When a clam gets an offer it can’t refuse

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Tridacna maxima in Eilat, Israel

I study the giant clams, bivalves which can grow over three feet long and and are willingly “infected” by a symbiotic algae which they house in an altered stomach cavity. They provide their algae partners with nitrogen, a stable environment and even funnel light in their direction, and the algae happily share the fruit of their labor in the form of sugars. Imagine yourself swallowing algae, storing it in your gut and developing windows in your flesh to let light into your stomach. You’d never have to eat again. This is the growth hack that enables the giant clams to grow to unusual sizes. But it turns out that this lovely, beautiful partnership may not have started so peacefully. The algae may have made an offer the clam couldn’t refuse.

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Top left: normal mussel. Top right: heavily infected L-shaped shell opening. Bottom: view of an algae-infected mussel, including close up of pearls. From Zuykov et al. 2018

A team from University of Quebec recently discussed what such a fresh infection looks like in mussels and it ain’t pretty. The mussels basically have their shells and bodies overgrown by parasitic Coccomyxa algae, leaving its flesh bright green and transforming its shell from the classic elongated, acute angled margin typical of Mytilus mussels into a strange L-shaped overhang. The more algae are present in the mussel, the more extreme this deformity becomes. The researchers propose that this is no accident, but that as they move in, the algae also manipulates the biochemical pathway that the mussel uses to create its shell.

Mussels, like all bivalves, create their shells by laying down calcium carbonate in layers at the outer edge of the shell. The calcium is sourced from salts in the water column and the carbon primarily comes from carbonate ions also available in the water. This reaction is easier when the pH of the clam’s internal fluid is higher (less acidic), and that is exactly what the algae may assist with. Algae like all plants take in carbon dioxide to use in photosynthesis, and in doing so they increase the pH of the mussel’s body fluid,

The authors note that the region of shell which experiences abnormal thickening in the infected mussels is also the most exposed to light. The Coccomyxa algae may be causing runaway calcification of shell in the regions that they infect, and even may be directly assisting with the calcification in an additional way through the action of an enzyme called carbonic anhydrase, which is used in both their photosynthesis and in shell production (I won’t get into the nitty gritty of that reaction here). But the calcification of the mussels does appear to be in overdrive, as infected mussels were also observed to make pearls!

The algae’s photosynthesis may be assisting the mussel’s shell formation, though overall these are still quite unhealthy organisms of lower weight than their uninfected brethren. Still, Coccomyxa is known to form symbioses with lichens and mosses, so it could be that with enough generations of collaboration and a bit of evolution, the harmful algal infection could become a much more mutually beneficial partnership. It’s not so far fetched to imagine that an ancestor of today’s giant clams got a bad case of gastritis and decided to make the best of a bad situation. Making a deal with their invaders, they became greater than the sum of their parts and evolved to be the giant hyper-calcifiers we know today.

Lessons of the Condors

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Condor 606 was born in Big Sur and regularly flies back and forth between there and Pinnacles.

Visiting Pinnacles National Park the other day, we were lucky to spot California condors several times. Their wingspan can reach up to 3 m. Their graceful flight is a sight to behold as they ride the warm updrafts of between the pinnacles of rock in the park, with their primary feathers bending up like a conductor’s fingers. Condors are the only remaining members of the genus Gymnogyps, which once contained five species. Four are only known from fossil specimens and went extinct at the end of the Pleistocene (~12,000 years ago), but Gymnogyps once ranged across the Americas. As such, the California condor (Gymnogyps californianus) is a relic species; a survivor of a long but mostly extinct lineage.

By 1987, poaching, lead poisoning and habitat destruction had reduced the population to 27 individuals, of which 22 were captured and put into an emergency captive breeding program. In the thirty years since this project began, the population has increased to around 450 individuals. The program was expensive, painstaking and a massive undertaking. To this day, all captive-bred individuals are individually numbered and continually monitored. They even have a very adorable directory on the Pinnacles site where you can look each bird up by its number. We saw #606 and #463, and a couple others from farther away that we couldn’t read.

