Savannahs of the Sea
Lewis Hawkins
Seagrass meadows are host to some of the most beautiful, diverse and charismatic creatures in our marine world – but as a habitat, you could say it is overlooked on a day-to-day basis, especially if you don’t know too much about it. Some may just think of it as a seaweed - scary slimy stuff that tickles your feet at the beach and makes you think some marine monster is about to swallow you whole. Many of you will have tried to find a way around the wet, stinking piles of ‘wrack’ that line the high tide-line after a storm. This is made up of seaweeds and seagrasses that have been battered by heavy waves – a process that is an important part of nutrient cycling in the intertidal zone. Well I’m here to tell you there’s more to seagrass than you might think. Soon you’ll be diving down to admire the toe-tickling meadows and exploring the funky piles of wrack to see what marine curiosities you may find.
A tropical seagrass bed in Sulawesi, Indonesia. Photo: Unsplash.
Seagrass is a wonderfully biodiverse and critically important habitat found along shorelines the world over that supports an abundance of marine life and therefore many coastal fisheries. All the while sucking carbon dioxide out of our atmosphere and swapping it for oxygen that we breathe. Moreover, it helps keep our favourite bathing waters crystal clear. Curious? Let’s dive in.
Back to the roots…
As its name suggests, seagrass is indeed related to grass. Monocotyledonous plants, or Monocots for short, are the group of plants encompassing grasses, palms and lilies. Described as ‘grass-like’ plants that flower, pollinate and germinate, examples we are all familiar with include wheat, rice, bamboo, sugar-cane, onions, and now: Seagrass.
Plant life left the oceans some 400 million years ago and began colonising the land. Some 300 million years later, the ancestors of today’s seagrasses decided to return to an aquatic way of life. What is truly unique about seagrasses, with the exception of one species, is that they complete their full life cycle underwater – flowering, pollinating and germinating, all beneath the waves.
Through the last 70-100 million years that seagrasses have evolved, we can today find 72 species that are assigned to 4 major groups.
A Green turtle swimming over a seagrass meadow, Aruba. Photo: Unsplash.
Some of these species are named after their appearance or the animals eat them, for example: turtle grass, the main food source of green turtles; eelgrass, resembling colonies of sandeels; and spoon grass which has spoon shaped leaves.
Seagrass meadows and where to find them
Over the millennia, seagrass has colonised nearly every coastline on earth, except for Antarctica. It exists in 6 defined ‘bioregions’: temperate North Atlantic, temperate North Pacific, the Mediterranean, temperate southern oceans (coastlines in the southern Atlantic and Indo-Pacific regions), tropical Atlantic and tropical Indo-Pacific.
A seagrass bed against the shore, Sulawesi, Indonesia. Photo: Unsplash.
Generally, seagrass is found on shallow, protected coastlines at depths of 1-3 metres, yet typically extending down to 15 metres. There are also deep water species that grow at depths of 60-70m, found in crystal clear, tropical waters, where light can penetrate greater depths.
Lush and thick seagrass in Fort de Soto Park, Tampa Bay, Florida, USA. Photo: Unsplash.
The global coverage of the seafloor by seagrass is up to some debate. There are a range of estimations from various studies with the best ranging from 160,000 – 600,000 square kilometres, but some habitat and oceanographic studies have suggested that there is 1,000,000 – 1,600,000 square kilometres of potential seagrass habitat the world over.
Seagrass Safari
To some, seagrass may seem a little drab and boring at times, especially in comparison to the millions of colours on a coral reef. Seagrass meadows are in fact vital habitats for a huge variety of marine wildlife. This includes some of the most cherished and charismatic animals in the sea. Green turtles, dugongs and manatees all rely on seagrass as their primary food source, grazing on the meadows much like cattle would on open grasslands. These three, amongst some others, are classified as keystone species. Their foraging activities help to keep seagrass meadows healthy and biodiverse.
Green turtles keep grasses cropped short, reducing old plant matter and encouraging fresh growth – which is beneficial for carbon absorption. The manatees and dugongs turf up the roots, known as rhizomes, causing structural disturbance to the meadow. This again encourages new growth, as well as opening up new microhabitats for other sea creatures to feed and shelter in. These keystone species also fertilise these ‘sea-vannahs’ with their faeces, aiding the cycle of nutrients throughout the food web.
A foraging Dugong, Red Sea, Egypt. Photo: Pixabay.
What we must not forget is the vast array of other creatures that make up the seagrass ecosystem, often smaller and harder to find due to their specialised camouflage. In southern Australia, you’re likely to find the seahorses’ weirdly wonderful relative, the Leafy Sea Dragon, where it lurks in seagrass and seaweed beds, blending perfectly with the plant matter. Likewise, in cooler waters such as the UK and Europe, you’ll find multiple species of Pipefish weaving in and out of seagrass meadows and kelp forests.
At the broader scale, seagrass provides shelter and food for vast quantities of wildlife and, when healthy, these meadows are bursting with life. This collection of creatures ranges from tiny invertebrates that live in-between the leaves and under the roots of the seagrass; juvenile fish using the meadows as a nursery whilst they are vulnerable; adult fish that graze or hunt amongst the swaying leaves; large predatory fish, sharks, rays and marine mammals that use seagrass as hunting grounds; as well as many shorebirds that feed on or in shallow seagrass beds. Studies suggest that 1 acre (0.004 square km) of seagrass can support more than 40,000 fish and 50 million small invertebrates.
A shoal of fish over Posidonia seagrass, Greece. Photo: Unsplash.
All of this wildlife means seagrass is a vitally important habitat for conserving our marine biodiversity, capturing carbon to combat climate change, and to help bolster stocks in near-shore fisheries.
There we have it, a whistle stop tour of the history, distribution, and biodiversity of the seagrass ecosystem. This habitat is truly a wonderful part of our coastlines, and deserves much greater appreciation and attention. So next time you’re on holiday or passing by the sea, grab a snorkel and hunt down some seagrass meadows – you never know what you might find roaming the savannah of the seas.
In Spotlight on Sealife we celebrate the creatures that inspire our love for the oceans and encourage us to fight for their homes. Follow this series to uncover lesser-known facts about well-known marine life, and discover weird and wonderful marine animals that you may have never come across before.
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