Why eating clams sometimes makes us sick (Part 1 of 2)

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Is eating these a gamble? Science can help improve our odds!

I am often asked if I eat clams. The answer is yes: while I love to observe live clams and appreciate their abilities, I will eat a good clam chowder or plate of grilled scallops if presented with the chance. While I’m generally not a fan of super fishy-tasting foods, I eat bivalves with a clear conscience because farmed mollusks represent a super sustainable way to get protein! However, as many of us have learned the hard way, shellfish can sometimes produce unwanted results later after the meal, if the animals are contaminated with food poison. Eating such “bad” clams can produce a spectrum of food poisoning symptoms ranging from vomiting and diarrhea to memory loss to even paralysis and death.

Humans have known the hazards of eating shellfish for a very long time. It has been suggested that the ban on shellfish present in kosher and halal dietary rules arose as a preventative measure to protect from food poisoning (though eating fish, land animals and even vegetables can poison people in numerous ways as well). Studies of oysters have determined that ancient peoples of modern day Georgia from 5000 years before present selected their season of harvest based partially on knowledge of the seasons when such poisoning was most prevalent in their area.

How and why does this happen, and what can we do to prevent it? It’s a billion-dollar question, because when flare-ups of shellfish food poisoning happen, they are hugely costly to fishermen and the food industry, costing millions of dollars a year in lost business when fisheries are forced to shut down and products are recalled. Such events are increasing in frequency and severity. Which makes it all the more strange that these shellfish poisoning events are not the fault of the bivalves per se, but rather what they’re eating.

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Note: people generally get annoyed when you start to point out the body parts of the oyster they’re about to swallow whole. Source

Almost all bivalves are filter-feeders, using their gills to gather small passing food particles, which they then either ingest or discard based on the quality of the food item. Clams are cows crossed with Brita filters, and for many species of clams which we eat, the reason they do all this filtration is to find phytoplankton food. Phytoplankton are microscopic algae suspended by ocean currents that make their living from photosynthesis. They are a hugely plentiful and high-quality food item, making up a huge amount of the biomass available in the ocean. Like plant-life on land, phytoplankton are highly seasonal in their appearance, rising and falling in abundance in periodic “bloom” events.

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Aerial view of a red tide off the Texas coast. Source: NOAA

But as Spongebob Squarepants taught us, plankton are not always peaceful. Many types of algae produce toxic compounds which may be integrated into the body parts of bivalves that eat them. Scientists call the blooms of algae which produce toxins “Harmful Algal Blooms” (HABs), and such events are growing in frequency and cause huge harm to marine life and sicken thousands of people per year. There are many algae species which cause HABs all around the world, sometimes visible as “red tides,” but not always. When HABs occur, they can lead to mass deaths of higher animals in the food chain that feed on clams such as marine mammals and seabirds. In fact, HABs are at their most dangerous to humans when they catch us by surprise.

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Who me? I’d never!
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Microscope view of the toxic dinoflagellate Karenia. Source: NOAA

When humans eat bivalves which have been dosed with such marine toxins, many types of poisoning can occur. Brevetoxin is produced by a type of dinoflagellate phytoplankton Karenia as well as other species, and when humans are exposed, we can suffer from Neurotoxic Shellfish Poisoning, which causes vomiting, diarrhea and even neurological effects like slurred speech. Saxitoxin is produced by a variety of plankton species including dinoflagellates and freshwater cyanobacteria. When ingested in clams (such as the butter clam Saxidomus which gave it its name), fish or other animals, it can cause Paralytic Shellfish Poisoning, a sometimes fatal syndrome which shuts down nerve signaling, leading to temporary paralysis.

So we know it’s bad for humans to ingest these toxins. What is it doing to the clams? Oddly enough, some types of toxins like saxitoxin are not that harmful to the clams or other plankton eating animals, allowing them to accumulate huge amounts in their bodies with little ill effect. Its presence does not seem to influence their feeding behavior much, or their growth after exposure. Its status as a neurotoxin in mammals might be a total chemical and evolutionary coincidence, as researchers suggest that it may actually serve as a signal in some part of the algae’s mating cycle. This also may be the case for brevetoxin, which appears to be produced when Karenia is under environmental stress. But there is not much agreement in the HAB and aquaculture research fields, because there are many types of algae, which may produce their toxins for many reasons, and it is very hard for us to zoom in to the scale of the microbe and out to the scale of the ecosystem at the same time, to find any kind of universal evolutionary role of these toxins. Some researchers insist that some bivalves are influenced negatively by brevetoxin, but only at the juvenile stage during major bloom events. The effects of the toxin may only influence certain species, or only become significant if the toxin reaches the digestive tract of the bivalve. Overall, research into impact of HABs on clams is still a topic of active research, and the idea that the microbes produce these toxins to defend against bivalve predators is definitely not a slam-dunk, easily proven hypothesis. While some clams are negatively affected by the toxins, it is not consistently observed across species in a open-and-shut way, and it can be a subtle effect to observe and quantify scientifically.

