What are the differences between Nymphoides peltata, Nymphoides indica, and Nymphalis? We explain how to distinguish between similar species! Why are they endangered? Did Nymphoides peltata and Nymphoides indica survive through a strategy of pleasing everyone?

Nymphoides peltata, Nymphoides indica, and Nymphaea tetragona are three species of floating-leaved plants with round leaves found in lakes and ponds in Japan. They look very similar, but Nymphoides peltata and Nymphoides indica belong to the Menyanthaceae family, while Nymphaea tetragona belongs to the Nymphaeaceae family, making them completely different species. They can be reliably distinguished if they have flowers, but it is believed that they can be distinguished with considerable accuracy even by their leaves alone. Nymphoides peltata and Nymphoides indica are designated as near-threatened species in Japan due to the reclamation and renovation of ponds and ponds, as well as the progression of water pollution, while in some areas they are considered invasive alien species. Their flowers have heterostylicity, with one type having long stamens and the other having long pistils, which prevents self-pollination. However, they also reproduce vegetatively to a considerable extent, and it seems that they actually use both seed reproduction and vegetative reproduction. The flowers of Nymphoides peltata and Nymphoides indica, which seem to play such a supplementary role, do not limit their role to being pollinated by specific insects, but have evolved to attract insects from a wide range of taxonomic groups. The fruits are capsules that disperse through water, and the seeds survive unfavorable environmental conditions by being buried in the soil underwater. This article will explain the classification, pollination ecology, and seed dispersal of Nymphoides peltata and Nymphoides indica.

Three species of floating-leaved plants with circular leaves found in lakes and ponds in Japan

Nymphoides peltatum, also known as Asaza (浅沙 or 阿佐佐), is a perennial floating-leaved plant widely distributed from Honshu, Shikoku, and Kyushu in Japan, to Korea and China, and across Eurasia, inhabiting relatively eutrophic lakes and ponds (Takagawa, 2006). Its status varies greatly depending on the region; in Japan, it is designated as Near Threatened (NT) on the Ministry of the Environment's Red List, and in Eastern Europe, it is an endangered species, while in North America and New Zealand, it is known as a harmful invasive alien species.

Nymphoides indica, also known as Gagabuta (mirror-lid), is a perennial floating-leaved plant distributed in Honshu, Shikoku, and Kyushu in Japan; Korea, China, Southeast Asia, Africa, and Australia, growing in colonies in lakes and ponds (Satake, 1999). It is listed as Near Threatened (NT) on the Japanese Ministry of the Environment's Red List.

The term "water lily" (水蓮) is not botanically a term referring to a specific species, but rather to the general genus Nymphaea. However, the term "water lily" is often used to describe cultivated varieties. Widely distributed in temperate and tropical regions, there are about 50 known species worldwide. In Japan, only Nymphaea tetragona is known to grow wild, although many cultivated varieties are actually found there.

All of these are floating-leaved plants with round leaves that can be found in lakes and ponds in Japan. They are also cultivated in horticulture, and may be easily confused based on their leaves alone.

What are the differences between Nymphoides peltata, Nymphoides indica, and Nymphoides natans?

However, in terms of classification, Nymphoides peltata and Nymphoides indica belong to the Menyanthaceae family, while Nymphaeaceae belongs to the Nymphaeaceae family, so they are completely different.

That difference is most evident in the flowers.

While Nymphoides peltata and Nymphoides indica have fused petals divided into five lobes with frills and thread-like decorations, Nymphaea species have separate petals with 4-5 sepals and numerous petals.

The difference between Nymphoides peltata and Nymphoides indica is that Nymphoides peltata has yellow flowers that are twisted, deeply divided into five lobes, and irregularly incised, while Nymphoides indica has white flowers with long, dense hairs on the inner surface and edges of the lobes.

If the flowers are in bloom, you won't mistake them for each other. However, depending on the time of year and location, you might be able to distinguish them by their leaves alone.

Regarding the differences in floating leaves, Nymphoides peltata and Nymphoides indica usually have leaves that are nearly ovate-circular with an ear-shaped base, while Nymphoides sanguinea generally have leaves that are nearly circular with a pointed, arrowhead-shaped base, often reminiscent of the Bandai Namco character "Pac-Man." Also, Nymphoides peltata and Nymphoides indica have entire leaf margins, but some Nymphoides varieties have wavy, serrated edges.

Furthermore, in Nymphoides peltata and Nymphoides indica, the veins are particularly white and conspicuous only in the center of the leaf, with the lateral veins being almost inconspicuous, whereas in Nymphaea species, fine lateral veins running throughout the leaf can be seen even from the outside.

The difference between Nymphoides peltata and Nymphoides indica is that in Nymphoides peltata, the base of the ear-shaped leaves is not very wide and the base of the petiole is reddish, while in Nymphoides indica, the base of the leaves is not very wide and the base of the petiole tends not to be reddish.

