A coral, a worm and some clams walk into a bar…

On May 31, 2022March 21, 2023 By Dan KillamIn Biology, clamsLeave a comment

The tree of life is often portrayed as a neatly branching structure, with each division point cleanly delineated and separated from its neighbors. The truth is that the various twigs of the tree of life often overlap and become tangled in a process we call symbiosis. I’ve talked about symbiosis before on this blog, which falls along a spectrum of wholesomeness. At one end we have mutualism, a partnership where both organisms benefit and achieve more than the sum of their parts. The other extreme is parasitism, when one organism benefits at the expense of the other. Between the two, there is a broad gray area including commensalism, when one organism’s presence doesn’t necessarily cost or benefit the other in any way. The tree of life is crowded and unpruned, and so sometimes the twigs might wrap around each other quietly and without much fuss. We live on a small planet, and have had to get used to living in uncomfortable intimacy with all sorts of creatures, such as the mites that are living on your eyelashes right now.

But things start to get really weird and tangled when the tree of life loops over on itself twice, or three times, or more. “Three-way” symbioses are surprisingly common, and the more you look for them, the more you realize that the tree of life is more of a knot than anything else.

A view of the *Heteropsammia* coral from the side

A recent paper from researchers in Bremen (Germany) and Saudi Arabia looked at such a three-way symbiosis between a coral, a worm and bivalves found off of Tanzania in East Africa. The relationship between solitary corals (Heteropsammia cochlea and Heterocyathus aequicostatus), a sipunculan worm (Aspidosiphon muelleri muelleri) and the clam Jousseaumiella, is a complex triangle of dependencies that had previously been noticed by other researchers, but never investigated at great depth. The worm lives with multiple tiny clams attached, all inside of a small solitary coral the size of a dime (1 cm long). Is the coral a willing host for this crowded boarding house, or has it been parasitized? Does the worm gain anything from the clams? The researchers sought to find out.

Part of the reason I enjoyed reading this study so much was that it had to take a narrative structure to describe the evolutionary ménage à trois of its focus. So much of modern science has moved away from anecdote to hard data, and while there is plenty of that to find in the study, it turns out that a lot of the study of symbiosis is storytelling. We need to know the setting and the characters.

In this case, the main characters are small solitary corals living in the tropical reefs of the Indo-Pacific. We denote them as solitary to distinguish them from their giant colony-forming compatriots that construct the coral reefs currently threatened by climate change and pollution. But like those giant reef-builders, these solitary corals get much of their food from sunlight through a mutualistic partnership with algae called Symbiodinium. The algae provide the host with sugars and other photosynthetic products, and the hosts give them nutrients and a safe cozy home in their tissue.

You might be thinking, “Wait! Dan just said this was a three-way partnership between a coral, a worm and some clams. So this is actually a four-way partnership between corals, worms, clams and algae?” You’d be exactly right. And I’m happy to say that the plot of this sordid story is about to thicken even further.

The side of the coral. See the little pores?

The Aspidosiphon worm is found in a spiral-shaped burrow inside of the skeleton of the coral. It is a pretty cozy home, with walls made of calcium carbonate by the coral, with breathing holes in the sides to allow the worm to breathe and release waste. The researchers wanted to know more about the structure of the burrow. Was it dug out by the worm using acid or an abrasive motion, like some clams use to dig into coral? So the researchers essentially gave the coral a CT scan to see its 3D internal structure. Inside they found growth features suggesting the coral grew around the worm, as if intentionally providing it a home.

Cross-sectional CT scans of the coral skeleton. In figure D, you can see the silhouette of the chambers of the snail shell where the worm made its first home!

Even more crazily, they found evidence that the worm had first settled inside an empty snail shell, like a hermit crab! The coral probably settled on a snail shell as a larva, and grew to engulf the whole snail shell, leaving growing space for the worm inside, with windows and all! So to review, this is now a five-way symbiosis between a dead snail, a worm that moved into its empty shell, the coral (powered by algae) that grew around it and encased the snail shell within its skeleton, and we haven’t even gotten to the clams. How many creatures are hiding stacked in this trench coat? Please bear with me as I explain!

An SEM image of Jousseaumiella. These are less than 1 mm long! Pinhead sized!

What are the clams doing in this picture? Jousseaumiella is part of a family of clams called Galeommatidae, which we previously mentioned on this blog in the context of some bivalves found growing in the gills of unfortunate sand crabs. Many members of the Galeommatidae family are parasitic or commensal with other marine organisms. In this case, Jousseaumiella are tiny flat-bodied clams less than 1 mm long, found attached to the body of the worm, squeezed inside the burrow in the coral’s skeleton. It feeds on the worm’s waste and potentially food particles coming through the pores in the sides of the burrow. Not the most dignified existence, but a more mobile home means more opportunities to eat a varied diet similar to that that the worm and coral are seeking out, and the clam also gets protection from predation tucked inside the coral. It is unclear if it benefits the worm directly to have clams attached to it.

