Mallotus thinii Kim Thanh & V. S. Dang, 2025

Kim Thanh, Nguyen Thi, Anh, Nguyen Diep, Anh, Tran Thi Thuy, Thuy, La Thi, Duc, Nguyen Anh, Long, Tran Duc & Dang, Van-Son, 2025, Mallotus thinii (Euphorbiaceae), a new species discovered from the coastal areas of the south-central provinces of Vietnam, PhytoKeys 265, pp. 193-208 : 193-208

Mallotus thinii Kim Thanh & V. S. Dang sp. nov.

Figs 1 , 2

Type.

Vietnam • Quang Ngai province, Sa Huynh commune, on sandy soils, at the hedge of local people’s houses, 100 meters from the beach, close to Ganh Mountain , 14°38'12.1"N, 109°04'01.4"E, 07 May 2023, Kim Thanh KT 230507-03 ( holotype: HNU 024793 !) .

Diagnosis.

The new species is similar to M. leptostachyus Hook. f. and M. peltatus (Geiseler) Müll. Arg. in shrub habit, in having alternate to opposite leaves, indumentum of simple and stellate hairs and yellow glandular scales, leaf margin with denticulate glandular teeth, racemose and unbranched inflorescences. However, it can be fully distinguished from those by the deciduous stipules (vs. present), oblanceolate leaf blade (vs. ovate to obovate in M. peltatus , and elliptic in M. leptostachyus ), penninerved veination (penninerved or palmate in M. peltatus , and triplinerved in M. leptostachyus ), extrafloral nectaries two larger near the base or sometimes 2–6 (– 8) along midrib (vs. 2–6 small one in M. peltatus and two in M. leptostachyus ), stamens 28–33 (vs. 25–35 in M. peltatus and c. 60 in M. leptostachyus ), pistillate flower with sepal 3 (vs. calyx urceolate, caducous in M. peltatus ), short style up to 0.4 mm long (vs. 2.8–4.5 mm long in M. peltatus , c. 0.2 mm in M. leptostachyus ), fruits without spines (vs. with spines in M. peltatus ).

Description.

Dioecious shrubs, rarely monoecious, 0.8–1.5 m tall. Young branches flattened, matured branches roundish with longitudinal ridges. Indumentum pubescent on most young parts and inflorescences, composed of simple and stellate hairs and yellow glandular scales. Stipules subulate, c. 2.2 mm long, pubescent outside, deciduous. Leaves simple, alternate, apically opposite, or whorled; blade oblanceolate to rarely narrow elliptic; 9.7–13.5 × 3.2–4.7 cm; apex bluntly apiculate up to 0.7 cm long; base truncate or slightly cordate, margin dentate for distal 1 / 2 to 2 / 3 rd, with distal-pointing glandular teeth; upper and lower surfaces become glabrous at maturity; venation penninerved, nerves 6–8 per side, lateral vein angle from 25–30 °, looped and joined at 1–3 mm from the margin; extrafloral nectaries 2, on basal nerves, elliptic, 1–1.5 × 0.3–0.5 mm, on the upper surface, sometimes 2–6 (– 8) extra ones, elliptic to orbicular, ca. 0.4 × 0.2–0.4 mm, along midrib, near the base; petiole sulcate, 0.5–3 cm long, covered with stellate hairs. Inflorescences erect, terminal to axillary, unbranched, pubescent with yellow glandular scales, petals, and disc absent. Staminate inflorescence racemes, up to 11 cm long, flowers in groups of up to eight per node; bracts triangular, c. 1 × 0.8 mm. Staminate flowers pedicels 1.3–1.5 mm long; sepal 3–4, free, ovate, c. 1.7 mm long, densely covered with stellate hairs outside and yellow glandular scales outside and inside; stamens 28–33, filaments 1–1.5 mm, thecae two separated by broad connective, ovoid to ellipsoid, 0.2–0.25 × 0.15–0.2 mm, extrorse, opening lengthwise; pistillode present, consisting of 2–3 wart-like appendices. Pistillate inflorescences racemose, up to 9 cm long, bracts triangular, c. 0.8 × 1 mm. Pistillate flowers c. 1.5 mm in diameter; pedicel up to 1 mm long; sepal 3, ovate, ca. 1 mm long, free; ovary 3 - locular, hairy, green; style absent to 0.4 mm long; stigmas 3, plumose, up to 1.5 mm long, persistent. Infructescence up to 17 cm long, peduncle 2–3 cm long. Fruits 3 - lobed capsules, curved downward when mature, green, subglobose (slightly flattened dorsiventrally), without spines, 0.8–1.2 mm in diameter, 3 - locular, densely hairy, and yellow glandular scales, opening into three valves, septicidally and incompletely loculicidally from the base, each valve slightly flattened dorsoventrally, wall ca. 0.7 mm thick. Seeds subglobose, ca. 5 × 4 × 5 mm, ecarunculate, surface smooth, brown.