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Karenia to mammals: Oops!

The more I read about this stuff, the more shocked I am at the incredible complexity of marine algae and their toxins. I only started reading about them trying how to to understand how they influence bivalves. I was hoping to find some evidence of their effects on bivalve growth that I could apply back in time in fossil shells to understand the historical occurrence of HAB events. It’s important to understand HABs because they hurt people, cost our society a lot of money and if we understand how to avoid them, we can help minimize such impacts in the future as HABs continue to become more common. In my next post, I’ll talk about some of the ways that researchers have come up with to measure and monitor HABs, so that we can eat clams as safely as possible.

A Make-Up Presentation!

Hi colleagues! Several weeks ago, I was supposed to present a talk at GSA’s annual meeting in Phoenix at the session “Advances in Ocean and Climate Reconstructions from Environmental Proxies”, but I shattered my wrist in a scooter accident the night before and was in emergency surgery during my talk time. So instead I’ve uploaded my talk with voice-over to Youtube! The whole video is about 15 minutes. You can view it above. Feel free to comment on this post or email me if you have questions!

This work is currently in the last stretch of drafting before submission, but I also discuss some ongoing research and am always open if you have your own ideas for collaborations!

Correction: we are working with geophysicists to understand the shell transport mechanism.

These are the references mentioned at the end:

Crnčević, Marija, Melita Peharda, Daria Ezgeta-Balić, and Marijana Pećarević. “Reproductive cycle of Glycymeris nummaria (Linnaeus, 1758)(Mollusca: Bivalvia) from Mali Ston Bay, Adriatic Sea, Croatia.” Scientia Marina 77, no. 2 (2013): 293.

Glycymeris nummaria (Linnaeus, 1758).” 2019. World Register of Marine Species. 2019. http://www.marinespecies.org/aphia.php?p=taxdetails&id=504509#distributions.

Grossman, Ethan L., and Teh-Lung Ku. 1986. “Oxygen and Carbon Isotope Fractionation in Biogenic Aragonite: Temperature Effects.” Chemical Geology: Isotope Geoscience Section 59: 59–74.

Gutierrez-Mas, J. M. 2011. “Glycymeris Shell Accumulations as Indicators of Recent Sea-Level Changes and High-Energy Events in Cadiz Bay (SW Spain).” Estuarine, Coastal and Shelf Science 92 (4): 546–54.

Jones, Douglas S., and Irvy R. Quitmyer. 1996. “Marking Time with Bivalve Shells: Oxygen Isotopes and Season of Annual Increment Formation.” PALAIOS 11 (4): 340–46.

Mienis, Henk, R. Zaslow, and D.E. Mayer. 2006. “Glycymeris in the Levant Sea. 1. Finds of Recent Glycymeris insubrica in the South East Corner of the Mediterranean.” Triton 13 (March): 5–9.

Najdek, Mirjana, Daria Ezgeta-Balić, Maria Blažina, Marija Crnčević, and Melita Peharda. 2016. “Potential Food Sources of Glycymeris nummaria (Mollusca: Bivalvia) during the Annual Cycle Indicated by Fatty Acid Analysis.” Scientia Marina 80 (1): 123–29.

Peharda, Melita, Marija Crnčević, Ivana Bušelić, Chris A. Richardson, and Daria Ezgeta-Balić. 2012. “Growth and Longevity of Glycymeris nummaria (Linnaeus, 1758) from the Eastern Adriatic, Croatia.” Journal of Shellfish Research 31 (4): 947–51.

