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Seagrass Meadows on the Ocean Floor

Primary food of the dugong, sea grasses cover thousands of kilometres of ocean floor, and are massive oxygen generators too

By Chetana Babburjung Purushotham

The Aquarium is a dive site in the Andamans where dive instructors would often take divers. The water was always a royal blue, the reef continued for as far as the eye could see, and the marine life glittered in the sun like jewels. But to experience the full extent of the magic, you had to leave the reef and swim over to the sand.

I would often ask the divers I was leading at the Aquarium if they had ever seen a mermaid. I was referring to the dugong (Dugong dugon) of course, the state animal of the Andaman and Nicobar Islands. This nine-foot-long, 400-kilo-heavy, air-breathing marine mammal, specialises in feeding on seagrass; a marine flowering plant that goes through its entire life cycle in the sea. What made the Aquarium special was the vast expanse of seagrass that grew in the white sand adjoining the reef.

The snake sea cucumber (Synaptidae) is the longest sea cucumber in the world. Here it uses the tentacles in its mouth to feed on detritus in a meadow of seagrass (Thalessia sp.). Photo: Umeed Mistry  Tunicates or ascidians also commonly known as sea squirts, are filter feeders and sometimes detritivores, those that feed on dead organic matter, finding plenty of food in seagrass ecosystems. Cover Photo: Umeed Mistry

The snake sea cucumber (Synaptidae) is the longest sea cucumber in the world. Here it uses the tentacles in its mouth to feed on detritus in a meadow of seagrass (Thalessia sp.). Photo: Umeed Mistry
Tunicates or ascidians also commonly known as sea squirts, are filter feeders and sometimes detritivores, those that feed on dead organic matter, finding plenty of food in seagrass ecosystems. Cover Photo: Umeed Mistry

GRASSES THAT WENT BACK TO SEA

Seagrasses are often mistaken for algae and talked about interchangeably with seaweed. Some traits are common — they can both be green, leafy, and photosynthesise to make their food, but beyond these superficial similarities, seagrasses and seaweeds are completely different sets of organisms. Seaweeds have a ‘holdfast’ to attach themselves to a surface and use diffusion to transport nutrients through their bodies. Seagrasses are monocotyledon plants like grasses and lilies. They grow roots, shoots, flowers, seeds, and have a vascular system built to distribute nutrients; air pockets called lacunae help the leaves to stay buoyant in the water. While we talk about how life began in the water and moved onto land, seagrasses quite extraordinarily evolved from terrestrial plants that moved back into the sea nearly 100 million years ago.

Before imagining seagrasses to be merely clumps of grasses arbitrarily sticking out of the sand, it is important to understand the scale at which these plants engineer themselves and entire ecosystems in otherwise loose and shifting sand. To steady themselves, seagrasses grow roots vertically into the sand as well as horizontally (called rhizomes). These buried rhizomes can also then send up new shoots, clones if you will, as they spread through a sand patch. We could be moving through a large patch of seagrass for several minutes and effectively still be following the same plant.

AN ECOSYSTEM IN THE SAND

Seagrasses trap particles from the water, increasing visibility, and stabilising the sediment in which they grow. They create nutrient-rich environments for large grazing animals like dugongs and turtles (Chelonia), by storing useful nutrients in their leaves and shoots. These nutrients are then made available to a host of other animals after they are shed and begin to decay.

Octopuses (top) in seagrass and sandy habitats have few structures to camouflage with and will often use empty mollusc shells as refuges to operate from. Holothuria scabra or sandfish (bottom left) is a variety of sea cucumber that prefers seagrass meadows, which offer nutrient-rich detritus. It is endangered by overharvesting though this species is protected by Indian law under the Schedule 1 of the Indian Wildlife Protection Act of 1972. What do dugongs, sapsucking slugs (bottom right) and juvenile filefish share in common? All three of these animals thrive when they can find a healthy patch of seagrass. Photos: Umeed Mistry

Sea cucumbers and snails amble through seagrass constantly devouring the rich detritus they find along the way. Sea stars use their hydraulically-powered tube feet to slide over the sand in search of hidden crustaceans. Octopuses stealthily move through the meadow, sometimes taking cover under abandoned snail shells, looking for other critters to satiate their carnivorous appetites.

Leafy seagrass blades play host to numerous types of ‘epiphytic’ algae that thrive in such sunny microhabitats, while also attracting a slew of foragers, from herbivorous fishes to slugs that are looking to have algae-rich meals. Tunicates of all shapes, sizes, and colours filter feed in the plankton-rich waters that constantly circulate through the meadow. What results is an intricate ensemble of life, where each organism focussed on its survival, has an important part to play in the running of this unique ecosystem constructed amidst fine sand.

