Tag: biogeochemistry

Research Explainer: How giant clams record their diet in their shells

On By Dan KillamIn Biology, Chemistry, clams, scicomm1 Comment
Two giant clams near Eilat in the Northern Red Sea. To the left is the small giant clam, Tridacna maxima, and to the right is a mature individual of the rare endemic giant clam Tridacna squamosina, only found in the Northern Red Sea.

You are what you eat, and clams are too. We’re made of atoms, which come in “flavors” called isotopes, relating back to the mass of the atoms themselves (how many protons and neutrons they have). Nitrogen, for example, comes in two stable (non-radioactive) forms called nitrogen-14 and nitrogen-15. Much like scientists can track the composition of a person’s diet from the isotopes of their hair, researchers have used the isotopes of clams to figure out their diet.

Nitrogen isotopes provide us with a useful and detailed record of food webs. Plants and algae tend to have more of the light isotope of nitrogen in their tissues than the animals that eat them (primary consumers), and the animals that eat those animals have even higher nitrogen isotope values. We can measure the amount of “heavy” atoms of nitrogen with a unit called δ¹⁵N (“delta 15 N”). A carnivore at the top of the food chain will have a very high δ¹⁵N, while plants will be the lowest. Clams, typically being filter feeders, will usually have an intermediate value, since they’re eating a lot of phytoplankton (tiny microscopic floating algae) and zooplankton (animal plankton that eat other plankton).

But I study a special kind of clam, the giant clams, which have a cheat code enabling them to become giant: they have algae *inside* of their bodies. The algae make food using photosynthesis and share it with their hosts! In exchange, the clams provide the algae with a stable environment free of predators, plenty of fertilizer in the form of their own waste, and even channel extra light to help the symbionts grow faster. This partnership is called photosymbiosis, and is pretty rare in clams, though it is common in other animals like the corals that build the reefs where giant clams are found! Previous researchers have shown that giant clams have very low nitrogen isotopic values in their tissue, like a plant, because they get most of their nutrition from the algae, rather than filter feeding.

I am a sclerochronologist. That means I study the hard parts of animals, in this case the shells of bivalves. Like the rings of tree, bivalves make growth lines in their shells which can serve as a diary of their lives. Some of my past work has looked at using chemistry of the growth lines of giant clams to measure the temperatures they grow at, compare the growth of ancient and modern clams, and even look at how much the clams grow in a day! Today though, I’m talking about my most recent paper, which looks at how we can use the shells of giant clams as a food diary.

But when they’re babies, the symbiosis in giant clams is not yet fully developed. During this early period of their lives, giant clams actually get more of their nutrition from filter-feeding like a “normal” non-photosymbiotic clam, until they’ve had a chance to grow in surface area and become a living solar panel. Like all bivalves, the shells of giant clams are made of calcium carbonate, bound together by a protein scaffold we call the shell organic matrix. Proteins are made of amino acids, which contain nitrogen! If we can get the nitrogen out of the shell from the early part of the clam’s life, and compare it to the nitrogen at the end of the clam’s life, it might record the clam’s transition from filter feeding to its mature plant-like lifestyle! If our hypothesis holds, we should record a decrease through its life in the shell δ¹⁵N values.

A model I made of the clams’ nitrogen intake, with the left plot how they switch from filter feeding to getting most of their nitrogen from dissolved sources around 5-6 years of age. Because the nitrogen isotopes of those two sources are different, that manifests in the expected values from the clam’s body (the right plot)!
A map made by my talented partner, Dana Shultz!