Reinhardt, Eduard G, Beverly N Goodman, Joe I Boyce, Gloria Lopez, Peter van Hengstum, W Jack Rink, Yossi Mart, and Avner Raban. 2006. “The Tsunami of 13 December AD 115 and the Destruction of Herod the Great’s Harbor at Caesarea Maritima, Israel.” Geology 34 (12): 1061–64.

Royer, Clémence, Julien Thébault, Laurent Chauvaud, and Frédéric Olivier. 2013. “Structural Analysis and Paleoenvironmental Potential of Dog Cockle Shells (Glycymeris glycymeris) in Brittany, Northwest France.” Palaeogeography, Palaeoclimatology, Palaeoecology 373: 123–32.

Sivan, D., M. Potasman, A. Almogi-Labin, D. E. Bar-Yosef Mayer, E. Spanier, and E. Boaretto. 2006. “The Glycymeris Query along the Coast and Shallow Shelf of Israel, Southeast Mediterranean.” Palaeogeography, Palaeoclimatology, Palaeoecology 233 (1): 134–48.

How does a scallop swim?

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Scallops spooked by divers’ lights and fleeing en masse to filter somewhere else

The ocean is a place of constant dynamic movement. Fish use their fins to push water away from themselves, and because every action has an equal and opposite reaction, they therefore move forward. Some cephalopods use jet propulsion, constricting their mantle cavity to push water out through siphons, allowing them to jet forward like a deflating balloon. And other life forms sail the seas on constantly moving currents , indirectly harnessing the power of the sun and earth.

Bivalves are a fairly sedentary bunch by comparison. While most bivalves have a planktonic larval form, when they settle they are constrained to a fairly small area within which they can burrow or scramble around with their muscular feet.

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But some bivalves have evolved to move at a quicker rate. The most famous swimming bivalves are the scallops, which have evolved to use jet propulsion, similar to their very distantly related cephalopod relatives. But unlike the cephalopods, scallops evolved to use their hinged shells to aid this process!

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Notice the expelled water disturbing the sediment below the scallop as it “claps” its way forward!

Many filter-feeding bivalves use their shell valves as a biological bellows to pull in water for the purposes of sucking in food, or even to aid in digging, but scallops have developed another use for this activity, to enable propulsion. Scallops draw in water by opening their valves to create a vacuum which draws in water to their sealed mantle cavity. They then rapidly close their valves using their strong adductor muscles to pull them together, which pushes the water back through vents in the rear hinge area, propelling the scallop forward.

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Don’t panic if a scallop swims toward you. They can see, but not super well. This one is just confused.

Using this strategy, scallops can evade predators and distribute themselves to new feeding sites. It’s a surprisingly effective swimming technique, with the queen scallop able to move 37 cm/second, or over five body lengths per second! Michael Phelps would have to swim at nearly 35 km/h to match that relative speed (his actual highest speed is around 1/3 of that). I’m sure sustaining that speed would be tiring for Mr. Phelps, though, and it’s the same for scallops, only using their swimming for short-distance swims.

(video from Supplemental Materials of Robertson et al. 2019)

A recent paper from a team in Switzerland just came out describing an effort to engineer a robot which imitates the scallop’s elegant and simple swimming method. The resulting totally adorable “RoboScallop” closely imitates the design of a scallop, using a pair of hinged valves with rear openings to allow the movement of water backward. The internal cavity is sealed by a rubber membrane draped across the front so that all water is forced through these rear vents when the Roboscallop snaps shut.

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Diagram from the Roboscallop paper (from Robertson et al. 2019)

As seen in the diagram above, the rhythm and relative velocity of opening vs closing is important to make sure the RoboScallop actually moves forward. If the scallop opened as quickly as it closed, it would just rock back in forth. It instead opens slowly so that it does not draw itself backward at the same rate that it can push itself forward. The researchers had to do quite a bit of calibration to get these rates right (equating to about 1.4 “claps” per second), but once they did, they ended up with a RoboScallop that can generate about the same force of forward movement (1 Newton) as a real scallop (1.15 Newtons), and similar rates of speed.

This paper really fascinated me because it is merely the latest in a long line of successful engineering projects imitating the ingenuity of evolution. Other marine robots have been made which emulate the locomotion of fish, manta rays, sea snakes and other forms of swimming. And now we have a clam! Let me know when I can buy one to play with in my pool.

