Costularia xipholepis (Baker) Henriette & Senterre, 2015

Henriette, Elvina, Larridon, Isabel, Morel, Charles, Goetghebeur, Paul & Senterre, Bruno, 2015, Revision of the genus Costularia (Cyperaceae: Schoeneae) for the flora of the Seychelles, including the rediscovery and resurrection of a rare endemic species, Phytotaxa 231 (1), pp. 31-41 : 34-37

Costularia xipholepis (Baker) Henriette & Senterre , comb. nov., Fig. 1

Cladium xipholepis Baker (1877: 424) . Type:— SEYCHELLES. Wright s.n. (holotype K!).

Schoenus xipholepis (Baker) Summerhayes (1928: 394) . p.p. quoad holotypus sed excl. Horne 626.

Perennial herbs, ca. 65 cm high, caespitose, forming dense clumps. Stems 15 cm long, 7 mm diam., upright, cylindrical, robust, woody and persistent, not stoloniferous, with one orthotropic branching along the stem, about midway from base to apex. Lateral roots along the stem, up to 8 mm below the uppermost internodes, growing through the dense cover of dead leaves. Basal leaves distichously arranged, densely set, numerous; dead leaves 8–28 on each side, persistent, the older ones abscising above the leaf sheath; green leaves 7–12 on each side, arcuate; leaf-sheath 40–60 mm long, 29–40 mm wide (measured at base when flattened), semi-cylindrical, thick, yellowish, margins dark red, ciliate distally; leaf-blade 75–123 cm long, 7–10 mm wide, not pseudopetiolate, linear, gradually tapering towards apex, upwardly concave in section, coriaceous, glabrous, smooth, mid-green, margin entire, with tiny ascendant prickles, apex acute, slightly rounded, not apiculate, midrib not distinct, longitudinally striate. Cauline leaves present; leaf-sheath 45–52 mm long, 12–15 mm wide (when flattened), cylindrical, notched up to 15 mm long (in the basal node), thin, dark red at base, yellowish distally; leaf-blade shorter than in basal leaves, decreasing in length towards apex of inflorescence, 40–74 cm long, 8–10 mm wide. Inflorescence a floriferous leafy shoot, 150–250 cm long, narrow (7–15 cm wide), with 4–5 orders of branching. Main axis axillary, inserted below the green basal leaves, cylindrical, smooth, nodes glabrous, 3–5 sterile (at base), 15–21 cm apart, fertile nodes 9–14. Secondary rachises 15–360 mm long (longer in basal fertile nodes), 1–7 per node, erect (held close to the main axis), compressed, slender, smooth, subtended by leafy bracts up to 17–27 cm long at basal nodes, 2.5–3.0 cm long at distal nodes. Spikelets bisexual, 2-flowered, densely clustered, 3–4 per fascicle, 7–8 mm long, 1.0– 1.2 mm wide, lanceolate, reddish-brown; pedicels 7.5–8.0 mm long, straight; bracts of ultimate order ca. 4.7 mm long. Rachilla sympodial, persistent, straight, internodes not elongated above fertile flower, 0.08–0.60 mm long, slightly decreasing towards the apex. Glumes 7–9 per spikelet, distichous, completely enclosing the rachilla at base, deciduous, lanceolate, smooth, reddish-brown on the sides and towards apex, margins glabrous, apices with a straight awn (longer in basal glume), midrib distinct. Basal glumes 5–7, empty, the lowest glume 2.5–3.7 mm long x 1.0– 1.5 mm wide, subsequent glumes 3.3–6.8 mm long x 1.4–2.0 mm wide. First fertile glume male only, 6.0– 6.5 mm long, slightly shorter than the last empty glume. Second fertile glume bisexual, 6.1–6.5 mm long, enclosed in the previous glume. Hypogynous bristles 6 per flower, well developed, 5.5–7.3 mm long, 2–3 times longer than achene (beak included), sparsely plumose. Stamens 3 per flower, 5.0– 7.6 mm long, not protruding or slightly protruding; anthers oblong, 1.7–4.2 mm long, yellow (in fresh specimen). Style protruding, trifid, 7.5–10.3 mm long (including stigmas). Nutlet stalked at maturity, trigonous, wingless, obovoid, 2 mm long (excluding beak), 0.8–0.9 mm diam., golden brown, beak with a constriction at the junction with the nutlet, 1.5 mm long, long-acute, 0.4 mm wide at base, ciliate.

Representative specimens examined: — SEYCHELLES. Mahé : Copolia, 500 m elev., - 4.64916°S, 55.45778°E, 17 July 2014, B. Senterre & E. Henriette 7101 ( GENT, P, SEY), 25 August 2006, M. Luceño & M. Guzmán 8406 ML ( UPOS) ; Pérard , pentes nord-ouest, 821 m elev., – 4.64011° S, 55.43617° E, 17 March 2013, B. Senterre & E. Henriette 6558 ( P, SEY), 19 May 2013, B. Senterre 6583 ( P, SEY), 18 June 2013, B. Senterre 6586 ( GENT, P, SEY), 26 July 2013, B. Senterre 6589 ( GENT, P, SEY), 29 January 2014, B. Senterre & E. Henriette 6964 ( GENT, P, SEY), 11 December 2014, B. Senterre & E. Henriette 7117 ( SEY) .

