Hymenophyllum senterreanum Dubuisson & Deblauwe, 2016

Dubuisson, Jean-Yves, Hennequin, Sabine, Droissart, Vincent & Deblauwe, Vincent, 2016, Hymenophyllum senterreanum Dubuisson & Deblauwe, sp. nov. (Hymenophyllaceae) and its relatives in western Central Africa, Phytotaxa 257 (3), pp. 287-294 : 288-292

Hymenophyllum senterreanum Dubuisson & Deblauwe , sp. nov., Figs. 1 & 2

Plants exhibiting the same gross morphology as the sympatric Hymenophyllum kuhnii C. Chr. , but being clearly distinct with rhizomes having thick whitish to translucent hairs, laminae that are more or less curled in the wild, sori appearing campanulate with a thick obconic base and valves/lips partially fused longitudinally along both margins, and receptacles sometimes exerted well beyond the sorus.

Type:— CAMEROON. Province du Sud: Parc National de Campo-Ma’an, aux environs des villages d’Ebianemeyong et Nyabissan, 02°29.59’N, 010°20.84’E, 934 m, 12 February 2015, Droissart V., Couvreur T. & Kamdem N. 1836 (holotype BRLU0000909 ; isotypes YA, P, MO) .

Epiphytic plants. Rhizomes long-creeping, filiform and branched, 0.3–0.5 mm in diameter, glabrous or with scattered long simple up to 6-celled whitish-translucent hairs 0.8–1.2 mm long, especially on young axes, at the base of stipes, and fully covering apices, and bearing sparsely distributed not well-developed roots. Fronds 1.5–3.0 cm apart. Stipes cylindrical, (0.7–)1.5–5.5(–7.5) cm long, glabrous or with the same hairs as on rhizomes localized at their base, narrowly winged (2–3 cells-wide) down to the rhizome. Laminae 3.0–8.0(–12.0) × 1.0–4.0(–5.0) cm, glabrous, green and diaphanous, slightly curled on fresh material, ovate to lanceolate or narrowly deltoid, bipinnatifid to tripinnatifid, with anadromous venation. Rachis glabrous, narrowly winged at the base, the wing significantly widening towards the frond apex. Pinnae in 4–8 pairs, alternate, deltoid to ovate or oblong, pinnatifid to bipinnatifid, with base broadly winged, and the proximal pinnules of basal-most pinnae often overlapping the rachis or overlapped by it. Segments linear, 0.9–1.2 mm wide, entire, with rounded or emarginate ends. Laminar cells thick-walled, mostly pentagonal to hexagonal, nearly isodiametric or twice as long as wide. Sori paratactic, borne apically on the frond, terminal on acroscopic basal-most segments of proximal pinnules, few (4–5) to up to 25 per fertile frond and up to 4–5 per pinna (for the largest specimens), more often 1 or 2 sori per pinna, often oriented upward and thus not in the same plane as lamina; indusia 1.4–1.8 × 1.0– 1.4 mm, appearing campanulate but with thick obconic base fully immersed in lamina and two valves or lips entire and longitudinally fused along both margins for at least half their length; valves apically well dilated or sometimes appearing adnate in young sori, the indusium apex becoming abaxially concave and forming an arch; receptacles thick and cylindrical, ranging from not exerted to exerting beyond the sorus by up to one soruslength.

Distribution and habitat: — Hymenophyllum senterreanum is endemic to the coastal part of the Lower Guinea Domain of the Guineo-Congolian Region ( White 1979). It is known from two localities in South Region of Cameroon and another two in the continental part of Equatorial Guinea ( Fig. 3 ). In these localities, the species was found in humid evergreen and sub-montane forests above 750 m elevation, growing epiphytically on canopy branches or on lianas in the understory (see Fig. 1A ). The presence of the species in neighbouring countries (e.g., Nigeria, Gabon, Congo and São Tomé-et-Príncipe) is possible and would require further investigation; see also comments in the section on conservation status.

Etymology:— The specific epithet refers to Bruno Senterre, a Belgian botanist, who greatly contributed to recently enrich collections of ferns from western Central Africa, and thus to revive the systematics of ferns in the region. He also collected one of the specimens of this study. He is currently working on Seychellois flora. The epithet senterreanum (as adjective form and dedication to a colleague) is here phonically preferred to senterrei (as genitive form, more usually attributed to the collector of a new taxon).

Specimens examined (paratypes):— CAMEROON. Akom II ( Est de Kribi ), 1000 m, 02°44.65’N, 10°31.80’E, 06 March 2004, Stévart & Droissart V. 2101 ( BRLU0000646 , BRLU0000647 , two sheets) . EQUATORIAL GUINEA. Rio Muni : Inselberg de Mungum, à 45 minutes de marche du village de Kuhumancoc, 750 m, 01°19’N, 010°49’E, 22 May 2002, Parmentier I. & Esono P. 3377 ( BRLU0000908 ) ; SO du Parc National de Monté Alen , 2 km au NE du site de traversée du rio Uolo pour aller au cataractas, 750 m, 01°36.56’N, 10°4.55’E, 26 June 2002, Senterre B. & Ngema N. 3061,1 ( BRLU0000648 ) .