The clams that sail the seas on rafts of kelp

 

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The streamlined shells of Gaimardia trapesina. Source: New Zealand Mollusca
Bivalves are not known as champion migrators. While scallops can swim and many types of bivalves can burrow, most bivalves are primarily sessile (non-moving on the ocean bottom). So for many bivalves, the primary method they use to colonize new territories is to release planktotrophic (“plankton-eating”) larvae, which can be carried to new places by currents and feed on other plankton surrounding them. Many bivalves have broad distributions because of their ability to hitchhike on ocean currents when they are microscopic. They don’t even pack a lunch, instead eating whatever other plankton is around them. But once they settle to grow, they are typically fixed in place.

Not all bivalves have a planktotrophic larval stage, though. Larvae of lecithotrophic bivalve species (“yolk-eaters”) have yolk-filled eggs which provide them with a package of nutrition to help them along to adulthood. Others are brooders, meaning that rather than releasing eggs and sperm into the water column to fertilize externally, they instead internally develop the embryos of their young to release to the local area when they are more fully developed. This strategy has some benefits. Brooders invest more energy into the success of their offspring and therefore may exhibit a higher survival rate than other bivalves that release their young as plankton to be carried by the sea-winds. This is analogous to the benefits that K-strategist vertebrate animals like elephants have compared to r-strategist mice: each baby is more work, and more risky, but is more likely to survive to carry your genes to the next generation.

Brooding is particularly useful at high latitudes, where the supply of phytoplankton that is the staple food of most planktrophic bivalve larvae is seasonal and may limit their ability to survive in large numbers. But most of these brooding bivalves stay comparatively local compared to their planktonic brethren. Their gene flow is lower on average as a result, with greater diversity in genetic makeup between populations of different regions. And generally, their species ranges are more constricted as a result of their limited ability to distribute themselves.

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A bunch of G. trapesina attached to kelp. Notice the hitchhiking clams have in turn had hitchhiking barnacles attach to them. Freeloaders on freeloaders! Source: Eleonora Puccinelli

But some brooding bivalves have developed a tool to have it all: they nurture their young and colonize new territories by sailing the seas using kelp rafts. The clam Gaimardia trapesina has evolved to attach itself to giant kelp using long, stringy, elastic byssal threads and a sticky foot which helps it hold on for dear life. The kelp floats with the help of gas-filled pneumatocysts, and grows in the surge zone where it often is ripped apart or dislodged by the waves to be carried away by the tides and currents. This means that if the clam can persist through that wave-tossed interval to make it into the current, it can be carried far away. Though they are brooders, they are distributed across a broad circumpolar swathe of the Southern Ocean through the help of their their rafting ability. They nurture their embryos on specialized filaments in their bodies and release them to coat the surfaces of their small floating kelp worlds. The Southern Ocean is continuously swirling around the pole due to the dominance of the Antarctic Circumpolar Current, which serves as a constant conveyor belt transporting G. trapesina across the southern seas. So while G. trapesina live packed in on small rafts, they can travel to faraway coastlines using this skill.

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The broad circumpolar distribution of G. trapesina. Source: Sealifebase

The biology of G. trapesina was described in greater detail in a recent paper from a team of South African researchers led by Dr. Eleonora Puccinelli, who found that the clams have evolved to not bite the hands (kelp blades?) that feed them. Tests of the isotopic composition of the clams’ tissue shows that most of their diet is made up of detritus (loose suspended particles of organic matter) rather than kelp. If the clams ate the kelp, they would be destroying their rafts, but they are gifted with a continuous supply of new food floating by as they sail from coast to coast across the Antarctic and South American shores. But they can’t be picky when they’re floating in the open sea, and instead eat whatever decaying matter they encounter.

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Falkland Islands stamp featuring G. trapesina. Source.

The clams are small, around 1 cm in size, to reduce drag and allow for greater populations to share the same limited space of kelp. Their long, thin byssal threads regrow quickly if they are torn, which is a useful skill when their home is constantly being torn by waves and scavengers. Unlike other bivalves, their shells are thin and fragile and they do not really “clam up” their shells when handled. They prioritize most of their energy into reproduction and staying stuck to their rafts, and surrender to the predators that may eat them. There are many species that rely on G. trapesina as a food source at sea, particularly traveling seabirds, which descend to pick them off of kelp floating far from land. In that way, these sailing clams serve as an important piece of the food chain in the southernmost seas of our planet, providing an energy source for birds during their migrations to and from the shores of the Southern continents.

