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Digging Deeper into Clam and Seagrass Science
A Q&A with Dr. Diana Chin
With support from the Link Foundation, the National Science Foundation’s Graduate Research Internship Program, and the Dutch Research Council, Dr. Diana Chin conducted research on lucinid clams at the Smithsonian Marine Station from September 2017 to January 2018. SMS spoke with Dr. Chin about the inspiration behind her work, her fascination with mutually beneficial relationships between organisms, as well as what she’ll be investigating as a National Science Foundation postdoctoral fellow at the University of Florida.
This interview has been edited for clarity and length.
What was the origin of your interest in lucinid clams?
As long as I can remember, I’ve been interested in symbiosis. I came to lucinid clams as a first-year PhD student, when I was looking for a project to present to my advisor, Bradley Peterson at Stony Brook University. I’d read one of my future collaborator’s papers, Tjisse van der Heide, on seagrasses and lucinid clams and mutualism and was intrigued by it—but there aren’t any lucinid clams in coastal New York. So I worked with a different species, Solemya velum, a really small, weak-shelled orange chemosymbiotic clam. But when it came time to do my dissertation a few years down the line, I was still interested in lucinids.
What makes these clams such an appealing subject of study for you?
I’m really attracted to things that aren’t well studied. Lucinids are everywhere, and though it’s been known for a while that these clams have unique energetic resources, we still don’t know a lot about their ecology. They live deep underground. You can’t just go on a scuba dive through a seagrass bed and see them. And whether there are 20 or 3000 of them per square meter, you have to dig them up, so it’s really hard to work with them. Some of the larger species can be as deep as 40 centimeters (16 inches).
Lucinids are quite different from the species you studied previously.
Yes! The species we worked with at SMS, Codakia orbicularis, is really big. Its maximum size can be around 9 centimeters across (3.5 inches), making it one of the larger lucinids. Their shells are really recognizable—its common name is the tiger lucine.
What do we know about their importance to seagrass ecosystems?
We know much more about the clams’ symbiosis with chemosynthetic bacteria than about how they fit into seagrass ecosystems. In the lucinids’ vicinity in sediments, sulfide levels do tend to be low. We think it’s partly due to the fact that the lucinids’ symbiotic bacteria consume sulfide. In areas where lucinid populations are denser, we have evidence that lucinids can play an important role in the food web, that they can be mutualistic with seagrasses, and that they probably help seagrasses grow and survive in times of stress. But it’s still not clear exactly how globally important the lucinids are to the seagrasses.
Why are sulfides important?
Sulfides, and sulfur compounds in general, are very common in marine sediments – they’re major byproducts of the breakdown of organic matter. One of the reasons lucinids may be so common in seagrass beds is that they’re hotbeds of organic matter, with abundant sulfides present.
With all that decaying organic matter around them, seagrasses need a defense to keep toxic sulfides out of their tissues, which they do by releasing oxygen through their roots. The clams need oxygen as well, and their symbiotic bacteria can use that sulfide, oxygen and carbon dioxide to make food that the clam can use.
How could human or climate-change impacts potentially throw that cycle out of harmony?
There are many ways to change the quality of seagrass sediments that are particularly human influenced. You get more sulfide production when temperatures are higher, or there’s a lot of organic matter breaking down. Nutrient runoff and an increase of fine particulates in the water column make the sediments muddier and the water cloudier, and you don’t have seagrasses without light. Seagrass beds are also easily damaged by trampling, scarring from boats, anchors and mooring lines.
Your research at SMS suggests that lucinid clams may help buffer seagrass against these kinds of negative environmental stresses. Does that mean that clams could be ‘planted’ in vulnerable seagrass beds to give them a boost?
The dream would be to be able to use lucinids in conservation plans, maybe as a co-restoration tool with seagrasses. But that’s a very long way off. We need to know much more before we can say for sure, but implications are that they might be pretty important, at least in certain areas of the world.
Lucinids aren’t a monolith—they’re an extremely diverse family of clams, and not always associated with seagrasses in coastal areas. Some like sandier conditions, while others prefer mangroves. And we also know that there are thriving seagrass beds with what appear to be very few lucinids. So it makes people more reluctant to think of them as a potential tool.
My intermediate dream is to raise awareness that the clams are even there. They’re a unique part of the seagrass ecosystem that we know almost nothing about compared to species that are more easily observed. I’d love to have some kind of an underground clam radar!
We’ve had other lucinid research at SMS since you were here—those experiments involve a lot of buckets.
All of the seagrasses, sediment and clams we collected from Key Biscayne, at Crandon Park Beach. The sediment was the hardest part—we had 25 five-gallon buckets of wet sediment, and it was a job getting it. We had to go back and forth, down into the water, and shovel wet sand and sediments into these buckets, and use a wheelbarrow to get them back up onto the beach. We actually had to do a calculation to make sure the axle weight of our transport vehicle was adequate! But it was important to have that sediment for the experiments, since exactly how and when lucinids acquire symbionts from their environment is also understudied. These clams need a source for their symbionts, so there was a risk that if we used other sediments we’d accidentally stress or kill the clams.
What’s next for your work with these clams?
I’ll be looking at the ability of lucinids to switch between carbon and nitrogen sources. They have their chemosymbionts and can also filter-feed – but the chemosymbionts can probably also fix nitrogen, which means they can garner their own nitrogen sources in oligotrophic (nutrient-poor) areas. I’d like to know whether anthropogenic nutrient inputs in coastal areas affect how they obtain carbon and nitrogen. I plan to investigate changes in the lucinids’ stable isotope signatures and any differences in symbiont expression of a particular gene involved in nitrogen fixation.
If we do find that the lucinids are filter-feeding more and not fixing nitrogen as much, the possibility is that they’re not drawing down sulfides as much, which isn’t as beneficial to seagrasses. Where lucinids are more abundant, this might actually have implications for the resilience of seagrass beds.
--September 2020