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.

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.

Lessons of the Condors

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

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

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

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Condor #463 was born at a breeding center in Idaho

 

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

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

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

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

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

 

Revenge of the Clams

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

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

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

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

 

 

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

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

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

 

 

 

 

The Snails that Farm

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Littoraria grazing on Spartina marsh grass. (source)

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

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The marsh periwinkle Littoraria irrorata (source)

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

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Close-up of a snail’s radula (source)

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

 

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

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

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

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

What is Conservation Paleobiology?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

A cold-water ecosystem dominated by Antarctic scallops. Source

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

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

A hinged shell does not a clam make (QUIZ)

Bivalves are so named for their two hard shell valves made of carbonate, linked by a soft ligament acting as a hinge. They use a strong adductor muscle to close their shell, and the relaxation of the muscle allows the springy ligament to reopen (you might be familiar with adductor muscles as the edible tasty part of a scallop). In deference to the bivalves, laptops and flip-phones are called “clamshell” designs. That satisfying snap into place when you spring the ligamen… I mean, hinge of a flip phone is an example of human design imitating the ingenuity of evolution. But it turns out that plenty of other members of the tree of life have also stumbled upon the durable idea of a hinged two-valve shell. On the other hand, plenty of bivalves have given up on the classic clamshell look. In fact, the ancestor of all bivalves had a one-part shell, and the hinge evolved later.

Test your knowledge by trying to identify which which pictures are bivalves and which aren’t. Answers and picture sources at the bottom!

A. brachiopod-semenov.jpg

B.IMG_20150808_120740529.jpg

C.

D.

E.

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F.

G.

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H.

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I.

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J.

K.

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L.

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ANSWERS

A. This is not even a mollusk, never mind a clam! It’s a different benthic (bottom-dwelling) invertebrate called a brachiopod, which make up their own phylum. To put that in perspective, brachiopods are as far from bivalves on the tree of life as you are (you’re in phylum Chordata)! Yet they evolved a similar look through a process called convergent evolution. If environmental needs are the same, organisms may come to the same solution multiple times. Much like wings for flight evolved independently in insects, birds and bats, bivalves and brachiopods both evolved a hinged shell as a form of protection from predation.

B. These are indeed bivalves: rock scallops commonly found off the coast of California (picture by me of an exhibit at Monterey Bay Aquarium). So similar to the brachiopods in their ridged, hinged shells. Like picture A, these guys specialize in living on hard, rocky nearshore bottoms. Some cultures do apparently eat brachiopods (I have not), but I have little doubt that rock scallops are tastier.

C. These are also bivalves: windowpane oysters. Also known as capiz shell, they are commonly used for decoration and art due to their beautiful, thin semi-transparent shells. A large industry harvests them off the shores of the Philippines, where they unfortunately are growing scarce due to overexploitation.

D. These are not bivalves! They are crustaceans called clam shrimp. They have little legs poking out of a hard hinged shell, and have been found in some of the harshest environments on earth, where they wait in extended hibernation, sometimes years, between bouts of rainfall.

E. Not a bivalve. These are another kind of crustacean called ostracods. Like clam shrimp, ostracods live in a hinged shell and swim around with the help of tiny legs, filter-feeding in the water column. Ostracods are everywhere in the oceans and in freshwater, but have undergone an extreme process of miniaturization from their ancestral form, and are now represent some of the smallest complex multicellular life known.

F. These are fossil ostracods. You can see why they are sometimes mistaken for bivalves! The givaway is that one valve is overhanging the other. Most bivalves have symmetry between the two halves of their shell, but ostracods and brachiopods do not.

G. This is a snail, so it’s a close molluskan cousin of bivalves. Some snails feature a hinged lid at their shell opening called an operculum. This operculum can be closed to protect from predators and also seal in water to help land snails from drying out between rains.

H. This is a bizarre bivalve called a rudist. They were common during the time of the dinosaurs but went extinct during the same extinction, 66 million years ago. While they come in many bizarre shapes, this elevator form (or as I prefer to call them, trash-can form) would have been stuck in the sediment with its small lid poking out at the surface. They could open and close the lid to filter-feed.

I. This is a giant clam, Tridacna crocea! Its shell is hidden, embedded in the coral that has grown to surround it on all sides. Only the mantle (soft “lips”) are exposed, and are brilliantly colored by the symbiotic algae in its tissue. It harvests the sugars made by the algae for food. Despite being embedded in the coral, the clam does have enough room to close and pull back its mantle if a predator approaches.

J. This is a one of the weirdest modern bivalves, called a hammer oyster. These guys are found in the tropics, and the hammerhead part of their shell is actually their hinge, extended at both sides. The hinge provides the surface area needed to “snow-shoe” on top of the soft sandy bottom where they live. Other bivalves sometimes take advantage and live on the oyster like a raft!

K. This is a different brachiopod. Notice the lack of symmetry between the valves which gives it away.

L. This is by far the weirdest modern bivalve, a shipworm. These guys live buried deep within wood and are the number-one killer of wooden ships. They secrete a long tube of carbonate and have largely given up the hinged lifestyle, looking more like worms than mollusks.