We have long known that certain insects, such as bees, help plants reproduce by spreading their pollen from plant to plant. However, biologists have recently identified a surprisingly similar process in the underwater world, involving red algae and tiny crustaceans. This is the first known case of an animal helping algae reproduce. This new discovery could also suggest that pollination first evolved in the oceans.
In flowering plants, pollination represents the transport of pollen from the male reproductive organs (stamens) to the female reproductive organ(s) (pistil), thus enabling sexual reproduction. The process can be operated by the wind, but 90% of flowering plants today remain pollinated by animals, mainly insects.
Until recently, we thought that the pollination of plant species by animals was an exclusively terrestrial process. Things changed in 2016 when scientists discovered zooplankton pollinating Thalassia testudinum, a species of seagrass found in the Caribbean. Seagrasses are the only flowering plants that grow in marine environments.
However, seagrasses are not the only ones concerned in the marine environment. According to a study, a tiny crustacean named Idotea balthica would indeed also play this role of pollinator for a species of red algae named Gracilaria gracilis. This work, led by doctoral student Emma Lavaut, from the Sorbonne University, and geneticist Myriam Valero from the CNRS, has been published in the journal Science.
The researchers made the discovery after pointing out an unusually high density of marine invertebrates visiting the red algae they were studying. These algae have always seemed popular with invertebrates, especially the isopod species Idotea balthica. We also know that Gracilaria gracilis produces spermatia which, like pollen grains, cannot move on their own. Previous studies have suggested that these spermatia were dispersed by ocean currents. As part of this new work, the team wondered if these small crustaceans could also play a role.
To test their hypothesis, the researchers placed male and female algae about 15 centimeters apart in tanks. Half the tanks were populated with tiny crustaceans, the others were not. At the end of their experiment, they found that fertilization occurred about twenty times more in the tanks with the isopods.
In another experiment, researchers took crustaceans that had spent time in tanks with breeding male Gracilaria gracilis and transferred them to tanks with unfertilized female algae. They then found that this process also resulted in high fertilization rates. By then examining the isopods under a microscope, the team observed that they had spermatia stuck on almost all parts of their body.
An oceanic origin?
Like bees with flowers, researchers believe that isopods have a mutualistic relationship with algae. On the one hand, the algae provides the isopods with food and shelter. In return, the isopods help to fertilize them. This discovery therefore extends the range of species that use this reproductive strategy. It also raises questions as to whether it first evolved on land or in the sea.
It is believed that the group to which Gracilaria gracilis belongs evolved about 500 million years before the appearance of the first land plants. Although isopods only appeared 300 million years ago, it is possible that red algae relied on other now extinct marine invertebrates to “pollinate” them. If so, this relationship between algae and animals would predate the evolution of the animal-plant relationship. However, this idea will be difficult to prove. Alternatively, it is also possible that animal-mediated fertilization strategies evolved independently and repeatedly in the terrestrial and marine environment.
Finally, this new knowledge could also be essential for maintaining biodiversity in our oceans. Many scientists are now documenting how pollution and climate change affect the relationship between plants and terrestrial pollinators. It would therefore be interesting to assess the impact of these forces on the relationship between algae and their marine “pollinators”.