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Condor #463 was born at a breeding center in Idaho

 

California condors can be distinguished by the much more common turkey vultures by their underwing coloration, with large white patches at the front of the wings. I think that turkey vultures are fun to watch as they fly in circles over the highways searching for roadkill, but when you see a condor fly low over your head, it is awe-inspiring. Even the smaller juveniles have much larger, more pronounced heads than turkey vultures, and seem to fly even more effortlessly with a more gently curving V shape in their wings. We had stopped on the trail to discuss field markings for condors with another hiker when #463 soared over our heads, and we couldn’t help but jump for joy and hug each other at the privilege to see one of the famous condors ourselves.

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Turkey vulture just checking if we were feeling ok as we sat in the shade.

As an advocate for invertebrate conservation, I have been known to unfairly poke fun at the human tendency to focus on large, charismatic megafauna for conservation as opposed to smaller and less exciting species that may make up more of an ecosystem’s biomass, or represent a more important link in the local food chain. Pandas have used up billions of conservation dollars, yet they are kind of an evolutionary oddball with their poorly evolved guts that can barely digest their chosen bamboo food, and their infamous failure to successfully mate. Koalas have a similar story. We reformed tuna fishing not out of concern for the fish, but because of concern for dolphins getting mistakenly caught. We tend to put a lot of time and effort into conserving species that we consider cute, or cool, or awe-inspiring. The condors are important decomposers of carcasses, ranging over huge distances in their search for food and efficiently returning the nutrients of dead animals back to the ecosystem. But the public outcry motivating their rescue was greatly helped by the fact that these creatures are incredibly impressive megafauna. If the turkey vulture was critically endangered, it might not get the same funding supporting its conservation.

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This condor was very far away, gracefully circling the canyon.

But looking at the condors, my skepticism melted away and I was left with only gratitude that I was able to witness the grandeur of these beasts as they soared through the air; gratitude that I was able to see them myself and not only read about them in a book. I will never see a Steller’s Sea Cow, or a Great Auk, or a thylacine, or a dodo, because they disappeared before we realized what we were doing, and that extinction is a real and irreversible loss. Perhaps Gymnogyps won’t be around in 10,000 years. Their lineage originally evolved to feed on the giant carcasses from a collection of North American megafauna that is believed to have been hunted to death by humans (though that is a topic of endless debate). But if we hadn’t intervened to save the condors, we would have had to live with the guilt of knowing that we as a species committed the killing blow and did nothing to stop when we knew the reality of our crime. Instead we went to extreme lengths to save these unusual and majestic creatures because we feel empathy for them. As I watched condor 463 soar over my head, I felt relief, and pride, and hope. Success stories are important motivators for conservation, and I couldn’t help but think his wings were spelling out a “V” for victory as he flew away.

 

Behold, my new favorite creature

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Porcellanopagurus nihonkaiensis wearing a bivalve shell (Source)

Some of you may be aware that I harbor great affection for hermit crabs. I own terrestrial Caribbean hermits. Your mental image of hermits may feature a wardrobe of gastropod (snail) shells, which are by far the most common mollusk contractor they use to construct their homes, but as I’ve discussed, they actually have great flexibility in their choice of abode. It turns out that there is yet another option which hermits take advantage of as a mobile home: the flat shells of bivalves and limpets!

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What a cutie, wearing a limpet like a hat (source)

Porcellanopagurus nihonkaiensis is a species of marine hermit found off the coast of Japan. It uses the relatively flat, unenclosed shells of clams (and also limpets) for protection. Though lacking the 360 degree protection afforded by a snail shell, bivalve shell valves can be more plentiful in the marine environment, and being able to utilize a different shell frees them from competition with other hermit species which are specialized to work with snail shells.

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Without the shell! From Yoshikawa et al, 2018

Hermits typically have a long, soft coiled body which fits in where the snail’s body once was, using “uropodal endopods” (little feet at the end of their bodies) to hold themselves in the shell. Some species like Porcellanopagurus, however hold a bivalve or limpet shell on their backs, which still provides protective cover for their bodies. One recent study talked about their method of acquiring and holding the shell. They actually took a cute little series of pictures showing how the crab picks up a shell it with its front claws, places it on its back and then holds it in place with their fourth pair of legs. So now I’ve found a creature that combines my beloved clams and hermit crabs in one fun package. Gonna have to keep an eye out if I ever dive off of Japan!

