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.

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).

Oh, the seasons they grow! [research blog]

My latest clamuscript is published in Palaios, coauthored with my advisor Matthew Clapham! It’s the first chapter of my PhD thesis, and it’s titled “Identifying the Ticks of Bivalve Shell Clocks: Seasonal Growth in Relation to Temperature and Food Supply.” I thought I’d write a quick post describing why I tackled this project, what I did, what I found out, and what I think it means! Raw unformatted PDF of it here on my publication page.

Why I did this project:

I study the growth bands of bivalve (“clam”) shells. Bivalves create light and dark shell growth bands as they grow their shells, much like the rings of a tree. The light bands form during happy times for the clam, when it is growing quickly and putting down lots of carbonate. The dark bands appear during times of cessation, when the bivalve ceases growth during a hibernation-like period. This can happen in the cold months, or the hot months, or both, or neither, depending on the clam and where it lives. It turns out that there are a lot of potential explanations for why these annual cessations of growth happen. Different researchers have suggested through the years that temperature (high or low) is the biggest control on the seasons that bivalves grow, but others have suggested that food supply is more important. Others say it’s mostly a function of the season they reproduce, when they’re putting most of their energy into making sperm/eggs and not growing their bodies. I wanted to try to see if I could find trends across all of bivalves which would shed light on which factors are important in determining their season of growth.

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Annual growth lines in the shell of a giant clam. The transparent spots are the times that it was growing more slowly and not happy. Was this because of temperatures? Or was it getting less to eat? I wanted to know.

What I did:

I read a ton of papers in the historical literature about bivalves. These were written by people in many fields: aquaculture, marine ecology, paleoclimate researchers (using the clams shells as a chemical record of temperature), and more. All of the papers were united by describing the seasons that the bivalves grew, and the seasons that they stopped growing. I ended up with nearly 300 observations of marine (saltwater) bivalve growth for dozens of species from all around the world. I had papers as old as the earliest 1910s, and some as new as last year.

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A map of all the places the observation of bivalve growth came from. Blue means they shut down in the winter, while red means they do not.

We have mussels, oysters, scallops, clams, cockles, geoducks, giant clams, razor clams, quahogs, and more in the database. Bivalves that burrow. Bivalves that sit on the surface of the sediment. Bivalves that stick onto rocks. Bivalves that can swim. With each, I noted data that the researchers recorded. If they grew during a season, I coded it as a 1. If they didn’t, I coded it as a 0. So a bivalve growing in summer but not winter would be recorded as 1,0. I also recorded environmental data including temperature of the location in winter and summer in the location, as well as seasonal supply of chlorophyll (a measure of phytoplankton, which is the main source of food for most clams). It turned out that not enough of the studies recorded temperature or chlorophyll for their sites, so I wanted to back these up with an additional data source. I downloaded satellite-based temperature and chlorophyll data for each location, as well as additional studies which directly measured chlorophyll at each site. I wanted lots of redundant environmental data to ensure that any trend or lack of trend I observed in my analysis was not due to a weakness of the data.

I then compared the occurrence of shutdown by season with these environmental variables using a statistical technique called regression. Regression basically involves trying to relate a predictor variable (in this case, latitude, temperature and chlorophyll during a certain season) to the response variable (did the clam grow in that season or not?). We wanted to see which environmental variable relates most closely to whether or not the clam grows or not. Because our dependent variable was binary (0 or 1), we used a technique called logistic regression, which tries to model the “log odds” of an event occurring in response to the predictor variable. That log odds can then be back-calculated to probability of the event occurring.

What we found:

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In a clamshell, we found that latitude (distance from the equator) is a very good predictor of whether or not a bivalve shuts down for the winter. As you’d expect, bivalves in the far north and far south of our planet are more likely to take a winter nap. However, bivalves at the equator mostly grow year round and are not likely to take a summer nap. In relation to temperature, the lower the winter temperature, the more likely the bivalve is to stop shell growth. High summer temperature is not as good a predictor for the occurrence of a summer shutdown, but the majority of summer shutdowns seem to occur at the low temperate latitudes, where the difference between the annual range of temperature is largest. Unlike at the equator, where bivalves likely can adapt to the hottest temperatures and be happy clams, they have to adapt to a huge range of temperatures in places like the American Gulf and Atlantic coasts, the Adriatic and Gulf of California. And if they are restricted at the northward end of their range, they may have no choice but to shut down in summer as there is nowhere cooler to migrate to.

