The Mystery of the Giant Clams of the Red Sea and Indian Ocean

I have always been fascinated by scientific discoveries that are hanging right in front of our noses. Cryptic species are one such surprise. Sometimes, researchers using genetic sequencing are surprised to discover that a group of animals that all look the same from the outside are actually reproductively isolated from each other; separate twigs on the tree of life. This surprise has happened over and over in the history of natural science.

It turns out such puzzles are frequent among the giant clams. These unusual bivalves are specialists in coral reef environments, growing to large size with the help of symbiotic algae that create sugars through photosynthesis. Within the genus Tridacna there are ~10 accepted species which vary in size, shape, color and mode of life.

Tridacna squamosina (right) sitting next to the small giant clam T. maxima (left) on the Israeli Red Sea coast

I specialize in the three species known (so far) from the Red Sea, including the small giant clam Tridacna maxima and the fluted giant clam T. squamosa, which are both found worldwide, all the way from the Red Sea to down past the equator along the Great Barrier Reef. The third local species, T. squamosina is more unusual, so far being only known from the Red Sea (an endemic species). T. squamosina is an example of a cryptic species, having previously been assumed to be a local variant of T. squamosa. It looks pretty similar, with long scutes (flap-like appendages) protruding from its shell, thought to help stabilize it on the flat bottom of loose coral rubble. But unlike T. squamosa, T. squamosina lives exclusively at the top of the reef in the shallowest waters closest to the sun. It has a very angular, zig-zag pattern in its plications (the wavy shapes at the edge of the shell) and a characteristic pair of green stripes where the soft tissue meets the edges of the shell. The soft tissue is covered with warty protuberances.

Pictures of details of T. squamosina from Richter et al. 2008

It was only first described in detail in the early 2000s, when an international team of researchers figured out using genetic sequencing that it was a distinct species and named it T. costata. They noted that in their surveys all around the shores of the Red Sea, they only found 13 live specimens, making it an extremely rare and possibly endangered species. Fossil specimens on local reefs appeared to be much more common, suggesting it had a much larger population in the past. Then in 2011, another team at the Natural History Museum in Vienna discovered a shell of one had been forgotten in its collection for over 100 years. Rudolf Sturany, the researcher on the 1895 research cruise who had originally collected the clam, had called it T. squamosina.

The T. squamosina shell in the collection of the Museum of Natural History in Vienna (from Huber and Eschner, 2011)

In taxonomy (the science of naming and classifying organisms), the first team to name the species wins, so the name T. costata was synonymized (retired) in favor of the earlier name T. squamosina, which became the name of record. It must be annoying to spend so much time working to name a species and then discover you had been scooped over a century before! But such is science.

A mystery clam thought to be T. squamosina, later identified as T. elongatissima found off of Mozambique by iNaturalist user bewambay

The strange part was that there were some murmurs over the last few years that T. squamosina was not only found in the Red Sea, but also had been seen along the coast of Africa as far south as Kenya, Mozambique and Madagascar. Divers and snorkelers had taken pictures of a giant clam that did indeed look strangely like T. squamosina, with a zigzag shell opening and green stripes at the edge of its tissue. But some aspects of these individuals seemed off. In the Red Sea, T. squamosina lives freely, not embedded in the coral as these pictures showed, and the geometry of the angles of the shell seemed a bit different. It also would be difficult for T. squamosina to be connected in population from the Red Sea all the way South to Mozambique, as there are natural barriers which would prevent its planktonic larvae from riding currents to intermix between the two regions. When populations are separated by a barrier, the flow of genes between them is cut off and evolution begins to separate the populations from each other until they are separate species, a process called allopatric speciation.

A large specimen of T. elongatissima observed by iNaturalist user dawngoebbels off of Kenya

I figured that someday, researchers would collect tissue samples from these mystery clams to settle whether they were actually T. squamosina or something else. And this year, a team did just that, traveling along the coast of Mozambique, Madagascar, Kenya and other places, collecting samples of tissue to compare how all the different clams they saw were related in a family tree. They genetically sequenced these “clamples” and in the process, found that the mystery clams were a new cryptic species, which they called T. elongatissima!

Shells of T. elongatissima from the Fauvelot et al. 2020 paper
For comparison, a shell of T. squamosina collected off of Sinai, Egypt. You can see why they’re easy to mix up!