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Play by play of how they pick up and carry away their new home (in this case a limpet shell) (source)

Revenge of the Clams

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Lampsilis showing off its convincing fish-like lure. Photo: Chris Barnhart, Missouri State.

Clams are traditionally the victims of the aquatic realm. With some exceptions, clams are generally not predatory in nature, preferring to passively filter feed. When they are attacked, their defenses center around their protective shell, or swimming away, or just living in a place that is difficult for predators to reach. They are picked at by crabs, crushed in the jaws of fish, and pried apart by sea stars. But some clams are sick of being the victims. They have big dreams and places to be. For these clams, the rest of the tree of life is a ticket to bigger and better things. These clams have evolved to live inside of other living things.

Pocketbook mussels, for example, have a unique problem. They like to live inland along streams but their microscopic larvae would not be able to swim against the current to get upstream. The mussels have adapted a clever and evil strategy to solve this problem: they hitch a ride in the gills of fish. The mother mussel develops a lure that resembles a small fish, complete with a little fake eyespot, and invitingly wiggles it to attract the attention of a passing fish. When the foolish fish falls for the trick and bites the mussel’s lure, it explodes into a cloud of larvae which then flap up to attach to the gill tissue of the fish like little binder clips. They then encyst themselves in that tissue and feed on the fish’s blood, all the while hopefully hitching a ride further upstream, where they release and settle down to a more traditional clammy life of filter-feeding stuck in the sediment.

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Very tiny Mytilus edulis living in the gills of a crab (Poulter et al, 2017)
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The tiny 2.5 mm long Mimichlamys varia, living on the leg of a crab (Albano and Favero, 2011)

 

 

Clams live in the gills of all sorts of organisms. Because they broadcast spawn, any passing animal may breathe in clam larvae which find the gills a perfectly hospitable place to settle. Sure, it’s a bit cramped, but it’s safe, well oxygenated by definition and there is plenty of food available. They also may just settle on the bodies of other organisms. Most of these gill-dwelling clams are commensal: that means that their impact on the host organism is fairly neutral. They may cause some localized necrosis in the spot they’re living, but they’re mostly sucking up food particles which the host doesn’t really care about. In addition, in crabs and other arthropods, these clams will get shed off periodically when the crab molts away its exoskeleton, so they don’t build up too heavily.

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Top: Kurtiella attached among the eggs of the mole crab. Bottom: aberrant Kurtiella living within the tissue of the crab (Bhaduri et al, 2018)

While being a parasite is often denigrated as taking the easy way out, it is actually quite challenging to pursue this unusual lifestyle. Parasitism has evolved a couple hundred times in 15 different phyla, but it is rare to find some organism midway in the process of becoming a true parasite. One team of researchers just published their observations of a commensal clam, Kurtiella pedroana, which may be flirting with true parasitism. These tiny clams normally live in the gill chambers of sand crabs on the Pacific coasts of the Americas. They attach their anchoring byssal threads to the insides of the chambers and live a comfortable life until the crabs molt, when they are shed away. The crabs mostly are unaffected by their presence, but the researchers noticed that some of the clams had actually burrowed into the gill tissue itself. This is an interesting development, because the clams would not be able to filter feed in such a location, so they must have been feeding on the crab’s hemocoel (internal blood). These unusual parasitic individuals are currently a “dead end” as they haven’t figured out how to get back out to reproduce, but if they ever do, they could potentially pass on this trait and become a new type of parasitic clam species. The researchers have potentially observed a rare example of an animal turning to the dark parasitic side of life, with some living in a neutral commensal way and other innovative individuals seeking a bit more out of their non-consensual relationship with their host crabs.  Considering the irritation that other bivalves suffer at the claws of pesky parasitic crabs, this seems a particularly sweet revenge.

 

 

 

 

The Snails that Farm

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Littoraria grazing on Spartina marsh grass. (source)

Us humans really like to talk up our skills at farming. And while it’s true that we have domesticated animals and plants to a degree not seen in other life forms, the act of nurturing and harvesting food is actually not really that special, and is broadly observed throughout the animal kingdom. Perhaps the most iconic invertebrate farmers are insects. Leaf-cutter ants, termites, and some beetles have been observed to actively cultivate fungus by gathering plant material to feed it, growing the fungus, protecting the fungus from competition, and then harvesting the fungus to feed themselves and their young. Ants are also known to keep livestock in the form of aphids, which they lovingly protect and cultivate for the sweet nectar they excrete. Such practices are called “high-level food production” because, like human farmers with their seeds and fertilizer, insects have evolved a highly complex symbiosis with their fungus. The fungus has shaped their behavior as much as the ants cultivate the fungus. 

