Life on a rocky coastline can be difficult. Even though there are lots of potential partners out there, stalks of these large algae are stuck in place, and possible mates often keep their reproductive structure hidden. This is a problem for the red seaweed Gracilaria gracilis. The spermatia lacks flexible flagella to propel them through the water. Myriam Valero is an ecologist at the French National Center for Scientific Research.
The seaweed's spermatia have a slim chance of landing on a female seaweed's reproductive organ, according to scientists. Valero and her colleagues discovered that the species Idotea balthica, a type of buglike crustacean called an isopod, can be used as a pollination tool. This is the first instance of an alga practicing pollination in the water.
Valero and her team collected stalks of Gracilaria from shorelines and dropped them into tanks to see if the isopods were involved in seaweed reproduction. In the first experiment, both males and females were dangled in tanks with and without isopods, which can grow to about the length of aAAA battery. The researchers counted the number of cystocarps that sprouted on the female seaweeds. They found that reproductive success was 20 times higher for the seaweed stalks exposed to the isopods than it was for the people in the tank.
The researchers ran a second experiment where they dropped only female stalks into two separate tanks to make sure the isopods were responsible for the spike in reproductive activity. One contained isopods that had been exposed to male seaweed, while the other had no isopods. Despite the absence of male stalks, the isopods boosted the number of cystocarps on the female seaweeds.
The isopods are bombarded by spermatia, which are coated in a tacky, mucuslike substance. Isopods spread spermatia throughout a tide pool as they travel between seaweed clumps. In exchange for being fertility nurses, the isopods use the seaweed for shelter and feast on a buffet of smalleralgae stuck to the stalks.
The discovery is intriguing because isopods and red seaweed have been studied for hundreds of years. It has been known for a long time that they interact with each other. Ollerton, who was not involved with the new study but provided comments and wrote an analysis of it for Science, believes the finding shows how many complex pollinationlike interactions may be hiding. He estimated that only 10 percent of the 300,000 species of animal-pollinated flowering plants have had their documentation.
"I think it's more common in the sea than we think," says van Tussenbroek. She found out in 2016 that plankton pollinate marine seagrass plants. There isn't a lot of information about the sexual reproduction of sea plants.
It is possible that the new finding has evolutionary implications. The emergence of flowering plants around 140 million years ago is thought to be when animal-driven pollination began. Recent findings that some mosses and fungi rely on animals for fertilization may change the origin of equivalents to pollination back to the early days of land colonization. The discovery that pollinationlike interactions happen below the waves could push this development back even further.
The ancestors of Gracilaria were predating complex animals by hundreds of millions of years. A mutually beneficial partnership may have been formed when ancestral arthropods scurried onto the scene around 500 million years ago. There is a chain of interactions between animals and plants that goes back hundreds of millions of years. It makes us think about pollination in a different way.
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