Underwater grasses are probably the most misunderstood plants on Earth. Tourists and hoteliers get upset because they stain green the turquoise blue of the Caribbean that looks so good in the background on selfies. Many of us simply ignore their existence or believe that they are populations of seaweed similar to sargassum that harm Caribbean beaches so much.
What is the use of having grass under the sea, some may ask, if you can't play soccer there, have picnics or fatten cattle? First of all, we must not let the names confuse us: although they grow like grass, seagrasses are more related to ginger and lilies, than to domesticated grasses such as wheat or oats; due to their fascinating evolutionary history (which we will tell later), they descend from terrestrial plants that adapted, with everything and flowers, to live exclusively in marine environments or estuaries. Seagrasses are essential ecosystem engineers: those responsible for creating dens and pantries for aquatic life on the coasts of all continents, except Antarctica.
Despite their importance, these plants have had only a few key moments in the history of science. The first when Aristotle described the seeds of what is now known as Posidonia oceanica, a Mediterranean grass that rose to fame in 2012 because a meadow over eighty thousand years old was identified as “the oldest living organism on Earth” on the coast of Spain. 1
After Aristotle, no one remembered pastures again until Linnaeus classified in 1779 the species Posidonia oceanica and Zostera marina, which Erasmus Darwin (Charles's grandfather) described in a poem from 1803 as the queen of coral reefs who sleeps surrounded by algae. 2
The technological advances of the 1970s in diving, such as autonomous underwater breathing equipment, known as SCUBA by its acronym in English, made it possible for seagrass beds to begin to be studied with more patience and determination.
Since then, it has been discovered that underwater grasslands, together with coral reefs, are the basis of the shallow marine ecosystem. The pastures are home to many species of fish that are responsible for cleaning and maintaining the coral reef, such as the parrotfish. The vast majority of fish and seafood consumed worldwide is caught in seagrass meadows and the livelihoods of the hundreds of millions of people around the world who live on fishing depend on these meadows 3. Although they are just as important as coral reefs, or perhaps more so, because without grasses the reef deteriorates, seagrasses are in the popular imagination as a nuisance and not as a charismatic and colorful aquatic world that is key to food security.
35% of Mexico's seagrass meadows are located in the Yucatan Peninsula, 4 and those currently most threatened by industrial activity, the lack of conservation policies and the proliferation of sargassum are those on the Quintanarroan coast. For this reason, it is very important to know and protect this treasure.
Engineer, babysitter, breaker and anti-acid, to serve you
Like other plants, tropical species of seagrass such as Thalassia testudinum, Syringodium filiforme or Ruppia maritima capture energy from the sun and use it to transform carbon and other minerals into organic matter and underwater grasslands that help fix coastal soil, reduce tidal force and serve as shelter for many animals and as food for others. 5
This transformation of carbon into food, both our own and foreign, makes seagrass fields work as “sinks” for that dreaded element that, due to the enormous emissions produced by fossil fuels, is making seawater more acidic every day and increasing the temperature of the atmosphere. An underwater meadow traps around 10% of marine carbon and does so 35 times faster than a tropical forest, so its role in mitigating climate change is extremely important. 6
In addition, seagrasses directly combat ocean acidification, as they have evolved chemical reactions to convert carbonic acid dissolved in water into CO2. A more acidic ocean hinders the production of external calcium carbonate shells and skeletons in mollusks and corals, prevents their reproduction and compromises their survival.
Like all good grass, these meadows are also foraged. Its biggest consumer is the manatee, a gentle coastal mammal that the first European explorers of the Caribbean identified with the mythical mermaids (which is why their taxonomic order is called Sirenia). If they are also known as sea cows, it is because they like to graze (they are not particularly picky, and to maintain their bodies they eat many species of grass, algae and even mangrove leaves).
