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Birds do it, bees do it, but until recently, no marine critters were thought to do it. Pollination, that is. Tides and currents do a great job of sweeping pollen from marine plant to plant, so scientists thought underwater pollinators were unnecessary. But now, researchers from the National Autonomous University of Mexico have discovered a species of Caribbean seagrass, Thalassia testudinum, that can be pollinated by zooplankton and bottom-dwelling invertebrates.
Marine biologist Brigitta van Tussenbroek first had her suspicions raised in 2012, when she and her colleagues found some seagrass covered by a diverse group of animals. She suspected the organisms were acting as pollinators, but to be certain she and her team needed to observe the plants in a controlled setting, away from ocean currents.
The researchers developed a system of aquariums that allowed them to the control flow of the water, added seagrass and potential pollinators, and watched for signs of seagrass reproduction.
The team found that even in the complete absence of current, pollen tubes grew on the female flowers, indicating that—thanks to the zooplankton in the tanks—male flowers were able to send their donation to the right destination. More interesting still, in the absence of both current and plankton, the male plants stopped flowering.
“The results are fascinating,” says plankton biologist Richard Kirby, who wasn’t involved in the study.
“When you think about it, why shouldn’t plankton play an important role?” says Kirby. “There is no reason not to expect similarly complex interactions that we observe between land plants and their pollinators. But, hindsight is easy.”
That marine plants can reproduce like their terrestrial counterparts also makes sense with hindsight. After all, much like how whales evolved from small, amphibious carnivores, seagrasses evolved from terrestrial plants. They adapted to a salty, aquatic environment over millions of years, yet much of their existing physiology persisted. Seagrasses have roots and leaves, for example, and a seagrass flower is either a pollen-producing male or pollen-receiving female.
Van Tussenbroek and her team found that plankton would visit both male and female flower parts, further supporting their hypothesis.
In fact, the researchers found that even the mode of interaction between the zooplankton and the seagrass is much like that between terrestrial pollinators and plants.
Plankton is drawn to the seagrass’s nutritious mucilage—a carbohydrate-rich substance that houses pollen, says the team. Mucilage is exceptionally gooey, so as the zooplankton munch away, excess pollen grains stick to their bodies. As the plankton move from seagrass to seagrass, the pollen spreads. The plants get a helping hand, and the plankton get food.
Thalassia testudinum isn’t wholly reliant on the zooplankton, however. The plant can also clone itself, so planktonic pollinators likely serve as a safeguard—a way to give genetic diversity a boost.
But before we can dub plankton the “bees of the seas,” we need to find out how many plant species rely on them. Seagrasses are true ecosystem engineers: they filter water, convert carbon dioxide, and provide critical food and shelter. Learning more about them—and their newfound pollinators—can only help secure a positive future for our oceans.