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.
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.
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.
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.
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:
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.
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.
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,
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.
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.
Visiting Pinnacles National Park the other day, we were lucky to spot California condors several times. Their wingspan can reach up to 3 m. Their graceful flight is a sight to behold as they ride the warm updrafts of between the pinnacles of rock in the park, with their primary feathers bending up like a conductor’s fingers. Condors are the only remaining members of the genus Gymnogyps, which once contained five species. Four are only known from fossil specimens and went extinct at the end of the Pleistocene (~12,000 years ago), but Gymnogyps once ranged across the Americas. As such, the California condor (Gymnogyps californianus) is a relic species; a survivor of a long but mostly extinct lineage.
By 1987, poaching, lead poisoning and habitat destruction had reduced the population to 27 individuals, of which 22 were captured and put into an emergency captive breeding program. In the thirty years since this project began, the population has increased to around 450 individuals. The program was expensive, painstaking and a massive undertaking. To this day, all captive-bred individuals are individually numbered and continually monitored. They even have a very adorable directory on the Pinnacles site where you can look each bird up by its number. We saw #606 and #463, and a couple others from farther away that we couldn’t read.
Condor #463 was born at a breeding center in Idaho
California condors can be distinguished by the much more common turkey vultures by their underwing coloration, with large white patches at the front of the wings. I think that turkey vultures are fun to watch as they fly in circles over the highways searching for roadkill, but when you see a condor fly low over your head, it is awe-inspiring. Even the smaller juveniles have much larger, more pronounced heads than turkey vultures, and seem to fly even more effortlessly with a more gently curving V shape in their wings. We had stopped on the trail to discuss field markings for condors with another hiker when #463 soared over our heads, and we couldn’t help but jump for joy and hug each other at the privilege to see one of the famous condors ourselves.
As an advocate for invertebrate conservation, I have been known to unfairly poke fun at the human tendency to focus on large, charismatic megafauna for conservation as opposed to smaller and less exciting species that may make up more of an ecosystem’s biomass, or represent a more important link in the local food chain. Pandas have used up billions of conservation dollars, yet they are kind of an evolutionary oddball with their poorly evolved guts that can barely digest their chosen bamboo food, and their infamous failure to successfully mate. Koalas have a similar story. We reformed tuna fishing not out of concern for the fish, but because of concern for dolphins getting mistakenly caught. We tend to put a lot of time and effort into conserving species that we consider cute, or cool, or awe-inspiring. The condors are important decomposers of carcasses, ranging over huge distances in their search for food and efficiently returning the nutrients of dead animals back to the ecosystem. But the public outcry motivating their rescue was greatly helped by the fact that these creatures are incredibly impressive megafauna. If the turkey vulture was critically endangered, it might not get the same funding supporting its conservation.
But looking at the condors, my skepticism melted away and I was left with only gratitude that I was able to witness the grandeur of these beasts as they soared through the air; gratitude that I was able to see them myself and not only read about them in a book. I will never see a Steller’s Sea Cow, or a Great Auk, or a thylacine, or a dodo, because they disappeared before we realized what we were doing, and that extinction is a real and irreversible loss. Perhaps Gymnogyps won’t be around in 10,000 years. Their lineage originally evolved to feed on the giant carcasses from a collection of North American megafauna that is believed to have been hunted to death by humans (though that is a topic of endless debate). But if we hadn’t intervened to save the condors, we would have had to live with the guilt of knowing that we as a species committed the killing blow and did nothing to stop when we knew the reality of our crime. Instead we went to extreme lengths to save these unusual and majestic creatures because we feel empathy for them. As I watched condor 463 soar over my head, I felt relief, and pride, and hope. Success stories are important motivators for conservation, and I couldn’t help but think his wings were spelling out a “V” for victory as he flew away.
UC Santa Cruz, like all UC schools, is in the midst of a massive regent-mandated effort to increase enrollment. In the face of a governor hostile to the idea of investing in education (despite his prior promises), the regents have decided that we are going to grow our way out of the budget shortfall. Much has been said about the foolishness of this plan from the standpoint of housing, student tuition and access to resources, but I thought I’d talk about the cost it has had on me personally as a Teaching Assistant (TA).
