Move over birds and bees, there is another pollinator on planet Earth, and it lives in the sea.
In a study published Thursday in the journal Science, scientists found that a small crustacean, Idotea balthica, played the role of pollinator for a species of seaweed. They do this by inadvertently collecting the algae’s sticky spermatia, the pollen equivalent, on their bodies and scattering it around as they move from leaf to leaf in search of food and shelter.
This is the first time an animal has fertilized an algae. Not only does this discovery expand the scope of species that use this reproductive strategy, it also raises questions about whether it evolved first on land or in the sea.
It was long thought that animals only pollinated plants on land. In 2016, however, scientists discovered that zooplankton pollinates Thalassia testudinum, a species of seagrass native to the Caribbean. Seagrasses are the only flowering plants that grow in marine environments, but they remain closely related to land plants. Seaweeds, on the other hand, although technically plants themselves, are not closely related to terrestrial plants.
The discovery that Thalassia testudinum was pollinated by animals was made after scientists noticed an unusually high density of marine invertebrates visiting seagrass flowers. Shortly after this discovery, Myriam Valero, a population geneticist at Sorbonne University in France, saw something similar happen in the red algae she studied.
The seaweed species she studied, Gracilaria gracilis, always seemed popular with invertebrates, especially the isopod species Idotea balthica. Because Gracilaria gracilis produces spermatia that, like pollen grains, cannot move on their own, Dr. Valero wonders whether the isopods could play a role in the spread of the spermatia. Previous studies suggested that the spermatia of Gracilaria gracilis were dispersed by ocean currents, but given their abundance in calm coastal pools, Dr. Valero that another dispersal mechanism was at play.
To test her hypothesis, Dr. Valero and Emma Lavaut, a graduate student at the Sorbonne, studied male and female Gracilaria gracilis and placed them 15 inches apart in seawater tanks. Half of the tanks were filled with the small crustaceans, the others were not. At the end of their experiment, they found that fertilization occurred about 20 times as much in the tanks with the isopods than in the tanks without them.
In a subsequent experiment, the researchers took crustaceans that had spent time in tanks with reproductive male Gracilaria gracilis and transferred them to tanks with unfertilized female algae. They found that this also resulted in high fertilization rates. They examined the isopods under a microscope and found that they had spermatia stuck to almost every part of their bodies.
The researchers think that the isopods have a mutualistic relationship with the seaweed. The algae provide the isopods with food in the form of a type of microalgae that grows on the surface as well as shelter. In return, the isopods help fertilize the algae.
“This is such a very fascinating study that is really shaking up our understanding of how seaweeds reproduce,” said Jeff Ollerton, a visiting professor at the Kunming Institute of Botany in China, who was not involved in the study but co-wrote a perspective article that accompanied the study in Science on Thursday. “This kind of interaction may have been going on before plants ever evolved, and using a third party for reproduction may have much deeper roots than we ever realized — if you’ll excuse the pun.”
The group to which the Gracilaria gracilis belongs is said to have evolved about 500 million years before the first plants appeared on land. Although isopods only appeared on the scene 300 million years ago, it’s possible that before their arrival there were red seaweeds that depended on other now-extinct marine invertebrates to “pollinate” them.
“It is possible that the relationship between seaweed and animals predates the evolution of the animal-plant relationship,” said Dr. Valero, who acknowledged that this hypothesis could not yet be proven. Another possibility, she said, was that animal-mediated fertilization strategies evolved independently and repeatedly in the terrestrial and marine environment.
dr. Valero added that it was important to find out whether other types of red algae depended on marine animals for fertilization, as this could be critical to preserving biodiversity in our oceans. While scientists document how pollution and climate change affect the relationship between plants and pollinators on land, we have no idea how these forces affect the relationship between algae and their “pollinators” in the ocean.
dr. Valero hopes to be one of the scientists to find out in the coming years.