Flowering and fruiting.

Flowering from April to October and fruiting from May to November.

Distribution.

This species is known from the coastal areas of south-central Vietnam: Sa Huynh beach in Sa Huynh commune, Quang Ngai province; Dang Vinh Loi beach of Phu My commune, Binh Dinh province; and Binh Ba Island of Nam Cam Ranh commune, Khanh Hoa province (Fig. 3 ).

Habitat.

Mallotus thinii grows on sandy soils near the sea, characterized by poor soil fertility, frequent strong sunlight, drought, and wind. This species is distributed at altitudes up to 100 m.

Vernacular.

The new species was discovered at three locations, all of which are near the sea. To clarify the difference in habitat with other species within the genus Mallotus , we propose the Vietnamese common name “ Ru ối bi ển Thìn ”.

Etymology.

The species is named in honor of Professor Nguyen Nghia Thin from VNU University of Science, who devoted his career to revising the Euphorbiaceae of Vietnam.

Uses.

No known uses.

Other specimen examined.

Vietnam • Quang Ngai province, Sa Huynh commune , on sandy soils in the local cemetery closed to the beach, 14°38'10.5"N, 109°03'59.5"E, 07 Oct 2018, Kim Thanh KT 181007-12 ( HNU!) • Binh Dinh province, Phu My commune, Dang Vinh Loi beach , 14°07'42.3"N, 109°12'42.5"E, 22 Jun 2007, Thin N. N. NT 070622-17 ( HNU!) • Khanh Hoa province, Nam Cam Ranh commune, Binh Ba Island , 11°50'07.7"N, 109°14'43.0"E, elevation 100 m, 30 Aug 2023, Kim Thanh KT 230830-04 ( HNU!, VNM!) .

Preliminary conservation status.

Data deficient ( DD). Mallotus thinii was recorded from three populations in three different provinces, including Quang Ngai, Binh Dinh, and Khanh Hoa. All populations are located outside protected areas and near beaches or tourist areas where there is a high risk of land use change or human impact. At Sa Huynh beach in Quang Ngai, there are only a few restaurants and seafood farms serving tourists, and finally close to Ganh Mountain ( 14°37'58.9"N, 109°04'06.0"E) is a local cemetery. About 20 individuals were observed. We interviewed local people and learned that there is a larger population near Ganh Mountain. The presence of young individuals in this population is an indicator of the possibility of sexual reproduction process is normal and ongoing. Although we have not had the opportunity to explore that mountain, we believe that the number of individuals in this area is much larger than what we observed. In Dang Vinh Loi beach ( Binh Dinh province) and Binh Ba Island ( Khanh Hoa province), each population contained approximately 30 individuals. Although these populations are potentially endangered because they are not located in a protected area, from what has been observed and what information is available, this species has adapted very well to the dry conditions of the sand beach and the hot, dry climate of the south central region. According to insufficient information on distribution range and population size, this species can be qualified as Data deficient ( DD) ( IUCN 2024).

Figures

Figure 1. Mallotus thinii Kim Thanh & V. S. Dang. A. Habit of new species at the type locality; B. Leaves adaxial surface; C. Leaves abaxial surface; D. Staminate inflorescence; E. Pistillate inflorescence; F. Extrafloral nectaries along midrib (arrows); G. Staminate flower; H. Pistillate flowers; I. Infructescence with fruits; J. Fruit with sepals persistent (all photos by Nguyen Thi Kim Thanh).

Figure 2. The dried flowers of Mallotus thinii Kim Thanh & V. S. Dang. A. Staminate flower in bud; B. Staminate flower when open; C. Stamens with separated thecae; D. Pistillode; E. Pistillate flower; F. Dried fruit without spine; G. Valves and seeds from dehisced capsule; H. Brown seeds (photos taken by Nguyen Thi Kim Thanh and Nguyen Thanh Son).

Figure 3. Geographical distribution of Mallotus thinii Kim Thanh & V. S. Dang (pink triangles; map created with http://www.simplemappr.net).