Similar to Asplenium salignum in its short creeping rhizome, tuft of simple or once pinnate fronds dentate with rounded marginal teeth but A. merapohense is distinct in its shorter, much narrower pinnae 3.4–7.1 × 0.2–0.3(–0.45) cm (vs. 6–27 × 0.9–3.7 cm) and shorter sori, which are 2–6 mm long and lie parallel to the midrib (vs. sori 4–13 mm long angled at 45° to the midrib).

Peninsular Malaysia. Pahang, Lipis District, Merapoh, opposite to Gua Gajah, 4°42.26'N, 101°58.46'E at 348 m, 28 May 2014, Imin et al. FRI 81479 (holotype KEP!; isotypes K!, SAN!, SING!, UKMB!).

Lithophyte. Rhizome short to 2 cm long, shortly creeping, slender (to 3 mm diam.), rhizome apex with narrowly triangular clathrate scales, 2–3 × 0.5–0.8 mm, dark brown with slightly paler margin with a few protuberances. Fronds tufted and quite close together, once pinnate with 1–2 pairs of pinnae or occasionally the lower pinna unequally forked; stipes 2.5–8.5 cm long, pale to mid-green when dry, grooved above, glabrous or with sparse stellate scales when young; rachis 0.8–4.3 cm long, grooved above, glabrous, slightly winged at the junction of the rachis and pinnules; lamina leathery, lower lamina surface paler than the upper surface, lower pinnae opposite, the upper pinnae (if present) alternate or only a single upper pinna present, terminal pinna conform to lateral pinnae; pinnae narrowly lanceolate to almost linear, 3.4–7.1 × 0.2–0.3(–0.45) cm, margin dentate, teeth minutely rounded giving the pinna a scalloped-edge look, base of pinnae gradually attenuate, midrib raised on upper surface and slightly raised below, apex attenuate; veins alternate, almost invisible, oblique to midrib, free and unbranched, terminating slightly below the lobe/tooth. Sori elongate and embedded on acroscopic veins, 2–6 mm long, indusium c. 1 mm wide, pale brown and opening towards the midrib of the pinna. Spores ellipsoid, 45–52 × 32–37 µm.

Figure 1. 

Asplenium merapohense R.Jaman & K.Imin, sp. nov. A habit B rhizome scale C rachis with slightly winged axils D pinna with sori E sori with sporangia F sporangia before and after dehiscence G 3- and 5-armed scales on lower pinna surface. (Drawn by Mohamad Aidil Noordin from Imin et al. FRI 81479).

Endemic in Peninsular Malaysia, from Pahang, Merapoh (Gua Gajah) and Kelantan, Gua Musang District (Gua Panjang).

It takes its name from the type locality.

Critically Endangered, CR B2ab(iii). Known from only two localities with an AOO of 4 km² (not far apart, so the two sites count as a single location), its small population (the population on Gua Gajah comprises about 100 individuals; the Gua Panjang site has not been relocated), and the threats to its habitat from quarrying for cement and burning for agricultural purposes, all contribute towards its critically endangered status. A few fertile plants are growing in the Herbarium Nursery at the Forest Research Institute Malaysia.

Restricted to karst limestone hills where it grows on steep rock faces in sheltered conditions at 180–348 m altitude. It is a rare and very local species (Figure 2).

Figure 2. 

Asplenium merapohense R.Jaman & K.Imin, sp. nov. A Gunung Gajah B habitat – in crevices on a shaded, mossy limestone vertical cliff C Undersurface of a pinna with sori showing indusia. (Photographs by A P.T. Ong, B, C K. Imin).

Peninsular Malaysia. Pahang: Merapoh, opposite Gua Gajah-28 May 2014 Imin et al. FRI 81479 (KEP!, K!, SAN!, SING!, UKMB!). Kelantan: Gua Panjang-10 Aug 1962 UNESCO Limestone Expedition 586 (K!, SING!).

Asplenium salignum is a widespread species in Peninsular Malaysia, most commonly encountered on limestone rocks but also on granite rocks in lowland forest and occasionally as an epiphyte in mountain forest. It is variable in the division of the fronds: it is sometimes found with only simple fronds, most commonly plants with simple fronds also bear fronds with one or two pairs of pinnae, while many plants have 1–2(–6) pairs of pinnae. However, even small fronds with simple or a single pair of pinnae have wider lateral pinnae (7–) 14–20(–28) mm wide with wider terminal pinnae (11–)15–25(–46) mm wide compared to the new species that has pinnae 2–3(–4.5) mm wide. Another conspicuous difference is in the size and position of the sori. In A. merapohense the sori are 2–6 mm long and lie parallel to the midrib, while in A. salignum they are 4–13 mm long and positioned at 45° to the midrib Asplenium merapohense is restricted to limestone, but while A. salignum is usually found on limestone, it also grows as an epiphyte in mountain forest.

The fact that this new species is extremely rare and known from a single population might suggest that it is a hybrid. Murakami et al. (1991) recorded that several non-fertile natural hybrids of Asplenium occur in Japan and that they were frequent where both parents occurred.

The hybrid status of apparently fertile hybrids was revealed by their misshapen spores. On the karst limestone in the Merapoh region, four other Asplenium species occur, A. batuense Alderw., A. macrophyllum Sw., A. normale Don and A. salignum, but A. merapohense does not display any characters intermediate between them and A. salignum, the most likely putative parent if it were a hybrid. In addition, examination of the spores of A. merapohense show them to be of regular size and shape, the fronds are not malformed, and the population numbers about 100 or more individuals in all life stages, indicating that it is fully fertile, all of which strongly indicate that it is not a hybrid.