HEART OF THE MATTER

You may be wondering why you have not have heard about seagrass ecosystems before, or in much detail. It is simply because we do not speak about them much. As far as marine ecosystems go, coral reefs are now globally recognised for their importance, but seagrasses on the other have not garnered as much appreciation. This is surprising because seagrass ecosystems cover a global estimate of 300,000-600,000 sq km of the ocean floor, across the tropics, the temperate zone, and every continent on Earth, except Antarctica. Since they are plants and require light to grow, they are found in shallow water and very often right off our beaches. Seventy-two species of seagrasses have been recorded globally, with at least 14 species found in Indian waters alone.

Seastars might appear relaxed at any given point of time. Their movements although slow, are powered by the action of thousands of hydraulically powered tube feet (left). The robust ghost pipefish (Solenostomus sp.) mimics a drifting blade of fresh to drying seagrass, an excellent camouflage as it hunts small crustaceans in the sand (right). Photos: Umeed Mistry

The value of seagrasses is well known to the inhabitants of the ocean. It is widely established in the marine realm that seagrass meadows make for excellent nurseries for offspring in their formative years.

Typically situated in shallow water, seagrass meadows are protected from larger predators that might be looming in their primary habitats such as coral reefs or out in the open sea. With a complex food web already in place, seagrasses provide ample food supply for juvenile animals of all kinds.

Seagrasses are also permanent residences for several wonderfully bizarre animals. Sea moths of the family Pegasidae are like small cars that have gone through every modification one can think of. With a pair of fins modified into wings, and another set of fins modified to walk on the seafloor, these fishes use a specialised toothless snout to suck out small invertebrates hiding in the sand. Equally clever in evolutionary design is the robust ghost pipefish (Solenostomus sp), a relative of the seahorse. The pipefish is so frequent a visitor to seagrass meadows that it is adapted to look exactly like a drifting blade of fresh to drying seagrass.

A snail (top) uses its proboscis to forage for food while paying close attention to its surroundings for looming predators. A juvenile pufferfish (bottom left) camouflages well in the leafy nurseries that seagrasses create, sustaining itself on smaller invertebrates that also use these habitats to feed and hide. Sea moths (bottom right) are also known as Pegasus, named for their resemblance with the divine winged-horse from Greek mythology. They are severely threatened by harvesting for Chinese medicine, aquarium trade, accidental fishing, and loss of habitat by bottom trawling. Photos: Umeed Mistry

In parts of India and around the world, humans have been a part of the seagrass ecosystem for centuries. Fishing communities living close to these habitats have long respected them for their high productivity and value, depending on it for their basic proteins and livelihood. Using various traditional tools and techniques, from free-diving to using intricate nets and spears, small-scale fishers harvest its resources keeping in mind their ecology and the need for balance.

The free diving shank fishers of Palk Bay have been diving in these areas for centuries. Today they use aluminium fins and dive masks as aids when they dive, although they report that their catches have significantly dropped due to overharvesting by large-scale mechanised fishing. Photo: Umeed Mistry

The free diving shank fishers of Palk Bay have been diving in these areas for centuries. Today they use aluminium fins and dive masks as aids when they dive, although they report that their catches have significantly dropped due to overharvesting by large-scale mechanised fishing. Photo: Umeed Mistry

BLUE LUNGS AND BLUE CARBON

As the Amazon forests continue to burn, we are learning about how much oxygen production we stand to lose, how much more carbon is being pumped into the atmosphere, and how much pressure will be put on other ecosystems from this ongoing catastrophe. We tend to forget that seagrasses and other vegetated coastal ecosystems produce a significant amount of oxygen too and they store more carbon than do forests. But just like the forests, seagrasses are being wiped out rapidly, but silently. From climate change, rapid coastal development, and destructive fishing we have lost 29 per cent of seagrass cover in the past century, and continue to destroy two football-field-sized meadows every hour.

DUGONGS AND US

Each time I dived in the seagrass meadow at the Aquarium, I would look up every few minutes in the hope of seeing a dugong (also called sea cow). After five years of diving at the Aquarium and countless upward glances, I saw it once. In last two decades, dugongs have disappeared from 60 per cent of their range in the Andamans due to hunting and entanglement in fishing nets. As conservationists work tirelessly to bring them back from the brink of extinction in this unique seascape, we must collectively recognise the need to protect seagrasses, before their obliteration pushes us to the brink as well.

Crabs(Portunus sps) love seagreass beds for the abundant matter they can feed on, but these shallow residences mean they are also easier tangled in a fishing net. Photo: Umeed Mistry

Crabs(Portunus sps) love seagreass beds for the abundant matter they can feed on, but these shallow residences mean they are also easier tangled in a fishing net. Photo: Umeed Mistry

Chetana Babburjung Purushotham
Chetana Babburjung Purushotham

is a wildlife biologist and PADI divemaster based in the Andaman Islands. She is also an avid writer, keen on sharing her experiences with wildlife to help more people reconnect with nature.

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