T. elongatissima closely resembles T. squamosina, and they are sister species on the bivalve family tree. It’s hard to tell them apart without training. Even a professional would probably mix some of them up if they were all placed sitting next to each other. The major differences appear to relate to shell shape, with T. elongatissima having a less symmetrical shell than T. squamosina, and a bigger opening at the rear hinge for a foot to poke through. The symmetrical shell and closing of the foot opening may represent changes that T. squamosina took on to adapt to be able to sit freely on the bottom, rather than embedding in the coral like T. elongatissima seems to prefer. If you’ve read this far, you may be thinking “Who cares? A clam’s a clam and these look practically the same. Aren’t you just splitting clams at this point?” At the end of the day, a species is a man-made concept; an organizing tool for use by us humans. Species are the characters in our reconstruction of the history of the world. What can we learn about the world by having identified this species T. elongatissima?

A giant clam family tree! Notice T. squamosina and T. elongatissima right next to each other.

The researchers behind the new paper discuss that based on statistical analyses of the genetic differences between the species, the most recent common ancestor for T. elongatissima and T. squamosina probably lived more than 1.4 million years ago! Some researchers have previously suggested that T. squamosina probably began its development as a separate species due to geographic isolation by low sea level, caused by repeated glaciations. With so much water trapped as ice on land during this period, the narrow Strait of Bab al Mandab, currently the gateway to the Red Sea, became a land barrier as sea level fell (kind of like opposite of the Bering Sea land bridge that formed allowing humans to migrate to the Americas). Ancestral clams trapped on the Northern end of this barrier were proposed to have evolved to become the rare T. squamosina.

This has occurred with a variety of species that became Red Sea endemics (meaning they are unique species that evolved in the Red Sea and are found nowhere else), including a unique crown of thorns starfish. The issue is that during this time of low sea level, the Red Sea went through periods where it was a rather unfriendly place for clams to live. All sorts of creatures went extinct in the period when the sea was repeatedly cut off, because the water became extremely salty, along with other unfriendly changes. So it’s unlikely T. squamosina would be present for us to see today if it only lived in the Red Sea throughout the entire length of time.

A map from Fauvelot et al. 2020 showing the distributions of different giant clams the researchers identified along the coasts of Africa and the Red Sea. Notice the bright red dots representing T. squamosina, only found in the Red Sea, while green dots represent T. elongatissima. Notice how the currents (arrows) seem to meet and then go offshore from Kenya. More on that in the next paragraph.

The researchers of this new paper propose that T. squamosina was more likely to have initially branched off due to the barrier of the Horn of Africa. The seas off of Kenya and Somalia harbor a meeting of southward and northward currents which then group and head offshore, away from the reefs that giant clam larvae are trying to get to. So any tiny floating planktonic clam larvae would experience a strong “headwind” preventing them from crossing that point. It would also mean that during times that the Red Sea was not a happy place to be a clam, T. squamosina may have found refuge on the coasts of places like Eritrea, Oman and possibly even as far as Pakistan. During times when sea levels rose and Red Sea conditions became friendlier, it recolonized the area.

As far as we know, the Red Sea is the only place T. squamosina is now found, but it may well be present elsewhere like Yemen or Oman. If T. squamosina was found in other regions, it would be tremendously important for its conservation. Right now, the species is thought to be extremely rare, with a very small native range. If it inhabited a broader area, that would mean more reservoirs of genetic diversity. This would reduce the odds that it will go extinct as reefs are put under stress from climate change, pollution and overharvesting. To survive as a species, it helps to not put all your eggs in one basket. If you’re only found in one small place, it increases the chances that a disaster (like climate change) will wipe you out.

The only way we will know for sure is to visit reefs in understudied places like Yemen, Oman, Pakistan, Eritrea and Somalia, to understand the richness of the giant clams present. These areas are understudied for various reasons: lack of research funding for non-Western researchers, lack of interest from the scientific community too focused on familiar places, and geopolitical situations that make it difficult to conduct research. But I hope someday to collaborate with people in these countries to better understand the giant clams present in such understudied regions of the globe. It is virtually certain that there are more species of giant clams, both alive and as fossils, waiting to be discovered.

Apps that Darwin would have loved

Was Charles Darwin first? Kind of depends – Harvard Gazette

Most people know that Charles Darwin was a cerebral, deep thinking type. He traveled the world, collecting data about those finches and other stuff on field expeditions, synthesizing big ideas over decades to form his magnum opus, On the Origin of Species, where he set out his theories on natural selection and its role in evolution. However, you might not know that on a day-to-day level, Darwin was an all-around nerd’s nerd who just wanted to learn everything he could about the world, motivated by an unending sense of curiosity.

Darwin was a man who pulled plankton nets, filtering seawater just to see what cool stuff would show up when he put the resulting sample of goo under the microscope. He fiddled for earthworms and wrote a book about them over 40 years (including an experiment where he watched their progress burying a bunch of rocks over a 30 year period). He kept a heated greenhouse where he studied orchids and carnivorous plants. I consider Darwin a role model, because it’s hard to find a topic in natural history that he didn’t write about. The dude was just an unfillable sponge of knowledge.