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The marsh periwinkle Littoraria irrorata (source)

Less well understood is the “low-level food production” that may occur more broadly throughout the tree of life. There is less direct evidence of such behavior because it is more indirect and less specialized than high-level food production, but it may be equally advantageous for the cultivator and the cultivated. One study published in 2003 uncovered a simple but powerful relationship between marsh periwinkles of the genus Littoraria and fungus which they cultivate and harvest.

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Close-up of a snail’s radula (source)

Marsh periwinkles are small and not particularly charismatic creatures. Like many snails, they are grazers with a shell, a fleshy foot and a rough, abrasive organ called a radula which they use like sandpaper to graze on pretty much whatever they can get into. Snails are not known as picky eaters. But researcher Brian Silliman of Brown University and Steven Newell of University of Georgia noticed that these innocuous snails regularly undertake the risky, low reward activity of grazing above the water on the blades of swamp grass, stripping off the surfaces of the blades of grass. The researchers were confused why the snails would expose themselves to predation and the harmful open air for such a low-nutrition food.

 

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A typical snail farm, complete with liberally applied fertilizer. Yum.

They discovered that the snails were investing in the future. By stripping away the protective surface of the swamp grass blades and liberally fertilizing the surface of the grass with their droppings, the periwinkles are ensuring that the swamp grass will be infected with an active and very prolific fungal infection. The fungus, unlike the plant it lives on, is of high nutritional value. The researchers demonstrated the active partnership between the snails and fungus by conducting caged experiments where they showed that snails which grazed on grass but not the resulting fungus did not grow as large as snails which were allowed to return and chow down on the fungus. The fungus loves this deal as well. They grow much more vigorously on grass that is “radulated” (rubbed with the snail’s sandpapery radula) than uninjured grass. The fungus grows even faster if the snails are allowed to deposit their poop next to the wounds. The researchers found that this same relationship applies at 16 salt marshes along 2,000 km of the Eastern Seaboard.

The periwinkles don’t really know what they’re doing. They aren’t actively planting fungus and watching proudly like a human farmer as their crop matures. But over millions of years, the snails have been hard-wired to practice this behavior because it works. Snails that abrade a leaf of swamp grass, poop on the wound and come back later to eat the yummy fungus do a lot better than snails which just stick to the safe way of life below the surface of the water. The fungus loves this relationship too. The only loser is the swamp grass, which the researchers unsurprisingly found grows much more slowly when infected with fungus. But marsh grass is the largest source of biomass in swamp environments, and the snails that partner with fungus are able to more efficiently use this plentiful but low-nutrient food source, to the extent that it is now the dominant way of eating for swamp periwinkles on the East Coast of the US, and probably in a lot more places too. The researchers noted that there are likely far more examples of low-level food production that we simply haven’t noticed.

Since this work was published, other teams have discovered that some damselfish like to farm algae, fiddler crabs encourage the growth of mangrove trees, and even fungus get in on the action of farming bacteria. We love to talk up our “sophisticated” high-level food production techniques, but such relationships probably got started at a similarly low level. Our activities as hunter-gatherers encouraged the growth of certain organisms, we stumbled upon them, ate them, kept doing what we were doing and eventually our behavior developed into something more complex. Next time you see a snail munching its way up a blade of grass, consider to yourself whether it knows exactly what it’s doing. Come back later to see the fruits of its labor.

What is Conservation Paleobiology?

In undergrad, I felt like my school and internship were training me to be two different types of researcher. At USC, I was majoring in Environmental Studies with an emphasis in Biology. It was essentially two majors in one, with a year of biology, a year of chemistry, a year of organic chem, a year of physics, molecular biology, biochemistry, etc. On top of that, I took courses on international environmental policy and went to Belize to study Mayan environmental history. Meanwhile, I was working at Jet Propulsion Laboratory in Pasadena researching trends in historical rainfall data. I loved both sides of my studies, but felt like neither was exactly hitting the spot of what I would want to spend my career researching. I love marine biology but am not particularly interested in working constantly in the lab, looking for expression of heat shock protein related genes or pouring stuff from one tube into another. On the other hand, I was fascinated by the process of untangling the complex history of rainfall in California, but I yearned to relate this environmental history to the reaction of ecological communities, which was outside the scope of the project.

