Isoetes divyadarshanii Shukla, Srivastava & Shukla, 2005

Singh, Sarvesh Kumar, Brunton, Daniel F., Shukla, Pradeep Kumar, Sokoloff, Paul C., Shukla, Santosh Kumar, Patil, Anant Prakash & Dubey, Nawal Kishor, 2025, Rediscovery of presumed extinct Isoetes divyadarshanii (IsoetaceaeLycopodiopsida) in India: New insight on its morphology, taxonomy and conservation status, Phytotaxa 697 (1), pp. 97-108 : 103-106

Isoetes divyadarshanii Shukla, Srivastava & Shukla ( Shukla et al. 2005:113).

Type:— INDIA. State of Maharashtra: Dist. Pune , Lonawala , 4 October 2000, P. K. Shukla 8400 ( holotype: CAL [8400]!; isotypes: DD [8400A]!, K [8400B]!)

Plant amphibious (shallow-water emergent), growing individually or in clumps, perennial; corm 3-lobed, globose; roots up to 115 mm long, branched; leaves 15–32 cm long, 2.0– 3.5 mm wide at mid-length, herbaceous, predominately megasporophylls, 15–75 per plant, tufted, spirally arranged, stiffly erect or occasionally recurved, light green with pale to hyaline broadened base surrounded by dark brown, partially sclerified, 6–9 mm long, 4–7 mm scale leaves; sporangia oblong to ovate with gray-tan to brown to dark brown faces and abundant small dark-brown to light-brown spots; ligules broadly triangular atop a prominent labium, tapering to an acuminate summit; velum short, 0-10% from the top of the sporangia and 9–11% in from the sides; megaspores globose or rarely irregular in shape (weakly triangular), averaging 485 µm in diameter; distal ornamentation in an open to congested, broadly tuberculate-pustulate pattern of 20–40 low projections occasionally with fused bases ( Fig. 1A ); proximal ornamentation similar but with fewer (3–7) projections per segment; suture ridges uniformly broad and round-topped ( Fig. 1B ); equatorial band narrowly obscure or absent and the equatorial ridge bold and straight ( Fig. 1C–D ); microspores oval, averaging 30 µm in length, with echinate or echinate-tuberculate ornamentation of broad-based acuminate projections and ± smooth-crested dorsal ridges ( Fig. 1E ). Chromosome number 2 n = 2x = 22 ( Fig. 1F ).

Additional specimens examined:— INDIA. State of Maharashtra: Dist. Pune, Lonawala , 7 September 2006, S.K. Singh & P.K. Shukla 601 ( BAN), same location, 11 September 2009, S.K. Singh & P.K. Shukla 901 ( Bck), same location, 7 September 2011, S.K. Singh & P. K. Shukla 1110 ( Uaa), same location, 10 September 2013, S.K. Singh & P. K. Shukla 1306 ( BAN). State of Karnataka State: Dist. Belagavi, Sada Plateau , near Goa border , 3.4 km northwest of Chorla, 15°40’38” N, 74°08’01” E and 15°40’17” N, 74°07’02” E, 28 October 2023, S.K. Singh & P.K. Shukla SPKS231028 ( DD, DFB, Ggg) .

Site ecology and environmental conditions:— Isoetes divyadarshanii on the Sada Plateau grows in open ephemeral low areas or water courses and margins of water channels in blackish soil over igneous bedrock within a tropical rain forest area at an elevation of approximately 820 m a.s.l. Various other wetland vascular plant species are present but no other Isoetes taxa. The population is luxuriant, growing within an area of approximately 28 hectares ( 281584 m 2; Fig. 3 ). The soil in which the Isoetes plants are rooted has a pH of 6.3 (N = 3) while the water has a pH of 6.2 (N = 3).

Annual plant growth commences with the emergence of new leaves at the start of the rainy season (mid-June), with spores maturing by mid-October and leaves disappearing when the area dries out in early November. Desiccated leaves separate from the corm during the dry season and are dispersed by wind or sometimes water. Sometimes the dormant corm remains intact in the substrate, awaiting the onset of the next rainy season to initiate a new set of leaves.