One of the oldest descriptions of Caribbean seagrasses is found in a passage from the New Journey Around the World (1681) in which pirate William Dampier is describing manatees: “They live on grass seven or eight inches long and with a narrow blade, which grows in many places in the sea, especially near islands close to the mainland”. 7 This proximity to the mainland is due to the fact that grasses need sunlight to photosynthesize and cannot live in very deep waters since the water column filters and attenuates light.
Other charismatic herbivores that usually feed on seagrass are turtles. In fact, the “most robust” grass species in the Caribbean, which “dominates meadows that extend for tens or hundreds of square kilometers” is commonly known as turtle grass. Its scientific name, Thalassia testudinum, is formed from the Greek name for the sea (Thalassos) and the scientific name given by Lineo to the order of turtles, Testudine.
Turtles are a bit pickier in their way of grazing, as they like fresh salad, sprouted grass. Their unique way of eating consists of pruning the grasses about four fingers above the ground, spitting out the pruned leaves and leaving to return 5 to 8 days later to eat the juicy and nutritious shoots that sprouted after pruning. Thus, turtles maintain a rotation of meadows that they exploit periodically to ensure an abundance of fresh grass. Mature grass, in addition to being less nutritious, is more like a nursery inhabited by small jellyfish, crustaceans and fish.
If we think of manatees and turtles as the cows and armored sheep of the sea, we may miss aquatic crickets. However, hedgehogs don't disappoint, as they are ferocious invertebrate herbivores that, like locust pests, can put grasslands in serious trouble if their appetite multiplies excessively.
Evolution: into the water, pastures!
The evolutionary history of seagrasses is a random odyssey, which after a long journey returns to its place of origin, the sea. Emerging from the sea, the ancestors of terrestrial plants had to overcome the challenges of the lack of water in the terrestrial environment, and when they returned, these pastures had to shake off those acquisitions and evolve new antidotes against excess water, salt and lack of air.
Living beings are a fairly dissolute gelatin whose main ingredient is water. Therefore, the scarcity of water was a significant stress factor for the organisms that undertook the conquest of the mainland. Although many algal lineages emerged from the water, only one was able to colonize the Earth: the streptophytes, from which all terrestrial plants descend.
In algae, reproductive cells are almost immediately dehydrated out of the water. Streptophytes coated pollen, zygotes and spores in sporopolenin (an incredibly resistant and durable substance that protects against desiccation and UV radiation) and managed to proliferate on earth. They also developed a water-repellent fatty cuticle that prevents the loss of fluid in their photosynthetic tissues, but allows gaseous exchange with the environment through pores or stomata.
Thus, 450-500 million years ago, plants began to conquer the Earth's environment, and 100 million years ago, several million years before the dinosaurs began to die out, a group of flowering plants set out on their journey back to the water. The first were mangroves that colonized bodies of fresh water, followed by pastures that adapted to shallow seas. 11
When they returned to the water, the pastures got rid of some terrestrial adaptations, such as the cuticle and their stomata, because desiccation was no longer a problem and the water had to flow freely to absorb nutrients, stay upright and ensure gas exchange. As there is almost no carbon dioxide in the sea, which is necessary for photosynthesis, seagrasses have evolved chemical reactions to convert carbonic acid into CO2, which is very convenient for curbing the acidification of the sea mentioned above.
The flowers (and their bees) in the sea
The sexual life of the plants also changed a lot when they returned to the water. The sporopolenin that covered the pollen against desiccation thinned out, and the spherical male grains were packed into gelatinous strands meant to join the stigmas of the female flowers.
Since water is capable of carrying pollen randomly, female and male flowers were separated into different unisexual individuals, avoiding self-fertilization.
For decades, it was thought that seagrasses had completely renounced the help of animal pollinators (such as insects, birds and bats that visit flowers on land), but thanks to research carried out in the seagrass meadows of Puerto Morelos, Quintana Roo, it was discovered in 2016 that there are also “bees” in the sea. 12 With this surprising finding (in which the coatura of this article participated), the belief that underwater pollination is totally abiotic, that is, carried by water currents, was put to rest, and it was understood that some plankton organisms function as sexual mediators between male and female plants.