 

Weird Clam Profile: Hammer Oysters

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Malleus malleus from Indonesia. Source: Wikipedia

Oyster. Reading that word, you probably formed an image in your mind of a rough-shelled creature with a shiny mother-of-pearl (nacreous) inside that someone pulled out of some silt in an estuary. And yes, that’s what most oysters look like. Some oysters are of additional economic value through their creation of pearls. These pearl oysters have long, straight hinge lines and live in the tropics in and around coral reefs.

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A pearl oyster. See the straight hinge? Source: Pearl Paradise on Flickr

The hammer oysters are another sort of oyster, not of the Ostreidae family that includes most of the bivalves we think of as oysters, but still closely related and in its own family, the Malleidae. Malleus is the latin word for hammer, and the most distinctive genus of hammer oysters indeed look just like a hammer sitting on the seafloor.

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In a typical life position in a seagrass bed. Notice all the algae, anemones and other encrusting creatures freeloading off the hammer oyster’s hard work. Source: Ria Tan on EOL

What the…that thing’s alive? How does that even work? This is an oyster? That’s how I imagine the first scientist to discover the hammer oyster reacting. Because they are weird and rather incomprehensible-looking. But when you know the way they live, it makes more sense.

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There is a small area of nacre (mother of pearl) in the area near the rear of the interior. Source: Archerd Shell Collection

The hammerhead part of the oyster is just a super elongated hinge. The creature has a long, straight hinge like other oysters, but it has evolved to instead have a relatively narrow set of valves attached to that ridiculously overbuilt hinge. Like other oysters, they secrete byssal threads from their backside to attach themselves to the bottom. The narrow valves commonly poke up out of sandy bottoms in tropical waters nearby coral reefs. They do particularly well in seagrass beds, and often live in large colonies similar to other oysters.

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Shell collectors seek out hammer oyster shells which have other bivalves attached. Here is a thorny oyster living on top of Malleus. Two for one! Source

The absurd hinge helps these creatures to stay anchored into the sediment, but also serves as “wings” that help it avoid sinking into the sediment over time. One thing us humans don’t realize sitting on sand is that it actually acts like a liquid. Over time, if we sat on wet sand, we would likely begin to sink in unless we spread out our arms and legs to increase our surface area. In the ocean, all sand is quicksand. Different organisms have different strategies to avoid being engulfed by the sediment they live on, and the hammer oyster has had good success with its strategy. It doesn’t care that you think it looks weird. It just sits there, filtering water for passing food particles and plankton. It’s very good at it, has been perfecting the strategy for over 250 million years, and doesn’t need your smartass remarks, thank you very much.

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Another shot of a happy hammer oyster doing what it does best, in a seagrass bed near Singapore. Source: Wild Singapore on iNaturalist

The boring giant clam is anything but.

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Tridacna crocea, bored into a coral head on a reef in Palau

There are many types of giant clam. Not all of them are giant; the boring giant clam, Tridacna crocea, only grows to 10 cm long or so. The boring giant clam is not so named because it’s dull; its main skill is its ability to bore into the coral of its coral reef home and live with its entire shell and body embedded in the living coral. They sit there with their colorful mantle edge exposed from a thin opening in the coral, harvesting energy from sunlight like the other giant clams. When disturbed by the shadow of a human or other such predator, they retract their mantle and close their shell, encased by an additional wall of coral skeleton. It’s a clever defensive strategy, and they are some of the most numerous giant clams in many reefs in the Eastern and Southern Equatorial Pacific.

But it’s always been a mystery of how they bore away at the coral so efficiently, and how they continue to enlarge their home as they grow their shell. There are other bivalves that are efficient borers, including the pholad clams (“piddocks”) which use sharp teeth on their hinge to carve their way into solid rock, and the shipworms, which have abandoned their protective shell and instead use their two valves as teeth to burrow into wood. Both of these methods of boring are pretty straightforward.

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Piddocks in next to holes that they made in solid rock. Source: Aphotomarine

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Shipworm embedded in wood. Source: Michigan Science Art via Animal Diversity Web

But the boring giant clam has no such adaptation. It does not have large teeth on its hinge to carve at the coral. Such abrasion of the coral would also not explain how they widen the opening of their cubby-holes to allow their shell to grow wider. This mystery has long confounded giant clam researchers. I myself have wondered about it, and was surprised to find there was no good answer in the literature about it. But now, a team of scientists may have cracked the problem once and for all.