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A view of Darwin’s greenhouse. Source: The Darwin Correspondence Project

I think Darwin would have marveled the online information that we have easy access to today. If Darwin were a researcher today, I could imagine him hosting a forum or listserv where he’d moderate, muse over the natural world and keep correspondence with the other leading scientific minds, much as he was a prolific letter writer with other researchers of his time. He might not be huge on Twitter: a bit too fast paced for his liking I bet; but I think he’d love two apps that I have also fallen in love with over the last few years.

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The Explore page on iNaturalist, where you can see what species other people have recently observed in your area!

The first is iNaturalist (Android, iOS), a website and social network where you can upload pictures of any lifeform, attach information about its location, time of day and other info, and the machine learning powering the site will try to identify it for you, with amazingly accurate results. If the AI can’t figure it out, experts are waiting in the wings to provide an identification or confirm yours. Think of it like a Pokedex or Pokemon Go, but for “collecting” real life creatures. And like Pokemon Go, it can be insanely addictive to find out about all the species that are all around us at all times. iNaturalist also has an app called Seek that makes the process even more gamified!

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a view of my observations

I am definitely an iNaturalist addict, and have accumulated a healthy collection of observations over the past couple years. My first was a red-shouldered hawk that I saw on campus during my PhD, but then I moved onward to fungi and plants I saw walking from my car to the office, and went back through my past pictures to identify bats I saw in Belize, fish I saw diving, and of course, my beloved banana slugs. I find it’s particularly satisfying to find out how life around you changes through the year, with the seasons. When I felt like pulling my hair out during my PhD, it kept me grounded to be able to know that the first spring flowers were opening, or the fledglings of birds were leaving the nest, or the winter rains were bringing all sorts of new fungus and banana slugs to life among the undergrowth.

I think being in touch with our natural world can help us now. While we’re sheltering in place, we don’t have to be trapped within ourselves. Even in the densest cities, there are so many bugs, flowers, birds and squirrels all around us going about their lives while we are effectively on pause. It brings me relief to know that life continues, and satisfaction to be able to understand them. Knowledge really is power, and if you know the relations between all the types of life both in evolution and ecology, it makes the world make sense in a way that provides a weird zen-like peace.

To that effect, there is a #citynaturechallenge happening starting yesterday, from 4/24-4/27/20. Take pictures of living things around you and upload them to iNaturalist, Twitter and elsewhere with that hashtag! My uploads there have been of interest to researchers studying rare snails of the Negev desert, writing books about tropical bats, and researching invasive beetles. I have been following uploads of giant clams on iNaturalist for quite some time, including the newest described species, Tridacna elongatissima, which users had been observing on the Eastern coast of Africa before it was formally described in a recent paper! I bet Darwin would have been a major, influential user on iNat.

Darwin was also a big nerd regarding rocks and fossils. Evolution is the story of life, and we can only understand that story by turning to the fossil record for information. Environmental changes provide a major motivating factor pushing life to constantly change. Geology in Darwin’s day was a developing field, with the first geological maps appearing only due to the work of William Smith and others, mere decades before Darwin’s work came to be. But his work would not have happened without the growing understanding of the massive passages of time needed to deposit the rocks all around us today. Evolution typically needs time, and fossils provide proof of how life has changed during those almost incomprehensibly long intervals.

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One of William Smith’s geologic maps

To find a fossil of an age we are interested in, we must know the rocks present in the area. And the second app I’ll mention today is RockD (Android, iOS), which we can use to figure out the type of rocks right under our feet, how they were made, how old they are and even what kind of fossils have previously been found within. The data in RockD is pulled from two sources that scientists have lovingly curated. Macrostrat is a database of stratigraphic (rock layer) data that scientists have aggregated into one of the most detailed and comprehensive geologic maps ever made. And the Paleobiology Database collects observations that scientists have made over the decades of almost every fossil that has ever been found and recorded.

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Of local geology when you first open RockD (my detailed coordinates obscured ;))

Rather than relying on only printed maps for his work, Darwin would have loved the ability to pull out his phone in the field and instantly know the combined work of dozens of previous researchers to understand the rock he was looking at. You can even “check in” and upload your own pictures of rocks to help researchers improve their databases, and go back in time to look at where the continent you live on was during the time of the dinosaurs!