In terms of classification, there are differences between *Nymphoides peltata* and *Nymphoides indica*, where the upper leaves are opposite and the petioles are shield-shaped, while in *Nymphoides indica*, the leaves are alternate and the petioles are basifixed (Kanagawa Prefecture Flora Survey Association, 2018). However, it may be difficult to recognize these characteristics from the outside.

Why did it become a near-threatened species?

Why have plants like *Nymphoides peltata* and *Nymphoides indica* become near-threatened species?

The main cause is thought to be the reclamation and renovation of ponds and reservoirs, as well as the progression of water pollution, similar to the situation with many rare floating-leaved plants (Kyoto Prefectural Government Environmental Department, Natural Environment Conservation Division, 2015).

However, as will be discussed later, they have various reproductive methods and, as mentioned above, have become harmful invasive alien species in North America and New Zealand. There is still a lack of research into why these two species, which at first glance seem to have high reproductive capabilities, have become endangered.

Why are there types with long stamens and types with long pistils?

Nymphoides peltata flowers from June to September. The corolla is deeply divided into five lobes, 3-4 cm in diameter and 2.5-3 cm long, yellow, twisted, and irregularly notched, giving it a frilly appearance. The sepals are five in number and 9-13 mm long.

Nymphaea tetragona flowers from July to September. It produces numerous flowers in clusters at the nodes. The corolla is deeply five-lobed, about 1.5 cm in diameter, white, and densely covered with long hairs on the inner surface and edges of the lobes. The sepals are ovate, 4-6 mm long.

As described above, their flowering periods are quite close, but the shapes of their flowers are very different.

There's something a little unusual about the flowers of the water lily and the water hyacinth.

This means that there are individuals with long stamens and individuals with long pistils (Takagawa, 2006). Why is this the case?

This is thought to be a mechanism to prevent self-pollination. In this way, when insects visit the flowers, in the case of flowers with long stamens, the pollen from the flowers with long stamens will land on the stigma of the flowers with long pistils. Conversely, in the case of flowers with long pistils, the pollen from the flowers with long pistils will land on the stigma of the flowers with long stamens.

This might be a little difficult to understand, but essentially, it means that the male and female sexes are spatially separated, and pollination occurs between plants with long stamens and plants with long pistils, but not between plants with long stamens or between plants with long pistils.

This characteristic is called "heterostyly" and can be found in a variety of plants.

The formation of heterostylicity is considered a rare phenomenon because it is genetically extremely complex (Watanabe, 2022). Furthermore, since it results in two distinct types, it effectively means that only half of the population can crossbreed. This is a significant disadvantage. Therefore, while it effectively prevents self-pollination, it remains a strategy for a minority group.

Did they use both seed propagation and vegetative propagation?

On the other hand, genetic analysis of Nymphoides peltata has shown that most of its natural habitats consist of only one or two clones, and that individuals with heterostylicity, which is necessary for seed production, grow only in Lake Kasumigaura in Ibaraki Prefecture, with only 61 individuals remaining in Japan (Uesugi et al., 2009).

Nymphoides peltata not only produces seeds through pollination, but also reproduces vegetatively by severing stolons. In Japan, where the population has declined, clonal reproduction may also be prevalent.

Subsequent studies have detected heterostylicity in Lake Inawashiro in Fukushima Prefecture and Lake Biwa in Shiga Prefecture, and have also shown that clonal populations exhibit genetic diversity (Fujii et al., 2015), suggesting that both methods are ultimately important means of reproduction.

Similarly, Nymphaea tetragona also reproduces vegetatively, forming banana-cluster-like buds at the base of the petiole.

Was the water lily a two-faced plant that attracted various insects for pollination?

What kinds of insects visit the flowers of the water lily (Nymphoides indica)?

Studies in Europe and Izunuma, Miyagi Prefecture, have shown that bees, especially bumblebees, are the primary visitors (Takahashi et al., 2011). On the other hand, studies in Lake Kasumigaura, Ibaraki Prefecture, suggest that the butterfly Parnas guttata is important. Why do these differences occur?

Regarding domestic conditions, the Common Skipper butterfly cannot survive the winter in Miyagi Prefecture, limiting its habitat, but it is commonly found west of the Kanto region. This difference seems to be the reason for the discrepancy. In other words, it appears that the important insects change depending on the surrounding environment, rather than the influence of the Nymphoides peltata flowers.

As evidenced by its large, flat, saucer-shaped corolla and its thick stamens and pistils that are easily visible from the outside , the Nymphoides peltata flower is a "generalist" that can attract a variety of insects. It has also been observed that aquatic flies such as the shore fly, as well as common hoverflies and moths, visit the flower. It may be able to live flexibly in its surrounding environment.

From a conservation perspective, it seems important to learn more about these kinds of interactions with animals.

The water hyacinth is also a people-pleaser, but it seems to be having trouble with pollination...

On the other hand, what kinds of insects visit Gagabuta?