So I gathered a team of talented collaborators and set out to test that hypothesis, using giant clam shells that I was able to get on loan from the Hebrew University of Jerusalem Museum. These shells had been confiscated from poachers at the Egypt-Israel border. While I would have rather known these clams were still alive in the waters of the Northern Red Sea, being able to use them for research to understand the biology of their species was the next best thing! I had originally planned on pursuing a postdoc undertaking this project with Rowan Martindale, a professor at UT Austin who has studied the nitrogen isotopes of photosymbiotic corals, but when I started up at Biosphere 2, we ended up continuing with the project anyway as a collaboration! We measured the nitrogen isotopes of the shell material in the lab of Christopher Junium, a professor at Syracuse University, who has developed an exquisitely sensitive method to measure the nitrogen from shell material by essentially burning the shell powder and then scrubbing out unwanted material to isolate the nitrogen, to measure the isotopes in a machine called a mass spectrometer. Katelyn Gray is a specialist in isotopes of biominerals and assisted with drilling out powder from the shells with a Dremel. Shibajyoti Das, now at NOAA, is a specialist in measuring the shell nitrogen isotopes of other bivalves and he was master at doing much of the mass spectrometer work, and assisting in interpretation. Adina Paytan is a professor at UC Santa Cruz. She first provided the funding and support for me to go to the Gulf of Aqaba and collect these shells as part of an NSF-funded student research expedition! She also provided environmental data which helped us to interpret what the clams were actually eating!

A figure showing the four shells we sampled from, with the sampling areas in each hinge area showing colored and matching with the corresponding isotope plot to the right (colored points). 3 of the 4 shells show declines in isotope values with age. Shaded ribbon behind the data shows the model output.

So what did our crack team of scientists find out? We found that three of the four tested giant clams did indeed measure a decline in nitrogen isotopes over the course of their lives. Their earliest growth lines in the hinge areas of their shells record elevated δ¹⁵N values similar to other filter-feeders from the region. But as they aged, their later growth lines show much lower δ¹⁵N values, more like photosymbiotic corals and plants from the region. So clams indeed recorded the transition in nutrition as they became solar-powered! This degree and directionality of change in nitrogen isotopes was much greater than has been observed in any other clams measured in this way, which made sense considering their unique physiology. The clams have another area of the shell, the outer shell layer, which is closer to the symbionts than the hinge area. In this outer shell area, we did not observe much of a consistent trend in nitrogen isotopes. It’s likely that the outer layer is highly influenced by the photosymbionts even at the earliest stages of life.

Growth lines in the hinge area of two of the shells lit from behind, with the drilled areas for this study visible as well. The outer shell layer is the opaque and was also sampled for this study.

There was one clam that differed from the others in showing low δ¹⁵N values through life in its hinge shell layer. To help explain these differences, I created an independent model of the clams’ internal chemistry based on their growth rate, which slows as they age, and also is faster in the summer. When the clams are young filter feeders, they get most of their nitrogen from plankton, debris and other material floating in the water column making up floating material we call Particulate Organic Matter (POM). Meanwhile, when they are in their photosynthetic life stage, they get most of their nitrogen from nitrate, which is essentially Miracle Gro for the symbionts. The model showed that the clams should record a flip from filter feeding to photosynthesis around 4-5 years of age, which was confirmed by three of the shells! But what about the one that didn’t show this trend? My colleague Adina had fortunately measured the isotopes of POM and nitrate in different seasons in the Gulf of Aqaba. We found that in summer, as expected, POM δ¹⁵N is lower than nitrate. In the winter, meanwhile, that relationship is flipped! So if a clam grew more in winter, it would not record the same transition as was seen by the other clams. We think the clam that was the exception to the rule might have been more of a winter grower.

The chaotic nutrient environment of the Northern Red Sea, showing how in different seasons, dissolved nitrate has higher or lower δ¹⁵N values than the Particulate Organic Matter that the clams filter-feed on.

But long story short, we were able to demonstrate for the first time that giant clams show nitrogen isotopic values in their shells in line with expectations from their diet. Other clams have been measured this way, but the fact that we were able to conduct these analyses at all is a testament to the sensitivity of the elemental analyzer in Chris’s lab. Giant clams have *very* low concentrations of organic matter in their shells, so the forward march of technology was a major factor enabling this study to be possible.