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Geologic map of my area. Darwin would have seriously nerded out checking out the locations of faults, formations and other features

These are only two of many apps and websites that I think would have blown Darwin’s mind. We are living in a golden age of digital science, with so many new discoveries being precipitated by the availability of easily accessible, free information in the palms of our hands. But more than that, it is fun to go outside and be able to decode the mysteries of the world around you without even being an expert in natural history. In the process, you might find yourself becoming an expert!

What good is a clam?

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When I mention to people that I study bivalves, I can sometimes sense from their facial expressions that they are secretly asking “why?” While clams are perfectly content to keep doing what they’re doing without being thanked, I think it’s important to enumerate all of the ways they make our world more livable and functional.

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Various roles that freshwater mussels can play in their local food webs (Source: Vaughn and Hoellein, 2018)

Bivalves are ecosystem engineers. While they may seem rather stationary and not up to much at any particular time, they are actually always working to actively maintain their habitat. The majority of clams are filter-feeders, meaning that they use their gills to gather particles from the water column for food. Some of these particles are ingested as food and later pooped out. Some inedible particles are discarded immediately by the clam as “pseudofeces”. Both mechanisms serve as a bridge between the water column and the benthos (the sediment at the bottom). In this way, clams are engines that take carbon fixed by algae floating in the water and transfer that material to be stored in the sediment. Their bodies also act as nutrition to feed all sorts of animals higher on the food chain like sea stars, lobsters, seabirds, sea otters and humans that depend on bivalves as food. They are literally sucking up the primary productivity (algae) to be used by the rest of the food chain.

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The filtration rate of oysters. Graphic from The Nature Conservancy

Different clam species vary in their precise filtration rate (how fast they can inhale and exhale water, filtering the particles within), but it is prodigious. Some freshwater mussels, for example, can pick-through 1-2 liters of water per hour for every gram of their own flesh. Since these individual bivalves can weigh over 100 g, they are capable of picking the food out of an immense quantity of water. In doing so, bivalves help improve the clarity of the water column, allowing more sunlight to reach deeper into the water body (the photic zone), providing more energy for additional photosynthesis to occur. While there are examples where invasive bivalves such as zebra or quagga mussels take this phenomenon too far, in well-functioning ecosystems, the filtration activity of clams helps improve the productivity of the community.

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An oyster reef. Source: The Nature Conservancy

Bivalves help make sediment through their filtration of material from the water column, and they also engineer and manipulate the sediment directly. Some bivalves, like oysters, are able to make huge mounds of dirt that serve as habitat for all sorts of life, increasing the diversity of the community. They do so both by excreting sediment, and also by passively trapping it between the shells of neighboring oysters (“baffling”). By doing so, they reduce rates of coastal erosion and increase the biodiversity of wetlands. For this reason, New York and other communities plan to seed oyster reefs to help fight sea level rise and reduce the threat of storm surges like the one that occurred during Superstorm Sandy.

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Comparison of sediments without bioturbation by digging animals, and with. Notice how the non-bioturbated sediment is layered and darkened due to activity by anaerobic bacteria, while the well-oxygenated, mixed sediment is light all the way through. From Norkko and Shumway, 2011

Other “infaunal” bivalves (burrowers) help to aerate the sediment through their tunneling, bringing oxygen deep under the surface of the dirt. This mixing of the sediment (also called bioturbation) ensures that nutrition from deep under the sediment surface is again made available for other organisms. Some bivalves can bore into coral reefs or solid rock, creating burrows which serve as habitat for other animals and can free up minerals for use by the surrounding ecosystem. Helpful shipworms assist in eating wood, assisting in returning nutrients stored in that tissue to the ecosystem as well.

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Enormous grouping of giant clams in a lagoon in French Polynesia. From Gilbert et al., 2005

Bivalves of course are also famous for their shells, and this activity also provides habitat to sponges, snails, barnacles and many other encrusting organisms specially adapted to live on bivalve shells and found nowhere else. Giant clams are the most legendary “hypercalcifiers,” and in some regions like New Caledonia can rival reef-building corals in terms of biomass. In areas where soft-bottoms dominate, bivalves like hammer oysters, adapted to “rafting” on the quicksand-like surface of the soft sediment, can assist by providing a platform for other animals to take refuge. In the deep sea, bathymodiolid mussels and other chemosymbiotic bivalves can feed directly on the methane and sulfur emitted from hot vents or cold seeps with the help of symbiotic bacteria, creating dense reefs which provide food and habitat for all sorts of life. Even once the clams die, their shells can continue to serve as homes for other creatures.

