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Lucinid clams: Seagrasses’ secret defense against stress
NSF Postdoctoral Research Fellow in Biology, University of Florida
Seagrass beds are favored coastal habitats for lucinid clams. These bivalves have evolved since ancient times to take advantage of several unique features of seagrass and mangrove habitats. One of these is an abundance of hydrogen sulfide, or sulfide, present in seagrass sediments as a result of decomposing leaf litter and other organic material.
The results of a new study, published in July in the Journal of Ecology and conducted at the Smithsonian Marine Station in 2017, highlights how environmental stress may actually boost the clams’ beneficial effects on seagrasses—and that they may even play an important role in future seagrass conservation and restoration efforts.
If you’ve ever taken a walk at low tide and been hit by a rotten egg odor near exposed mud flats, you’ve smelled hydrogen sulfide. The compound is a natural and widespread byproduct of decomposition in low-oxygen conditions, but is toxic to most plants and animals.
Seagrasses have evolved several ways to deal with the consistent threat of sulfide toxicity. But when seagrass is stressed by low light availability in turbid water, high temperatures, high salinity, or other factors, it can lose the ability to detoxify sulfide and can die. This process is thought to be responsible for several large-scale seagrass die-offs in Florida Bay and other areas in recent decades.
Previous research has shown that the presence of lucinid clams can enhance seagrass growth and survival, and that this effect is tied to the clams’ ability to drastically decrease sediment sulfide levels. One reason is the symbiotic chemosynthetic bacteria that live in the clams’ gills, which use sulfide from the surrounding sediment to make food.
However, little is known about how the beneficial effects of lucinid clams on seagrass may change as environmental conditions change.
To test the effect of environmental stress on the lucinid-seagrass relationship, we set up an outdoor laboratory experiment that compared seagrass growth and sulfur accumulation under several different conditions. Some seagrass was shaded, while the rest received full light. Some seagrass sediments received regular infusions of additional sulfide, while no sulfide was added to the rest. Finally, some seagrass was grown with lucinid clams, while the rest was grown without.
We found that lucinid clams were more beneficial to seagrasses overall when environmental conditions were stressful relative to typical conditions. If lucinid clams were present, seagrass leaves grew slightly faster, regardless of increased shade or sulfide stress. Underground rhizomes maintained their growth rates whenever lucinid clams were present, but grew more slowly in the absence of lucinid clams if shading or sulfide stress was added. Additionally, leaves and rhizomes that grew more slowly tended to be those that had accumulated more sulfur, suggesting that a major cause of slow growth was toxic sulfide impacting those plants.
All of this suggests that while lucinid clams can benefit seagrass merely by being present, the clams could even help seagrass survive and continue growing during periods of intense environmental stress, such as during a harmful algal bloom or after a storm.
“This study suggests that the clams are most beneficial any time conditions become more stressful for the seagrasses, because of the detoxification effect they have on sediments,” said Dr. Valerie Paul, SMS director and co-author on the study.
Many questions remain, but this study provides more evidence that seagrass conservation and restoration efforts might benefit from considering these inconspicuous clams. For example, knowing more about how lucinid clams are distributed across seagrass beds (or whether they are even present) could help resource managers predict which beds are most vulnerable to die-off, or may recover more quickly following die-off events. Another tantalizing possibility is that co-transplanting lucinids along with seagrasses could enhance the success of restoration projects.
This study would not have been possible without the cheerful patience of the Smithsonian Marine Station staff, who hosted our research team for four months and allowed us to fill a large amount of laboratory space with buckets of sand and syringes full of odiferous water. We were generously funded by a Smithsonian/Link Foundation Fellowship, a National Science Foundation Graduate Research Fellowship, and a grant via our two Dutch colleagues.
Chin, DW; de Fouw, J; van der Heide, T; Cahill, BV; Katcher, K; Paul, VJ; Campbell, JE; Peterson, BJ. 02 July 2020. Facilitation of a tropical seagrass by a chemosymbiotic bivalve increases with environmental stress. Journal of Ecology. https://doi.org/10.1111/1365-2745.13462