Like Bees of the Sea, Plankton Pollinate Plants

On land, flowers are pollinated by a wide range of animals, from bees and bats to lemurs and lizards. Under the sea, however, things work a little differently.

Flowering plants that grow in the ocean, known as seagrasses, are typically pollinated by the water itself. They don't seem to need as much hands-on help as land plants do, and it was long thought animals aren't involved. But as a team of scientists recently found, a species known as turtlegrass has a secret: It's pollinated at night by tiny crustaceans, copepods and other animals that act like bees of the sea.

"They visit both female and male flowers, carry pollen grains on their bodies, and transfer pollen between male and female flowers in aquaria experiments," the researchers write in their study, published in the journal Nature Communications.

This shows that marine invertebrates can be pollinators, they add, "revoking the paradigm that pollen in the sea is only transported by water."

Underwater oasis

Turtlegrass forms vibrant underwater meadows in the Caribbean Sea and Gulf of Mexico, providing food for sea turtles, manatees, fish and various invertebrates. It's considered "the most important habitat-forming seagrass species in the Caribbean," according to the International Union for Conservation of Nature (IUCN).

In 2012, scientists reported that turtlegrass flowers off Mexico's Caribbean coast receive nocturnal visits from small invertebrates. Led by Brigitta van Tussenbroek, a marine botanist at the National Autonomous University of Mexico, they recorded hundreds of creatures foraging at both male and female flowers after dark. As van Tussenbroek tells Emily Benson of New Scientist, it looked like pollination.

"We saw all of these animals coming in," she says, "and then we saw some of them carrying pollen." They caught the behavior on video, as seen in the clip below:

They decided to investigate further, starting a new study in an aquarium setting. For the animals to be confirmed as pollinators, four conditions would need to be met: both male and female flowers were visited, the visitor carried away some pollen, the visitor transferred pollen between male and female flowers, and that pollen transfer resulted in successful fertilization of the plant.

Haulin' pollen

To test this, the researchers placed the invertebrates and flowers together in tanks without water flow. The animals were seen on both male and female flowers, and the researchers used light traps to prove the visitors carried pollen away when they left. To see if that pollen was transferred, they also counted pollen grains on the stigmas of female flowers before and after the experiment began.

Within just 15 minutes, several extra grains of pollen had appeared on many of the flowers. "Only fauna could have moved the pollen," the study's authors write, "because there was no water flow in the aquaria." In the control tanks, which contained flowers but not animals, there was no gain or loss of pollen.

The researchers captured these images of pollen grains being carried by tiny invertebrates. The top three are various crustacean larvae, and below them is a young polychaete worm. (Photos: Van Tussenbroek et al./Nature Communications)

Finally, the pollen that was transported like this led to successful pollination, as most female flowers developed pollen tubes. This confirms that turtlegrass is pollinated by its tiny visitors, the authors conclude, and suggests these important seagrass meadows are more ecologically complex than anyone realized.

Seawater is nearly 800 times denser than air, and animals smaller than 1 millimeter are easily swept around. But the study still revealed directional movements when they approached male turtlegrass flowers, likely because they're attracted to sweet globs of pollen. The flowers release their pollen at night, the researchers note, which also happens to be when these invertebrates are normally active.

Fragile grass

Exposing the secrets of seagrass isn't just interesting; it's also an important part of protecting the ecosystems seagrass creates. Meadows of turtlegrass and other species are highly biodiverse and productive, often providing critical nursery habitats and feeding grounds. They're also carbon sinks, and play a key role in global nutrient cycling — a service worth roughly $1.9 trillion per year to humanity.

Yet these oases are now dwindling in many parts of the world, with at least 1.5 percent of Earth's seagrass meadows lost every year, and possibly as much as 7 percent. This is partly due to the direct effects of coastal development and dredging activities, experts say, and partly to indirect impacts of low water quality.

It's still unclear how important the pollinators are for turtlegrass, and if any other species of seagrass might also be pollinated by animals. More research will be needed to answer those questions, but they are worth answering. As we've learned on land, it's often easier to protect an ecosystem if we understand how it works.