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Crabs feeding on Bathymodiolus in the deep sea (NOAA)

The shells of clams provide great scientific value in understanding our world. Much like tree rings serve as a record of environment thousands of years into the past, growth rings in clam shells serve as a diary of the animal’s life. These rings can be yearly, lunar, tidal or even daily in rhythm, with each ring serving as a page in the diary. The chemistry of those “pages” can be analyzed to figure out the temperature the clam experienced, what it ate, whether it suffered from pollution, and even the frequency of storms! The study of rings in the hard parts of animals is called sclerochronology, and it’s what first drew me to study bivalves. I was so fascinated by the idea that our beaches are covered with high-resolution records of the ocean environment, waiting to be cut open and read.

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This giant clam shell recorded an interruption in the animal’s daily growth caused by a typhoon! From Komagoe et al., 2018

While they don’t owe us anything, clams provide a lot of value to humans as well, serving as a sustainable and productive source of food. Humans have been farming bivalves for thousands of years, as evidenced by “oyster gardens” and shell middens which can be found all over the world. Particularly in seasons when food is scarce on land, native peoples could survive by taking advantage of the wealth of the sea, and bivalves are one of the most plentiful and accessible marine food sources available. But they aren’t just the past of our food; they may be part of the future. Bivalves are one of the most sustainable sources of meat known, requiring very little additional food to farm and actively cleaning the environment in the process. Mussels grown out on a rope farm are an easy investment, growing quickly and with very little required energy expenditure. Someday, giant clams may provide the first carbon-neutral meat source, as they gain their food from symbiotic algae within their flesh. I have never eaten one, but I’ve heard they’re delicious.

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A shell midden in Argentina. Photo from Mikel Zubimendi, Wikipedia

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Mussels being farmed on ropes

Clams are heroes we didn’t know we needed and maybe don’t deserve. They ask for nothing from us, but provide vast services which we take for granted. So the next time you see an inconspicuous airhole in the sand, thank the clam that could be deep below for aerating the sediment. The shell of that long-dead mussel at your feet may have fed a sea star, and now is a home for barnacles and many other creatures. While that mussel was alive, it sucked in algae to improve water quality on our beaches. And the sand itself may contain countless fragments of even more ancient shells. Clams silently serve as an important cog in the vast machine that makes our oceans, rivers and lakes such amazing places to be. Thank you clams!

 

Thoughts of a clam

To us active, dynamic mammals, the humble clam can appear positively…inanimate. Their nervous system is decentralized relative to ours, lacking any sort of brain, and to the untrained eye, it can appear that their only discernible reaction to the outside world is opening or closing. Open = happy, closed = not happy; end of story, right? Some vegans even argue that the clams are so nonsentient that it is okay to eat them and think of them as having no more agency than a vegetable!

You might already have predicted I intend to tell you about just how animate and sentient clams can be. But let’s start out by describing the nuts and bolts of their nervous system. As with many invertebrates, their nervous system is distributed throughout their body as a system of ganglia. Ganglia are clumps of nerve cells which may have local specialization, and transmit messages within neurons using electrical potentials. At the connection between cells (called a synapse), neurotransmitters are used to pass signals to the next cell. Researchers have found that bivalves use “histamine‐, octopamine‐, gamma‐aminobutyric acid‐ (GABA)…like immunoreactivity” in their central and peripheral nervous systems, much like us vertebrates do, and other studies have even found that the response to serotonin and dopamine is localized in nervous tissue linked to different organ systems.

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Nerve cells (bright green) highlighted in a larval oyster with fluorescent dye (from Yurchenko et al 2018)

These systems of chemical nerve transmission are truly ancient, likely dating back to the formation of complex animal body plans in the earliest Cambrian. Researchers have great interest in studying these nervous and hormonal signaling systems in mollusks because they can shed light on the relative flexibility and limitations of these systems throughout the animal tree of life. Characterizing these systems can also allow us to understand the mechanisms that bivalves and other animals use to react to environmental stimuli.

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Electron microscope view of gill cilia, zoomed in 1000x (from Dan Hornbach)

Like humans, bivalves spend a lot of time and effort eating. Most bivalves eat by filtering food from passing water with tiny cilia on their gills. These cilia work to capture food particles and also act as a miniature rowing team moving water along the gill surface. The bivalve needs a way to control this ciliar activity, and researchers found they could directly control the speed at which oysters move their cilia by dosing them with serotonin and dopamine, which respectively increased and decreased activity.

