If you are called « pollination », you probably think of a bee covered with pollen twirling from flower to flower. This representation is quite correct as this insect is known to be the best pollinator among many other terrestrial pollinator animals. But in this article, you will be surprised to discover that this image is not really complete. Pollination is indeed a fundamental mechanism that also exists under water. For a long time, scientists themselves underestimated, or even ignored, this marine phenomenon. When you have finished reading, you will know the essentials of this relatively recent discovery.
Marine flowering plants
Algae, the most famous plants in the marine environment, are not flowering plants. However, there are real marine plants in this very particular environment. These are marine herbariums, also called marine phanerogams.
The adapter of precise information will be pleased to know that these herbariums include flowering plants belonging to one of the following four families: Zosteraceae, Posidoniaceae, Cymodoceaceae and Hydrocharitaceae. They can form real underwater grasslands in shallow coastal areas.
These plants come from terrestrial plants that recolonized the sea millions of years ago. They therefore have roots, leaves, flowers and produce seeds, just like terrestrial plants.
Marine herbariums provide multiple ecological services: oxygenation of water, carbon storage, stabilization of the bottoms and reduction of the speed of currents, which mechanically limits coastal erosion, production of organic matter and food, spawning grounds and refuge for many organisms such as fish, echinoderms and marine mammals.
How to pollinate without insects or wind?
On land, pollination is mainly based on wind or animals, especially insects. Under water, it necessarily happens differently. Marine plants have therefore developed their own strategies.
The best known is hydrophilia, that is, pollination is done through water. The male flowers release their pollen and the sea currents take care of transporting it to the female flowers. It was even long thought that it was the only mechanism of reproduction of these flowers.
Obviously, the pollen of these flowers is very different from that of terrestrial plants. It is often filamentous, elongated or surrounded by a gelatinous envelope that prevents it from flowing too quickly and increases its chances of encountering a receptive stigma.
The synchronization of flowering is just as important as on earth: plants of the same species bloom at the same time in order to maximize the probability of fertilization.
The surprising discovery of underwater animal pollination
If for decades biologists thought that animal pollination was impossible in the marine environment, a study published in 2016 highlighted a phenomenon called zoobenthophilia.
The researchers have identified the action of small crustaceans, marine worms and other invertebrates feeding on the mucus produced by the flowers of certain marine plants. By moving from flower to flower, these animals involuntarily carried pollen, playing a role comparable to that of terrestrial pollinating insects.
You can imagine that this discovery has profoundly changed the scientific vision of marine pollination. It shows that the interactions between plants and animals underwater can be just as complex and specialized as those observed on land.
A discreet but essential process for coastal ecosystems
Marine pollination is obviously not a simple biological detail. It is the basis of the reproduction of marine grasses, allowing genetic mixing and the adaptation of populations to environmental changes. Without this, herbariums would be more vulnerable to diseases, pollution and global warming.
Many species of fish, molluscs and crustaceans depend directly on herbariums for feeding or breeding. The disappearance of these plants would therefore lead to a domino effect on the entire coastal ecosystem. In this sense, understanding and protecting marine pollination is equivalent to preserving a complete ecological chain, often invisible from the surface.
Marine pollination and climate change
It is widely accepted that climate change is exerting increasing pressure on the oceans. Increased water temperature, acidification and pollution directly affect the flowering of marine plants and the behavior of underwater pollinating animals. Recent studies show that the shift in flowering periods or the decrease in invertebrate populations can disrupt pollination, compromising the reproduction of herbariums.
At the same time, these same grass lands play a key role in the fight against climate change thanks to their exceptional ability to store carbon in marine sediments, a phenomenon often referred to by the expression « blue carbon« . Preserving their pollination therefore also means strengthening a natural ally against global warming.
Zoom on some shadow pollinating animals
The discovery of animal pollination in the marine environment has highlighted discrete organisms, often unknown to the general public. Among them are tiny amphipod crustaceans, including species of the genus Gammarus. These animals, only a few millimeters long, move constantly on the leaves and flowers of herbariums. Attracted by the mucus rich in sugars and organic compounds secreted by male and female flowers, they come to feed on it.
Benthic copepods, such as Tigriopus fulvus, small crustaceans very abundant in coastal areas, represent other important actors. Their opportunistic eating behavior leads them to frequent the flowers of marine phanerogams.
Polychete verses also play a significant role. Some species, such as Nereis diversicolor, crawl up herbariums in search of organic matter.
These animals are not specialized pollinators in the strict sense, but their repeated interaction with the flowers is enough to improve the reproductive success of the plants.
As poetic as it is, the sea is not just a decor or a resource. It is a living system that turns out to be, study after study, decidedly very complex. And understanding underwater pollination is probably not adding one more curiosity to our collection of knowledge but refining our relationship to this fascinating environment.
source : le mag des animaux