TAs are usually graduate students working on a doctorate or master’s degree. We are paid 50% wages on the assumption we spend 20 hours a week working on TA stuff to support our other 20 hours (ha!) of work a week on our thesis and classes. Our tuition is waived and we receive health benefits.
In the last two years, my department’s academic division has been cutting TAships while pressuring the department to enroll more students. For four years, I’ve been teaching the lab section for Environmental Geology (EART 20), which has given me a natural laboratory to note the impact that increased class size has had on my instruction. I’d also note that my experience has been less extreme than other TA’s because EART 20’s enrollment has been flat overall during the time I’ve taught. But the lab section has doubled in size from 22 to 45 students, possibly because we have more majors declaring and they need the lab credit for their degree.
Here are some ways my instruction has been impacted by doubling of students:
As a TA, I am supposed to only spend 20 hours a week on instruction. I’m literally not allowed by my union to spend more time than that. And fortunately, because I have been able to slowly tweak my lesson plans over many years, I now have a lot less prep time than I did when I started teaching this class. The lesson plans are already put together, and I can just focus on polishing and perfecting the lessons. However, the time I need to grade has ballooned to at least double what it used to be. I say at least double, because I usually get tired after the 30th lab and start to slow down.
My detail in grading is also impacted because I cannot spend as much time looking and commenting on each student’s assignment. So while they’re paying higher tuition than their compatriots from four years ago, they are getting less instructor time dedicated to feedback on their work. Note that our department has tried to make up for this by hiring graders to assist TAs, but I insist on grading my own labs because I need to understand how students are learning and responding to my assigned material.
Less physical space for students
Our building used to have large, luxurious desks perfect for specimen-rich lab sections. But they unfortunately couldn’t fit more than 20 students in a room with those so they have put in smaller, flimsy desks to stuff more students in. These desks are narrow and crammed together to allow up to 30 students in the room. As a lab instructor, I prefer to walk around and answer student questions looking at the specimen we’re talking about until they understand and have that light-bulb moment. But I can’t do that anymore because the desks are so close together. So instead I now sit at the front of the room and they come up to me. I hate this and I know I get less questions than I would if I could walk around. It is another way that they aren’t getting their money’s worth.
The UC claims to be a big cheerleader for the active learning style of teaching. Active learning is different than the classic lecture-based format in that exercises are designed to maximize student participation and interactivity between the student and instructor, hopefully leading to learning by experience rather than example. But active learning requires more grading time and a different classroom layout than the classic lecture format. And I have had to revise my labs over the years to reduce interactivity out of necessity. In the past, I’ve made a landscape out of play-dough for students to map out topographic profiles. This year, there were just too many students for it to work. They scrunched together around the model, with some deciding to wait until my office hours to get time doing it. It was sad to watch this. Next time, I’ll make two models to space through the room so it isn’t so claustrophobic.
These problems are only going to get worse, as our department is currently under pressure to increase enrollment and has less TAships to offer every year. We are often criticized for our low student to instructor ratio! Yet tuition is increasing. Students are getting less value for the same course offered four years ago. I’ve observed it with my own eyes. I feel a pang of sadness each lab section seeing the ways it reduces the quality of instruction. I spend more time to try to lessen the impact of these creeping changes, but something’s got to give. I hope Californians realize that the value of our legendary UC schools is under attack. I hope we invest more into education and don’t forget that the UCs helped make our state great. I hope Jerry Brown cements his legacy by increasing UC funding.
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!
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.
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!
It was hard not to feel irritated as I opened the homepage of East Meadow Action, a grassroots group recently formed to defeat the development of Student Housing West (SHW), particularly regarding the Family Student Housing project on East Campus. The group has its heart in the right place. They believe that the field at the base of campus is a setting deserving of preservation, and are trying to prevent the building of new student housing to achieve this goal. I’m writing this to counter a number of points brought up on their site about the planning and design of SHW. I feel qualified to discuss this matter because I’ve been serving as a graduate student rep on the planning committee for the project since its early days last summer. From the start, I’ve been dedicating my efforts to keeping the project economical to ensure lower rent and as dense as possible, to prevent campus from sprawling into land needed by wildlife. I’m sorry to say that East Meadow Action’s efforts to defeat construction will harm both students and the environment if they succeed.
