Sicydium nereoi Pozner, Phytotaxa, 2017

Sicydium nereoi Pozner, Phytotaxa View in CoL 197 (2): 153. (Figs, 1, 2)

Type:— ARGENTINA. Prov. Corrientes: Dpto. Esquina, Isla frente a Esquina, 30 November 1974, A. Krapovickas, C. L. Cristobal, J. Irigoyen & A. Schinini 26905 (holotype CTES!).

Dioecious, herbaceous climbing perennial up to 5 m high, with tendrils; roots slender, sparsely branched, reddish. Stem 5-sulcate, densely villous-glandulose. Glandulose simple trichomes, uniseriate, patent, of variable length up to 0.8 mm long, mixed with simple, less frequent, eglandulose trichomes of 1.5–2 mm long, more abundant and denser at the nodes. Leaves entire, simple, cordate, up to 5 cm wide × 11 cm long, 7–9-pedately nerved, apex acute-acuminate, margin entire, adaxial surface densely villose, most trichomes simple eglandulose, abaxial surface villous-glandulose with mixed simple glandulose and eglandulose trichomes. Petioles 0.3–1.3 cm, densely villous-glandulose. Tendrils distally bifid, coiling below and above the branching point, villose-glandulose at the base, becoming almost glabrous towards the distal end. Staminate inflorescence (3–) 7 to 12 (–16) cm long, densely glandular-pubescent to glandularpuberulent, bracts in the axils ovate-cordate, ovate-lanceolate or, the smaller ones, narrowly lanceolate, bracts and rachis villous-glandulose like the stem, but with shorter trichomes, floral pedicels glabrous, articulated at their very base. Staminate flowers up to 3 mm, hypanthium saucer-shaped, sepals 5, triangular to long triangular, ca. 0.2 mm wide × 0.9 mm long, margin entire, with some trichomes on the abaxial side, petals 5, triangular to long triangular-acuminate, 0.3–0.5 mm wide × 1–1.8 mm long, with some trichomes on both sides, and along the margin in a ciliated arrangement, inner side of hypanthium densely covered by glandular trichomes forming a cushion-like trichomatous nectary, stamens 3, one monothecate with horizontal theca, and two bithecous with vertical thecae, filament 0.1–0.15 mm thick, 0.3–0.4 mm long, usually erect in the bithecous stamens and incurved in the monothecous stamen, connective usually wide up to divaricate, thecae 0.2 mm long, longitudinally dehiscent. Pistillate inflorescences up to 5 cm long. Pistillate flowers up to 3 mm long, with hypanthium 0.6–1 mm wide × 1–2.5 mm long, glabrescent towards maturity/ fruiting or with simple glandulose and eglandulose trichomes mixed, floral pedicels up to 1 mm long, with glandulose simple trichomes; corolla 0.5–0.8 mm wide × 1–2 mm long, triangular-acuminate, with glandular trichomes, ovary ovoid, unilocular, with a single pendulous ovule. Styles 3, up to 1.5 mm long, linear, with apically spirally coiled stigmas. Staminodes 3, measuring 0.1 mm wide × 0.2 mm long, generally present. Fruits globose, indehiscent, up to 1.5 cm long × 1 cm wide, green when young, glabrous, with robust peduncles. Seeds 3–4 mm in diameter, spherical, beaked, strongly ornamented with abundant angular ridges of varying sizes.

Observations: our new added data on pistillate flower, fruit and seed morphology definitely confirm the position of this species within Sicydium .

Phenology: The species flowers and fruits from November to February.

Geographic distribution: Sicydium nereoi is only known from the province of Corrientes, on the islands located to the west of the city of Esquina, between the Corriente and Paraná rivers.

