Seagrass: The meadows of the sea

  Jan 13, 2023  |    Ainhoa Hermida

As we presented in our previous blog, underwater forests can be formed by a group of organisms called marine phanerogams or seagrasses. Seagrasses are marine plants (not algae) that cover shallow seabeds with lots of light, forming what we usually know as seagrass meadows. 

Now, you may be wondering: what does a plant with flowers, stems, leaves, and roots living underwater? Are seagrasses an imitation of terrestrial plants? Or are they not? To answer these questions, it is necessary to take a short trip to the start of the Cambrian era.

If we put our heads under the water of the ocean 550 million years ago BP (before present), we would observe algae of various groups, many of them not very different from those we find on our coasts today. Red, green algae... all of them covering the rocky bottoms and filling the underwater landscapes with life. However, outside the water the situation was very different. Dunes, mountains, and bare rocks created a desolate and inanimate landscape because the emerged life had not yet had the opportunity to develop, but that was about to change.

At that time, planet Earth was in constant change. Variations in the planet's climate, continents, and geology have led to major changes in sea level, tides, and the rhythm of coastal waves. Thus, little by little, the earth's surface retained water in the form of salt marshes, estuaries, and basins which, by remaining stable for a while, gave rise to new spaces for the spread of life.

It was very likely in environments like those that the colonization of the terrestrial environment by plants and animals began. In particular, green algae were the primitive pioneers that gave rise to the great diversity of land plants that inhabit our forests today. But when did our beloved seagrasses appear?



Moving a little closer to the present, long after the appearance of land plants, a peculiar phenomenon occurred in the Cretaceous: plants undertook a journey back to the sea. They began to live submerged again and diversified with adaptations to live under the sea. The roots allowed them to live anchored to the sandy bottoms and the stems (or rhizomes), which join their bundles of leaves, now carry out photosynthesis under the water, actively capturing CO2 from it. 

Parts of Posidonia © Pelopanton 


And how did they spread? In addition to reproducing asexually by cuttings (small pieces from which a new plant emerges), seagrasses are capable of reproducing sexually through flowers and fruits, just like their terrestrial relatives. However, underwater flowers are much more modest: no colored petals, sweet aromas, or fleshy fruits. As their flowers and seeds are pollinated and dispersed through marine currents, they do not need to attract any type of organism to expand their habitat. For instance, sexual reproduction allows Posidonia to colonize new areas and asexual reproduction allows them to expand through the seabed. Moreover, thanks to the latter property we can carry out active restoration by planting Posidonia!

In the Mediterranean Sea we find 5 species of seagrasses. The most emblematic specie is, without any doubt, Posidonia oceanica, and constitutes one of the most recognized symbols of our sea. Despite what its name indicates – oceanica -, Posidonia is endemic to the Mediterranean and is not found anywhere else on the planet. Its little sisters are Cymodocea nodosa, Zostera marina, and Zostera noltii; each with a particular distribution area. Last, but not least, Halophila stipulacea is an exotic seagrass recently introduced to the Mediterranean Sea through the Suez Canal coming from the Red Sea.



Posidonia oceanica occupies sandy bottoms and has the largest size of the aforementioned species. It has roots, rhizomes, and green leaves that can measure between 20 and 100 cm in length. Its rhizomes can grow vertically and/or horizontally and its flowers are green and pretty simple. Although these usually appear in autumn and we do not know much about how their reproduction times work, this year has occurred an episode of massive flowering on the coasts of the Spanish peninsula and the Balearic coast. It has not been proven yet, but it is believed that the phenomenon is related to the high temperatures and the excellent visibility of the waters in this previous summer of 2022. Thanks to the online platform of Observadors del Mar, you can contribute with evidence of the latter phenomenon by uploading photos in their “Seagrass in Reproduction”  project.

Posidonia flowers © Silvia Mus

Finally, seagrass meadows provide us with numerous ecosystem services, in other words, benefits for all humans and society. Some of them are the creation of a space for marine life to grow and refuge, coastal protection against storms, the creation and stabilization of our beaches’ soil, or the storage of anthropogenic carbon, among many others. If you want to learn more about these services, don't miss our next blog!

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