Student Housing West is an enormous planned development on the West side of campus designed to house ~2600 undergraduates, 200-220 graduate students, nearly 140 apartments for student families and a childcare facility. Well, it was initially planned to be on the west side of campus, beginning below Kresge College and continuing down the hill to the current site of Family Student Housing. That was the site university administrators provided for developers pitching their concepts for the project in an extended series of meetings last summer. We selected one developer, Capstone Development Partners, because of their proven track record of student housing construction and management. I personally voted for them because their plan had the highest goals for sustainability and the team made it clear that they would work to adapt to any contingencies which would surely arise during the future planning process.
It turned out that their promised adaptability was put to the test when we were informed that nearly half the site would not be usable for construction. It turns out that a very cute and endangered species, the Red-Legged Frog, uses that corridor for their annual migration from the northerly forests to the grasslands near the Arboretum. Capstone suddenly had to work with half the space that they had initially anticipated for the 2800 students destined to live at West Campus. In addition, it meant there would be no housing prepared for current families living in Family Student Housing (FSH) when it was torn down to make room for the new development.
This news could well have killed the project. SHW would not be built in time. Students would be left to fend for themselves and find housing in town on Craigslist. But members of the planning committee discussed another site for the new FSH, one which I have long personally considered a waste of space: the “meadow” near Hagar and Coolidge Drive. As an environmentalist and earth scientist, I bristle at the suggestion that the field at the base of campus is some kind of natural grassland. This space is heavily damaged; if it was left alone, it would return in a few decades to being redwood forest much like North Campus. But it hasn’t been left alone. It is instead a haven for domestic cows which graze and trample the space year-round. The animals, while endearing, continuously roam mowing every blade of grass to a stub, happily emitting methane on a prime piece of real estate.
Perhaps the current ongoing environmental review of the site will find that the land is needed by some sensitive species other than domestic cattle. If that turns out to be the case, then I will shut up and the project will probably need to restart at the drawing board. But some of the “alternative sites” that East Meadow Action assures us are available on campus (but never get specific about) seem far worse to me. Some possibilities include the forested land north of campus or the trailer park on the Northwest of campus. Both of these places seem much worse candidates to pave over to me in terms of ecological value (redwood and oak woodland) or their need for people (the trailer park is some of the most affordable housing on campus and much beloved by the students living there). I would much rather pave over cow pasture than a forest or someone’s home.
When I read through the website of East Meadow Action, I am struck that the centerpiece of their argument is the aesthetic value of lower campus as open space. I confess that I find this extremely irritating. It’s irritating to me because aside from being an advocate for conservation of sensitive species like the red-legged frog, I am also a student who has to get by living in this town, and I don’t have the luxury of worrying about aesthetics. When they mentioned Ranch View Terrace as an example of “building done responsibly”, my opinion was sealed. Take one look at the floor plans described for the single family homes of Ranch View Terrace, “a large housing complex set back from the road and mitigated by vegetation and topography.” How exactly are we going to house 2500 students in single family homes, artfully hidden behind trees? East Meadow Action is not motivated by environmentalism. They’re motivated by the same Not In My BackYard mentality that is choking the development of dense housing in town. We are in a housing crisis and their ocean views are a luxury that we can’t afford.
I agreed to be SHW rep because I want to make sure future graduate students will have an affordable housing option on campus. I currently am “lucky” to spend 40% of my stipend on rent in town. When I first came to attend grad school here I had to deal with Craigslist and ended up paying over 60% of my income for the first two years I lived here. Students are pursuing extreme solutions. I know people considering living in the woods, or in unzoned bedrooms in town that are another source of tension with the community. It will only continue to get worse as rents rise in the coming years.
