ABy definition, barnacles are immobile: these crustaceans congregate on surfaces such as whale heads, sea turtle shells, coastal rocks and ship hulls. Once mature, barnacle larvae move on ocean currents in search of an anchorage point, preferably settling in places with good water flow, which often happen to be marine animals. moving. Barnacles adhere to host locations by secreting an adhesive cement, usually from a rigid plate on their underside, which sticks them to the surface. The general consensus has long been that barnacles then remain permanently cemented to their chosen surfaces, never to move again.
But a 2008 paper described an exception in the immobility of the barnacle in the tortoise barnacle Chelonibia testudinaria. This unexpected sighting inspired John Zardus, marine biologist and professor at the Citadel in South Carolina, and his colleagues to investigate the locomotion of barnacles on turtle shells. In a study published on October 6 in Proceedings of the Royal Society B, they report that not only Chelonibia testudinaria mobile, but it deliberately navigates to places with high water flow where barnacles can feed more easily.
courtesy of john zardus
Barnacles are classified as filter feeders, extending a small, fan-shaped, fiber-shaped appendage that captures particles and tiny organisms from the surrounding seawater. They may be able to feed more efficiently if they are placed so that the currents push the food particles into their fan. Zardus and his colleagues hypothesized that Chelonibia testudinaria position themselves to capture more food, and tested this idea in several aquariums around the world. The researchers observed that barnacles moved up to 78.6 mm over the course of a year and could change direction. They also found that barnacles on turtle shells tended to move towards the turtle’s head, against the current of the water that passes when the turtle swims, giving them access to stronger currents. and therefore more food.
The scientist spoke with Zardus about the importance of these barnacles being able to move and how these slow animals move on turtle shells.
The scientist: What interested you in the study of animal mobility?
Jean Zardus: Originally, I was interested in barnacles that live on other animals. My real question was, how did these barnacles find the host they were to live on? There are some [barnacles] specialized for sea turtles, some for whales. It was by chance that we came across this moving species. We were not the first to find out. There was an article published in 2008 by Julia Moriarty. She contacted me and sent me pictures asking, “Can barnacles that live on turtles move?” I said, ‘No, no, barnacles don’t do that. Once they’re stuck, they’re there. She sent these photos of a time series of turtles over time with barnacles on them. She used barnacles to find out which turtle was which. She realized that the barnacles seemed to be moving. When I looked at his photos, I was really blown away. Since then, we have been examining how this phenomenon could occur. We were finally able to run some lab experiments that completely confirmed this, and then we started to wonder why they are doing it.
ST: According to the article, the study found that barnacles directed their movements for feeding purposes. Could you expand on what this means on the current understanding of barnacles?
You need to understand a bit of how a barnacle works. Basically, they’re like a shrimp in a shell, living upside down with their heads down, and they kick their legs to capture food. They are filter feeders, feeding on whatever is in the plankton. When they get attached to a turtle, that’s probably the big advantage [of attaching] feeds the currents. If you take a barnacle and it lives in calm water, it will take its appendages and very actively stroke them in the water. But once you provide some flow, they’ll just sit there and passively capture their food. It is much easier for them to do it. They don’t have to spend energy. This species and others that live on mobile hosts probably take advantage of the flow for food.
The bottom [of a barnacle shell] is very flat. They’re sitting right on the surface. What [move around]? He doesn’t have little paws there. We don’t know how it goes. We just know it’s [moving] very slowly. It takes him weeks or months to cover a significant distance. Most barnacles, on their bottom, secrete a basal plaque of calcium carbonate and it is solid. But this one doesn’t. It has a membrane. There are some [barnacles] which are like that with a soft bottom. We’re pretty sure it’s an important part of the movement.
ST: What surprised you about this ability of barnacles?
JZ: We were pretty sure it was breeding, but we were wrong. It turned out to be for food.
Let me tell you a bit about reproduction in barnacles. They are mostly hermaphrodites. They have both male and female components, but they cannot fertilize on their own. They need to have a neighbor. It is very common for barnacles to live in clusters so that they can fertilize their neighbor next door. They have direct insemination, with the largest penis in the world by body size. They reach out and copulate with their next door neighbors. We thought, “Ah, these guys are moving so they can get closer for breeding.”