Bivalves also work very hard to make babies. Most bivalves reproduce by releasing sperm and eggs to fertilize externally in the water column. To maximize their chances to find a mate, they typically save up their reproductive cells in gonads for multiple months and release them in a coordinated mass spawning event. It appears that this process is controlled by hormonal releases of dopamine and serotonin. Researchers have determined that serotonin concentrations vary through the year, with mussels in New England using it to regulate a seasonal cycle of feeding in summer, followed storing of that energy for winter. During the winter when food is less available, they use that stored energy to bulk up their gonads in time for reproductive release in spring months, when their larvae have plentiful access to food and oxygen, ensuring them the best chance of survival. In recent decades, aquaculturists have learned to use serotonin injections to induce spawning in cultured clams, to ensure they will have a harvest ready at a certain time of year.

So bivalves are very sensitive to the seasons. How about shorter term sources of excitement? You might have observed this yourself through the clam’s most iconic activity: opening and closing its shell. Clams close their shells with powerful adductor muscles which pull the two valves together. A springy ligament at the hinge pulls the shell open when the muscles relax. Just like us, the clam needs to use nerve cells to signal the muscle to do its thing. In addition, two different sets of ganglia act to control the foot that some bivalves can extend to dig into sand, with one ganglion acting to extend the foot and the other causing it to contract. While clams don’t have a centralized brain with specialized regions for different uses like we have, this represents a sort of specialization of neural systems with a similar result.

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This iconic gif is often shared along with the claim it shows a clam “licking” salt. It is actually using its foot to search for a place to dig. The salt was not needed.

When a certain neuron is used repeatedly, it can form a cellular memory allowing the organism to acclamate (ugh sorry) and moderate its response to a particular stimulus over time. Giant clams, for example, close their shells when their simple eyes detect a shadow overhead. This behavior can protect them from predation. When I conducted some of my PhD research, sampling body fluid of aquarium and wild giant clams with a syringe, I noticed that captive clams didn’t close up in response to my shadow overhead, while wild clams required me to sneak up and wedge their shells open with a wooden block to do my work. I suspected that after exposure to frequent feedings and water changes by aquarists, the clam had “learned” that there was no reason to expend energy closing its shell. Meanwhile, in the process of proving that our sampling technique was not harmful to the animal, I discovered that clams which detected my shadow would quickly reopen within seconds when I hid from them, while those that were stuck by a syringe would stay closed for minutes before opening and beginning to feed again. Makes sense!

Other researchers noticed this phenomenon as well. One group found that giant clams repeatedly exposed to shadows of different sizes, shell tapping and even directly touching its soft tissue began to habituate (become accustomed) to the stress, opening more quickly and staying open longer each time the stimulus occurred. Even more interestingly, they did not transfer that habituation between stress types; for example, the clams that saw a shadow again and again would still react strongly to a different stress like tapping its shell. This suggests the animal can distinguish between different threats along a spectrum of seriousness, with touching of tissue (similar to a fish pecking at its flesh) being the most serious threat with the most dramatic response.

Another study determined that larger giant clams stayed closed longer than smaller ones in response to the same threat. They proposed this was related to the greater risk large clams face as they have more tissue area vulnerable to attack. While the clams might not have made a “conscious” decision in the way we do as thinking creatures, they were able to place their individual risk in context and vary their response. This ability to tailor a response to different risk levels is a sign of surprisingly complex neurology at work.

Inside the Scallop
Close up of the eyes of a scallop. Each is a tiny crystalline parabolic mirror (photo by Matthew Krummins on Wikipedia)

Scallops show some of the most complex bivalve behaviors. This relates back to their unique adaptations, including simple eyes that can resolve shapes and the ability to swim away from danger. Scallops have been found to discern between predator types by sight alone, to the extent that they did not initially recognize an invasive new predatory seastar as a threat. When swimming, they are capable of using this vision to navigate to places where they can hide, such as seagrass beds. It would be very interesting to compare the behavior of scallops in marine protected areas to those that can be freely harvested. Do they vary their behavior in response?

I hope I’ve made clear that while clams are not exactly intellectual powerhouses, their behavior is much more complicated than simply sucking up water and opening or closing their shells. Like us, they inhabit a complex environment that requires a multitude of responses. Their nervous systems have evolved to allow them to survive and adopt nuanced behaviors which they can vary on the fly, and which us “higher” animals are only just beginning to comprehend.

Mystery of the “spurting” mussels

If you’ve read any of my posts, you should realize by now that clams are pretty weird. Some catch live prey. Some have algae in their bodies that they “farm” for food. Some can bore into hard rock. Some sail the seas on rafts of kelp. Clams live in a competitive world and have had hundreds of millions of years of time to evolve to try out all sorts of weird, unlikely ways of life.

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U. crassus in a Slovenian river (Alexander Mrkvicka)

The thick shelled river mussel (Unio crassus) is known from many rivers and streams of Central Europe. As this is a very well-studied region of the world, many generations of academics have noted an unusual, seemingly inexplicable behavior undertaken by these mussels at certain times of year.