Why does it matter that we can measure the transition of the clams from filter-feeding to photosymbiotic in their shell records? Well, giant clams are not the only bivalves which have photosymbionts. There are other clams in the fossil record which have been proposed to have had symbioses with algae, but until now we’ve never had a definitive geochemical way to measure this in fossils. We hope that this approach can be applied to the organic material in fossil shells, which is often well preserved, to see if huge clams in the Cretaceous and Jurassic had a similar way of life to the modern giant clams! If we can demonstrate that was the case, we can see how such species responded to past intervals of climate change, which will help us understand how giant clams will fare in the warming, acidifying ocean of the present.

These results also help explain the lives of giant clams themselves. We hope this kind of data can be used to measure the symbiotic development of giant clams in different places, with different types of food and nitrogen available, where we’d have the potential to measure pollution. Interestingly, the time that the model shows the clams transitioning to photosynthetic maturity is right around the time that they reach reproductive maturity (5-10 years of age). We’d like to investigate whether the time of clam maturity is controlled by the development of their symbiosis, which itself might relate to nutrients in the clams’ environment. If clams can grow faster, then they can mature faster, and potentially reproduce sooner in life. Will giant clams be able to thrive in the presence of increased nitrate, which is a common pollutant in coral reef environments? Like all worthwhile research projects, we have dozens of new questions to pursue as a result of this work, so stay tuned for the next installment in this journey of clam knowledge!

Research Explainer: Comparing the daily shell diaries of giant clams and scallops

On By Dan KillamIn clams1 Comment
Figure 1 from our paper, showing a comparison of a scallop, its growth increments and where it came from in France, to a giant clam shell section (dyed blue to show its growth lines), and where it came from in the Northern Red Sea

In 2020, I got an interesting email in my inbox from another mollusk researcher! Niels de Winter had emailed me, who I was familiar with from his past work on big Cretaceous rudist bivalves and giant snails. Niels had seen my paper published that year on giant clam shell isotopes from the Gulf of Aqaba in the Northern Red Sea, and was interested in teaming up on a new study to compare the daily growth of giant clams with another bivalve that has daily growth: scallops! I was intrigued because I had similar work underway to study the shells of clams I was growing at Biosphere 2, but I didn’t have any plans to measure my collected wild clam shells that way. So this sounded like a win-win opportunity to work together on a study that neither of us could do alone! Plus, I liked his work and had cited it in the past.

The shells of bivalves are very useful as each produces a shell diary consisting of growth lines, similar to the rings of a tree. Giant clams keep a very detailed diary, with a new growth line forming every night, which previous research has suggested was due to the control that the symbiotic algae inside giant clams have on their host. When the algae conduct photosynthesis, they use CO₂ in the fluid the clam makes its shell from, which increases the pH and accelerates the formation of the shell mineral crystals! The symbionts also directly assist by pumping calcium and other raw materials for the clam to use! Niels had found such daily lines in an ancient rudist bivalve from over 66 million years ago, and proposed it as a sign that the rudists might have had similar algae! I used the daily lines to compare giant clam growth before and after humans arrived in the Red Sea, finding that the clams are growing faster!

But it turns out that giant clams aren’t the only bivalves that make daily lines. Some species of scallops do it too, but that’s a bit confusing, since scallops have no symbionts that could be producing this daily growth period! One way we could investigate this is by bombarding the shells with very tiny laser beams only 20 µm across: the width of a hair is a flawed unit of measurement but 20 microns is as narrow as the narrowest type of hair you can think of! The laser would carry across the cross sections of the shell in a line, literally burning away tiny bits of shell, with the resulting gases captured by a machine called a mass spectrometer, which can figure out the concentrations of elements in the gas.

So we’d basically create a very detailed wiggly graph, where the wiggles represent years, months, days and even tides, depending on how fast the clams and scallops grew! I’m happy to report the paper was published earlier this year, so I thought I’d switch it up a bit and have a conversation with Niels through this blog post. Let me open it up to Niels, who I decided to bring in for this post in a kind of conversation!