Distribution: — Costularia xipholepis is endemic to the Seychelles and restricted to Mahé and has been found in only three localities all situated in the Morne Seychellois National Park: Congo Rouge (B. Senterre & T. Stévart, observation record, 20 July 2014: – 4.6512°S, 55.44126°E, 610 m elev.), Copolia (where specimens were also collected by M. Luceño & M. Guzmán in 2006, but identified as Costularia hornei ) and Pérard ( Fig. 2 ). It has been searched for in suitable sites on Silhouette (Mont Pot à Eau) and around Morne Blanc but was not found. Other localities that should be checked on Mahé include Glacis Sarcelles and Trois Frères.

Ecology: —This species is restricted to the herbaceous fringe of lower montane inselbergs. It has been observed from 500 to 821 m elev. but was more abundant on the site at the higher elevation, which corresponds to an altitudinal belt named the tree fern lower montane belt ( Senterre 2011, Senterre & Wagner 2014, Senterre et al. 2009). At Copolia, it has a more patchy distribution, growing on rock crevices and along fissures where the soil is damp. In all sites, it grows in association with Costularia hornei .

Conservation: — Costularia xipholepis is rare and highly localized, known from only 3 small sub-populations representing 3 localities, 1.4 to 2.4 km apart, separated from each other by unsuitable habitat (i.e. wet forests). The three sub-populations are within the Morne Seychellois National Park, appear healthy, with limited risks from invasive species. The area of occupancy (AOO) for the Congo Rouge population is 10 m 2, Copolia 6,000 m 2, and Pérard 20,000 m 2, totalling approx. 26,000 m 2 (0.026 km 2) and its extent of occurrence (EOO) was estimated at approximately 1 km 2. Both AOO and EOO fall within the limits of Critically Endangered status under criteria B. The total number of mature individuals is unknown but probably around a thousand. According to IUCN criterion B (EOO <5000 km 2, AOO <500 km 2, number of locations ≤5, and a projected decline of the quality of the habitat as a result of climate change ( IUCN 2012), this species can be classified as Endangered (EN B1ab(iii)+2ab(iii)).

Vernacular name: —Due to the rarity of Costularia xipholepis , the species did not receive a local name yet. Because of the remarkable distichous disposition of the leaves, very much fan-shaped, we propose to name it ‘Lerb levantay’, meaning fan-shaped grass in Creole.

Taxonomic note: —Among the known species of Costularia subgenus Costularia , only one presents some similarity with Costularia xipholepis , i.e. Costularia pantopoda ( Baker 1885: 451) Clarke (1894: 658) var. baronii ( Clarke 1894: 658) Kükenthal (1939b: 67) (type: Baron 3316, K000244885!), endemic to Madagascar. Both species have long leaves and hypogynous bristles much longer than the nut, with relatively few empty glumes, but C. xipholepis differs most obviously in its clearly stipitate nutlet ( Table 1).

The identity of Costularia xipholepis has been the subject of much confusion. It was initially described as Cladium xipholepis , a new species endemic to the Seychelles, by Baker (1877), based on the single specimen Wright s.n. (K). Nevertheless, later studies treated that specimen sometimes as belonging in Costularia subgenus Lophoschoenus (as a species synonym of Costularia hornei : Clarke 1894: 657; Summerhayes 1928: 394), and sometimes in Costularia subgenus Costularia (as a species synonym of Costularia melicoides : Kükenthal 1939b: 72). Our rediscovery in the field of plants of Baker’s true Cladium xipholepis allowed us to collect new specimens with more developed spikelets and to correct important errors made in the original description of that species. It also allowed us to confirm the distichous position of the leaves, the absence of a midrib and to confirm diagnostic characters distinguishing it from the Mascarene C. melicoides . The latter is quite different from C. xipholepis , even at first sight, and can be distinguished by its much less robust habit, its basal leaf sheaths which are smooth (long ciliate in C. xipholepis ), and its spikelets with the terminal flower sterile (fertile in C. xipholepis ).

Figures

FIGURE 1. Costularia xipholepis (all photos by B. Senterre, mostly on the living population at Copolia:B. Senterre & E. Henriette 7101). A. Part of a clump showing many stems branching from one original plant; B. Closer view showing stem branching and distichous basal leaves; C. Basal leaf-sheath; D. Cauline leaf-sheath; E. Dissected stem (after removing the basal dead leaves) showing the insertion of the inflorescence below the green basal leaves; F. Developing nutlet; G. Fully ripe nutlet (seen only in B. Senterre & E. Henriette 7117) showing basal stipe and hairy beak; H. Mature stamens in flowering spikelets (seen only in B. Senterre 6589); I. Maturing spikelets showing persistent rachilla and deciduous glumes; J. Dissected spikelet showing the male flower below the terminal hermaphrodite flower still partly in its glume.

FIGURE 2. Distribution maps on the island of Mahé (Seychelles) for Costularia hornei (A) and Costularia xipholepis (B), showing also the distribution of their respective habitats (based on Senterre & Wagner 2014).