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U. crassus propped up on its foot (UCforLife)

Using its muscular foot, U. crassus pulls itself to the edges of the streams and rivers it lives in until it is partially exposed to air. It orients itself at a right angle with the surface of the stream with its siphons (two little snorkels coming out of the shell) facing out towards the water. Like all bivalves, U. crassus can act as a bellows by opening and closing its shell to pull in and push out water through those siphons. It has one siphon above the water and one below, and it proceeds to suck in water and spray it into the center of the stream using the power of its suction. The water can travel over a meter away and they continue this spurting about once a minute, sometimes for hours.

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Squirting water into the stream! (Vicentini 2005)

Needless to say, this is a very strange and unlikely behavior to observe in a mussel. It is exposing itself to potential dessication or suffocation from exposure to air. It is vulnerable to predation from terrestrial mammals and birds. There has to be a very powerful benefit from this behavior to outweigh those risks. And why squirt water into the air?

Some researchers proposed that the mussels were traveling to shore to harvest from the more plentiful food particles deposited there. But why would they face their siphons away from the shore then? Other workers suggested that it was a way to reduce heat stress through evaporation, though that also seems unlikely, considering the water is warmest in the shallows. The question persisted for decades in the minds of curious malacologists.

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Top-down view of spurting behavior (Vicentini 2005)

In 2005, Heinrich Vicentini of the Swiss Bureau for Inland Fisheries and Freshwater Ecology decided to try settle the question of why these mussels spurt. He observed several dozen of the mussels crawl to the edge of the water and diligently begin squirting into the streams. In the name of science, he put himself in the path of these squirts, caught the water and used a hand lens to observe that the squirted water was full of mussel larvae (glochidia).

Lifecyle of U. crassus (Rita Larje via UCforLife)

U. crassus falls in the order Unionida, a group of freshwater mussels distinguished by a very unusual method of reproduction. They are parasites! Because they can’t swim well enough to colonize upstream against the current, they need to rely on fish to hitch a ride. Some have evolved elaborate lures to convince fish to take a bite, then allowing them to release their larvae, which attach to the fish’s gills like binder clips and ride all the way upstream. Once they have reached their destination, they detach and grow up into more conventional burrowing mussels. It’s a weird, creepy and wonderfully brilliant strategy that enabled the mussels to invade the inland rivers which would otherwise be inaccessible to them.

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Loach (type of freshwater fish) gills with unionid larvae attached (UCforLife)

The mussels appear to be spurting out not only water, but their babies. They gain a couple of advantages from this. For one, their larvae can distribute further than would be possible from the bottom of the creek. Instead, they are released at the center of the surface of the stream, where they can be carried for a much longer distance by the current before they settle at the bottom. In addition, the splash of water on the surface may mimic the behavior of insects and other fish food falling in the water. A curious minnow might venture to investigate the source of the splash, where it would promptly breathe in a cloud of larvae that get stuck on its gills. A pretty rude surprise, but a brilliant trick to give the baby mussels the best chance of surviving.

So again, clams prove themselves to be far more clever and interesting than they might initially seem. U. crassus and other members of the Unionida are an ancient and globally distributed lineage which have evolved all sorts of weird and wonderful ways to maintain their river lifestyle. Unfortunately, rivers are some of the most widely damaged environments in the world. A majority of freshwater mussel species worldwide including U. crassus are endangered by habitat loss, overharvesting and pollution. But more research into their unusual biology can help us understand ways we can enhance their conservation, with the hope of providing more habitat for them to recover populations in the future. New projects in Sweden and other countries aim to recover habitat for their larvae to settle along 300 km of rivers, and research the fish species which their larvae prefer to hitch a ride on. With more work, we can hopefully ensure that the streams of Europe will harbor little mini super-soakers for millennia to come.

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: Pinna nobilis

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A fan mussel among the seagrass it calls home (Arnaud Abadie on Flickr)

The fan mussels (Pinna nobilis) are a species of enormous mussel which live in seagrass beds of the Mediterranean Sea. They can grow to nearly 4 feet long (though most are 1-2 feet in size at maturity), and live with most of their bodies protruding straight up out of the sediment, anchored down into the sand with long rootlike byssal threads which grow out of their rear hinge.

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They are really enormous (Marc Arenas Camps on WordPress)

These mussels grow up to 20 cm per year, almost entirely in the vertical direction. As they gain in mass, their bodies start to sink in the sand beneath them, so it is believed this extremely fast growth rate evolved in order to stay above the sediment. It also helps them to remain elevated above the seagrass around them, where they can access passing phytoplankton and organic particles in the current.