Niels, what did you expect to find heading into this experiment? For me, I figured the giant clams would have greater amplitude of variation on a daily basis than the scallops, due to the influence of the symbionts. Is this what you expected?
More or less. To be honest, that is what I was hoping to find, because if the daily lines were so much stronger in photosymbiotic shells than in the non-photosymbiotic scallops, it would make it easier to recognize photosymbiosis by studying modern and fossil shells. Also, a finding like that would obviously support the hypothesis we had about the ancient rudist bivalve. However, I was a bit skeptical as to whether the reality would be so clear-cut.

I mailed samples from six juvenile giant clams to Niels for analysis. We went with juveniles for a couple reasons: they grow faster at this life stage than they do as adults: 2-5 centimeters per year for the species we were studying, which meant the greatest opportunity to record a very detailed record from their shells! Scallops also grow extremely quickly, up to 5 cm/year, and so we would be able to get a similar resolution for both types of bivalves, since each page in their diaries would be a similar width.

Niels brought in our collauthors Lukas Fröhlich, a scallop expert, as well as other geochemists like Lennart de Nooijer, Wim Boer, Bernd Schöne, Julien Thébault, and Gert-Jan Reichart. Could you tell us about the other members of the team and how you brought them in?

When I start a new study like this, I always like to “outsource” the expertise about the topic a bit. Our work in sclerochronology often involves bringing together several fields of research and interpreting the results of complex measurements like these requires input from several people who look at them from different viewpoints. I had just finished a research stay at the University of Mainz in 2019, where I worked with Bernd Schöne and Lukas Fröhlich. I know Lukas was working on scallops together with Julien Thébault, whose team collects them alive in the Bay of Brest and keeps a very detailed record of the circumstances the scallops grow at. To carry out the laser measurements, I needed geochemistry experts, and Lennart de Nooijer, Wim Boer and Gert-Jan Reichart came to mind because I was already working with them on other topics and they run a very good lab for these analyses at the Royal Netherlands Institute for Sea Research (NIOZ). This is how the team came together.

Niels conducted a series of laser transects across the clam shells. He used some sophisticated time series analysis approaches to try to quantify the different periodic cycles that appeared in the clam and scallop growth. This was a different approach to how other workers have gone about finding daily growth cycles in giant clams and scallops, where they have often started by zooming in to find the wiggles, and work backwards from there. Niels instead tried to agnostically dissemble the growth records across each clam shell using mathematical approaches, based on the idea that this would be how future workers have to go about identifying daily growth patterns in fossil clams, where we often don’t have a real “growth model” up front to work with. By growth model, I mean the way that we convert the geochemical observations, which are arranged by distance along the shell, into units of time, which requires us to know how fast the clams grew. For the scallops, the age model was made by counting daily “striae” they form on the outside of their shells. For the giant clams, I helped with this by counting tiny growth lines inside the shell made visible by applying a dye called Mutvei’s solution. Because the growth lines weren’t visible all the way through the shell, I used a von Bertalanffy model to bridge across and create a continuous estimate of how old the clams were at each point along their shells.

Niels found some interesting results! I personally expected that the daily variation in giant clams would dwarf what was seen in the scallops, because of the impact of the daily activity of the symbionts. But it turned out that while the clams had a more regular pattern of daily shell growth than the scallops, likely controlled by the symbionts, that was still a minority of the variance across the clams’ records. Yet again, these clams destroyed my hypothesis, but in an interesting way!

Niels, what were your expectations going into this, and how did the results confirm or go against your hypotheses? What challenges did you run into in the course of your analysis, and how did you end up addressing those challenges?

