They are the only flowering plants that live in the sea and they migrated from land to the aquatic world more than 70 million years ago. Seagrasses are the marine plants that have adapted to life underwater and belong to the large group of angiosperms (Angiospermae). Unlike algae, they have developed roots, stems, leaves, and flowers – just like terrestrial plants.
According to UN data, seagrasses are present in coastal areas of 159 countries across six continents. They cover a total area of around 300,000 km² and are considered one of the most valuable ecosystems in the world.
So important, yet so poorly protected
Seagrasses play a key role in maintaining the health of oceans and seas. They improve chemical conditions in the sea by absorbing CO₂ and releasing oxygen. Their roots stabilize the seabed, while their leaves slow currents and allow particles to settle, which helps clear the water and reduce coastal erosion. They provide shelter and spawning grounds for numerous marine species. These marine plants are beneficial not only to the sea but also to land! As storms become more frequent and intense, their role in coastal protection grows ever more crucial – they buffer wave energy and protect coastal communities. In this way, they offer a natural solution to mitigating the effects of climate change – yet another reason to preserve and restore them.
Unfortunately, nearly 30% of global seagrass meadows have been lost since the late 19th century. The causes vary: urban, industrial, and agricultural runoff; coastal development and dredging; unregulated fishing and free anchoring; and climate change. According to the UN, seagrass meadows are among the least protected coastal ecosystems. Only 26% of seagrass habitats fall within marine protected areas. Compared to 40% of coral reefs and 43% of mangroves, this is extremely low.
Although the focus of Sunce’s efforts is on the most significant and widespread species in our region – Posidonia oceanica – there are three other seagrass species found in the Adriatic Sea: Cymodocea nodosa, Zostera marina, and Zostera noltii. Worldwide, there are a total of 72 species!
Posidonia
Posidonia oceanica, also known as Neptune grass or Mediterranean tapeweed, is endemic to the Mediterranean Sea and one of the most important seagrass species in the Adriatic. Named after the Greek god of the sea, Poseidon, it’s often referred to as the “lungs of the sea” due to the large amount of oxygen it produces. This perennial underwater plant has long, ribbon-like leaves (30–120 cm long, 1–1.5 cm wide) of a dark green color that grow in dense clusters from an underwater stem (rhizome). The rhizomes spread both horizontally below the sediment and vertically, creating dense meadows that can last for thousands of years. Posidonia thrives from the surface down to 40 meters deep on sandy, muddy, or mixed seabeds. It forms thick and stable meadows that buffer wave action, prevent coastal erosion, and provide refuge for thousands of marine species. It rarely flowers and mostly reproduces vegetatively. In the Adriatic, Posidonia plays a crucial ecological role: it purifies water, stores carbon, and supports biodiversity. However, it’s highly sensitive to physical damage such as anchoring and trawling, as well as to pollution and rising sea temperatures. Its presence is a direct indicator of a clean and healthy marine environment. Because of its importance, Posidonia is strictly protected in many Mediterranean countries, including Croatia.
Cymodocea nodosa
Cymodocea nodosa is a Mediterranean-Atlantic seagrass. The name Cymodocea derives from a Nereid sea nymph named Cymodoce, while nodosa refers to the Latin word nodus, meaning “knot”. It has a branched root system just a few millimeters thick and thin, serrated leaves (15–40 cm long, 3–4 mm wide). Its flexible, smooth stem runs parallel to the seabed and is often covered in sediment. The species is named after the densely packed “nodes” along its stem. Cymodocea nodosa typically grows on sandy-muddy seabeds with organic debris, from the surface down to 10 meters. In some areas, it forms underwater meadows alongside Zostera noltii. It flowers between April and August, but reproduction more commonly occurs vegetatively through the rapid spread of horizontal rhizomes, which can grow several meters per year. This plant is more tolerant to wave energy than Posidonia and can handle moderate levels of organic load, pollution, and reduced light. However, it prefers clean, well-lit waters with temperatures between 10°C and 30°C.
Zostera marina
Zostera marina, also known as eelgrass, gets its name from the Greek word zoster, meaning “belt” or “strap”, referring to the shape of its leaves. Its bright green, narrow, ribbon-like leaves (30–100 cm long, 3–9 mm wide) resemble those of Cymodocea nodosa, though their ends are rounded and fringed (not serrated). This seagrass has creeping rhizomes (2–6 mm thick) with roots up to 20 cm long and only 0.2–1 mm thick, featuring root hairs. When the fruit matures, the flower stalk detaches and floats freely, spreading the seeds. Zostera marina inhabits sandy or muddy bottoms, often influenced by freshwater, from the intertidal zone to a few meters deep. It’s more common in the northern Adriatic and areas such as the Novigrad and Karin Seas, but very rare in the central and southern Adriatic. Remarkably, it can survive short periods out of water. It tolerates lower salinity and colder waters.
Zostera noltii
Zostera noltii, also known locally as dwarf eelgrass, features light to dark green, very narrow leaves (up to 1 mm wide and 5–30 cm long) with heart-shaped tips. It grows on sandy-muddy seabeds under freshwater influence, from the surface down to 5 meters. New leaves emerge in spring, while older ones die off in autumn, leaving only the rhizome buried in the sediment to overwinter. It reproduces both via seeds and vegetatively, though the latter is more common. Zostera noltii is not tolerant of strong water movement, turbidity, or pollution, so it inhabits clean, shallow brackish to salty waters in quiet lagoons and river estuaries. It can survive in intertidal zones – from mid to low tide levels. Like other seagrasses, it purifies water, stabilizes the substrate, and provides shelter for numerous marine species.
And where do you anchor?
Seagrasses hold immense ecological value, and protecting them is vital for preserving our seas. That’s why this summer, through our campaign And where do you anchor?, we aim to raise awareness among coastal communities and their visitors – since ignorance is often the main cause of irresponsible behavior toward nature.
Thanks to funding from the Interreg Italy–Croatia BIOPRESSADRIA project, in partnership with Municipality of Monfalcone (lead partner), SELC Cooperative Company, CORILA – Consortium for coordination of research activities concerning the Venice lagoon system, Management consortium of Porto Cesareo Marine Protected Area, Public Institution National Park Kornati, Public Institution for the Management of Protected Nature Areas of Dubrovnik-Neretva County, Public Institution for the Management of Protected Areas of Split-Dalmatia County – Sea and Karst, we are developing strategies and proposing solutions to reduce the negative impact of coastal and nautical tourism on marine and coastal biodiversity. As part of the project, we’ve created informational leaflets about Posidonia and responsible anchoring practices. You can download them from our website, and if you work with boaters, feel free to request materials by emailing us at [email protected].
