Flowers under the sea

Past the sandy beaches and rocky shores, beneath the lapping of breaking waves, coasts around the world are (were) carpeted with green underwater meadows of seagrasses. Contrary to what you may be thinking, seagrasses are not a form of seaweed or even vaguely related, though they do often co-occur. Seagrasses are in fact angiosperms, flowering plants whose ancestors adapted to life in salt water back when dinosaurs roamed on land. Is anybody else picturing an plesiosaur gliding over a bed of seagrass? The ancestors of seagrasses took many biochemical and physical shifts in order to adapt to a wet and salty lifestyle. However, amazingly, an important feature that seagrasses have retained despite millions of years of evolution under the sea is their ability to flower.

Turtlegrass flowering shoot ©Dawn Witherington

I won't go on too much about what seagrasses are or why they are important, as I have in a previous post. Instead, here I want to focus on their flowers because, well, underwater flowers – need I say more?

Turtlegrass flower. ©F. T. Short

Just like their terrestrial and freshwater cousins, seagrasses display a range of different flower sexualities. For instance, seagrasses like the Neptune grass (Posidonia oceanica) of the Mediterranean, have hermaphroditic flowers – each flower possesses both female and male sexual organs. As bizarre as that can seem to the non-biologist, many of the plants most are familiar with share this type of flower sexuality, such as roses. Other seagrasses have separate male and female flowers. These flowers can be on the same or on separate plants. The eelgrass (Zostera marina) that dominates the Northern hemisphere has male and female flowers on the same plant (they are monoecious), whereas Caribbean turtlegrass (Thalassia testudinum) has separate male and female plants (they are dioecious).

Flower sexuality. © X O'Reilly Berkeley

Unlike many of their well-endowed terrestrial cousins, however, seagrass flowers are rather drab and don't exibit the colourful and vibrant range of flower morphologies seen on land. Marine flowers are usually small, white, and sparse amongst watery meadows of green. This fact is not really surprising. On land, pollinators have been one of the key drivers behind the evolution of the weird and wonderful colours, shapes, and scents of flowers. However, animals are not the only vectors plants use to spread their pollen, in many plants this is can be done by wind or water.

Until very recently, it was believed that all seagrasses were pollinated by the movement of the surrounding sea. A few years ago, however, a team of researchers working in Florida noticed a flurry of invertebrate activity around the flowers of turtlegrass when they opened at night. The very fact that the flowers only open at night is enough to raise suspicions, as after dark is when small marine invertebrates are most active.

However, just because something hangs around a flower, doesn't mean it plays a role in the plant's pollination. In order for a potential vector to be considered a pollinator, it must satisfy four functions:

1. The female and male organs of the plants must be visited by the potential pollinator.
2. The visitor must carry pollen following its visit.
3. The carrier must transfer the pollen onto the female organs.
4. The pollen deposited by the animal must result in germination.

Through a carefully planned set of creative experiments, the Van Tussenbroek and colleagues set out to test whether or not the invertebrates were indeed assisting seagrass pollination. Each experiment consisted of separate aquaria and wild-caught marine invertebrates.

Simplified illustration of Van Tussenbroek et al.'s experimental set-up. © X O'Reilly Berkeley
I am ashamed that I didn't take the time to draw the rhizomes!

Based on these results, the authors concluded that turtlegrass is probably pollinated both by water movement and invertebrates, mostly small crustacean larvae and polichaetes. This does not mean all seagrasses are pollinated by invertebrates – after all, the different families of seagrasses adapted to life in the water separately and so have different adaptations and occupy a diverse range of environments.

Nonetheless, this discovery is a sweet reminder of the intimate and inseparable interactions between plants and animals. It also goes to show that there is yet much to learn about the natural world, even when it comes to relatively common or abundant organisms.

Neptune grass (Posidonia oceanica) meadow in the Mediterranean © M. San Felix, from livescience.com

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Bibliography

Van Tussenbroek, B. I. et al. (2016) Experimental evidence of pollination in marine flowers by invertebrate fauna. Nature Communications 7,12980 doi: 10.1038/ncomms12980

For a very brief and general article about seagrass: http://tambarikosy.blogspot.co.uk/2015/12/seagrass-ecosystem-services-grass-roots.html