This was honestly one of the most difficult shell-datasets I have worked with so far. The laser technique we used measures the elemental composition of the shells in very high detail, but while this is ideal for funding daily rhythms, it is both a blessing and a curse! In a dataset like this it becomes quite hard to separate the signal we are interested in from the noise that occurs due to measurement uncertainty. I ended up using a technique called spectral analysis, which is often used to detect rhythmic changes in successions of rocks. I guess this is where my geology background was helpful. With this technique, we were able to “filter out” the variability in the records of shell composition that happened at the scale of days and tides and remove the noise and the longer timescale variations. It turns out that, when you do this, you have to remove a surprisingly large fraction of the data, which shows us that the influence of the daily cycle on the composition of both the scallops and the clams is not very large (at most 20%). We did find a larger contribution in the giant clams, as expected, but the difference was much smaller than anticipated. I also find it interesting that most of the variability was not rhythmic. This shows that there are likely processes at play that control the composition of shells on a daily basis which we do not understand yet.

We were measuring a suite of different elements across both bivalve species, including strontium, magnesium, manganese and barium. All of these were reported relative to calcium, the dominant metal ion in the shell material (they’re made of calcium carbonate). This is why we call them “trace” elements; each is integrated into the material of the shell due to a variety of causes, including the temperature, the composition of the seawater, the growth rate of the clams, and also simply due to chance.

Examples of the time series of trace elements from a scallop shell (to the left) and giant clam (to the right), showing the very intricate wiggles in trace element values on a on a tidal and daily basis in each bivalve

In the giant clams, the elements that varied most on a daily basis were strontium and barium. Prior workers had found strontium was the strongest in terms of daily variation, but barium was more unexpected! Normally, barium is thought of as a record of the activity of plankton in the environment, and since there is very little plankton to be found in the Red Sea, it was not expected to see that element vary on a daily basis. It could be that barium gets included in the shell more as a function of the growth rate of the animals. Meanwhile, the scallops (from the Bay of Brest in France) were measuring strong tidal variability in barium and strontium, which makes sense because that location has huge tides compared to the Red Sea. Tides happen on periods of ~12.4 and 24.8 hours. The scallops showed swings lining up with both, and the tidal variability might be the main explanation for how scallops form daily lines. Because the lunar day is so close to a solar day, they would be hard to tell apart from each other! Interestingly, the giant clams also showed some sign of a ~12 hour cycle. While the Red Sea has pretty tiny tides, I had noticed that some of the clams make 2 growth lines a day, and if some clams in the shallowest waters were exposed on a tidal basis, that could explain why they’d make 2 lines: one at low tide, and one at night! Even in places without tides, like the Biosphere 2 ocean, I’d noticed evidence of 12-hour patterns of activity in the clams. It’s so nice (and rare!) when one of my hypotheses is confirmed!

A nice schematic Niels put together showing all the environmental factors that influence the shells of scallops and giant clams, and how much different elements vary as a function of sunlight, tides and other more irregular events like storms. Mn stands for manganese, Ba for barium, Sr for strontium and Mg for magnesium.

Both the giant clams and scallops recorded large irregular swings in all of the studied elements, likely due to non-periodic disturbances. In the case of the scallops, these included storms and the floods of sediment from rivers. For the giant clams, these probably included algae blooms that affect the Red Sea, as well as potentially dust storms that also come every 1-2 years. Both giant clams and scallops have a lot of potential to measure paleo-weather, which is something that other researchers have observed as well!


Niels, where do you see this work heading next?

The recent work looking at very short-term changes in shells is very promising, I think. I agree that there might be a possibility to detect weather patterns in these shells, but that would require some more work into understanding how these animals respond to changes in their environment on an hourly scale and what that response does to their shell composition.

In the meantime, I was intrigued to find that we were not the only people looking for daily cycles in the chemistry of giant clam shells. I had the pleasure of reviewing this paper by Iris Arndt and her colleagues from the university of Frankfurt (Germany). Iris took a similar approach to detecting these daily cycles by using spectral analysis, but she a smart tool called a “wavelet analysis” to visualize the presence of daily rhythms in the shell, which I think was more successful than my approach. She even wrote a small piece of software which can be used to (almost) automatically detect the days and “date” the clam shell based on them. This is quite a step forward, and if I were to do a project like this again, I would certainly try our Iris’ method.