Based on the methodical observation of the grasslands by diving biologists, and with different experiments carried out in the laboratory, the necessary evidence was gathered to talk about the underwater biotic pollination of turtle grass (Thalassia testudinum): at nightfall, the male flowers open and release pollen spaghetti surrounded by mucilage rich in sugars.
The invertebrates that float in the water at the swaying of the currents (isopods, tiny crustaceans, polychaete worms and other bugs) make enormous efforts to swim towards these spaghetti, which in addition to being nutritious, are sticky, so pollen grains adhere firmly to their bodies. As soon as the bugs are close to another flower, they swim back with enthusiasm to land on it. If they fall on a female flower, their unsuccessful search for more mucilage will leave the pollen attached to them clogged in the stigmas. Thus, without knowing it, they have fulfilled their function as sexual mediators between the male and female plants of the turtle grass.
Hours and hours of slow-motion videos show this underwater waltz of tiny creatures transporting pollen in a semi-orchestrated manner. They are seen walking, eating and almost enjoying the sweetness of their pollen-filled rewards, a strategy that was originally developed in the terrestrial grandparents of seagrasses with the function of attracting pollinators (evolution, as always, does not know who it works for...).
Challenges: The Prairie Calvary
Underwater grasslands share with the rest of the biosphere an existential crisis caused by human activity. Based on a review of 215 regional studies, a team of researchers discovered that the global area of underwater grasslands had decreased by 29% between 1879 and 2006, and that land loss was increasingly rapid 13.
Many threats to terrestrial ecosystems also plague aquatic ecosystems, and in particular underwater grasslands, which are affected by deforestation (deforested soils erode and rivers deposit more sediment in the sea every day), the abuse of fertilizers and herbicides in industrial agriculture, the accelerated development of coastal areas at the hands of the tourism industry and the release of other harmful pollutants into the water that intoxicate pastures.
In Quintana Roo, one of the main threats to underwater grasses is the proliferation of sargassum, macroalgae that, when decomposed in coastal seawater, release substances that block the passage of light, decrease the availability of dissolved oxygen in the water and directly affect plant tissues.
According to Rosa Elisa Rodríguez Martínez and Brigitta Ine van Tussenbroek, researchers at the Institute of Marine Sciences and Limnology who have documented the health of coastal ecosystems in the Mexican Caribbean for many years, “after the brown sargassum tide of 2015, the changes in vegetation close to the beach line were evident; the most obvious feature was the replacement of seagrass by macroalgae”. 14
The effects of climate change, such as the acidification of seawater and the increase in the strength of hurricanes and tides, also harm these grasslands. According to researcher Tania Cota Lucero, from the Center for Research and Advanced Studies of the National Polytechnic Institute (IPN), Mexico has lost more than 100,000 hectares of seagrass in the last 17 years due to climate change. 15
Faced with these challenges, the conservation of seagrass meadows is, from the point of view of our anthropocentric interest, a crucial strategy to guarantee food security and face the pernicious consequences of climate change; on the other hand, to respect and restore these pastures is an ethical duty to all animal species whose lives depend on them.
We hope that knowing the importance of seagrass meadows as carbon sequestrators, beach protectors, biodiversity refuges and food sources for the most charismatic animals in the Caribbean will help promote their conservation and help us to appreciate the beauty of these modest and generous plants that flourish under the sea.
To learn more about ecosystems, ecosystem services and citizen participation, visit www.serviciosecosistemicos.mx
This article was produced as part of the Conservation and Sustainable Use of Marine Biodiversity in the Mexican Caribbean project of the National Commission for Protected Natural Areas and the German Agency for International Cooperation GIZ, in collaboration with the Mexican Network of Science Journalists.
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