Malaysia • Pahang, Cameron Highlands (precise coordinates withheld for conservation purposes), ca. 2,000 m elevation, 30 October 2025, Rusea, Nasrul & Besi EDW160 (holotype: UPM! dry and wet-preserved specimens).

Ceratostylis wanrosdyi is most similar to C. radiata, C. pulchella, and C. longisegmenta in having white flowers with narrow petals but differs by its shorter, narrowly elliptic leaves, 2.4–3.5 × 0.3 cm (vs. narrowly lanceolate, 16.0 × 0.9 cm in C. radiata; linear, 12.0–20.0 × 0.5–0.7 cm in C. pulchella; narrowly oblong, ca. 10.5 × 1.0 cm in C. longisegmenta), peduncles ca. 2.3 cm long, nearly as long as the leaves at anthesis (vs. ca. 1 cm, 4 cm, and 3 cm long, respectively, and much shorter than the leaves), and an entire, broadly lanceolate labellum (vs. trilobed in C. radiata, C. pulchella, and C. longisegmenta).

Plant epiphytic, ca. 6 cm tall. Rhizome ca. 3 cm long, 0.6 mm wide, pubescent. Stems clustering, ca. 1.5–2.0 cm, covered with imbricate brown sheaths, 1- or 2-leaved; sheath narrowly lanceolate, ca. 9–12 mm long, acute to subacute, minutely punctate, veins parallel. Leaves narrowly elliptic, 2.4–3.5 cm long, 2.7–3.0 mm wide, gradually narrowing toward the base, unevenly bilobed at the apex, apex rounded to obtuse; petiole 1.0–1.3 cm long. Inflorescence terminal, single-flowered, 2.5–2.8 cm long; peduncle filiform, 2.3 cm long, 0.6 mm in diameter, reddish brown, pubescent. Floral bracts ovate, acute, 5 × 2 mm, glabrous. Flowers solitary, ca. 9.2 × 12.2 mm; perianth pure white, partially translucent. Pedicel and ovary 4 mm long, 1 mm diam., reddish brown, pubescent. Dorsal sepal oblong-lanceolate, 9.5 × 2.4 mm, subacute, 3-nerved. Lateral sepal elliptic-lanceolate, 10.5 × 2.9 mm, subacute, 3-nerved or 5-nerved, united basally into a blunt, globose mentum. Petals oblong-lanceolate, 9.0 × 1.7 mm, subacute, 3-nerved. Labellum entire, white, broadly lanceolate, 5.6–6.2 × 2.2–2.4 mm, 3-nerved, fleshy, basal part concave, lateral margins at basal part incurved, clawed, apical part somewhat decurved, obtuse; claw enclosed within the mentum, short, broadly falcate in outline, margin incurved. Column 1.9 mm long, base 0.9 mm wide, slightly arcuate, exalate; arms concave; foot short, conspicuous at the base; anther cap ovoid, at front emarginate, 0.8 × 0.7 mm, 2-valved; pollinia 8, pyriform, ca. 0.3 × 0.2 mm.

The species is currently known from two localities in Cameron Highlands, Pahang, Malaysia (Fig. 3).

Ceratostylis wanrosdyi occurs as an epiphyte in montane cloud forest at elevations of 1,800–2,000 m, on exposed mountain ridges and in partially shaded areas. The species grows on moss-covered trunks and lower branches of Clethra hendersonii Sleumer and several Lithocarpus species, including L. elegans (Blume) Hatus. ex Soepadmo, L. hendersonianus A.Camus, L. neorobinsonii A.Camus, and L. suffruticosus (Ridl.) Soepadmo. Individuals on C. hendersonii were observed ca. 2 m above the ground. Ceratostylis wanrosdyi also occurs sympatrically with C. eria Govaerts, another epiphytic orchid characteristic of this montane habitat.

Flowers in August and October.

