Sea creatures pollinate marine plants and algae, surprising scientists

Once thought to be a land-only phenomenon, pollination may have existed in the ocean millions of years before terrestrial plants appeared.

By Lina Zeldovich
Published 9 Aug 2022, 11:39 BST

This colorised confocal microscopy image shows the body of an isopod covered in the germ cells of a red algae, Gracilaria gracilis. New research shows that these animals spread around the algae’s spermatia, effectively pollinating them.

Photograph by Sebastien Colin

About a decade ago, Vivianne Solis-Weiss, a marine biologist at Universidad Nacional Autónoma de México who studies marine worms, talked to a colleague studying seagrasses, flowering plants that grow in the ocean.

“‘Every time we gather the flowers, we see these small animals all over them,’” she recalls her colleague telling her. Both wondered why these little worms and tiny shrimp-like crustaceans would gather there. Could they be pollinating the plants—the marine equivalent of bees and butterflies?

Solis-Weiss and her colleagues hypothesised that the creatures might indeed play a role in ocean pollination and outlined their idea in a study, which appeared in the small journal Inter-Research Science Publisher in 2012.

An isopod climbing on a bit of red algae, Gracilaria gracilis, which it has been shown to pollinate.
Photograph by Wilfried Thomas

“It was very hard to publish that first paper, because no one would believe us,” she recalls.

The role of pollinators on Earth is well-established. Hundreds of thousands of flowering species depend on animals and insects to procreate. The plants provide nectar or the promise of something to eat, and the pollinators facilitate the plants’ sexual reproduction. But until recently, it was believed to be a terrestrial-only phenomenon that didn’t exist in the ocean.

“There is a dogma that in marine environments all the fertilisation is done by water movements,” says Emma Lavaut, a marine biologist at Roscoff Marine Station of Sorbonne University in France, who studies Gracilaria gracilis, a seaweed commonly called red algae that grows in coastal rock pools. Indeed, in many marine organisms, males and females release their eggs and sperm into the water, letting the currents mix and fertilise them.

Yet, in the past few years, new evidence has emerged suggesting that the ocean has its own pollinators. These creatures can be likened to the “bees of the sea,” and they may be more common than we imagine. As scientists learn more about their symbiotic relationships, it changes how they think about the evolution of all involved—algae, plants, insects, and crustaceans. It also highlights the complexity of these mutually beneficial relationships.

Grass and seaweed mysteries 

To prove their hypothesis, Solis-Weiss’s team set up research fields of seagrass Thalassia testudinum on the ocean shore and in aquariums, capturing the pollination process with photos and videos. Every sunset when the male flowers of T. testudinum opened, worms and other invertebrates would swarm among them, covering themselves in pollen.

“We made experiments to show that they will go to forage in the masculine flowers, get the pollen, which sticks to their body, and then go to the feminine flowers and leave the pollen there,” Solis-Weiss says. In 2016, the team published these findings, along with pictures of marine worms covered in pollen in the journal Nature—the first ever study to demonstrate pollination in the sea.

Lavaut was next to observe a similar phenomenon while working on her PhD thesis about the G. gracilis’ reproductive mysteries. Rather than spewing its eggs into the waves like many other ocean inhabitants, the female alga keeps them inside its funnel-shaped filaments called thalli. The males release the sperm, but the tiny cells don’t have the tails to swim to the female plants and get inside the filaments.

That seeming disadvantage doesn’t affect the reproductive success of the seaweed: The taxonomic group to which red algae belong evolved around 1 billion years ago. Lavaut and her advisor Myriam Valero, a population geneticist at the French national research agency CNRS, wanted to understand how these organisms reproduce.

Over the years of studying the algae in the tidepools around Europe, Valero noticed that most fertilization happens at low tide, when there is little water. At that time, swarms of little isopods called Idotea balthica—crustaceans that look like a cross between a shrimp and a pill bug—swim within the algae. Valero and her team wondered if they were transporting the sperm on their bodies.

To test that idea, the team used virgin G. gracilis that has never been fertilised before and didn’t have any fruiting structures called cystocarps. The scientists placed male and female plants inside multiple aquariums and added 20 crustaceans to some but not others. When cystocarps developed, the aquariums with the creatures had 20 times more of them.

“I was surprised by the fact that there was much more fertilisation,” Lavaut says. The team also gathered crustaceans that swam in the tanks with the male alga for some time, then releasing them into tanks with virgin female plants, which also increased the number of cystocarps. Under the microscope, the isopods were covered in tiny specks of sperm just like marine worms in Solis-Weiss study. Lavaut’s group reported its findings on July 28 in the journal Science.

In this case, both organisms are helping each other. Algae provide shelter for the isopods, but also food in the form of an algal biofilm that grows to coat G. gracilis. That cleanup helps these red algae photosynthesise.

“If there’s too much [biofilm] growth, the red algae start to die,” Lavaut says—and the isopods help keep it clean.

Ancient roots

But while two teams described a seemingly similar phenomenon, evolutionary biologists and pollination ecologists point out that the two studies have major differences.

Seaweed and seagrass may sound similar, but they are two very different organisms with divergent evolutionary trajectories. Seagrasses have only been around for about 130 million years, says Jeff Ollerton, an ecologist who studies pollination at the Kunming Institute of Botany at the Chinese Academy of Sciences who was not involved in either study.

Seagrasses evolved from terrestrial plants that returned to sea yet retained some of their land features, such as flowering. And, apparently, relying on animals for pollination.

“It’s so interesting to see how they found in the water different kinds of animals to replace bees and butterflies,” says Solis-Weiss—which she means in a figurative, not literal sense.

In comparison, seaweeds are only distantly related to plants, and are neither plants nor animals, but their own thing—a type of algae, Ollerton explains. They are ancient organisms that evolved eons before plants left the ocean and began to grow on land. That means that pollination may predate plants, says Zong-Xin Ren, botanist and pollination ecologist also with the Kunming Institute.

“This finding totally changed our idea [of] what is pollination,” he says. “We may even redefine what pollination is.”

Save the isopods

This discovery prompted Ren and Ollerton to write a perspective paperDid pollination exist before plants?—in which they reflect on the significance of such mutually beneficial relationships between animals and photosynthetic organisms stretching back much further in evolutionary history than thought.

Such relationships between species are what allows ecosystems to function, and “to understand when such interaction[s] began will greatly increase our understanding of the original biodiversity,” Ren says.

“We know so little about our world, so little about what’s happening on land and even less about what’s happening in the water,” he adds. “The paper gave us the tip of the iceberg.”

The new research suggests that the important and previously unknown relationships between animals and aquatic plants and algae could make them more vulnerable. In the case of red algae, for example, most pollination takes place in shallow tide pools, where the delicate dance between animals and those they pollinate could be disrupted by pollution, climate change, and development.

On land, bees are under threat from pesticides and other toxins, much of which washes into the sea. Will the sea’s bees find themselves in a similar peril one day? On his blog, Ollerton cautions about this possibility.

“In the same way that ‘Save the Bees’ has been a rallying call for conserving interactions between species on land,” he writes, “we may soon hear this message echoed in ‘Save the Isopods.’”

Looking forward, the researchers are excited to see if more examples of pollination can be found in nature—and they suspect there will be.

“There are no doubt many more revelations awaiting the careful observer,” Ollerton and Ren write.


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