At the back of T. crocea‘s shell at the hinge, there is a large “byssal opening” with a fleshy foot which they can extend out of the opening to attach themselves to surfaces. Giant clams that don’t embed in coral (“epifaunal,” resting on the surface of the coral rather than “infaunal,” buried in the coral) lack this opening. The researchers suspected that the foot was the drilling instrument the clam used to create its home.

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Byssal opening of T. crocea with the foot retracted. Source: NickB on Southwest Florida Marine Aquarium Society

How could a soft fleshy foot drill into the solid calcium carbonate (CaCO3) skeleton of corals? I can confirm from experience that my own foot makes for a very ineffective drilling instrument in such a setting. But T. crocea has a secret weapon: the power of acid-base chemistry. CaCO3 can be dissolved by acids. You may well have taken advantage of this chemistry to settle your acid stomach by taking a Tums, which is made of CaCO3 and reacts with the excessive hydrochloric acid in your stomach, leaving your tummy with a more neutral pH. pH is a scale used to measure acidity, with low numbers indicating very acidic solutions like lemon juice, and high pH indicating a basic solution like bleach.

Scientists are well aware of the hazards corals face from decreasing pH (increasing acidity) in the oceans. All the CO2 we are emitting, in addition to being a greenhouse gas, dissolves in the ocean as carbonic acid and gets to work reacting and dissolving away the skeletons of corals and any other “calcifying” organisms that make shells. It makes it harder for corals to form their skeletons and is already worsening die-offs of corals in some areas. The researchers suspected that the clams use this phenomena to their advantage at a small scale, lowering the pH with their foot somehow to dissolve away the coral to make their borehole.

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Using a wedge to keep open a Tridacna shell in my Red Sea work. We took a small blood sample with permission of local authorities. This caused no lasting effects to the clams.

But they needed to prove it, and that was a challenge. Giant clams can be unwilling research participants. I myself have observed this in trying to take samples of their body fluid for my own research. When they sense the presence of a predator, they immediately clam up in their protective shell. I used a small wedge to keep their shells open to allow me to take a sample of their body fluid, but the researchers working on T. crocea needed to convince the clam to place its foot on a piece of pH-sensitive foil, keep it there and do whatever acid-secreting magic allows it to burrow into coral. They would then be able to measure whether it indeed is making the water around its foot more acidic, and by how much.

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Diagram from Hill et al., 2018 showing their experimental design.

In what I can only assume was an extended process of trial and error and negotiation with a somewhat unwilling research subject, the researchers found exactly the right angle needed to convince the clam that it was safe enough to try making a coral home. But it was not in coral, instead sitting in an aquarium, on top of a special type of foil that changes color when exposed to changing pH, like a piece of high-tech litmus paper. The researchers discovered that their suspicions were correct: the clams do make the area around their feet significantly more acidic than the surrounding seawater, as much as two to four pH units lower. Where seawater is around a pH of around 8, the clams were regularly reducing pH to as low as 6 (about the level of milk) and sometimes as low as 4.6 (about the pH of acid rain). Small differences in pH can make a big difference in the power of an acid because each pH unit corresponds to 10x more protons (hydrogen ions, H+) in the water. The protons are the agent that dissolves CaCO3. Each proton can take out one molecule of coral skeleton. The clams are dissolving away coral skeleton to make holes with only their feet!

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Footage of the pH- sensitive foil, with darker areas corresponding to lower pH. The areas of low pH (high acidity) correspond exactly to the “footprint” of the clam!

But what in T. crocea‘s foot allows them to make acid? I know that my foot does not do this, though that would be a very entertaining and obscure superpower. The researchers found the enzymes called vacuolar-type H+-ATPase (VHA) present in great quantities in the outermost cells of the clam’s feet. These enzymes are found throughout the tree of life and are proton pumps that can quickly reduce pH through active effort. Other prior researchers like the influential Sir Maurice Yonge, a legendary British marine biologist who worked extensively with giant clams, had suspected that the clams had used acid but had never been able to detect a change in pH in the seawater around the clams’ feet through more conventional methods. It was only because of new technologies like the pH paper that this research team was able to finally solve this issue. And now, I suspect other groups will want to re-investigate the importance of VHA in their study organisms. Many branches of the tree of life may be utilizing acid-base chemistry to their advantage in ways we never had previously imagined.

