figures

Fig. 1. – Didymoplexis avaratraensis P. J. Cribb, Nusb. & L. Gaut. A. Habit; B, C. Flower, side view; D. Dorsal sepal abaxial view; E. Lateral sepals, front view; F. Lip (labellum), flattened, front view; G. Labellum and column details, front view; H. Flower, front view. Didymoplexis recurvata P. J. Cribb, Nusb. & L. Gaut. I. Flower, front view; J. Lip (labellum), flattened, front view. [A-H: Nusbaumer & Ranirison1763, G; I-J: Nusbaumer & Ranirison 1763bis, G] [Drawing: Juliet Beentje]

Fig. 2. – Didymoplexis avaratraensis P. J. Cribb, Nusb. & L. Gaut. A. Flower, front view; B. Labellum and column details, front view; C. Flower and pedicel, side view; D. Habit; E. Tuberous rhizome. [Nusbaumer & Ranirison 1763] [Photo: L. Nusbaumer,12.XII.2005]

Fig. 3. – Didymoplexis avaratraensis P. J. Cribb, Nusb. & L. Gaut. A. Habitus; B. Flower, front view; C. Habitus; D. Flower, front view. [Photos: A-B: Mark Clements and nne Mackenzie,18.XI.2006; C-D: Jean-Michel Hervouet, 8.XII.2007]