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GIF of the satellite data showing white as hotspots of phytoplankton ability. Notice that the food is more available in summer months for each hemisphere. We were trying to see if this relates back to when the bivalves grow in every place we had data for.

Food supply, on the other hand, is not a good predictor of when bivalves shut down. When we went into this project, we expected food to be a powerful control on seasonal growth because it is intuitive and well understood that the better fed a bivalve is, the larger it will grow overall. But the seasonal low amount of chlorophyll (and therefore the amount of photosynthesizing plankton) in the bivalves’ areas had no relationship to whether or not the bivalve shut down in a certain season. To double check that this wasn’t a weakness in my satellite data, I downloaded additional direct observations from the same places as many bivalve studies in the dataset, but I still couldn’t find the relationship. We propose that the seasonal supply of phytoplankton is not well related to seasonal growth of bivalves because: 1) phytoplankton supply isn’t very seasonal in nature in most of the sites we studied. There are peaks in multiple seasons rather than a clean up and down wave shape like temperature. 2) Bivalves are pretty flexible in what they eat. They also eat other types of plankton and suspended particles that are even less seasonal. It may be pretty difficult to find bivalves that are seasonally starving. One of the most probable places to find such starvation shutdowns might be the poles, where seasonal ranges of temperature are quite small but plankton does really have a seasonal pattern of availability. More research will be needed to describe the nature of polar bivalves and why they shut down growth.

What’s next?
This is the first chapter of my PhD. I have two more chapters I’m working on, both related to the geochemistry of bivalve shells. I am writing those manuscripts this summer and looking for postdoctoral fellowships in the fall related to geochemistry of marine organisms in the fossil record. I hope to pursue more projects looking at the season of growth in bivalves, switching to understanding the role that changing seasonal cycles in their environment and biology play in their evolution. Do bivalves that live closer together tend to reproduce at different times? Can we track season of reproduction in relation to temperature and food supply? There are a lot more clam stories to be told and I look forward to sharing them all with you. Until the next research blog,

Dan

 

When a clam has a stowaway

My mussel contained a tiny half-eaten crab! - Imgur
Source: jeredjeya on Reddit

Bivalves put a lot of energy into their shells. These hardened, hinged sheaths of carbonate are an effective defense against many predators looking to get at the squishy clam’s body encased inside. Parasitic pea crabs have evolved to free-ride on the bivalves’ hard work.

 

(video courtesy Dana Shultz)

Pea crabs are small (pea-sized), very specialized parasites which live in the mantle cavity of many bivalve groups including oysters, mussels, clams and more. The mantle is the wall encasing the soft body of the bivalve, and the cavity is the space between this soft gooey tissue and the shell itself.

For a pea crab, there is no better place to be than this tiny, claustrophobic space. In fact, they can’t live anywhere else, though some species have been found in other unusual places such as inside the anuses and respiratory tracts of sea cucumbers (link SFW, fortunately, unless you’re a sea cucumber). In a bivalve host, the crab is protected from predation by the shell, and the bivalve provides a constant buffet of food as it sucks in suspended particles with its gills. The crab steals some of this food from itself before the bivalve can digest it.

As you might imagine, having a crab living in you taking your food and pinching at your gills is not an ideal arrangement for the bivalve. Pea crabs damage their hosts’ gills with their constant picking, and bivalves infected with crabs suffer slower growth than uninfected individuals, particularly for those unlucky enough to play host to the larger female pea crabs. At a certain point, the males will sneak out of their hosts and find a bivalve with a female crab inside. At this point, they mate inside the host’s shell, adding great insult to injury. The female releases her larvae, which swim out to infect new hapless bivalves and start the cycle over again.

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Aww, she’s expecting! The papers refer to this as being “with berry” which I find amusing for some reason. (photo from Dana Shultz)

 

You might think that commercial oysters with crab parasites would be thrown out, but to the contrary, finding a pea crab or its close relative the oyster crab with your meal is a cause for celebration in some areas, such as the Cheasapeake Bay. The crabs are eaten whole and often raw, and are said to have a texture akin to shrimp, with notes of sweetness and umami. Personally I prefer surprises in my Kinder eggs rather than in my shellfish, but to each their own.

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A pea crab serves as a nice side dish for this lunching sea otter. Source: Brocken Inaglory on Wikipedia