The House That Snot Built
Meet the giant larvacean. A relative of the sea squirt, this marine invertebrate lives in the upper hundred meters of the ocean, is about the size of your pinky, and every day builds itself a mucus “house” more than a meter across. By filtering water through its house, the organism traps food particles that drift down from above. Then, when the structure becomes too heavy and clogged with debris, the larvacean lets its house drop to the seafloor and builds anew.
Writing in Science Advances, researchers have now identified exactly how much seawater a giant larvacean can filter as it eats. The amount is surprisingly large: on average, about 45 liters per hour. On a good day? As much as 80 liters per hour.
All of this points to a creature that could play a significant role in cycling carbon from higher in the ocean to deeper reaches, says Kakani Katija, a principal engineer at the Monterey Bay Aquarium Research Institute and lead author on the paper. She describes the strange life of the giant larvacean, and where a better understanding of its biology could take ecologists and engineers alike.
Kakani Katija is Principal Engineer at the Monterey Bay Aquarium Research Institute in Moss Landing, California.
Bruce Robison is a senior scientist and midwater ecologist at the Monterey Bay Aquarium Research Institute in Moss Landing, California.
FLORA LICHTMAN: This is Science Friday, and I’m Flora Lichtman. And now we visit with a creature you definitely should know about. It’s the giant larvacean, which is pinky-sized, but giant for a larvacean. It’s translucent, it’s an invertebrate, and it has a huge footprint made of mucus. Yes.
These little creatures build elaborate gooey homes up to three feet across. They use this mucus to catch food, filtering bits of detritus out of the ocean. And now researchers have an idea of how much water they sift through in a given day. Just one giant larvacean can filter up to 20 gallons per hour. Monterey Bay’s community of giant larvaceans can filter all the water around them in less than two weeks.
You can see a video of these giant larvacean at work on our website, sciencefriday.com/mucus, appropriately. And all of this has some exciting implications for their role in marine ecology and carbon cycles. Here to tell us more are my guest. Dr. Kakani Katija is a principal engineer at the Monterey Bay Aquarium Research Institute in Moss landing, California. She’s the lead author in the new research. Welcome to the show.
KAKANI KATIJA: Thank you very much.
BRUCE ROBISON: And her co-author, Dr. Bruce Robison, is a senior scientist and mid-water ecologist at the same institute. Welcome, Dr. Robison.
BRUCE ROBISON: Hi, Flora. Thanks.
FLORA LICHTMAN: Kakani, I want to know everything there is to know about giant larvaceans.
KAKANI KATIJA: (LAUGHING) Me, too.
Where do you want to start?
FLORA LICHTMAN: Well, you know, here’s the thing that first caught my interest. They’re pinky-sized, but they’re a type of plankton. Is that right?
KAKANI KATIJA: Right. Well, so by definition, plankton is essentially an organism in the ocean that just drifts with the current. And so larvaceans are, I think, exceptional of the plankton in that they create these really elaborate mucus houses. And as you said earlier, those houses serve to separate food and particles from the water around them, and do so by concentrating all that food. And what’s really interesting is the animal–
FLORA LICHTMAN: Before you go on, I want to give people a visual image of these mucus houses.
KAKANI KATIJA: Sure.
FLORA LICHTMAN: So what do they look like?
KAKANI KATIJA: That’s a– it’s hard to describe. It looks a little bit like a diaphanous cloud. And imagine that that’s just basically built by creating strands of mucus. And you know, if you were to take these things out of water– I mean, it looks like a snot ball, essentially.
FLORA LICHTMAN: Delightful.
KAKANI KATIJA: But in water, they’re just– I know, it really is delightful. But I mean, in water they just have so much complexity to them. They look like they have different chambers in them. And they all play a role in essentially feeding this organism.
FLORA LICHTMAN: Where can we find these larvaceans, these giant larvacean?
KAKANI KATIJA: Well, we can find them really easily in Monterey Bay. And I know that there are also reports of them in other oceans around the world. I’m sure Bruce has something to say about that.
BRUCE ROBISON: Sure. They’ve been reported from the Atlantic, the Indian Ocean, and all through the Pacific Ocean. The issue is that while we know that they occur in many different places, we have no idea of how abundant they are in other parts of the world ocean. In Monterey Bay, they’re quite abundant, and we’ve measured that and reported on it in this paper. What’s curious, and of interest to us in the future, is are these patterns of abundance that we see here reflected elsewhere in the world ocean?
FLORA LICHTMAN: And they– you know, mucus houses aside, and I actually do want to talk more about them, but they play an important role in the ecosystem. Is that right?
BRUCE ROBISON: They sure do, in a number of ways. By filtering out particles from the water column in order to feed, they select a certain size range that’s appropriate for their mouth and their digestive system. Anything larger than will fit into a relatively small animal is excluded by the outer filter of their house. And that means all of this big stuff collects on the outside of the filter. So when the animal has passed enough water through the system that the outer filter becomes clogged with these big particles, the animal just swims away and builds a new house. The old house collapses on itself and compresses into a relatively dense package and sinks very rapidly to the sea floor.
FLORA LICHTMAN: And then it becomes a snack.
BRUCE ROBISON: Maybe more than a snack.
