In many underwater ecosystems, seagrass meadows act as a food source, a safe haven, and an ecological lynchpin. But until now, we knew very little about how these plants reproduce — critical information for conserving the meadows. New research shows that seagrass reproduces sexually, delivering new genetically diverse seedlings that disperse on ocean currents to form new meadows. To preserve genetic diversity and increase meadows' resilience to environmental shocks, baby seedlings could be scooped up and used to plant new meadows or rescue struggling ones.
"Seagrass meadows are like underwater gardens that quietly do a lot of work for us," said Prof Jennifer Verduin of Murdoch University, lead author of the article in Frontiers in Conservation Science. "They provide nursery habitat and shelter for many types of critters such as fish and invertebrates, they provide food for dugongs and turtles, they provide oxygen, they help keep water clearer by trapping sediments — thereby reducing coastal erosion by holding sand in place — and they store carbon in their sediments. Healthier, more resilient seagrass meadows support better fisheries, clearer water, and more stable coastlines."
The seeds of success
Seagrass reproduction is complicated. Some seagrasses can reproduce asexually, growing shoots which are genetically identical to the parent plant — but which can be less robust than plants with more genetic diversity. Other seagrasses can produce flowers for sexual reproduction: some have distinct male and female flowers, while others have hermaphrodite flowers. Amphibolis antarctica, the species the scientists studied, is a viviparous seagrass: it 'gives birth' to seedlings rather than releasing seeds.
"Viviparous is a word we usually hear with animals, but it can apply to plants too," explained Verduin. "It means that the young plant starts to grow into a seedling while it is still attached to the mother plant. A good way to picture it is that instead of dropping a small seed and hoping for the best, the plant releases a bigger starter plant that's already partway grown."
Scientists suspected these seedlings were the product of sexual reproduction, but the details were unclear. If the seedlings formed through sexual reproduction, they could be an important source of genetic diversity for conserving struggling meadows.
To investigate, the team set out to observe seagrass reproduction in the wild and in the lab. In the spring, they dived to two different meadows off the coast of Western Australia to collect 200 male and female shoots. To test if female plants would form seedlings without male plants, they set up two seawater tanks: one containing male and female plants, and one containing only female plants.
The scientists also continued observing plants at the meadows to track the relationship between pollination and seedling formation. As the season progressed, they collected female shoots and mature female flowers to see how reproduction developed underwater.
Maternity tests
The scientists found that only the female shoots housed alongside male shoots began to form seedlings. The female shoots in the tank without male shoots grew normally but developed no seedlings.
Meanwhile, in the oceanic seagrass meadows, all male flowers had released their pollen 60 days after the first pollen release was observed. 50 days later, the scientists saw the first signs of seedlings forming. 70% of the female flowers formed new seedlings, and these seedlings were from sexual reproduction, not cloning.
"Seedlings emerge after underwater pollination and fertilization," said Verduin. "We observed pollen landing on female flowers, then using microscope testing of field-collected flowers we observed pollen tubes growing toward the ovary and then embryos developing. That means the drifting seedlings seen in the ocean aren't just copies of the parent plant, they are new genetic individuals. Sexual reproduction matters because it creates genetic variety, which helps seagrass populations cope with disease, heatwaves, storms, and such-like."
This research offers a conservation opportunity and a warning. It shows that seedlings could be collected, without damaging the original meadow, to form new meadows or rejuvenate struggling ones — but it also shows that relying too heavily on seagrass plants from specific patches could leave replanted meadows inbred and vulnerable.
"This information can be used to design better restoration strategies," said Verduin. "For example, by making sure restoration projects do not rely on a single local patch, and by protecting the conditions that allow flowering, pollination, and seedling release.
"I hope that this research encourages conservation that protects the seagrass and the life cycle processes that keep seagrass meadows healthy in the long term. This could be as easy as not dredging or anchoring in key seagrass areas."