A time lapse of the coral+worm moving from the paper’s supplement! It would be handy to navigate to greener pastures, if it became too muddy in a certain place!

It is, however, clear how living inside a coral would be a pretty good deal for the worm, which gets a stable, protective suit of carbonate armor to protect it from predators, and grows to fit it as it gets larger. They are normally found inside of rocks, shells and other hard inanimate objects, but having a living home is a cool upgrade. What is the coral getting out of the deal? The researchers note that the corals are often found in the crevices between other large reef-building corals, in areas of the reef that receive high supplies of nutrients and turbidity (dirt that blocks out light). These sorts of environments aren’t necessarily friendly places for a coral to be, since they reduce the light and therefore the food that the coral can receive from photosynthesis. These crevices also have a lot of variability in other conditions like temperature and water flow. But because the coral has hitched a ride on the back of a worm, it can actually move in the sediment to react to changing conditions and avoid being buried by piles of sediment floating by! The worm can also act as a sort of anchor preventing the worm from sinking in the sediment underneath, which would be a big hazard for the small, stubby coral on its own. The coral seems to go to great pains to make its partner comfortable, not growing its skeleton to cover the pore windows to the outside. The researchers note that as coral reefs worldwide are subject to increasing human-made pollution and climate change, it would be interesting to research whether this complex three-(five?) way symbiosis provides the various participants with an advantage compared to other corals.

So like any good story, this symbiosis features complex, growing characters, a dynamic setting, and still plenty of mystery demanding a sequel! To that end, there are lots of other great three-way symbioses to investigate. Snails which farm fungus that parasitizes plants. Bryozoans living on snail shells that have a hermit crab inside. Gobies serving as lookouts at the entrances of burrows built by shrimp, with a crab freeloader along for good measure. Algae and bacteria teaming up to attack mussels. The list keeps going! I could see this becoming quite a franchise!

What good is a clam?

On September 4, 2019September 4, 2019 By Dan KillamIn Biology, clamsLeave a comment

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.

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.

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.

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.

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.

https://media.springernature.com/original/springer-static/image/art%3A10.1007%2Fs00338-005-0494-2/MediaObjects/338_2005_494_Fig1_HTML.jpg — 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.

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.

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.

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

Image result for mussel farm — 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

On June 2, 2019June 2, 2019 By Dan KillamIn Biology, clams, Uncategorized12 Comments

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.

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.

poly-gill-detailsem — 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.

Clam licking salt - Imgur — 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.

The clams that sail the seas on rafts of kelp

On October 19, 2018October 19, 2018 By Dan KillamIn Biology, clams3 Comments

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.

gtrap2 — 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.

pic_w-msc-197185 — 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.

shell-gaimardia-trapesina — 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

On July 22, 2018 By Dan KillamIn Biology, clamsLeave a comment

8175041434_a8b6c7e573_o — 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.

muerte-masiva-nacra-pinna-nobilis1 — 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.

SANYO DIGITAL CAMERA — A sea silk glove (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.

IMGP2909 — 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

On July 22, 2018July 22, 2018 By Dan KillamIn Biology, clams1 Comment

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.

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.

Screenshot_2018-07-22 Clams.png — 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.

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.”

*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!

*Lyonsiella* going after a doomed copepod (Morton 1984).

Weird Clam Profile: Hammer Oysters

On July 1, 2018July 2, 2018 By Dan KillamIn Biology, clams1 Comment

naturalis_biodiversity_center_-_zma-moll-1264_-_malleus_malleus_28linnaeus2c_175829_-_malleidae_-_mollusc_shell — *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.

4880314958_03e3a2b85c_o — 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.

73494_orig — 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.

maleus_albus — 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.

hammer-oyster-with-natural-attachment-seashell — 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.

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

Weird Clam Profile: The Heart Cockles

On June 13, 2018June 13, 2018 By Dan KillamIn Biology, clams2 Comments

corculum_cardissa_002 — 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.

corculum-cardissa-heart-cockle-solomon-islands-1 — Two heart cockles embedded in the top of a *Porites* coral. Source: Reefbuilders

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.

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.

Behold, my new favorite creature

On April 6, 2018April 6, 2018 By Dan KillamIn Biology1 Comment

https://i0.wp.com/www.izuzuki.com/Zukan/Other/images/large/kaigaraKG.jpg — *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!

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.

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!

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

A hinged shell does not a clam make (QUIZ)

On August 16, 2017August 16, 2017 By Dan KillamIn BiologyLeave a comment

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!

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.