Article: Martínez-Crego B, Arteaga P, Ueber A, Engelen AH, Santos R, Molis M (2015) Specificity in Mesograzer-Induced Defences in Seagrasses. PLoS ONE 10(10): e0141219. doi:10.1371/journal.pone.0141219
Most people don’t think of plants as fighters. But they fight to survive the same as every other creature on the planet! Until now, no one had documented seagrasses chemically defending themselves! But they do!
Plants Defend Themselves
Plants fight back in a number of ways. They’ve evolved structural protection (think thorns!) and chemical defenses (like nicotine in tobacco). Chemical defenses make plants poisonous and/or less nutritious or palatable (less tasty) to grazers. And many plants have evolved inducible chemical defenses (they only produce them if they’re being grazed). However, not all plants have inducible defenses and not all defenses work against all grazers.
Optimal defense theory states that inducible defenses are good for the plant if grazing reduces the plant’s fitness (the ability to survive and reproduce). It takes energy to produce defense structures and chemicals. That energy is better used for growth if grazing is not severe. There’s only so much energy so it has to be allotted properly for the best overall outcome for the plant. Some tissues are more valuable than others (for example, young and actively growing plant parts), so it’s not just the amount of tissue eaten by grazers, but also the type that matters. To complicate matters further, plants can also express “compensatory growth” to reduce the impact of herbivores: they simply increase their growth rate when being grazed upon instead of defending against the grazing.
Seagrasses are plants that grow underwater! They’re a foundation species that provide a number of ecosystem services, such as oxygen production and sinking carbon and support a large number of organisms as both habitat and food. Previous studies on some seagrasses have shown no induced defenses in response to large grazers (ex. sea turtles and manatees) or simulated herbivory. But mesograzers (small herbivores) may be more likely to induce such responses since they remain in small areas and gradually consume seagrass.
In this study, researchers looked at the effects of grazing by three species of small herbivores on two species of NE Atlantic seagrass (Zostera noltei and Cymodocea nodosa). To explore the effects of grazing, the researchers questioned whether different grazing intensities and preferences for young or old seagrass tissues cause seagrass to produce defenses. They also wanted to see whether the plants would compensate with growth or produce defenses – or both!
The researchers used a number of feeding trial assays and chemical analyses to determine the answers. (If you would like to learn more about their methods, please click the DOI link to the paper! It’s open access!)
What They Found:
Grazing impact varied by grazer:
Some grazers consumed more seagrass than others (Fig. 3). By far the greatest consumer was the isopod Synischia hectica, which consumed 60% of the C. nodosa blades after only 10 days (Fig. 2). 2 grazers removed about 50% of Z. noltei blades in 12 days, with others consuming well below those levels.
S. hectica preferred younger blades of C. nodosa (Fig. 4). Younger blades are more linked to fitness since they contribute more to growth and energy production. Other grazers also showed preferences: two for young Z. noltei, one of which then preferred older C. nodosa tissues. And one grazer preferred young C. nodosa but any age of Z. noltei.
Evidence for Chemical Defenses:
NO for Z. noltei: In feeding trials, no grazer showed any preference for previously grazed or ungrazed Z. noltei, indicating that grazing does not induce defenses in that species.
YES for C nodosa: S. hectica preferred ungrazed blades over previously grazed ones, even when blades were ground up (Fig. 5). Preference for ungrazed blades even after removing the physical structure of the blades means that there is a chemical reason for the preference. In addition, the researchers found increased phenolic content in C. nodosa after grazing by S. hectica. There were no differences in nutritional content to deter consumption. Therefore, the chemical deterent is of the less tasty/poisonous variety!
Evidence for Structural Defenses:
Again, NO for Z. noltei.
But YES for C. nodosa. S. hectica preferred ungrazed blades regardless of whether they were intact or ground-up. But, they were more deterred by previously grazed intact blades than previously grazed ground-up blades, indicating that while chemicals are part of the defense, that structural defenses may also be important (Fig. 5).
Evidence for Growth Compensation:
YES for Z. noltei: it compensates by increasing growth rate when grazed by one species (Fig. 6).
NO for C. nodosa: it may not need to compensate due to a consistently faster growth rate. It is already able to outcompete other seagrasses when it comes to growth.
All told, what does that mean? Grazing by S. hectica induces chemical (and possibly structural) defenses in C. nodosa! Grazing on Z. noltei did not induce any defenses, but that is likely because it can compensate for consumption with increased growth. These two seagrass species therefore show different ways of dealing with grazing: resistance in the case of C. nodosa and tolerance in that of Z. noltei. This is likely due to the difference in grazing impact on the plants’ fitness. Interestingly, only one grazer induced any sort of defense. Therefore grazer identity is extremely important. S. hectica consumed the greatest amount of seagrass and preferred younger blades, which are more linked to fitness since they are more photosynthetically active and contribute more to growth and energy production. Low grazing intensity and preference for older tissues on the part of the other grazers may explain why they do not induce defenses.
Why should we care?
It’s a new discovery! It’s cool that seagrasses can defend themselves against grazers. And it’s really interesting to learn about the specifics of when and how they do! There are still so many discoveries to be made in the world! Research like this expands our knowledge and opens up new fields of inquiry at the same time! It’s a beautiful thing.
Some cool videos about plant defense: