Like sea bees, these crustaceans pollinate seaweed

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 pollinators for a type of seaweed. They do this by inadvertently collecting the sticky algae sperm, which is equivalent to pollen, on their bodies and spraying it as they go from frond to frond in search of food and shelter.

This is the first time an animal has been observed fertilizing algae. This discovery not only expands the range of species using this breeding strategy, but also raises the question of whether they first evolved on land or at sea.

It has long been believed that animals only pollinate plants on Earth. However, in 2016 scientists discovered that zooplankton pollinate Thalassia testudinum, a type of seaweed found in the Caribbean. Seaweeds are the only flowering plants that grow in marine environments, but they remain closely related to terrestrial plants. On the other hand, seaweeds are not closely related to terrestrial plants, although they are technically self-cultivating.

It was discovered that Thalassia testudinum was pollinated by animals after scientists noticed an unusually high density of marine invertebrates visiting the kelp flowers. Shortly after this discovery, Myriam Valero, a population geneticist at the Sorbonne University in France, noticed something similar happening among the red algae she was studying.

The species of seaweed she was studying, Gracilaria gracilis, appeared to be common to invertebrates, specifically the isopod species Idotea balthica. Since Gracilaria gracilis produces sperm that, like pollen, can’t move on their own, Dr. Valero wondered if isopods play a role in sperm dispersal. Previous studies indicated that the sperms of Gracilaria gracilis were dispersed by ocean currents, but due to the abundance of these sperms in calm coastal rock pools, Dr. Valero suspected another dispersal mechanism.

To test their hypothesis, Dr. Valero and Emma Laffot, a graduate student at the Sorbonne, grew male and female Gracilaria gracilis and placed them six inches apart in seawater tanks. Half of the reservoirs were inhabited by small crustaceans, while the rest were not. At the end of their experiment, they found that fertilization occurred about 20 times in tanks with the isosceles compared to in tanks without them.

In a later experiment, the researchers took crustaceans that had spent time in tanks with a reproductive male Gracilaria gracilis and moved them to tanks with unfertilized female algae. They found that doing so also led to higher fertilization rates. They examined the isosceles under a microscope and found that they had sperm attached to almost every part of their body.

Researchers believe that isopods have a correlation with seaweed. The isopods provide food in the form of a type of microalgae that grows on their surface as well as shelter. In turn, isopods help in the fertilization of algae.

“This is a very fascinating study that really rocks our understanding of how seaweed reproduces,” said Jeff Ollerton, visiting professor at the Kunming Botanical Institute in China, who was not involved in the study but co-wrote the perspective article, who accompanied the study in Science Thursday. “This kind of interaction may have been going on long before plants evolved, and the use of a third party for reproduction may have much deeper roots than we ever realized — if you’ll excuse the pun.”

The group to which Gracilaria gracilis belongs is believed to have evolved about 500 million years before the appearance of the first plants on Earth. Although isopods only arrived on the scene 300 million years ago, it is possible that prior to their arrival, there were red seaweeds that relied on some other now-extinct marine invertebrates to “pollinate”.

“It may be possible that the relationship between seaweed and animals preceded the evolution of the relationship between animal and plant,” said Dr. Valero, who acknowledged that this hypothesis could not be proven yet. Another possibility, she said, is that animal-mediated fertilization strategies evolved independently and repeatedly in the terrestrial and marine environment.

Dr Valero added that it is important to know if other red algae species depend on marine animals for fertilization because they may be necessary to maintain biodiversity in our oceans. While scientists document how pollution and climate change affect the relationship between plants and pollinators on Earth, we have no idea how these forces affect the relationship between algae and their “pollinators” in the ocean.

Dr. Valero hopes in the coming years to be one of the scientists who discover it.

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