What good is a clam?

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

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

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

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The filtration rate of oysters. Graphic from The Nature Conservancy

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

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An oyster reef. Source: The Nature Conservancy

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

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

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

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

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

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Crabs feeding on Bathymodiolus in the deep sea (NOAA)

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

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

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

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A shell midden in Argentina. Photo from Mikel Zubimendi, Wikipedia
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Mussels being farmed on ropes

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

 

Mystery of the “spurting” mussels

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

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U. crassus in a Slovenian river (Alexander Mrkvicka)

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

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U. crassus propped up on its foot (UCforLife)

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

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Squirting water into the stream! (Vicentini 2005)

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

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

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Top-down view of spurting behavior (Vicentini 2005)

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

Lifecyle of U. crassus (Rita Larje via UCforLife)

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

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

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

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

Weird Clam Profile: Pinna nobilis

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

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

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They are really enormous (Marc Arenas Camps on WordPress)

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

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

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

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

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

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.