Seagrasses retained their reproductive strategies by gluing pollen to plankton
The ancestors of seagrass moved into the ocean at least 40 million years ago. They likely started living in coastal environments, like mangrove swamps, where they began adapting to briny water from the sea. A few key adaptations later, these plants were eventually able to live entirely submerged on the ocean floor. Many features from seagrasses’ time as a terrestrial plant remained intact, including the slightly puzzling use of pollen as a reproductive strategy.
For a long time, it was assumed that the ocean’s currents moved enough pollen from one flower to the next. It certainly didn’t seem as efficient as the pollination strategies employed by plants on dry land, but even that took a lot of work and specialization to get bees and birds to reliably move pollen from one flower to the next. Some species of plankton were known to eat seagrass pollen, but it wasn’t clear how much of an impact that had on the plants’ reproduction. Otherwise, some seagrass species could clone themselves along their root systems, and others likely fertilize themselves if necessary.
Pollination with a partner
To try to narrow down just how these aquatic plants were putting their flowers to work, researchers built a series of interconnected tanks that could simulate different currents that seagrass would encounter in the ocean. The flow of water turned out to be of little importance, as flowers were still being pollinated even when the currents were deactivated. What made a big difference was when some of the local zooplankton were removed from the tanks— without the tiny animals visiting the flowers, pollination basically came to a standstill.
Much like the relationship between the flowers and pollinators in our neighborhoods, this pollination strategy is beneficial to both parties. The zooplankton feeds on a nutritious, sticky substance called mucilage. As they eat, some of that mucilage acts as a glue to stick pollen on the plankton. The plankton can then passively deliver that pollen to the next flower it visits when looking for its next meal. The relationship is so familiar it may seem obvious, but until being proven, nobody thought that this system had ever been transplanted to the seas.
Source: Scientists Discover an Underwater Pollinator by Sarah Keartes, Hakai Magazine