During my gap year post-USC, I thought long and hard about how I could reconcile these disparate interests. I read a lot, and researched a bunch of competing specialized sub-fields. I realized that paleobiology fit the bill for my interests extremely well. Paleobiologists are considered earth scientists because they take a macro view of the earth as a system through both time and space. They have to understand environmental history to be able to explain the occurrences of organisms over geologic time. I really liked the idea of being able to place modern-day changes in their geologic context. What changes are humans making that are truly unprecedented in the history of life on earth?

But it doesn’t have to be all zoomed out to million-year processes. A growing sub-field known as Conservation Paleobiology (CPB) is focused on quantifying and providing context of how communities operated before humans were around and before the agricultural and industrial revolutions, in order to understand the feasibility of restoration for these communities in this Anthropocene world. Sometimes, this means creating a baseline of environmental health: how did oysters grow and build their reefs before they were harvested and human pollution altered the chemistry of their habitats? I’m personally researching whether giant clams grow faster in the past , or are they reacting in unexpected ways to human pollution? It appears that at least in the Gulf of Aqaba, they may be growing faster in the present day. Such difficult and counterintuitive answers are common in this field.

Sometimes, CPB requires thinking beyond the idea of baselines entirely. We are realizing that ecosystems sometimes have no “delicate balance” as described by some in the environmental community. While ecosystems can be fragile and vulnerable to human influence, their “natural” state is one of change. The question is whether human influence paves over that prior ecological variability and leads to a state change in the normal succession of ecosystems, particularly if those natural ecosystems provide services that are important to human well-being. In a way, the application of paleobiology to conservation requires a system of values. It always sounds great to call for restoring an ecosystem to its prior state before humans. But if that restoration would require even more human intervention than the environmental harms which caused the original damage, is it worth it? These are the kinds of tricky questions I think are necessary to ask, and which conservation paleobiology is uniquely suited to answer.

At the Annual Geological Society of America meeting in Seattle this year, the Paleo Society held the first-ever Conservation Paleobiology session. The room was standing room only the whole time, investigating fossil and modern ecosystems from many possible angles. This field is brand new, and the principles behind it are still being set down, which is very exciting. It’s great to be involved with a field that is fresh, interdisciplinary, and growing rapidly. I look forward to sharing what my research and others find in the future.

Hard shells aren’t actually that hard to make (yet)

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One of the Antarctic bivalve species featured in this study. Source

Like all organisms, bivalves have a limited budget governing all aspects of their metabolism. If they put more energy into feeding (filtering the water), they can bring in a bit more food and therefore fuel more growth, but sucking in water takes energy as well, particularly if there isn’t enough food to be filtered out. Bivalves also periodically have to grow gonadal material and eggs for reproduction, expand their body tissue (somatic growth) and of course, grow their shells (made of of a mineral called carbonate). All of these expenditures are items in a budget determined by the amount of energy the bivalve can bring in, as well as how efficiently they can digest and metabolize that energy.

If a bivalve is placed under stress, their scope for growth (the max amount of size increase per unit time) will be decreased. Because they’re cold-blooded, bivalves are limited by the temperature of their environment. If temperatures are low, they simply can’t sustain the chemical reactions required for life at the same rate that endotherms like us can. They also may have to shut their shells and stop feeding if they’re exposed by the tide, or are tossed around by a violent storm, or attacked by predators or toxins from the algae that they feed on.

When their budget is lower, they have to make painful cuts, much like a company lays off employees if their revenues are lower. The question is which biological processes get cut, and when? My first chapter (submitted and in review) has settled temperature being the primary control on seasonal shell growth. Bivalves at high latitudes undergo annual winter shutdowns in growth, which create the growth bands I use to figure out their age, growth rate, etc. We’d be a lot closer to accurately predicting when bivalves suffer from “growth shutdowns” if we had hard numbers on how much energy they actually invest in their shells. A new study from a team led by Sue-Ann Watson of James Cook University attempts to do just that.