Weird Clam Profile: The Heart Cockles

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Corculum cardissa (from Wikipedia)

The heart cockle (Corculum cardissa) is so named because of its heart shaped shell. It is native to warm equatorial waters of the Indo-Pacific. While many bivalves sit with the their ventral valve facing down, the heart cockle sits on its side, with one side of both valves facing downward. The valves have adapted to resemble wings and are flat on the bottom, providing surface area that allows the bivalve to “raft” on the surface of soft sandy sediment and not sink. They may also sit embedded in little heart-shaped holes on the tops of corals.

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Two heart cockles embedded in the top of a Porites coral. Source: Reefbuilders
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A particularly green heart cockle from Singapore. Source: orientexpress on iNaturalist

Heart cockles are a member of a small club of bivalves which partner with symbiotic algae for nutrition created by photosynthesis. Most of the modern photosymbiotic bivalves are in the family Cardiidae, the cockles. The giant clams (Tridacninae) are also in this family and have a similar partnership with the same genus of Symbiodinium algae. This algae is also found in many species of coral.

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The dark circles in these microscope images are Symbiodinium. The top is a view of giant clam body tissue. The cells are present throughout the tissue in giant clams. The bottom shows heart cockle “tubules” which contain their symbiotic algae. The algae are restricted to narrow tubes that run through the tissue of the cockle. Source: Farmer et al. 2001

So when you find a live heart cockle, it is often green in color, because of the presence of this algae near the surface of its tissue. Its shell has adapted to be “windowed” (semi-transparent) to allow in light for the algae to harness to make sugars. The algae are housed in networks of tubes within the soft tissue of the cockle. They trade sugars with their host in exchange for nitrogen and carbon from the clam.

As I’ve mentioned before regarding the giant clams, this is a very productive partnership and has evolved separately several times in the history of bivalves. However, we don’t know why almost all examples of modern bivalve photosymbiosis occur in the cockles. Why aren’t the heart cockles giant like the giant clams? What features are necessary to allow this symbiosis to develop? These are the kind of questions I hope to help answer in my next few years of work.

Oh, the seasons they grow! [research blog]

My latest clamuscript is published in Palaios, coauthored with my advisor Matthew Clapham! It’s the first chapter of my PhD thesis, and it’s titled “Identifying the Ticks of Bivalve Shell Clocks: Seasonal Growth in Relation to Temperature and Food Supply.” I thought I’d write a quick post describing why I tackled this project, what I did, what I found out, and what I think it means! Raw unformatted PDF of it here on my publication page.

Why I did this project:

I study the growth bands of bivalve (“clam”) shells. Bivalves create light and dark shell growth bands as they grow their shells, much like the rings of a tree. The light bands form during happy times for the clam, when it is growing quickly and putting down lots of carbonate. The dark bands appear during times of cessation, when the bivalve ceases growth during a hibernation-like period. This can happen in the cold months, or the hot months, or both, or neither, depending on the clam and where it lives. It turns out that there are a lot of potential explanations for why these annual cessations of growth happen. Different researchers have suggested through the years that temperature (high or low) is the biggest control on the seasons that bivalves grow, but others have suggested that food supply is more important. Others say it’s mostly a function of the season they reproduce, when they’re putting most of their energy into making sperm/eggs and not growing their bodies. I wanted to try to see if I could find trends across all of bivalves which would shed light on which factors are important in determining their season of growth.

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Annual growth lines in the shell of a giant clam. The transparent spots are the times that it was growing more slowly and not happy. Was this because of temperatures? Or was it getting less to eat? I wanted to know.

What I did:

I read a ton of papers in the historical literature about bivalves. These were written by people in many fields: aquaculture, marine ecology, paleoclimate researchers (using the clams shells as a chemical record of temperature), and more. All of the papers were united by describing the seasons that the bivalves grew, and the seasons that they stopped growing. I ended up with nearly 300 observations of marine (saltwater) bivalve growth for dozens of species from all around the world. I had papers as old as the earliest 1910s, and some as new as last year.

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A map of all the places the observation of bivalve growth came from. Blue means they shut down in the winter, while red means they do not.

