Over the past few weeks, we’ve posted a few blogs providing an introduction to oysters and kelp as a way of introducing the subjects of some of our upcoming estuary projects. The final species we want to introduce you to is seagrass, specifically Dwarf Eelgrass (Zostera marina). But before we explain the importance of seagrass and its current situation around UK coasts, We want to explain the differences between seagrass and seaweed (marine algae). Both are marine plants, so how different can they be? Is seagrass like seaweed?
The simple answer is no.
Seagrasses are the only flowering plant that exists under water and are the only marine plant to reproduce using pollination like many terrestrial plants. In evolutionary terms compared to seagrass, seaweed is a relatively primitive group of plants with several key differences between the two groups.
Seaweed doesn’t have roots, instead attaching itself to rocks on seabed using a holdfast, a gnarled structure that effectively glues the seaweed to the sediment and preventing it from getting washed away. Seagrass on the other hand grows an extensive root system below each plant, stretching out into the soft sediment on which it grows and collecting nutrients from the soil. You may have spotted that unlike the root systems I made no mention to holdfasts collecting nutrients from the sediment, and that’s because they don’t. Unlike seagrass, seaweeds do not absorb any nutrients from the seabed, instead absorbing nutrients from the surrounding water column using diffusion.
Nutrient collection in this way is one of the reasons seaweeds is considered evolutionary primitive. When further compared to seagrass we see that within the structure of a seagrass plant is a complex network of veins running the length of the plant from roots to leaves, allowing for nutrient transport throughout the plant. While this initially may not seem like major difference (just different ways of getting nutrients, right?), this simple difference has allowed seagrass cells to evolve and specialise to fulfil a variety of roles to help the plants survival. While all the cells in seaweed need to be able a bit of everything from nutrient collection to photosynthesis, each group of cells in a seagrass plant has evolved to do one job and do it very well.
As I mentioned seagrass reproduces using pollination, a reproductive method employed by terrestrial plants, and that seagrass is the only plant that grows underwater which uses this method, but what’s the alternative? That would be spores! When seagrass reproduces the male plants release sperm into the water column which when it makes contact with a flowering female plant it will fertilise the egg and seeds will be produced by that female plant. Similar to this, seaweed releases sperm into the water to fertilise eggs located on the seaweed blades, and once fertilised the spore is released to settle on the seabed. However, unlike seagrass plants a single seaweed plant is cable of producing both male and female reproductive cells.
There you have it! You now know the major differences between seagrasses and seaweed. As we continue to post blogs to build to our upcoming projects, keep your eye out for the next seagrass post where we’ll detail the ecological and environmental contributions that come as a result of seagrass habitats as well as addressing the current UK seagrass situation.
Also, despite its name the closest terrestrial relations to seagrass are actually lilies and ginger. While not that reelevate to this post if you’ve read this far you definitely need to know this fact. Enjoy!!