Ecology: The species inhabits riverine islands associated with the Corriente and Paraná Rivers, at altitudes ranging between 28 and 30 meters above sea level ( Fig. 3 View FIGURE 3 ). These islands are subject to both ordinary and extraordinary floods, which significantly influence the ecosystem’s dynamics. In lower areas (26–27 m), plant communities establish and disappear rapidly, responding to variations in water levels, creating a mosaic of transient habitats. The soils on these islands are results of sedimentary processes linked to the Paraná River, consisting of a mix of sands, silts, and clays. In higher areas (28-30 m), where drainage keris moderate and water saturation is lower, the species is found in association with a riparian forest characterized by a mosaic of tree and shrub species. Dominant species include Celtis chichape ( Weddell 1852:193) Miquel (1853:181) ( Cannabaceae ), Croton urucurana Baill. (1864: 335) and Sapium haematospermum Müller Argoviensis (1865:217) ( Euphorbiaceae ), Inga uraguensis Hooker & Arnott (1833:202) , and Pseudalbizzia inundata ( Martius 1823:555) E.J.M. Koenen & Duno ( Peraza 2022:395) ( Fabaceae ). This riparian forest provides suitable conditions for Sicydium nereoi , such as shade, humidity, and shelter, especially during flood periods when the lower areas of the islands are submerged. Nearby vegetation includes smaller plants (1–4 m tall), such as Sesbania virgata (Cav.) Persoon (1807:316) ( Fabaceae ), Byttneria filipes Martius ex K. Schumann (1886:95) ( Malvaceae ), and Urera aurantiaca Weddell (1852: 201) ( Urticaceae ). These species may create complex ecological interactions, particularly in terms of competition for space and resources. In this context, taller trees play a key structural role in the habitat of Sicydium nereoi , acting as essential elements for its conservation (supplementary material: https:// figshare.com/s/84b4d86ecec46c2a201a)

Conservation status: — AOO: 4 km 2. GeoCAT points: 34 (six plants of pistillate flowers, 18 plants of staminate flowers, and 10 plants without flowers). Cell size: 2 km 2 IUCN (Figs 3,4).

Within the area of occupancy (4 km ²), zones where Sicydium nereoi is more abundant have been identified, covering 0.48 km ² (16% of the total area). These zones stand out for providing optimal conditions for the species’ survival, such as tall trees, loamy-clay-sandy soils, and shaded areas with minimal sun exposure. However, other areas with similar characteristics, spanning 0.76 km ² (25% of the total area), do not host the species, highlighting habitat fragmentation and limitations. Altogether, habitable areas account for only 1.24 km ² (44% of the total area), while the remaining 2.76 km ² correspond to unsuitable habitats dominated by vegetation typical of flood-prone environments. These areas are predominantly occupied by species such as Persicaria glabra ( Willdenow 1799:447) Gómez (1896:278) , Ambrosia tenuifolia Sprengel (1826:851) , Pluchea sagittalis ( Lamarck 1786:94) Cabrera (1949:36) , Solanum americanum Miller (1768:5) , Sesbania virgata ( Cavanilles 1795:47) Persoon (1807:316) , and Aspilia silphioides ( Hooker & Arnott 1841:314) Bentham & Hooker ex Baker (1884:197) , which do not meet the ecological requirements of S. nereoi .

This fragmentation underscores the importance of habitable areas as critical refuges for conservation. However, the species faces significant threats from infrastucture, deforestation of essential trees for its habitat, such as Pseudalbizzia inundata , and the presence of livestock (goats, sheep, and cattle), which exert substantial pressure on vegetation. Overgrazing reduces vegetation cover, exposes the soil to erosion, and constant trampling compacts the ground, hindering water infiltration and the regeneration of key plants. These threats are particularly severe in areas near water, where ecological conditions are already especially vulnerable. The island’s proximity to the city, just 300 meters away, facilitates human access, especially during fishing seasons and recreational activities, intensifying pressure on the ecosystem. With a population of only 34 individuals, of which only six produced pistillate flowers with low reproductive success, the species’ viability is seriously compromised. These threats, observed continuously in recent years, justify classifying Sicydium nereoi as Critically Endangered (CR) under IUCN criteria B2ab (ii, iii, iv, v), C2a (i), and D. https://figshare.com/s/c3aa699f47f131b5b2aa

Therefore, it is crucial to make rapid and effective decisions for the conservation of this species, which is unique and has a restricted distribution. So far, it has not been recorded in other areas near the adjacent sampled islands. In the future, it would be essential to assess the viability of its seeds and their conservation in a germplasm bank to ensure the species’ preservation. Additionally, measures should be implemented to mitigate the threats it currently faces in order to maintain stable populations.

Specimens examined: ARGENTINA. Corrientes, Isla enfrente de Esquina, Río Corriente , 05 December 2023, Florentín J.E., Medina W. & Piñeiro J. M. 156 and 157 (CTES). Ibid., 06 December 2024, Florentín J.E. & Medina W. 277 (CTES).