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Diagram relating the growth bands of Antarctic soft-shelled clam with a chart showing the widths of those bands. Source

Collecting a database of widths for the annual growth rings of bivalve and gastropod (snail) species from many latitudes, Watson and her team were able to get a global view of how fast different molluscs grow from the equator to the poles. Because the unit cost of creating carbonate is determined by well-understood chemistry, they were able to create an equation which would determine the exact number of Joules of energy used for every bivalve to grow their shells.

They still needed a total energy budget for each species, in order to the percent of the energy budget that each bivalve was investing in their shells. They drew on a previous paper which had calculated the standard metabolic rates for each species by carefully measuring their oxygen consumption. We could do the same for you if you sat in a sealed box for an extended period of time while we measured the exact amount of oxygen going in and CO2 going out. Dividing the amount of energy needed to grow the shell by the total amount of energy used in the organism’s metabolism would give us a percent of total energy that the bivalve dedicates to adding growth layers to its shell.

That number is…not very large. None of the bivalves or gastropods they looked at put more than 10% of their energy into shell growth, and bivalves were the lowest, with less than 4% of their energy going into their shell. Low-latitude (more equatorward) bivalves have the easiest time, putting less than 1% of their energy into growth but getting way more payoff for that small expenditure. High-latitude polar bivalves have to work harder, because the lower temperatures they experience mean the reactions needed to create their shells are more expensive. In addition, most of that energy is going into the protein-based “scaffolding” that is used to make the shell, rather than the crystals of carbonate themselves. Organisms right now don’t have to put a whole lot of effort into making their protective shells, which could explain why so many organisms use shells for protection. That is good, because if shells were  already breaking the bank when it came to the bivalves’ growth budget, they wouldn’t have a lot of room to invest more energy in the face of climate change. Unfortunately, as the authors note, these budgets may need to change in the face of climate change, particularly for bivalves at the poles. As the oceans grow more acidic due to human CO2 emissions, growing their shells will start to take up more of their energy, which is currently not a major part of their budget.

A cold-water ecosystem dominated by Antarctic scallops. Source

Right now, the cold waters of the poles are refuges for organisms that don’t deal well with shell crushing predators. As polar regions warm, such predators will begin to colonize these unfamiliar waters. Polar bivalves may encounter the double whammy of needing to spend more energy to make the same amount of shell, but also find that it is no longer enough to protect them from predators that easily crack open their protective coverings.

I found this study to be an elegant and thoughtful attempt to fill in a gap in our current understanding of how organisms grow and how energy budgets are influenced by environmental variables like temperature. I instantly downloaded the paper because it answered a question that has long been on my mind. Maybe can sneak its way into my manuscript during the review process!

You are Isotopes (Part III)

This is the third part of a series about isotopes and why they’re useful and interesting to scientists.

Isotopes are the flavors of elements. And because our universe is made up of atoms of elements, every object can be thought of as a delicious smoothie of flavors. Scientists like me are trying to reverse engineer those mixtures and pick out individual tastes, in order to answer questions about our world.

For example, I work with giant clams. These guys build enormous shells made of a mineral called calcium carbonate: CaCO3. That means that every molecule in a clam’s shell contains a calcium atom, a carbon and three oxygens. But as you might know from reading the previous entries in this series, not all of those atoms are the same. They are a mixture of different flavors. We have some carbon-12 and 13 in there (so named for their atomic weights), and some oxygen-16, 17 and 18. Here I’m focusing on the stable isotopes, which are not radioactive and are called “stable” because they’re not going to self-destruct. There are radioactive isotopes in there too, but I don’t use those nearly as often in my work.

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Officer, this is a pile of giant clam powder, I swear!

I am measuring stable isotopes of carbon and oxygen in my shell samples. To do this, I take a sample of powder, grind it up, weigh it, and put it into tiny little cups. We only need a very small sample: about 50 micrograms of shell material. A typical pill of tylenol contains over 300 mg of active ingredient, so about 6,000 of my samples will fit in a single tylenol regular strength pill, if you suddenly decided you needed a giant clam prescription.

