May 6th 2025
Fruits of Posidonia seagrass on the Shores – Help Document This Rare Phenomenon
Over the past several days, citizens and organisations along Greece’s coastal regions, particularly in the Ionian Sea, have reported unusually large accumulations of Posidonia oceanica fruits along the shoreline. This rare natural event, which occurs only every few years, presents a valuable opportunity for scientific research, conservation efforts, and public engagement. Similar events were previously documented in the spring seasons of 2013 and 2023.
The current deposits of Posidonia fruits and seeds are the result of the flowering that occurred last autumn. Fruiting in this species is a rare phenomenon, dependent on a precise combination of favourable environmental conditions. As a seagrass, Posidonia is not a type of seaweed, but a higher plant more closely related to terrestrial species, where its evolutionary roots lie. It flowers, bears fruit, and forms complex root systems, making it a cornerstone species in Mediterranean marine ecosystems. Its presence plays a vital role in supporting biodiversity, stabilising the seabed, and maintaining clean, oxygen-rich waters.
The last time a similar accumulation was observed, researchers from the Archipelagos Institute of Marine Conservation collected over 12,000 fruits and seeds from islands in the eastern Aegean. These were used to create experimental underwater seagrass gardens—one of the few such initiatives attempted globally, due to the immense complexity involved.
Establishing underwater Posidonia gardens requires careful preparation, both in the laboratory and at sea. Seeds must be processed and planted in carefully selected marine sediments with specific nutrient content. Site depth is also critical—neither too shallow nor too deep—and the gardens demand continuous maintenance over several years. This includes regular underwater work by divers, often multiple times per month, throughout the year.
Given the difficulty and precision required for such restoration efforts, it is important to stress that this work is not suitable for superficial or symbolic “greenwashing” actions. Unfortunately, such trends are increasingly common in Greece and elsewhere, and must be addressed with urgency and seriousness.
Posidonia Meadows – Our Ally in Combating the Effects of Climate Change
These underwater forests can absorb up to 35 times more carbon than tropical forests. They form the foundation of Mediterranean marine productivity and fisheries, providing habitat for over 300 species of algae and more than 1,000 species of marine animals.
Despite their critical ecological role, Posidonia meadows in the Mediterranean have already declined by at least 34%. The primary driver of this decline is the widespread, uncontrolled anchoring of recreational boats—a problem that grows more severe each year. Additional threats include destructive fishing practices and the expansion of intensive aquaculture near sensitive seagrass habitats. The loss of a single meadow can lead to the collapse of its entire ecosystem and the release of centuries’ worth of stored “blue carbon” from the seafloor.
To illustrate the scale of destruction caused by uncontrolled anchoring of recreational boats, consider this: as part of a multi-year study, during one single survey period, divers from the Archipelagos Institute collected on average approximately 500 fragments of uprooted Posidonia rhizomes every hour during the summer months at just one site of heavy anchoring activity.
As human activity continues to destroy one natural resource after another —and as we persist in creating more problems rather than solving existing ones — it is no longer sufficient to simply catalogue the causes. At the Archipelagos Institute, we believe it is critically important to move beyond merely identifying and documenting each factor that threatens and destroys our seas. We must take the next steps: addressing them through evidence-based knowledge and tangible solutions.
Send Us Your Observations
In a country like Greece, with over 18,000 kilometres of coastline, public participation is invaluable in tracking the extent of Posidonia seagrass fruits on the shores. If you find yourself on a beach or coastal area, we encourage you to keep an eye out for these rare fruits and seeds.
If you do spot any, please take a photo and send it to observations@archipelago.gr, along with the exact location (GPS coordinates if possible) and the date of your observation. Including a reference object in the photo (such as your hand or a coin) can help indicate size and support accurate identification. These contributions are crucial to our ongoing monitoring and research efforts.