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A sea silk glove (Wikipedia)

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Close up view of the hairlike byssus. I definitely am feeling some beard envy here. (Wikipedia)

Because they are exposed to the current like a giant fan, they need a very strong anchor. So they create huge quantities of byssal threads which root them down in the sand. These byssal threads are known as as “sea silk” and communities around the Mediterranean have used the silk to sew clothing for thousands of years. The material is extremely fine but strong, and has historically been of immense value as a result. Sea silk or sea wool is mentioned in writings of the ancient Egyptians, Greeks and Romans.

Unfortunately, the fan mussels are considered critically endangered due to overharvesting, pollution, climate change and destruction of their native seagrass habitats. However, they are now protected and active conservation efforts are underway. When the cruise ship Costa Concordia ran aground off of Italy in 2012, a community of fan mussels were rescued from a seagrass bed next to the wreck and moved to another nearby site. I hope someday to study the fan mussels because I find them to be a truly charismatic bivalve with many interesting mysteries still waiting to be uncovered about their unique lifestyle.

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Huge pen shell I saw at the Hebrew University Museum in Jerusalem. My lens cap is only 6 cm to give you a sense of scale! The shells are fragile and easily break.

Killer Clams

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Some shells of the carnivorous genus Cardiomya. Notice the protuberance off one side, making space for the overdeveloped siphon they use to capture prey (Machado et al. 2016)

You might think of clams as rather pacifistic creatures. Most of them are; the majority of bivalves are filter-feeding organisms that suck in seawater and eat the yummy stuff being carried by the currents. This mostly means phytoplankton, tiny single-celled photosynthetic plankton which make up most of the biomass in the world’s oceans. Most bivalves could be considered exclusively herbivorous, but as I’ve learned happens throughout evolutionary biology, there are exceptions to every rule. We already talked about parasitic bivalves that have evolved to hitch a ride on other hapless marine animals. But there is an even more sinister lineage of bivalves waiting in the sediment: yes, I’m talking about killer clams.

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View of the oversized siphon (Machado et al. 2016)

Carnivory in bivalves has evolved multiple times, but the majority of known carnivorous bivalves fall within an order called the Anomalodesmata. Within that order, two families of clams called the Poromyidae and Cuspariidae have a surprising number of species which are known to eat multicellular prey.

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Evil clams are also the star of my favorite Spongebob episode

Now, you can rest easy because there are no clams that eat people. You’re safe from the Class Bivalvia, as far as we know. But if you were a small crustacean like a copepod, isopod or ostracod, you would be quite concerned about the possibility of being eaten by a poromyid clam in certain regions of the world. These clams lie in wait in the sediment like a sarlacc, with sensory tentacles feeling for passing prey and a large, overdeveloped siphon ready to suck up or engulf their helpless targets.

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Until we catch the feeding behavior of poromyids on video, these whimsical artist’s depictions will have to do (Morton 1981).

Because they spend their lives under the sediment, these clams aren’t very well studied, and the first video of them alive was only taken in recent years. In addition, many of these killer clams live in deeper water, where their murderous lifestyle provides an advantage because food supplies can be much more sparse than in the sun-drenched shallow coastal zone. Much like the venus flytrap and carnivorous plants have arisen in response to the low nutrient supply of boggy swamp environments, the ability to eat alternative prey is valuable to the killer clams in all sorts of unconventional environments.

The siphon which these clams use to suck up their prey is a repurposed organ. In most other bivalves, the siphon is usually a snorkel-like organ which enables the clam to safely remain buried deep in the sediment and still breathe in oxgyen and food-rich water from open water above. But for the poromyids, the siphon is instead a weapon which can be used like a vaccum cleaner hose, or even be enlarged to engulf hapless prey. The poromyids have also evolved to have a much more complex, muscular stomach than any other bivalves. It takes a lot more energy to digest multicellular food, while most other bivalves simply just feed from the single-celled food they catch on their gills, expelling the other un-needed junk as “pseudofeces.”

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Dilemma, another strange carnivorous bivalve which eats marine isopods (pill bugs), found from deep waters off the the Florida Keys, Vanuatu and New Zealand (Leal 2008)

Hopefully soon we will have video of this predatory activity in action. But until then, you can imagine that somewhere on earth, tiny copepods foraging on the surface of the sediment pass by a strange field of squishy tentacles. Suddenly, out of nowhere a hellish giant vacuum hose appears in view and sucks them in like Jonah and the whale. Then it’s just darkness and stomach acid. What a way to go!

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Lyonsiella going after a doomed copepod (Morton 1984).

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.