Female seagrass flowers uncoil to reach pollen in undersea saga

For the underwater-dwelling tape grass Enhalus acoroides, pollination is an adventure where female flowers have their work cut out.

The pinkish-green male flowers produce plenty of pollen, encased in long lines of slime. Females have stigmas shaped like tentacles, and the flower itself is supported by a coiled stalk. Male flowers release their mucilage-covered pollen onto the water surface at night.

“…Then the female flowers open their petals to “catch” the drifting male flowers moving by surface currents or wind. To catch the male flowers, the peduncle [stalk] straightens, and then coils and sinks down to the bottom,” write Hisao Ogawa and Nobuyoshi Nanba in their study on Indonesian E. acoroides.

The coils are permanent and can be nearly half a metre (45-50 cm) in length, said Vardhan Patankar, a researcher with the Nature Conservation Foundation and Wildlife Conservation Society, who has been working on the marine ecosystem of the Andaman and Nicobar Islands. Patankar and colleagues Tanmay Wagh and Zoya Tyabji have documented this remarkable behaviour in a recent study.

Tape grass belongs to a group of plants called seagrasses, which are found in shallow salty waters around the world, on all continents except Antarctica. These underwater plants live in much the same way as their land-dwelling cousins, the monocotyledons we know so well — grasses, lilies and palms.

Land plants have small openings on their leaves, called stomata, through which they ‘breathe’ by exchanging gases with the atmosphere. Seagrasses have a thin cuticle which allows for gaseous exchange — but that’s about the only major difference between them and their land relatives.

Seagrasses form an important part of the coastal ecosystem. Shallow waters along the coast have seagrass meadows, seaweed beds (these are the algae that go into sushi), mangrove forests and coral reefs, the “tropical rainforests of the sea”. An undisturbed stretch of coast would be “lined by a thick mangrove forest, whereas coral reefs and seagrass meadows are found in the shallow coastal waters surrounding the island,” write Patankar, Wagh and Tyabji in their paper.

Underwater meadows along our coasts

Of the 72 species of seagrasses found globally, 14 (and counting, experts feel) are found along India’s coasts. India (and the Indo-Pacific region as a whole) houses the highest diversity of seagrasses in the world. All seagrass species occur in shallow, clear waters along the coast. Along Indian shores, species occur in both the intertidal zone, the area between the high tide and low tide lines that gets periodically inundated, and up to a depth of 15 metres in the sub-tidal zone, which is always submerged underwater.

The Gulf of Mannar and Palk Strait regions along the southeast coast of the Indian peninsula have all the 14 species found in the region. Cymodocea serrulata is dominant in the sub-tidal region, and flowering was observed to occur between January and April and in September, reports Tanaji Jagtap from the National Institute of Oceanography in his review on Indian seagrasses.

The Lakshadweep archipelago in the Arabian Sea and the Andaman and Nicobar Islands in the Bay of Bengal have eight and nine species of seagrasses respectively. Lakshadweep, composed of about 36 islands, is India’s only coral atoll: an array of coral reefs that surrounds a body of shallow water, called a lagoon. Among the seven species of seagrasses found in these islands, Thalassia hemprichii and Cymodocea rotundata are dominant.

The high diversity of seagrasses in Gulf of Mannar and the islands is because of “the high degree of clarity of the water, sandy substratum and calm waters,” reports Jagtap in his review. The underwater visibility was a stunning 20 metres in Lakshadweep, and 8-10 metres in the Gulf of Mannar between January and March, the scientists report.

The seagrass communities in the three areas of highest diversity in India: Gulf of Mannar, Lakshadweep and Andaman and Nicobar Islands, are of very different composition due to the dramatically different biogeographies. Lakshadweep’s coral atoll nature, and proximity to the Maldives atolls, versus Andaman and Nicobar’s proximity to Malaysia and Indonesia, account for different plant communities and as a result, different fish and megaherbivores as well.

Dugongs (Dugong dugon) swim the waters around Andaman and Nicobar Islands, though not in great numbers. A 2015 study counted just 15 dugongs from the entire archipelago. The seagrasses growing among the coral reefs of Lakshadweep seem to attract more of the green sea turtle (Chelonia mydas). Both are megaherbivores that feed on seagrasses, and act as important ecosystem modifiers.

The dynamics of meadow maintenance

Enhalus acroides — the tape grass with uncoiling female flowers we met earlier — is a dominant seagrass in the Andaman and Nicobar Islands. It is the tallest species of seagrass in the world, reaching heights of 150 cm. Its large size means it contributes heavily to the biomass of the region. Other studies have shown that tape grass can bury carbon sediments 40 times faster than tropical forests bury it in the soil, write the authors in a press release.

