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.

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).

When a clam gets an offer it can’t refuse

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Tridacna maxima in Eilat, Israel

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

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

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

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

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

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

Behold, my new favorite creature

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Porcellanopagurus nihonkaiensis wearing a bivalve shell (Source)

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

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

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

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Without the shell! From Yoshikawa et al, 2018

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

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

Revenge of the Clams

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

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

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

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

 

 

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

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

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

 

 

 

 

The Snails that Farm

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Littoraria grazing on Spartina marsh grass. (source)

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

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The marsh periwinkle Littoraria irrorata (source)

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

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Close-up of a snail’s radula (source)

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

 

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

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

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

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

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!

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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.