The specific epithet is named in honour of the 15^th Menteri Besar of Pahang, YAB Dato’ Sri Diraja Haji Wan Rosdy bin Wan Ismail, in recognition of his commitment and support for biodiversity exploration, conservation, and environmental stewardship in the state. This honour also acknowledges his role in supporting and approving the proposal to establish the UNESCO Man and the Biosphere (MAB) Mossy Forest, Cameron Highlands, in 2024, which encompasses the habitat of this new species.

Malaysia • Ceratostylis wanrosdyi. Pahang, Cameron Highlands, 23 August 2025, S.H. Tan & R. Go C466 (UPM!); Pahang, Cameron Highlands, 2 August 2026, E.E. Besi, D. Sandin, & S. Mahesvaran BW001 (UPM!). • Ceratostylis eria. Pahang: Cameron Highlands, Summit Batu Brinchang, 1 September 1956, H.M. Burkill 809 (K001638289!). • Ceratostylis pulchella. Pahang: Raub, Fraser’s Hill, January 1953, R.E. Holttum SFN39464 (type: K000810594!). • Ceratostylis radiata. Kedah: Langkawi, Gunong Raya, 13 November 1921, M. Haniff 7103 (holotype: SING0047107-image!). • Ceratostylis longisegmenta. Sabah, Mt. Kinabalu, Kiau, 2 November 1915, J. Clemens 93 (holotype: AMES00061186-image!). Indonesia • Ceratostylis longipedunculata. Lombok: Mt. Rinjani, south side, slope above Kembang Kerang, 25 June 1909, J. Elbert 2145 (K001638376!). • Ceratostylis anjasmorensis. East Java: G. Anjasmoro, 24 November 1983, J.B. Comber 1543 (holotype: K000220023-image!). Philippines • Ceratostylis philippinensis. Province of Bataan Luzon: Lamao River, 21 September 1905, H.N. Whitford 1359 (K000810579-image!); Province of Bataan Luzon, Mt. Mariveles, Lamao River, October 1903, E.D. Merill 3247 (K000810580-image!). • Ceratostylis pristina. Mindanao: Bukidnon, Kalatungan Mountain Range, 21 May 2016, M.D. De Leon MDL 007/2016 (holotype: CMUH00010858).

Ceratostylis wanrosdyi is currently known from two geographically distinct subpopulations in Cameron Highlands, Pahang (Fig. 3). With only two known occurrence records, the EOO cannot be estimated meaningfully, while the AOO is estimated at 8 km², corresponding to two occupied 2 × 2 km grid cells following IUCN (2024). The AOO falls within the threshold for Critically Endangered (CR) under subcriterion B2; however, the two subpopulations are treated as two locations sensuIUCN (2024) and therefore do not meet the CR threshold of a single location under subcriterion B2a.

Although the type locality lies within a protected forest reserve, it is situated in a popular recreational and ecotourism area that receives frequent visitor activity. The second subpopulation likewise occurs in accessible montane forest. Localised habitat disturbance associated with tourism infrastructure, trail maintenance, and recreational use is considered the main plausible threat to Ceratostylis wanrosdyi, as these activities could alter the humid microclimatic conditions required by epiphytic orchids. The localised nature of this threat supports the treatment of the two subpopulations as two locations sensuIUCN (2024), since a single threatening event affecting individuals at one location would not be expected to rapidly affect those at the other. The accessibility of both sites may also increase the risk of illegal collection, a recognised threat to wild orchid populations, particularly rare epiphytic species occurring near recreational facilities (Ballantyne and Pickering 2012; Wraith and Pickering 2017). Based on its restricted AOO, occurrence at two locations, and an inferred continuing decline in habitat quality due to ongoing anthropogenic pressures, C. wanrosdyi is provisionally assessed as Endangered [EN B2ab(iii)] following IUCN (2024).