“Due to the sheer number of plants and total forest cover, we think of forests as a major carbon sink,” said Patankar. “However, once the tree dies the carbon gets released into the atmosphere. Whereas, in seagrass meadows, carbon gets stored in oxygen-depleted sea beds and even if seagrass dies carbon stays trapped inside the sediment.”

Apart from studying E. acoroides in detail, the researchers also documented the difference between two islands with relatively intact coastal ecosystems: Henry Lawrence and Tarmugli. While Henry Lawrence had dense mangrove forest along the coast, making for a more sheltered coast with multiple seagrass species, Tarmugli had a smaller seagrass patch, exposed to the open ocean.

The difference between the two patches was significant, with the more sheltered Henry Lawrence having taller plants, with more number of shoots, flowers and fruits. The meadow location, size, depth and exposure seem to have had an effect on the seagrass meadows.

The devastation of the 2004 tsunami

The 2004 tsunami dramatically modified the coastlines of India’s islands, and the seagrass meadows abutting the islands were no exception. The northern islands of the Andaman and Nicobar archipelago were uplifted by almost a metre, and the southern islands were submerged by a drastic three metres.

In the Andaman & Nicobar Islands, before the tsunami of 2004, E. acoroides was reported from Paschim in Bihar, North Reef, Inglis, Henry Lawrence, Havelock, and Cinque islands in the Andaman group and Camorta, Trinket, Nancowry, Katchal, Pilomilow, Little Nicobar, and Great Nicobar islands in the Nicobar group. Post-tsunami, however, its presence was reported only from Henry Lawrence and Tarmugli in the Andaman Islands and Kamorta and Nancowry in the Nicobar Islands, write Patankar and others in their study.

The shift in distribution could be because there were no focussed studies on the species for comparison before and after the tsunami, the authors write. It is however possible that the tsunami-wrecked havoc and wiped out seagrasses from vast sections of the coastline.

“When catastrophic events (cyclones, tsunamis) affect seagrasses, the recovery of a meadow will be dependent first on the remnant seagrasses that will propagate vegetatively and regenerate the meadow or on the seed bank that is present in the sediment to give rise to new shoots,” said Elrika D’Souza from the Nature Conservation Foundation, who has been studying dugong-seagrass interactions in the Andamans.

“I do not have a map to show the extent of damage done by the tsunami and am unsure if the composition of seagrasses has changed. Based on comparisons with previous studies, it does not seem like. What could have changed, is the dominance of one species over the other, but this I do not have any data for,” she added.

A 2010 book chapter on the biodiversity of the Andaman and Nicobar Islands has recorded what is known about the shift in seagrass distribution and meadow composition before and after the tsunami. The researchers studied seagrass communities in 11 locations from the northern part of Andaman to the southern tip of Great Nicobar island. They found nine species of seagrasses (which is unchanged from before the tsunami), but differences in distribution could be spotted.

In Chidiyatapu close to Port Blair, for instance, the extent of seagrass cover was lesser and the remaining stand “physically destructed and not showing any sort of regeneration”, write the authors. The number of species found in Little Andaman dropped from five to three. In the Nicobar group, fewer species were found, and were sparsely distributed, with many seagrass meadows buried in sand and sediments.

Unusually destructive events like the tsunami and cyclones are set to increase across the world, as per the latest climatic predictions. A recent, serious threat to seagrasses across the world are marine heatwaves. Similar to the heatwaves on land, these are not a gradual increase in sea temperature but rather a sudden spike in temperature, a discrete warming event. Such events are set to increase during this century. A 2019 study examined data since 1925 and found that the number of marine heatwaves has increased around the world, throughout the twentieth and early twenty-first centuries.

The study predicts that taxa that cannot move, like coral reefs, seaweed beds and seagrass meadows, are at maximum risk from marine heatwaves. Longterm time series data from Western Australia have shown that an increase in the number of days with marine heatwaves is correlated with decreased seagrass density.

Seagrasses are the nurseries of the ocean ecosystem, but they are usually under-appreciated next to their more charismatic seabed neighbours, the coral reefs. However, destruction of the seagrass ecosystem would have a domino effect on the ocean food web and ultimately affect ocean productivity, reports a 2010 review on Indo-Pacific seagrass conservation.

Even if protected areas were established for seagrasses, they would survive only if clear water conditions are maintained, said Frederick Short, a marine biologist from the University of New Hampshire and director of SeagrassNet, a global seagrass monitoring network.

“The challenge is to protect the coastal ocean’s waters from human pollution and activities, in order to provide a sustainable environment for seagrasses,” he said.

Read more on seagrass biology in this explainer article: What is seagrass?