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Simplified representation of what’s happening in a mass spec. Source

This tiny sample is one of thirty that I can measure at a time. Those samples are reacted with acid and the CO2 gas that is released as a result of the reaction can be processed by a machine called a mass spectrometer. The mass spec, which is in the Stable Isotope Laboratory in my building, ionizes the molecules in that gas (gives them a bit of electric charge) and then those ions are flung through an electromagnetic field. That beam of charged gas is flung around a curve. That curve is where the magic of making a mass spectrum happens.

Think of the atoms in the CO2 gas from my sample as a bunch of racecars exiting the straightaway and starting around the curve on the racetrack. Only these racecars vary in weights. And the race organizers have greased the track at the curve so that they fling into the sides of the track when they try to turn. As the racecars fling into the sides of the track, they will separate according to their mass. The lighter cars will be able to make it further around the curve before they meet their demise because they have less inertia forcing them forward, whereas the SUVs in the race will barrel forward straight into the sides of the track. At the end, you have a spectrum of racecars poking out of the walls of the track, with SUVs first, then the coupes, then the compact cars and then the motorcycles, which almost made it around the bend, but not quite. Atoms in the mass spectrometer act the same way, and we measure how many collisions happen along each point of the bend in order to not only “weigh” the sample of gas, but also figure out how many molecules of each weight there are!

It turns out that it is quite difficult to measure the exact number of atoms of a particular isotope in gas, however. It is much more economical and feasible for the purposes of most researchers to simply compare our mass spectrum to the results from a standard. Much like there is a literal standard kilogram and standard meter in a lab somewhere in France which is used to keep track of how much mass is actually in a kilogram, there is a standard used by all researchers like me to describe our samples of carbonate.

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A collection of belemnite fossils from the Pee Dee formation, similar to the one used for the PDB standard. Source

The most common standard used is from a belemnite fossil from the Pee Dee formation in North Carolina. Belemnites are extinct squid-like creatures that formed an internal shell, and one of those internal shells was fossilized, unearthed by a researcher and ground up to become the reference for all other researchers following. Samples of the carbonate in its fossil had more carbon-13’s per unit mass than most other fossil specimens known.  Almost everything you measure will be “lighter” in terms of carbon, because carbon-12 is naturally so common on our planet.

Scientists needed a convenient way to put a number on this, so a simple formula was developed which would allow us to quickly communicate to each other how isotopically “heavy” or “light” a particular sample is in comparison to the Pee Dee Belemnite. The formula isn’t that important for our purposes but the units of its output are in parts per thousand, or “per mil” for short (same idea of how we shorten parts per hundred to “percent”).

The symbol for per mil is a percent sign with an extra little loop at the end: ‰. To make the shorthand complete, we also need to note that this is how much the carbon-13 to carbon-12 isotope ratio of a sample differs from the Pee Dee Belemnite. We do so, we use the Greek delta symbol (δ), commonly used in science and math to represent “difference or change from.” So a sample that has a carbon-13 to carbon-12 ratio which is 20 parts per thousand less than that of the Pee Dee Belemnite is written -20 ‰ δ13CPDB. There are other samples that can be used as well, including Standard Mean Ocean water (SMOW), and the Vienna Pee Dee Belemnite (VPDB). It’s important to note which you are using so that people know the scale of your measurement!

Phew, hopefully that didn’t confuse the hell out of you! Next time, I’ll talk about how different δ13C (and for oxygen isotopes, δ18O) can tell us different details about the life of an organism. Here’s a cute gif of a scallop as a chaser after all that science you read.

Local thief spotted in my backyard

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The brown-headed cowbird (Molothrus ater) is a notorious thief. This is merely a young thief which I observed in my yard; it’s still in training, but when it is an adult it will look more like this:

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Source: Wikipedia

It will search out a nest of an unsupecting bird, perhaps a sparrow or other songbird, and it will sneakily lay an egg when the parents are away. The egg will hatch and the other birds will raise it as their own. This practice is known as brood parasitism. The unsuspecting sap that raises the young cowbird will unfortunately feed its own young proportionately less and its fitness will suffer as a result.

Sometimes, the victim figures out that one of the eggs isn’t its own and disposes of it. If the cowbird sometimes returns to the scene of the crime and if it discovers its egg is missing, it may destroy the nest in a darkly Darwinian form of payback called “mafia behavior.” Sometimes, a cowbird egg is an offer you can’t refuse.