Ceratostylis wanrosdyi shares white flowers and narrow petals with the widespread C. radiata, the Peninsular Malaysian endemic C. pulchella, and the Bornean endemic C. longisegmenta. The petals differ in shape among these species, being oblong-lanceolate in C. wanrosdyi, narrowly oblong-lanceolate in C. radiata, lanceolate in C. pulchella, and linear in C. longisegmenta. Differences are also evident in perianth texture: C. radiata has fleshy petals and a fleshy apical portion of the labellum, whereas in C. pulchella, both structures are membranous and as thin as the sepals. In C. longisegmenta, all perianth segments are thickened towards the apex, with the thickening most pronounced in the labellum (Ames and Schweinfurth 1920). In contrast, C. wanrosdyi has membranous sepals and petals but a fleshy labellum. A further noteworthy feature of C. wanrosdyi is the progressive elongation of the peduncle during floral development; it is shorter than the leaf at the budding stage (Fig. 2A) but becomes nearly as long as the leaf or slightly exceeds it at anthesis.

A further comparison with another Peninsular Malaysian endemic, Ceratostylis eria, reveals a superficial similarity in the elongate peduncles (ca. 3 cm long), which exceed half the length of the leaves (ca. 5.5 cm long), and in the white flowers. Nevertheless, C. eria is readily distinguished by its longer and wider oblanceolate leaves (ca. 5.5 × 0.5–1.2 cm), compared with the narrowly elliptic leaves of C. wanrosdyi (2.4–3.5 × 0.3 cm). It also differs markedly in labellum morphology and colouration. In C. eria, the labellum (0.4 × 0.2 cm) is much smaller than the tepals (ca. 1.4 × 0.9 cm) and is green with two dark brown blotches. In contrast, the labellum of C. wanrosdyi is comparable in size to the tepals, and all floral segments, including the labellum, are uniformly white.

Among species from neighbouring countries such as the Philippines, Ceratostylis philippinensis and C. pristina share white flowers and an entire labellum with the new species. However, both differ in having short peduncles, less than 1 cm long, relative to leaves measuring 1.9–3.8 cm long in C. philippinensis and ca. 3.0 cm long in C. pristina. Moreover, C. wanrosdyi is distinguished from C. philippinensis by its larger petals (oblong-lanceolate, 9.0 × 1.7 mm vs. narrowly lanceolate, 5.0 × 1.0 mm) and labellum (broadly lanceolate, 5.6–6.2 × 2.2–2.4 mm vs. lanceolate to broadly lanceolate, ca. 4.0 mm long). It further differs from C. pristina in petal morphology (oblong-lanceolate, three-nerved vs. ovate, five-nerved) and labellum shape (broadly lanceolate vs. ovate).

The new species also resembles Ceratostylis longipedunculata from the Lesser Sunda Islands (Bali and Lombok) and C. anjasmorensis from Java in having peduncles nearly as long as the leaves. Nevertheless, C. wanrosdyi differs markedly from C. longipedunculata in floral colouration (pure white vs. white with a purple tint) and labellum morphology (broadly lanceolate, pure white vs. spathulate, white at the base and yellow from the centre towards the apex). Compared with C. anjasmorensis, C. wanrosdyi has larger petals (oblong-lanceolate, 9.0 × 1.7 mm vs. narrowly elliptic, 0.8–1.0 × 0.5 mm) and a larger labellum (5.6–6.2 × 2.2–2.4 mm vs. 3.0 × 0.8 mm). The two species also differ in floral colouration. All perianth segments of C. wanrosdyi, including the labellum, are uniformly white, whereas in C. anjasmorensis, the sepals and petals are translucent white, tinged pink towards the apices, while the labellum is white at the base and yellow towards the apex (Wood and Comber 1986).

A comparative summary of the new species with selected congeners is presented in Table 1. The comparison includes species with predominantly white, star-shaped flowers, namely C. eria (Hooker 1891; Seidenfaden and Wood 1992), C. radiata (Smith 1905; Seidenfaden and Wood 1992; Odyuo et al. 2013), C. pulchella (Holttum 1953; Seidenfaden and Wood 1992), C. philippinensis (Ames 1905), and C. pristina (de Leon et al. 2017), together with C. longipedunculata (Smith 1925; Atmaja and Tirta 2019), which is included because of its similarly elongate peduncles. Morphological data for the selected congeners were compiled from the respective literature cited above and verified against herbarium specimens examined in this study wherever possible.