Right now, all we can do to remedy the issue in Santa Cruz is pass rent control ordinances and increase supply. SHW is the best project we have to increase supply and do so ensuring maximum density. I want UCSC to continue to be a forest campus, which necessitates not chopping down trees. I am trying to ensure that students have an affordable housing option on campus. I want student families to have guaranteed housing that comes online right when their previous outdated complex is torn down. The sentimental affection that some may feel for the aesthetics of a cow pasture strikes me as the privilege of a comfortable and vocal minority. I value the croaking of frogs and the laughter of children over the yammering of NIMBYs and, yes, even the mooing of cows.
Fourth Year PhD Candidate, UCSC Earth and Planetary Sciences
University of Southern California ’12, BS Environmental Studies
Clams are traditionally the victims of the aquatic realm. With some exceptions, clams are generally not predatory in nature, preferring to passively filter feed. When they are attacked, their defenses center around their protective shell, or swimming away, or just living in a place that is difficult for predators to reach. They are picked at by crabs, crushed in the jaws of fish, and pried apart by sea stars. But some clams are sick of being the victims. They have big dreams and places to be. For these clams, the rest of the tree of life is a ticket to bigger and better things. These clams have evolved to live inside of other living things.
Pocketbook mussels, for example, have a unique problem. They like to live inland along streams but their microscopic larvae would not be able to swim against the current to get upstream. The mussels have adapted a clever and evil strategy to solve this problem: they hitch a ride in the gills of fish. The mother mussel develops a lure that resembles a small fish, complete with a little fake eyespot, and invitingly wiggles it to attract the attention of a passing fish. When the foolish fish falls for the trick and bites the mussel’s lure, it explodes into a cloud of larvae which then flap up to attach to the gill tissue of the fish like little binder clips. They then encyst themselves in that tissue and feed on the fish’s blood, all the while hopefully hitching a ride further upstream, where they release and settle down to a more traditional clammy life of filter-feeding stuck in the sediment.
Clams live in the gills of all sorts of organisms. Because they broadcast spawn, any passing animal may breathe in clam larvae which find the gills a perfectly hospitable place to settle. Sure, it’s a bit cramped, but it’s safe, well oxygenated by definition and there is plenty of food available. They also may just settle on the bodies of other organisms. Most of these gill-dwelling clams are commensal: that means that their impact on the host organism is fairly neutral. They may cause some localized necrosis in the spot they’re living, but they’re mostly sucking up food particles which the host doesn’t really care about. In addition, in crabs and other arthropods, these clams will get shed off periodically when the crab molts away its exoskeleton, so they don’t build up too heavily.
While being a parasite is often denigrated as taking the easy way out, it is actually quite challenging to pursue this unusual lifestyle. Parasitism has evolved a couple hundred times in 15 different phyla, but it is rare to find some organism midway in the process of becoming a true parasite. One team of researchers just published their observations of a commensal clam, Kurtiella pedroana, which may be flirting with true parasitism. These tiny clams normally live in the gill chambers of sand crabs on the Pacific coasts of the Americas. They attach their anchoring byssal threads to the insides of the chambers and live a comfortable life until the crabs molt, when they are shed away. The crabs mostly are unaffected by their presence, but the researchers noticed that some of the clams had actually burrowed into the gill tissue itself. This is an interesting development, because the clams would not be able to filter feed in such a location, so they must have been feeding on the crab’s hemocoel (internal blood). These unusual parasitic individuals are currently a “dead end” as they haven’t figured out how to get back out to reproduce, but if they ever do, they could potentially pass on this trait and become a new type of parasitic clam species. The researchers have potentially observed a rare example of an animal turning to the dark parasitic side of life, with some living in a neutral commensal way and other innovative individuals seeking a bit more out of their non-consensual relationship with their host crabs. Considering the irritation that other bivalves suffer at the claws of pesky parasitic crabs, this seems a particularly sweet revenge.
Imagine you had bone spurs coming out of your eyebrows that fluoresced under UV light in a way that attracted mates. That’s what chameleons do, according to a new study in Scientific Reports! Chameleons have a lot of ways to talk to each other, including most famously with their ability to change skin color. But this bone fluorescence strategy is a more subtle way to show off their sexiness to each other, and it may appear in other places in the tree of life. I can assure you that many, many researchers are now looking at their specimens to see if their study organisms do this as well.