We set up experiments where we placed them in different arrangements, tightly grouped together or separated from each other. We thought that in those who were far from each other, that in time they would come closer to each other. But they didn’t. They ended up moving at random. It wasn’t until we put the stream on them from the jet in the aquarium that they started to move and they moved towards the stream.
When you observed them on sea turtles in the wild, they were mostly heading towards the head of the turtle or towards the front part of the turtle shell. This will put them in a higher rate. It seems they are really reacting to the feed, and it’s probably more for food, and they don’t seem to do so much for breeding.
I also have another study that’s in press right now with this species showing that it’s the only barnacle we know of that isn’t actively feeding. If you don’t give them any feed, a barnacle will usually stroke its appendages for food. If you put this one without a stream, it doesn’t do anything. It will die. It will never even actively feed. It relies 100 percent on passive power, so it should always be in a high-speed environment. Which makes sense if you’re living on a turtle you’re going to have a lot of flow.
One of the reasons he might not be so keen on getting closer to other people is [this barnacle] has dwarf males that live there attached. They are really tiny, only a few millimeters, and a single hermaphrodite. . . may have 30 to 40 of what they call complementary males. They are tiny individuals and they live in these crevices. They settle there specifically and then they simply supply sperm to the central hermaphrodite. Yes [the hermaphrodites are] carrying their own males, they probably don’t care about being near a neighbor or not because they already have what they need. This one has some really cool biology. All kinds of interesting things are going on.
Checked in Chelonibia testudinaria barnacle movement on plexiglass plate
courtesy of john zardus
ST: When you observed these barnacles in aquariums, were there any difficulties with the experiments?
JZ: We had to figure out how to attach them to surfaces in the lab. We have tried two approaches in my lab here in South Carolina. I started raising them from the larval stage, and I could tie them to PVC pipe and then grow them in the lab just fine. But for some reason, those who lived on PVC did not budge.
My colleague in Taiwan, Benny Chan, tried a different approach. He found crabs that this barnacle lived on. He would catch the crabs, bring them to the lab, euthanize them, cut the crab shell around the base of the barnacle, then let it sit for a few days until the crab shell dissolved. Once dissolved, he could take the intact, healthy barnacle, lay it on a plexiglass panel, and let it sit for a few days. It would create a new glue and cement itself on it. It was the real success. Once we glued them to glass panels, they were the ones we could put into circulation and move them from tank to tank for the different experiments and do time-lapse studies on them.
ST: Do you think your findings could be extended to perhaps other types of barnacles or sessile animal models?
JZ: I think we should definitely look for it. I don’t think we’re going to find that happening much. Do I think other barnacles do this? I doubt. Perhaps in very specific circumstances. We would need to examine them. I think some of the prerequisites for this guy to move is that he has this very flat, wide base with a membranous bottom. There aren’t many barnacles that are like that. These are the candidates that I think we could look at.
Regarding other animals, there has been a recent paper who came out about deep water sponges they found moving. Again, they don’t know how, but they could see traces in the sediment or on the surface where the sponges had traveled. It was unexpected. I think it would be interesting to look elsewhere, but I don’t think it will be a widespread phenomenon.
ST: Where do you see the future of research? What would you like to explore next with these barnacles?
JZ: I think the big question people really want to know is, how do they do this? What is the mechanism? This is where we are headed. We don’t have a lot of hard evidence of how it is currently.
ST: Do you have any ideas or suspicions about how barnacles might move?
JZ: I think it has to do with the glue. Barnacles, when they first attach to the substrate – almost all species – begin to secrete a glue that fixes them permanently in place, and then continue to secrete that glue throughout their lives as they grow. ‘they are getting bigger and bigger. This one does the same thing, but it also has to dissolve its glue. We want to take a look at this and see if somehow the glue is being deposited, then the animal somehow cuts that connection and then reapplies the glue at periodic intervals. .
We also want to take a closer look at this glue. We know it’s a little different from some of the other glues we’ve seen in barnacles. It settles in different layers and the composition is different just by looking under an electron microscope. This has never been reported in barnacle glue before. This is a part that we think will be important to understand.