According to research conducted in Hyogo and Okayama prefectures, the area was visited by a wide range of insects, including the Common Skipper, several species of the genus Notiphila (spine skipper), the hoverfly Mesembrius flaviceps, the European honeybee Apis mellifera, the rusty-colored bee Lasioglossum mutilum, and several species of the genus Bagou (weevil) Bagou (spine weevil), indicating that a broad range of insects from the orders Lepidoptera, Diptera, Hymenoptera, and Coleoptera were visiting the site (Shibayama & Kadono, 2003).

It is thought that Nymphoides peltata, like Nymphoides indica, has evolved as a generalist plant.

However, these insects flew less than 20 cm from flower to flower and frequently performed cross-flower pollination. This means that, due to heterostylicity, the insects were not reaching individuals with different types of flowers. Therefore, it is thought that the plant's natural seed production capacity was reduced.

I don't know why this is happening, but it might be due to a decrease in the population size.

The fruits of Nymphoides peltata and Nymphoides indica are capsules, and the seeds are dispersed by water.

When Nymphoides peltata and Nymphoides indica flowers, they produce fruit. The fruit, common to all species in the Nymphoides genus, is a capsule.

The capsules of the water lily are oblong-ovate. The seeds are flattened and have columnar projections along the edges.

The capsules of *Nymphoides indica* are oval-shaped and 4-5 mm long. The seeds are broadly oval-shaped, glossy, and 0.8 mm long.

The method of seed dispersal for Nymphoides peltata is well understood. Because the seeds of Nymphoides peltata are covered with a volatile seed coat, they are buoyant and can be dispersed by water, even over long distances (Takagawa, 2006).

However, you might think that it would be difficult to move to a different river system under these circumstances.

In addition to this, it is said that "animal dispersal" also occurs, where the seeds of the water lily attach to waterfowl.

Furthermore, the seeds are dormant, forming a permanent soil seed bank. The seeds remain buried and dormant in the soil underwater. This means they won't force themselves to germinate when environmental conditions are unfavorable.

Experiments have shown that seed germination is promoted by cold-humidity treatment and variable temperature conditions, and inhibited in water with low oxygen partial pressure. This means that germination occurs in a warm, dry environment with oxygen. From this, it is hypothesized that germination and establishment occur in open, dry environments that are not flooded in early spring.

This might be a bit surprising considering they normally live in water, but they also have a "terrestrial" form adapted to bare ground environments. It's strange that they are endangered despite appearing to be so adaptable.

References

Fujii, Shinji; Uesugi, Ryuji; and Yamamuro, Masumi. 2015. A follow-up study on the growth environment, flower type, escape status, and genetic diversity of *Nymphoides peltata*. Conservation Ecology Research 20(1): 71-85. https://doi.org/10.18960/hozen.20.1_71

Kanagawa Prefecture Flora Survey Association. 2018. Kanagawa Prefecture Flora 2018 (Electronic Edition). Kanagawa Prefecture Flora Survey Association, Odawara. 1803pp. ISBN: 9784991053726

Kyoto Prefectural Government, Department of Environment, Natural Environment Conservation Division. 2015. Kyoto Prefecture Red Data Book 2015 (Volume 2: Wild Plants and Fungi). Kyoto Prefectural Government, Department of Environment, Natural Environment Conservation Division, Kyoto. 611pp. https://www.pref.kyoto.jp/kankyo/rdb/index.html

Satake, Yoshisuke. 1999. Wild Plants of Japan (New Edition, Herbaceous Plants 3, Sympetalous Plants). Heibonsha, Tokyo. 259pp. ISBN: 9784582535037

Shibayama, Y., & Kadono, Y. 2003. Floral morph composition and pollen limitation in the seed set of Nymphoides indica populations. Ecological Research 18(6): 725-737. https://doi.org/10.1111/j.1440-1703.2003.00591.x

Takagawa, Shinichi. 2006. Conservation ecological study on the regeneration of Nymphoides peltata populations using a soil seed bank. [Doctoral dissertation, University of Tokyo]. https://doi.org/10.15083/00004276

Takahashi, Mutsumi; Yoshida, Masayuki; Tamura, Masayoshi; Nakai, Shizuko; Shimada, Tetsuro; Yokoyama, Jun. 2011. Insect fauna of * Nymphoides peltata (SG Gmel.) Kuntze* growing in Izunuma. Research Reports of Izunuma and Uchinuma 5: 5-11. ISSN: 1881-9559, https://doi.org/10.20745/izu.5.0_5

Uesugi, T., Nishihiro, J., & Washitani, I. 2009. Current status and genetic diversity of the endangered aquatic plant *Nymphoides peltata* population in Japan. Conservation Ecology Research 14(1): 13-24. https://doi.org/10.18960/hozen.14.1_13

Watanabe, Kenta. 2022. Current Ecological and Evolutionary Biological Perspectives on "Heterostyle." Okinawa National College of Technology Bulletin 16: 31-45. https://doi.org/10.51104/nitokinawacollege.16.0_31

Source

This article is a significantly expanded version of a piece originally published in the following book.