Interesting, too, is that the fossil giant clams studied by Iris showed the daily cycles in magnesium concentration instead of strontium and barium. This shows that the incorporation of trace metals into clam shells is still not fully understood. So one of the things to do, in my opinion, would be to try to see if we can use shells grown under controlled conditions to link the shell composition to short-term changes in the environment. This would require a complex experimental setup in which we simulate an artificial day and night rhythm or an artificial “storm”, but I think it can be done using the culture experiments we do at the NIOZ.

This study represented a unique opportunity to collaborate with my colleague Niels on a topic that interested both of us, which we wouldn’t have been able to pursue on our own. I enjoyed collaborating with him on this work and we have some ideas for further studies down the road, so stay tuned for the next co-clam-boration!

Research Explainer: The chemistry across a “forest” of giant clams

On By Dan KillamIn clams8 Comments
T. squamosa near Eilat, Israel, 2016

Another one of my PhD chapters is published in the journal G-cubed, resulting from work I did in the summer of 2016 in Israel and Jordan around the Red Sea. This is my first geochemistry article in a journal, so it is a big deal to me! I thought I’d write up a clamsplainer about what I was looking for and how we went about achieving the paper.

A slice of giant clam shell. You can see the difference between the inner and outer layers. The inner layer has visible annual growth lines.

I study the chemistry of giant clam shells. You might already be familiar with the concept of tree rings, a field called dendrochronology. It’s like reading the diary of a tree, where every “ring” is a page in the record of its life. The related field of sclerochronology looks at rings in the hard parts of shelled organisms. We can count those rings to figure out the ages of clams, or their health, and we can measure the chemistry of those rings to understand the temperature the clam grew at, and even what it ate.

A giant clam growing on the reef flat in Eilat

Giant clams are bivalves of unusually large size which achieve a very rapid growth rate through the help of symbiotic algae in their flesh. The clams are farmers, and their crop is inside their tissue! They grow their shells very quickly (sometimes up to 5 cm a year, equivalent to if a six foot tall man grew a foot every year from birth), and live a very long time, up to 100 years (their growth slows later in life). A whole bunch of talented researchers have measured the chemistry of giant clams all around the world to reconstruct past climate and even measure historic storms!

If we want to understand the ecology of a forest, we can’t measure just one tree!

But if you come back to the analogy of tree rings, we essentially have measured the rings and chemistry of individual “trees” in a bunch of different places, but don’t have as good an idea of how the chemistry varies within a “forest” of giant clams in a particular place. In our new study, we set out to describe exactly that, focusing on the Northern Red Sea.

A map of the Northern Red Sea. The right “toe” is the Gulf of Aqaba
Sites where we sampled shells along the northernmost tip of the Gulf of Aqaba

The Gulf of Aqaba represents the northernmost toe of the Red Sea, bordered by Egypt, Israel, Jordan and Saudi Arabia. It hosts some of the northernmost coral reefs in the world, aided by tropical temperatures and clear waters due to the lack of rainfall in the surrounding deserts. Here, we can find three species of giant clams including the small giant clam Tridacna maxima, the fluted giant clam T. squamosa and the very rare T. squamosina, which is found only in the Red Sea and nowhere else (as far as we know). In summer 2016, I went all around the Gulf of Aqaba collecting shells of clams from the beaches, fossil deposits, and even were able to work with shells confiscated from smugglers at the Israel-Egypt border. We cut these shells into slices and used tiny drill bits to sample powder from the cross section of their shells, which we could then conduct geochemistry with! We sampled large areas in bulk from the inner and outer portions of the shell (more on why later) using a Dremel tool, and also sampled more finely in sequential rows with a tiny dental drill bit (same brand your dentist uses!) to see how the measured temperatures varied through seasons. By “we”, I mean my coauthor and friend Ryan Thomas, who spent every Friday morning for several weeks milling out most of the powder we needed for this study. This data became part of his senior thesis at UCSC!

