Disperis ngeziensis, a New Critically Endangered Orchid from Pemba Island, Tanzania

INTRODUCTION

The genus Disperis Sw. (Orchidaceae) comprises about 80 species of terrestrial, and only rarely lithophytic or epiphytic, herbs that predominantly inhabit wooded and forested areas, grassland and fynbos. The highest diversity is found in tropical East Africa and Southern Africa, which harbour more than three quarters of the known species (Kurzweil & Manning, 2005). Madagascar and the Mascarene Islands constitute a secondary centre of diversity (la Croix et al., 2002), whereas a single variable and widespread species, D. neilgherrensis Wight, is distributed across tropical Asia (Kurzweil & Manning, 2005; Kurzweil, 2005). Since the publication of the comprehensive synopsis of the genus Disperis listing 74 species (Kurzweil & Manning, 2005), only a few additional species have been described, mainly from Central West Africa (Szlachetko & Kowalkowska, 2008; Szlachetko et al., 2018) and the Albertine Rift Valley (Fischer et al., 2011; Fischer et al., 2012; Fischer et al., 2015). In East Africa, 17 taxa were reported by Summerhayes (1968) and following the description of two new species from the Udzungwa Mountains by Verdcourt (1986), no further species have been added to the flora of Tanzania.

Pemba Island lies approximately 50 km off the northern coast of Tanzania and forms part of the Zanzibar Archipelago in the western Indian Ocean. The principal forest in the island is the Ngezi–Vumawimbi Natural Forest Reserve. It is situated at the island's northeastern extremity; it covers 2031 ha and constitutes the largest extant tract of old-growth coastal rainforest within the archipelago. The Ngezi–Vumawimbi Forest Reserve comprises a diverse assemblage of natural habitats, including lowland moist forest, riverine and coastal forest, Erica mafiensis (Engl.) Dorr heathland, mangrove stands, and coral rag vegetation (Beentje, 1990; Nahonyo et al., 2005; Orsenigo et al., 2026).

During our study of the floristic diversity of Ngezi-Vumawimbi Forest Reserve on Pemba Island, we collected a few specimens of Disperis, previously reported by Beentje (1990) and Nahonyo et al. (2005) under the name Disperis johnstonii Rchb.f. ex Oliv. The small plant grows in the understorey of the evergreen coastal forest of Vumawimbi. At first sight, it appeared distinct from all other known Disperis species. In this manuscript we described this species as new to science.

MATERIAL AND METHODS

Plant material was collected from the evergreen coastal forest of Vumawimbi in Ngezi-Vumawimbi Forest Reserve, northern Pemba Island (Tanzania), and processed according to standard procedures for botanical specimens. Photographs were taken of both living and dried material, using a mirrorless camera (Canon R50) with a macro lens (Canon EF 100mm f/2.8L Macro IS USM). Morphological characters were examined, and flower dissections were performed. Plant specimens were also matched with herbarium vouchers viewed directly (NHT) or virtually (K, P) (herbarium acronyms follow Thiers, 2025), and with literature. The morphological comparison with D. neilgherrensis was based on literature description by Kurzweil & Manning (2005) and Kurzweil (2005). All morphological characters were examined and measured using an Evident Olympus SZ61TR stereomicroscope equipped with an Evident Olympus EP50 camera.

TAXONOMIC TREATMENT

Figure 1.

Disperis ngeziensis S.Orsenigo, A.Bianchi & Baldesi, sp. nov.: a) flowering plant; b) column and lip; c) flower, lateral view; d) lateral sepals. Drawing by Laura Tomasi from pictures and dry (Bianchi 925).

Figure 2.

Disperis ngeziensis S.Orsenigo, A.Bianchi & Baldesi, sp. nov.: plant habit and flower close-ups from its locus classicus. Photos by Andrea Bianchi.

Figure 3.

Disperis ngeziensis S.Orsenigo, A.Bianchi & Baldesi, sp. nov., clockwise from left: a) flower close-up, b) lip blade and rostellum arms, c) lip appendage. From (Bianchi 925) Photos by Simone Orsenigo.

DISCUSSION

The description of D. ngeziensis endemic to Pemba Island brings the total number of Disperis species in the Flora of Tropical East Africa to twenty. The discovery of a new Disperis species on Pemba Island is of particular biogeographical significance. Disperis ngeziensis, due to the callose and stalked lip blade, clearly belongs to subgenus Dryorkis (Thouars) Kurzweil & Manning and in particular to the so called “Johnstonii group”, the largest group in Disperis that includes 22 species, mostly distributed in Madagascar, the Comoros, the Mascarenes and the Seychelles (Kurzweil & Manning, 2005); an interesting distribution pattern shared with plant genera such as Typhonodorum Schott, Chrysalidocarpus H.Wendl. and animal genera such as Pteropus Brisson, 1762 and Ebenavia Boettger, 1878, amongst others (Beentje, 1990).

Disperis ngeziensis is morphologically related to the only Asiatic representative of the genus, D. neilgherrensis (table 1), which is regarded as a single variable species including all species previously described from that region. In fact, D. neilgherrensis is widely distributed from southern India and Sri Lanka eastwards through Southeast Asia to New Guinea and the western Pacific islands, typically inhabiting moist forest understoreys and shaded montane slopes (Kurzweil, 2005). However, the populations of D. neilgherrensis closest to Pemba are those from Kerala (India), located over 4000 km away.

A sample collected by Beentje in Ngezi Forest Reserve in 1989 and identified as D. johnstonii (K000766849) is clearly referable to this new species. Indeed, despite Beentje indicated on the herbarium label the presence of two leaves, in both specimens the structures described as leaves are clearly the bracts subtending the flower. Disperis ngeziensis differs from D. johnstonii in the number of leaves (1 vs 2-3), the colour of the petals, which in D. johnstonii should be pink or yellow, and the shape of the lip (in D. johnstonii the spur-like structure at the end of the stalk is absent).

The presence of a small, solitary leaf suggests that D. ngeziensis primarily obtains its carbon nutrition from fungal symbiosis. In fact, mycoheterotrophic abilities may offset the reduced photosynthetic carbon production in orchids with small leaves and growth habit in heavily shaded forest understories, as recently shown for D. neilgherrensis (Muthukumar et al., 2013; Suetsugu et al., 2025).

Based on molecular evidence, Disperis appears to be more closely related to Brownleea Harv. ex Lindl. than to members of the subtribe Coryciinae (Bytebier et al., 2008; Waterman et al., 2009). Consequently, Chase et al. (2015) transferred Disperis to the subtribe Brownleeinae. Nevertheless, a comprehensive, species-level phylogeny of the genus remains unavailable. At present, no molecular data are available for the newly described species; however, future molecular analyses will be important to clarify its placement within the genus and its relationships with other species, particularly with D. neilgherrensis.

Table 1.

Comparison of Disperis ngeziensis with some other related species of the ‘Johnstonii group’ .