Small crustaceans carry sperm-like bodies from male to female members of the red seaweed Gracilaria gracilis, hinting that pollination could have evolved in the sea rather than on land
A small woodlouse-like crustacean seems to help fertilise red seaweed in rock pools, much as a bee pollinates flowers. This suggests that such behaviour is more common in the oceans than we thought, and animal-mediated pollination may even have first evolved there.
We tend to associate pollination with insects like bees and flowering plants on land, but crustaceans have been found to pollinate seagrass underwater. Now, Myriam Valero at the Sorbonne University’s Roscoff Biological Station in France and her colleagues have shown that a very similar thing happens with the red seaweed Gracilaria gracilis, which is actually a type of algae.
During the seaweed’s life cycle, you can have individuals that are either male or female. It was already known that water currents can carry spermatia – the seaweed’s version of sperm – from male individuals to reproductive organs on nearby female seaweed. Each fertilisation event generates a bulb-shaped structure that is visible to the naked eye, called a cystocarp, on the female seaweed.
To test whether a woodlouse-like marine isopod called Idotea balthica can pollinate the seaweed, Valero’s team placed 20 of the animals into an aquarium containing one male and one female member of the seaweed that were 15 centimetres apart. I. balthica is commonly found on G. gracilis. The researchers also used tanks without the crustaceans.
By counting the number of cystocarps on female seaweed, the team discovered that there were 20 times more fertilisation events in the presence of the little crustaceans than in their absence. This suggests that the animals pollinated the seaweed.
The team then imaged the crustaceans that interacted with the seaweed and found spermatia attached to the abdomen and legs of the animals, showing that they could carry it around when moving from male to female seaweed.
The findings hint that animal-mediated pollination could have evolved in the sea rather than on land.
“Until recently, fertilisation with the help of animals was believed to have emerged among plants when they moved ashore 450 million years ago. Red algae arose over 800 million years ago and their fertilisation via animal intermediaries may long predate the origin of pollination on land,” says Valero.
“However, we cannot rule out that different animal-mediated fertilisation mechanisms evolved independently and repeatedly in terrestrial and marine environments.”
Journal reference: Science , DOI: 10.1126/science.abo0661