Phytogeography:— Isoetes divyadarshanii was considered endemic to the Northern Western Ghats Phytogeographical Division (NWGPD) of India ( Nayar 1996, Shukla et al. 2005), previously being known only from its type locality at Lonawala, Pune district of Maharashtra state at an elevation of approximately 628–900 m a.s.l.. The Sada Plateau population, situated within the Leeward Decan Plateau Phytogeographical Division (LDPPD at an elevation of approximately 820 m a.s.l.; Fig. 3A–B ), represents a significant disjunction (more than 465 km).

The NWGPD is characterised by two types of forests vegetation including moist deciduous forest and montane rain forest. The LDPPD, however, supports tropical grassland vegetation dominated by drought-resistant trees and shrubs that shed their leaves during the dry season to conserve water. Physiographically both the NWGPD and LDPPD are characterized by a series of hills on an extensive plateau ( i.e. both have lowland and upland areas) and include the Krishana and Tungabhadra River valleys (Singh 2021). The type locality of Isoetes divyadarshanii is situated at the western side of the Westen Ghats in NWGPD, while the Sada Plateau population is at the eastern side of the Western Ghats in different Phytogeographical Division (LDPPD). These phytogeographical divisions are separated by the Krishana and Tungabhadra River valleys which may act as a biogeographical barrier (Singh 2021) between the two I. divyadarshanii populations.

Isoetes divyadarshanii likely once occurred over a wider range across southwestern India. In the recent geological past, changes in the physical structure of these phytogeographical regions, along with habitat change and environmental shifts, and/or impacts of human interactions, may have fragmented the species into scattered populations within its originally broader range. Such isolated populations of single Isoetes species have been reported worldwide ( Brunton & Britton 1999, Troia 2016, Brunton & McNeill 2015, Amoroso et al. 2022, Shukla et al. 2024). Systematic field investigations of suitable wetland habitats in the two areas of confirmed occurrence as well as other parts of southern India, could test if this distributional interpretation applies to I. divyadarshanii as well.

Conservation status and sustainability:— The previous IUCN (International Union of Conservation of Nature) conservation designation for Isoetes divyadarshanii was established as A1ace; B2ab(ii,iii,iv); C1; D Critically Endangered ( IUCN 2019, Singh et al. 2021a). The discovery of a second, extant population of this species permits us to revise its conservation designation to A1cd; B2ab(ii,iii) Endangered ( Table 2 ).

There are no protection strategies in place for Isoetes divyadarshanii . The Sada Plateau population presently is adversely affected by many activities. These include the excavation and consumption of corms by wild boar ( Sus scrofa cristatus), grazing by domestic animals such as goats ( Capra aegagrus hircus ), cows ( Bos taurus ) and sheep ( Ovis aries ), human recreational activity, road development, tree plantation, stone quarrying, and site erosion during heavy rain events. The collection of plants by laboratory suppliers (pers. obs.) may also be having a negative impact on the population size.

The total number of individuals in the Sada Plateau Isoetes divyadarshanii population has not been precisely estimated. However, based on our field observation we roughly estimate it to be between 75000 and 125000 individuals. Even after animal grazing of the aerial leafy portions of the plants, the deeply buried corm remains intact and undisturbed (pers. obs.). The large number of individuals in the Sada Plateau population may also be due to the high frequency of occurrence of both megasporangia and microsporangia within the population, contrary to the situation with many other Indian Isoetes species ( Singh et al. 2021a).

Figures

FIGURE 1. Isoetes divyadarshanii spores and chromosome (S.K. Singh and P.K. Shukla SPKS321028). A. Distal surface of the megaspore showing an open to congested ornamentation pattern with tuberculate-pustulate projections/segment. B. Proximal surface of megaspore showing tuberculate-verrucate projections and uniformly broad suture ridges. C–D. Lateral view of the megaspore showing a uniformly wide equatorial ridge and narrow or absent equatorial band. E. Microspore ornamentation of broad-based acuminate projections (spines). F. Karyotype (2n = 2× = 22). Scale bars: A–D, 100 µm and E. 10 µm. Photos:A–E, P.C. Sokoloff & D.F. Brunton; Photo: F, S.K. Singh.

FIGURE 3. A. Phytogeographical divisions of India with locations noted for Isoetes divyadarshanii (star) and I. udupiensis (square). B. Location of Isoetes divyadarshanii populations (base image obtained from https://earth.google.com/web/). C. The open igneous bedrockbased habitat of the Sada Plateau Isoetes divyadarshanii population. D. Dense growth of Isoetes divyadarshanii plants at Sada Plateau population. Photos: S.K. Singh.