Two giant clams thriving on the shallow reef near Eilat, Israel

What kind of chemistry did we measure? The shells of clams are made of calcium carbonate, the same stuff Tums is made of. Calcium carbonate contains one calcium atom, one carbon atom, and three oxygen atoms. It turns out that all of those atoms come in “flavors” that we call isotopes, relating to the weight of those atoms. When you take the shell powder and put it into a machine called a mass spectrometer, you can figure out the proportions of isotopes of different elements present in the samples

The first isotope “flavors” we were interested were carbon-12 and carbon-13. The ratio of the two is thought to relate back mostly to the action of the algae inside (its symbionts) and outside the clam’s body (the floating algae the clam filters out of the water as an additional meal). This happens because as algae take carbon from the environment and bind it into sugars through photosynthesis, they naturally weight the dice in favor of carbon-12 making it into the sugars. So carbon-13 is left out in the water, and potentially in the clam’s shell. When photosynthesis is more active, it would leave the shell with proportionally more carbon-13. At least that’s what other researchers have confirmed happens in corals, and suspect happens in clams. In the world of isotope chemistry, this phenomenon is called “fractionation,” when a process causes isotopes to form fractions separated by mass. We wanted to test if that was true for giant clams, and could do so by comparing T. squamosina and T. maxima, which have more active photosynthesis, to the less photosynthetic T. squamosa.

Comparing carbon isotopes across different species and shell layers. The results are fairly flat all the way across.

It turns out that the more symbiotic species don’t have more carbon-13 in their shells. We set out several reasons that might be the case, including that the symbionts of these clams are actually more carbon-limited than many researchers might expect. Essentially, the algae lack an excess of carbon atoms to choose from, so they can’t be picky with which isotopes they use to make sugars. Therefore, the fractionation effect weakens and becomes possibly too subtle to manifest in the shell, even in the best-case scenario of three closely related species living the same area. This represents what I’d term a “null result.” We had a hypothesis and we demonstrated that hypothesis was not the case in our data. It was important to publish this result, because other researchers will know not to try the same thing. This means that when we try to search for evidence of symbiotic algae in fossil clams, we will likely need to use other types of chemistry to figure it out. But don’t worry, as finding such a “smoking gun” for algal symbiosis in fossil bivalves is part of my life’s work! I have a few projects in the works looking for exactly that kind of evidence! 😉

A look at how temperatures measured via oxygen isotopes vary through the lives of the animals. This is how scientists can use very old shells to figure out how temperatures varied through a year in prehistoric times!

But we had additional data we collected in addition to the carbon isotopes which actually turned out to provide some interesting results. This other type of measurement regarded the oxygen isotope ratio of the shells. Previous research has shown that the ratio of oxygen-18 to oxygen-16 in carbonate skeletons directly relates to temperature, a principle that has birthed a field known as paleothermometry. There are thousands of papers which use shells of corals, clams, cephalopods, microbes and more to reconstruct temperatures in ancient times. Giant clams have proven to be effective weather stations going all the way back to the Miocene epoch, millions of years ago! Because they grow so quickly (putting down a new layer every day), live for a long time, and don’t stop growing, they form very complete, high-resolution, and long records of past climate.

But no past studies had ever compared different species of giant clams from the same place. There would be interesting new lessons to draw from such a comparison, including seeing if one species preferred to grow at warmer parts of the reef. As complex, three dimensional structures, there are many remarkably different micro-environments throughout a reef, from the hot, sun-exposed reef flat and crest to the cooler, current-swept, deeper fore-reef. Do any of the species of giant clams show a consistently higher temperature than the others, and what would that mean if they did?

T. squamosina records higher temperatures than the other species. Outer shell layers also record higher temperatures than inner shell layers. More on that later in the post 😀

It turns out that the rare T. squamosina, only found in the Red Sea, does record a higher average temperature, almost 3 degrees C higher than the other two species. This is of interest because this species had been proposed by prior researchers to only live on the sun-drenched reef crest, at the shallowest part of the reef. We believe these results corroborate that observation. The previous research on the habitat of T. squamosina was limited to a single study which only was able to find 13 live animals along the coast of the Red Sea. But by independently confirming this life habit, we can ask further questions that may be borne out by further research.

