Mutualism of the Month: Turtles and eelgrass - a mutualism or commensalism?

Burrs attach to unaware animals and take a ride. Photo source.

Burrs attach to unaware animals and take a ride. Photo source.

Plants have to get their seeds from place to place. Some use forces of nature like wind or water currents. Others have developed relationships with animals to transport their seeds (called zoochory). These relationships span from commensalism, where the plant benefits at no cost to the animal, to mutualism, where both animals and plants benefit. Commensalisms most frequently come in the form of plants utilizing unaware animals without reward (think burrs and other sticky seeds). Mutualisms involve the plants encouraging and rewarding their animal transports through enticing fruits full of important nutrients. Plants have evolved to use animals as a dispersal agent because animals are inherently mobile; animals need to find food to survive so they travel from one patch of plants to another. This brings seeds beneficially into contact with more suitable habitats.

The process of transporting seeds is typically begins by an animal eating a fruit containing the seeds or eating the seeds directly. Then, while processing its meal, it travels to the next patch of plants (to find more food) and at some point defecates, releasing the seeds back to the earth. Sometimes this increases the success of the seeds germinating, while other times it reduces the likelihood. Either way, this trade-off is worth it. Seeds are dispersed beyond where they’d be able to reach solely on forces of nature and into satisfactory locations.

The unassuming eelgrass flower, chock full of seeds. Photo source.

The unassuming eelgrass flower, chock full of seeds. Photo source.

Zoochory is common in terrestrial environments. There are relatively few examples of this process happening in the aquatic world; perhaps because there are fewer flowering aquatic plants. The relationships between aquatic seed dispersers and seed producers, until recently, had been thought to occur only in freshwater environments where the diversity of plants is highest. The diversity of plants that can live submerged in salt water is extremely low (algae ≠ plants), but seagrasses have made the foray.

Seagrasses produce seeds within unassuming flowers. These seeds (about the size of a grain of rice) are released into the water and transported via currents, with some luck, to another location where they develop a new bed or grow up in another established bed. Sometimes they land where they can’t develop and the seed dies.

Recently, scientists at Virginia Institute of Marine Science found seeds of the eelgrass Zostera marina (a type of seagrass) in fecal samples of the diamondback terrapin Malaclemys terrapin. At the time only waterfowl and fish species had been considered seed dispersers of eelgrass.

The diamondback terrapin Malaclemys terrapin incidentally consumes seeds of eelgrass, increasing germination. Photo source.

The diamondback terrapin Malaclemys terrapin incidentally consumes seeds of eelgrass, increasing germination. Photo source.

To look at the capacity of terrapins as transporters of eelgrass seeds, researchers collected 118 turtles and examined their feces. About 30% of terrapins had seeds within their feces. Lab studies found that on average 14% of the seeds ingested germinated. The germination rate of wild (non-ingested) eelgrass seeds is about 10%, indicating that there may be a higher potential germination rate after passing through the terrapins gut. The scientists didn’t suggest a mechanism for the improved germination rate.

Scientists think that the terrapins are incidentally consuming the seeds as they consume animals growing on the eelgrass. It’s still unknown if terrapins are directly benefitting from consuming the seeds, as no nutritional study has been done. Either way, it seems that the eelgrass is benefiting by gaining access to a seed transporter, being able to distribute its seed farther than seeds would travel just by current.

To read the original scientific article from the open-access journal PLOS One click here.

Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!

More mutualisms of the month!