Corybaspapillatus (Orchidaceae), a new orchid species from peninsular Thailand

Abstract A new species, Corybaspapillatus, is described and illustrated from peninsular Thailand. The new species is easily recognized through a combination of the following characters: the purplish flower, the rounded apex of the dorsal sepal, the outer surface of dorsal sepal covered with irregular papillae in the upper half, the lateral sepals adnate laterally at the base to the connate petals, the V-shaped throat, the labellum bearing short hairs, dentate to erose labellum margins, and well-developed conical spurs. A key to the species of Corybas in Thailand is presented.


Introduction
Corybas Salisb. is a genus of terrestrial orchids comprising about 120 species, and is widely distributed from India, South China, peninsular Thailand, the Malesian region, to New Zealand and the Western Pacific Islands (Dransfield et al. 1986;Pridgeon et al. 2001;Pedersen 2011;Tandang et al. 2020). The members of the genus are easily recognized by being small in size with a single cordate leaf and 1-2 underground tubers, dorsal sepal and labellum which together form a tube with expanded mouth, lateral sepals and petals often antenna-like, and labellum usually bearing two spurs (Dransfield et al. 1986). Only two Corybas species are currently known from Thailand, namely Corybas ecarinatus Anker & Seidenf. (Anker and Seidenfaden 2001;Pedersen 2011), and C. geminigibbus J.J. Sm. (Chantanaorrapint and Chantanaorrapint 2016).
During a recent visit to Khao Luang Mountain, Khao Luang National Park, by staff of BKF herbarium, an interesting taxon of the genus Corybas was collected with a unique combination of characters that did not match any of the known species. It is therefore described here as a species new to science.

Materials and methods
This study is based on material collected during July 2018 from Khao Luang National Park, Nakhon Si Thammarat province, southern Thailand. Specimens were preserved in alcohol (70% ethanol) and deposited in BKF herbarium. Morphological characters were studied using a stereo microscope Olympus SZX7 and the distinctive characters of the species were illustrated with the aid of an Olympus drawing tube. Measurements were taken from spirit material. The specimen details were compared in detail with original drawings and descriptions given in the protologues of Corybas species in the Malaysian region (e.g. Dransfield et al. 1986;Anker and Seidenfaden 2001;Tandang et al. 2020). Small terrestrial herb with underground tubers. Tubers globose or ovoid, fleshy, 3-4 mm diam. Stem erect, whitish-green, 5-12 mm long, 1-1.5 mm diam., with a basal sheath; stolon whitish, hairy, up to 2 cm long, 1-1.2 mm diam. Foliage leaf sessile, cordate, long acuminate at apex, glabrous, 6-10 mm long, 6-8 mm wide at the widest point, flat, only slightly undulate along margin, pale green with paler veins, the veins scarcely conspicuous. Inflorescence one-flowered, terminal; bract pale green, lanceolate-triangular, long acuminate, 5-6 mm long, recurved. Flower dark purple. Dorsal sepal purplish, erect below, then strongly curved above, hooded and clasping labellum throughout its length, spathulate, ca. 15 mm long, 8-10 mm wide, rounded at apex, apical margins denticulate, abaxial (dorsal) surface bearing irregular papillae in the upper half. Lateral sepals greenish-white, linear-triangular to antenna-like, ca. 25 mm long, laterally adnate to the petals in the basal ca. 1 mm. Petals greenish-white, connate at the base for ca. 1 mm, free above, similar to lateral sepals but shorter, ca. 22 mm long. Labellum differentiated into 2 parts, erect in basal half and strongly reflexed above, ca. 17 mm long in total length; the basal half with margins strongly inrolled throughout, and forming a tube with the dorsal sepal, ca. 7 mm long; the upper half reflexed and expanded, orbicular, ca. 10 mm diam., margin erose, reddish-purple excepted for a white zone just below the margins; mouth of throat  very deeply V-shaped; adaxial surface bearing short hairs; spurs 2, divergent, conical, ca. 3 mm long. Column very short, ca. 1.5 mm long; stigma rounded, ca. 0.5 mm diam.; anther erect, smooth; pollinia not seen. Ovary green, glabrous, ca. 2 mm long. Capsule (immature) erect, ellipsoid or fusiform, 5-7 mm long, 2.5-3 mm diam. Seed not seen.

Corybas papillatus
Phenology. Flowering and fruiting observed in July.
Distribution. Corybas papillatus is only known from the type locality (Fig. 3), however, it may also occur in other areas in peninsular Thailand with similar vegetation type.
Etymology. The specific epithet 'papillatus' alludes to occurrence of irregular papillae in upper portion of abaxial (dorsal) surface of dorsal sepal.
Conservation status. We consider it likely that if a formal assessment were performed, this species would be categorized as Critically Endangered (CR (D)) based on a preliminary risk of extinction assessment using the IUCN red list categories and criteria (IUCN 2019). This species is known from only four individuals from the type locality which attracts high numbers of camping tourists. Although we returned to the same locality and tried to find more specimens in 2020 it could not be found again. The species is, however, easily be overlooked in the field because of its small size.

Discussion
Corybas papillatus is most similar to C. villosus, which is endemic to Peninsular Malaysia (Dransfield et al. 1986;Go et al. 2015). These two species share several common features, viz. flower coloration being purplish, the dorsal surface of dorsal sepal covered with irregular papillae in the upper half, the hairiness of the labellum, the V-shaped throat, irregularly dentate to erose labellum margins, and well-developed conical spurs. Corybas papillatus is distinguished from C. villosus by lacking dorsal sepal keel (strongly keeled in C. villosus) and the adnate lateral sepals and connate petals (all free in C. villosus).
Corybas papillatus might be confused with C. ridleyanus Schltr., another endemic to Peninsular Malaysia (Dransfield et al. 1986;Go et al. 2015), which also has purplish flowers and reddish-purple labellum excepted for a zone below the margin, and a Vshaped throat. Corybas ridleyanus, however, differs from C. papillatus by the truncate apex of the dorsal sepal and the free lateral sepals and petals.
Together with the recent discovery of a new species and new records of orchids from peninsular Thailand, especially in the Nakorn Si Thammarat mountain range (e.g. Ormerod et al. 2012;Tetsana et al. 2014;Chantanaorrapint and Chantanaorrapint 2016;Chantanaorrapint et al. 2017) it is clear that peninsular Thailand is an important region for orchid diversity and that further new species records can be expected to be found from many unexplored areas in this part of the country.
There are now three species of Corybas known from Thailand. A key to distinguish these is given below.