Unlike seaweeds, seagrasses are true flowering plants, they are related to the grasses we have on land but have adapted to live in the sea. Like their terrestrial relatives, seagrasses produce flowers and seeds to reproduce sexually. They can also reproduce clonally, expanding the meadow from the edges by growing new shoots from the rhizomes, which link the seagrass under the sediment. This dual reproduction strategy allows them to be more resilient in the face of small scale disturbance; the feeding trails left by dugongs will be gradually covered back over by new shoots while seeds will reach nearby areas where another meadow can grow.
So how can pollination and seed development possibly work underwater? Well, like plants on land the seagrass flowers are male and female and need to be pollinated for viable seeds to be produced. For some groups of seagrass the pollen floats and pollination takes place just above the sea surface, but most species carry out all aspects of their life history below the waves. All seagrasses have highly waterproof pollen which is also very long compared to land plants. A recent study which showed plankton help to pollinate seagrass, adds another interesting dimension to a process we don’t actually know that much about (it’s much more difficult to study plants when you can’t observe them in their natural environment without growing gills!).
At CQUniversity we’ve been collecting seagrass flowers in order to facilitate pollination and seed development, to try and use the seeds for restoration work. The seagrass beds in Gladstone Harbour used to be more extensive and lots of work is going on at CQUniversity into understanding how we can restore these beds to their former glory. In the natural environment, survival of seagrass seeds is comparatively low because they may get eaten (before or after they are released from the plant), and fast currents or strong winds might cause them to be washed away far from a good spot for a new meadow. But by collecting the seeds and starting off the germination process, we can give them a better chance of survival in the right place.
We collect seagrass flowers from sites around the harbour so that we have seeds which will be genetically adapted to cope with a whole range of physical conditions. Seagrass flowers look a bit like the flowers you see on some grasses on land with spathes (adapted leaf blades that look a bit like a zipper) which contain many seeds. You might not notice them at first glance, but once you get your eye in they are quite easy to spot, as long as you go at the right time of year (around late spring to mid summer). We keep these flowers in bags with a very fine mesh inside our mesocosm tanks which have conditions just like the natural environment, but without any predators to eat the seeds! After a few weeks we check the bags and pick out all the seeds which have dropped out from the spathes, we also dissect all the remaining spathes under a microscope to ensure there are no seeds trapped. These seeds are stored in sea water and put in the fridge, ready for some experimental trials next year to understand the best method to ensure they survive in the wild. A pulse of fresh water will help to prompt the seeds to start germinating.
You might think that once we have the seeds we can just go to the perfect location and spread them to plant a new seagrass bed but unfortunately it’s not that simple. You need to spread quite a few seeds to have a good chance of a meadow growing, but if you simply throw the seeds into the water or onto the beach at low water they will probably just get washed away, especially in an area like Gladstone Harbour where the water flows in and out very quickly. This new research will help us to understand the methods which work best to plant seeds and could be really important in helping to undo the damage and declines in seagrass caused by human activity. The Port Curtis Seagrass Restoration Project is always looking for volunteers to help with this important work, from joining us on the beach collecting flowers to helping us take care of the seagrass in the tanks and extracting the seeds – if you are in Gladstone please get involved!