Conservation status:— IUCN Red List Category: Endangered [EN B2ab (i,ii,iii,iv,v)]. The estimated Extent Of Occurrence (EOO) of Hymenophyllum senterreanum is 6441.8 km ² (which falls within the limits for Vulnerable status under criterion B1) and its Area Of Occupancy (AOO) is 16 km ² (which fall within the limits for Endangered status under criterion B2). The species is known from four subpopulations: two subpopulations in Cameroon and another two in Río Muni ( Equatorial Guinea). In Equatorial Guinea and Cameroon, the species has been collected inside the effectively protected areas of the Monte Alén National Park and the Campo-Ma’an National Park respectively, representing two locations sensu IUCN (2012). The four subpopulations thus represent a total of three locations, and this qualifies Hymenophyllum senterreanum for EN (number of locations ≤ 5 and falls within the limits of an Endangered status under criteria B2). The main known threats to the species outside protected areas are shifting agriculture and small-scale timber exploitation; which are gradually transforming native habitat into secondary forest. The ongoing loss of its habitat leads us to predict a continuous decline in the number of subpopulations and mature individuals, and therefore of the extent of occurrence and area of occupancy of the species. Nevertheless, considering the general habitat preferences of Hymenophyllum senterreanum , it is to be hoped that more specimens and additional sites will be found in the high-elevation, humid forests of adjacent protected areas in Equatorial Guinea or in the Monts de Cristal National Park in Gabon ( Fig. 3 ).

Discussion:— The genus Hymenophyllum is represented in Central Africa by three subgenera (see Table 1): Hymenophyllum , Mecodium C.Presl ex Copeland (1937: 93) and Sphaerocionium ( Presl 1843: 33) Christensen (1934: 5) . Sphaerocionium species always display trichomes on frond margins, whereas H. senterreanum is glabrous. All Mecodium species (including sympatric H. kuhnii ) display glabrous laminae, but never exhibit long whitish hairs on the rhizome, and the sori are all clearly bilabiate, never with a thick obconic base. Many species from subgenus Hymenophyllum show fronds with toothed to serrulate or dentate margins, as observed in local H. triangulare Baker (1867: 69) , and sori with lip margins that are sometimes toothed. But as for H. senterreanum , some species exhibit entire margins both for segments as well as for sorus lips. The thickness of the sorus base observed in H. senterreanum could be due to the development of the receptacle bearing sporangia. This receptacle extends from the terminal vein and is initially enclosed within the sorus. It sequentially and basipetally produces and bears the sporangia. It can be short, more or less thick to globose, or filiform and sometimes long as observed in subgenus Hymenophyllum and in H. senterreanum where the receptacle can exert outside the sorus. Such a thick obconic base with or without partially fused lips is frequently observed in the subgenus Hymenophyllum and could thus support the inclusion of H. senterreanum in it. Closer affinities for H. senterreanum within this subgenus could be with H. perrieri Tardieu (1941: 90) , H. sibthorpioides (Bory ex Willdenow 1810: 498) Mett. ex Kuhn (1868: 41) and H. tenellum ( von Jacquin 1789: 287) Kuhn (1868: 42) from the western Indian Ocean; all these taxa exhibiting entire lamina and more or less curled fronds. However, they lack the long, whitish hairs on the rhizome. In addition, we cannot exclude the possibility of an affinity to a subgenus not yet reported for continental Africa. In particular, abundant long and whitish hairs on the rhizome are also observed in species of the subgenera Hymenoglossum ( Presl 1843: 32) Tryon & Tryon (1981: 134) and Pleuromanes ( Presl 1849: 17) Ebihara & Iwatsuki (2006: 232) . Further phylogenetic studies involving H. senterreanum are required in order to specify the evolutionary relationships of this new African species and to verify its probable inclusion in the subgenus Hymenophyllum and proximity to western Indian Ocean taxa.

With Hymenophyllum senterreanum , west-central Africa would host at least six species of Hymenophyllum . We thus present an updated key for the genus in the region.

Figures

FIGURE 1. Hymenophyllum sp. nov. in situ in Cameroon, Campo-Ma’an National Park, as observed as epiphytic in rainforest understory at 934 m elevation, firstly identified as a trichomanoid (Droissart V. et al. 1836, BRLU0000909).A. Population; B. Detail of fertile fronds with campanulate-like shape sori; C. Detail of sori more or less oriented upward and showing the thick obconic base, here sori are young and adjacent lips appear adnate, the indusium apex becoming abaxially concave and forming an arch; D. Rhizome apex covered by long translucent whitish trichomes, scale = 0.5 mm; E. Detail of the bilabiate sorus with thick obconic base, scale = 0.5 mm; F. Detail of sorus showing the lips longitudinally and partly fused by their both margins, and an exerted cylindrical receptacle, scale = 1 mm.

FIGURE 2. Hymenophyllum senterreanum Dubuisson & Deblauwe, sp. nov. Habitus and detail of a sorus (Droissart V. et al. 1836, BRLU0000909) (drawing by J.-Y. Dubuisson).

FIGURE 3. Distribution of Hymenophyllum senterreanum specimens mentioned in the text in western CentralAfrica. Elevation above sea level (gray scale) highlights its sub-montane habitat.