Us humans really like to talk up our skills at farming. And while it’s true that we have domesticated animals and plants to a degree not seen in other life forms, the act of nurturing and harvesting food is actually not really that special, and is broadly observed throughout the animal kingdom. Perhaps the most iconic invertebrate farmers are insects. Leaf-cutter ants, termites, and some beetles have been observed to actively cultivate fungus by gathering plant material to feed it, growing the fungus, protecting the fungus from competition, and then harvesting the fungus to feed themselves and their young. Ants are also known to keep livestock in the form of aphids, which they lovingly protect and cultivate for the sweet nectar they excrete. Such practices are called “high-level food production” because, like human farmers with their seeds and fertilizer, insects have evolved a highly complex symbiosis with their fungus. The fungus has shaped their behavior as much as the ants cultivate the fungus.
Less well understood is the “low-level food production” that may occur more broadly throughout the tree of life. There is less direct evidence of such behavior because it is more indirect and less specialized than high-level food production, but it may be equally advantageous for the cultivator and the cultivated. One study published in 2003 uncovered a simple but powerful relationship between marsh periwinkles of the genus Littoraria and fungus which they cultivate and harvest.
Marsh periwinkles are small and not particularly charismatic creatures. Like many snails, they are grazers with a shell, a fleshy foot and a rough, abrasive organ called a radula which they use like sandpaper to graze on pretty much whatever they can get into. Snails are not known as picky eaters. But researcher Brian Silliman of Brown University and Steven Newell of University of Georgia noticed that these innocuous snails regularly undertake the risky, low reward activity of grazing above the water on the blades of swamp grass, stripping off the surfaces of the blades of grass. The researchers were confused why the snails would expose themselves to predation and the harmful open air for such a low-nutrition food.
They discovered that the snails were investing in the future. By stripping away the protective surface of the swamp grass blades and liberally fertilizing the surface of the grass with their droppings, the periwinkles are ensuring that the swamp grass will be infected with an active and very prolific fungal infection. The fungus, unlike the plant it lives on, is of high nutritional value. The researchers demonstrated the active partnership between the snails and fungus by conducting caged experiments where they showed that snails which grazed on grass but not the resulting fungus did not grow as large as snails which were allowed to return and chow down on the fungus. The fungus loves this deal as well. They grow much more vigorously on grass that is “radulated” (rubbed with the snail’s sandpapery radula) than uninjured grass. The fungus grows even faster if the snails are allowed to deposit their poop next to the wounds. The researchers found that this same relationship applies at 16 salt marshes along 2,000 km of the Eastern Seaboard.
The periwinkles don’t really know what they’re doing. They aren’t actively planting fungus and watching proudly like a human farmer as their crop matures. But over millions of years, the snails have been hard-wired to practice this behavior because it works. Snails that abrade a leaf of swamp grass, poop on the wound and come back later to eat the yummy fungus do a lot better than snails which just stick to the safe way of life below the surface of the water. The fungus loves this relationship too. The only loser is the swamp grass, which the researchers unsurprisingly found grows much more slowly when infected with fungus. But marsh grass is the largest source of biomass in swamp environments, and the snails that partner with fungus are able to more efficiently use this plentiful but low-nutrient food source, to the extent that it is now the dominant way of eating for swamp periwinkles on the East Coast of the US, and probably in a lot more places too. The researchers noted that there are likely far more examples of low-level food production that we simply haven’t noticed.
Since this work was published, other teams have discovered that some damselfish like to farm algae, fiddler crabs encourage the growth of mangrove trees, and even fungus get in on the action of farming bacteria. We love to talk up our “sophisticated” high-level food production techniques, but such relationships probably got started at a similarly low level. Our activities as hunter-gatherers encouraged the growth of certain organisms, we stumbled upon them, ate them, kept doing what we were doing and eventually our behavior developed into something more complex. Next time you see a snail munching its way up a blade of grass, consider to yourself whether it knows exactly what it’s doing. Come back later to see the fruits of its labor.