We have mussels, oysters, scallops, clams, cockles, geoducks, giant clams, razor clams, quahogs, and more in the database. Bivalves that burrow. Bivalves that sit on the surface of the sediment. Bivalves that stick onto rocks. Bivalves that can swim. With each, I noted data that the researchers recorded. If they grew during a season, I coded it as a 1. If they didn’t, I coded it as a 0. So a bivalve growing in summer but not winter would be recorded as 1,0. I also recorded environmental data including temperature of the location in winter and summer in the location, as well as seasonal supply of chlorophyll (a measure of phytoplankton, which is the main source of food for most clams). It turned out that not enough of the studies recorded temperature or chlorophyll for their sites, so I wanted to back these up with an additional data source. I downloaded satellite-based temperature and chlorophyll data for each location, as well as additional studies which directly measured chlorophyll at each site. I wanted lots of redundant environmental data to ensure that any trend or lack of trend I observed in my analysis was not due to a weakness of the data.

I then compared the occurrence of shutdown by season with these environmental variables using a statistical technique called regression. Regression basically involves trying to relate a predictor variable (in this case, latitude, temperature and chlorophyll during a certain season) to the response variable (did the clam grow in that season or not?). We wanted to see which environmental variable relates most closely to whether or not the clam grows or not. Because our dependent variable was binary (0 or 1), we used a technique called logistic regression, which tries to model the “log odds” of an event occurring in response to the predictor variable. That log odds can then be back-calculated to probability of the event occurring.

What we found:

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In a clamshell, we found that latitude (distance from the equator) is a very good predictor of whether or not a bivalve shuts down for the winter. As you’d expect, bivalves in the far north and far south of our planet are more likely to take a winter nap. However, bivalves at the equator mostly grow year round and are not likely to take a summer nap. In relation to temperature, the lower the winter temperature, the more likely the bivalve is to stop shell growth. High summer temperature is not as good a predictor for the occurrence of a summer shutdown, but the majority of summer shutdowns seem to occur at the low temperate latitudes, where the difference between the annual range of temperature is largest. Unlike at the equator, where bivalves likely can adapt to the hottest temperatures and be happy clams, they have to adapt to a huge range of temperatures in places like the American Gulf and Atlantic coasts, the Adriatic and Gulf of California. And if they are restricted at the northward end of their range, they may have no choice but to shut down in summer as there is nowhere cooler to migrate to.

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GIF of the satellite data showing white as hotspots of phytoplankton ability. Notice that the food is more available in summer months for each hemisphere. We were trying to see if this relates back to when the bivalves grow in every place we had data for.

Food supply, on the other hand, is not a good predictor of when bivalves shut down. When we went into this project, we expected food to be a powerful control on seasonal growth because it is intuitive and well understood that the better fed a bivalve is, the larger it will grow overall. But the seasonal low amount of chlorophyll (and therefore the amount of photosynthesizing plankton) in the bivalves’ areas had no relationship to whether or not the bivalve shut down in a certain season. To double check that this wasn’t a weakness in my satellite data, I downloaded additional direct observations from the same places as many bivalve studies in the dataset, but I still couldn’t find the relationship. We propose that the seasonal supply of phytoplankton is not well related to seasonal growth of bivalves because: 1) phytoplankton supply isn’t very seasonal in nature in most of the sites we studied. There are peaks in multiple seasons rather than a clean up and down wave shape like temperature. 2) Bivalves are pretty flexible in what they eat. They also eat other types of plankton and suspended particles that are even less seasonal. It may be pretty difficult to find bivalves that are seasonally starving. One of the most probable places to find such starvation shutdowns might be the poles, where seasonal ranges of temperature are quite small but plankton does really have a seasonal pattern of availability. More research will be needed to describe the nature of polar bivalves and why they shut down growth.

What’s next?
This is the first chapter of my PhD. I have two more chapters I’m working on, both related to the geochemistry of bivalve shells. I am writing those manuscripts this summer and looking for postdoctoral fellowships in the fall related to geochemistry of marine organisms in the fossil record. I hope to pursue more projects looking at the season of growth in bivalves, switching to understanding the role that changing seasonal cycles in their environment and biology play in their evolution. Do bivalves that live closer together tend to reproduce at different times? Can we track season of reproduction in relation to temperature and food supply? There are a lot more clam stories to be told and I look forward to sharing them all with you. Until the next research blog,

Dan

 

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.