An example of T. squamosina showing signs of possible bleaching (light parts at the center of its body).

Being restricted to the shallowest waters, is T. squamosina at greater risk of harvesting by humans along the shores of the region than its counterparts? Illegal poaching of giant clams along the Red Sea is believed to be a major stressor on their population size in the area. Could this explain why T. squamosina is so rare today, despite being proposed to have been more common in the past? In addition, being restricted to the top few feet of depth in the water could leave the species more vulnerable than the others to atmospheric warming. As with corals, when giant clams overheat they will “bleach”, expelling their symbiotic algae as a stress response. While the clams can recover, it is sometimes a fatal form of stress that leads to their death.

An excellent cartoon of the different shell layers in giant clams. From a peer of mine who also studies them, Michelle Gannon!

More research is needed to answer those questions. But the last aspect of this study relates to what is happening inside of the bodies and shells of the clams themselves. Giant clam shells have two layers. The outer layer grows forward away from the hinge, increasing clam’s length. The clam also makes an internal layer, growing inward to thicken the shell and add weight. We can read the growth lines of the clam’s diary within either layer, and different studies have used one or the other to make records of climate change. But very few studies have compared the two layers of the same individual. Do they record the same temperatures? Figuring it out would be important to determine how studies with just the inner layer or outer layer can be compared to each other across time and space.

A vividly blue example of the small giant clam, T. maxima. From user arthur_chapman on iNaturalist

In our studied clams, it turns out that the outer layer records warmer temperatures on average than the inner one! After ruling out other possible explanations behind this difference (the details are complicated and hard for even shell nerds to wrap our heads around), we settled on the idea that the outside of the clam is indeed warmer on average than the inside! This means that the outer layer, recording temperatures of the outer mantle, is indeed forming at a higher temperature than inside! Why is this?

Unlike us, clams are ectothermic. They generally stay the same temperature as their surrounding environment and don’t use their metabolism to generate internal heat. But that doesn’t mean that the clam doesn’t have hotter and cooler spots in its body. It makes sense that it would be hotter at the outer part of its body, facing the sun, as the solar rays hitting its outer mantle would then radiate out again as heat. The outer mantle is also darker in color than the inner mantle, allowing it to absorb more solar energy, much as you might feel hotter wearing a darker t-shirt in the sun than a white one. Photosynthesis itself produces a warming effect, a phenomenon known as non-photochemical quenching, and so the outer mantle, which contains the vast majority of the symbiotic algae, may be partially warmed by the activity of the symbionts!

More research is needed to confirm if this is true. As of yet, no researcher has ever stuck a temperature probe in multiple parts of a clam to see if the outside of it is indeed warmer than the inside. But until that day, it is interesting to think of how this would influence comparisons of diaries from the inner and outer layers of different bivalves. The effect is on the small side, so it doesn’t really mean one layer or the other should be preferred for future shell-based studies of climate change. But it could be an additional aspect to consider in the future as a way to record temperature differences within the body of an animal, and look into how those differences influence its overall level of stress.

Examples of juvenile smooth giant clams, T. derasa, that we’re growing at Biosphere 2. Photo by Katie Morgan.

So I hope this long explanation of my paper helps you to have a better idea of the work I did during my PhD thesis. There were other aspects to the paper that are too wonkish to get into here, particularly concerning the correlation we found between carbon and oxygen isotope ratios, but if you have questions or want a copy of the PDF, please message me! I have more clam papers in the pipeline, and my new postdoc at Biosphere 2 involves growing three species of giant clams in a controlled environment, where I hope to answer some of the physiological questions I mentioned above! But until then, stay hinged and happy as a clam (as much is possible in this chaotic time), and take comfort knowing there are colorful bivalves out there all at this very moment, harvesting sunlight for food and growing huge shells.