FLORA LICHTMAN: Oh.
BRUCE ROBISON: A big meal for a lot of animals that live on the bottom.
FLORA LICHTMAN: How important is that mucus house to feeding those bottom dwellers?
BRUCE ROBISON: We think that it’s quite important, because it’s high-quality nutrition. Most of the detritus that sinks out at the upper layers of the ocean and goes down to the deep sea floor sinks very slowly and, as a consequence, it can be decomposed by microbes on the way down. These discarded larvacean houses sink really fast. So instead of taking a month or more to reach the deep sea floor, these sinkers, as we call them, get down there in a day or two.
FLORA LICHTMAN: If you have a question about mucus houses or giant larvaceans, you can give us a ring– 844-724-8522. 844-SCI-TALK. Kakani, you built a device to help us understand these mucus houses better. Tell us about it.
KAKANI KATIJA: Right, exactly. Well, so my background is– I’m an engineer. And one of the things that I have done, or spent several years doing, is studying fluid mechanics. And so there’s a technique that’s very common to laboratory measurements of fluid mechanics, and what I wanted to do was take that technique and reapply it in the oceans, where the organism or where the system you’re interested in studying will be.
And the reason for that is you can imagine the ocean is a fluid world. And understanding the physics of that motion in that fluid world or in nature is really important. And so this instrument we call DeepPIV really was a result of that, whereby we can affix this instrument to a remotely-operated vehicle or an underwater robot and look at these kinds of questions– you know, maybe relating to larvacean feeding, or something else.
FLORA LICHTMAN: This is when I really love science, when we build tools to help us understand mucus houses. You know?
KAKANI KATIJA: [LAUGHS]
FLORA LICHTMAN: This is so awesome.
KAKANI KATIJA: Exactly. I appreciate it. I’m sure other people do, too.
FLORA LICHTMAN: Did you imagine that that’s– those are the kind of tools you would be inventing?
KAKANI KATIJA: You know, it really is often a question of the science question, as well. I think I was more shocked at how successful it has been. Because when we started the development of the instrument, I think that was the summer of 2014. And within a year, we were deploying the first version of this instrument on a remotely-operated vehicle here. And so that’s a really rapid cycle of development and then building and then deploying. So it’s been a really wonderful experience, then, seeing for the first time what the instrument then reveals when you’re looking at something like a larvacean house.
FLORA LICHTMAN: Why can’t you just pull the larvacean out of the water and study it in the lab?
KAKANI KATIJA: Well, I’m sure Bruce could speak to this, too. But I guess for years, scientists have been collecting giant larvaceans and actually bringing them into the lab. But because they make these mucus houses that are really sticky– and these houses are quite large, right? They’re on the order of a meter across. It’s difficult– in fact, they haven’t been able to get them to replicate and build homes or houses in the lab. And so because of that, we essentially were driven to try and develop some technologies that will allow us to study them in their natural environment.
FLORA LICHTMAN: Because I guess scooping them out, the mucus house just turns into a glop of mucus?
KAKANI KATIJA: Essentially. Essentially, yeah. It just balls up, and then it can’t separate. The walls can’t separate from each other.
FLORA LICHTMAN: And what does the tool actually do? What are you looking at, how fast the water’s filtered through? Or what were you trying to understand?
KAKANI KATIJA: Essentially, yeah. So the instrument really is made up of an imager– so in our case, a high-definition video camera– and a light source, which in our case is a laser with some optics that changes the laser beam. You know, think of a laser pointer you might use with your cat. But instead, instead of a single beam, you’ll have something that looks like a sheet, a sheet of light.
And so what we’re seeing or what we’re capturing when we shine this sheet of light on a larvacean, let’s say, is that the gelatinous or the mucus tissues light up in the laser light, but also the ambient suspended particulate that’s present in the ocean. And so we can actually track the motion of these particles to infer the motion of fluid around these animals.
FLORA LICHTMAN: Bruce, how often do giant larvaceans make these homes for themselves? Like, how often are they cycling through homes?
BRUCE ROBISON: We’re pretty sure that they make about one each day.
FLORA LICHTMAN: (SHOCKED) What?
KAKANI KATIJA: We did– yeah. We’ve measured them in the water column for more than 20 years, counting the number of active houses with animals in them, filtering away, and how many were sinking as abandoned houses. And over a 20-year stretch, it turned out that there was about one active house for each sinker each day. And so the math is pretty simple. It looks like, on average, they’re building one house a day.
FLORA LICHTMAN: This seems like a– like an existential kind of torture, to build your house over and over again every day?
BRUCE ROBISON: But the bottom line is that it works. And mucus is pretty cheap stuff.
FLORA LICHTMAN: Well, I was wondering, where does all– that’s a lot of mucus to produce. These houses are three feet wide.
BRUCE ROBISON: That’s true. But mucus is mostly water, just sort of held together with a mucopolysaccharide kind of network. So it’s really cheap to build. And again, these animals wouldn’t be doing it this way if it were inefficient. Obviously it works, because it’s been that way for quite a long time.
FLORA LICHTMAN: How does the mucus house– so it feeds animals at the bottom. But does it also impact the amount of CO2 that’s sequestered by the ocean?
BRUCE ROBISON: We think that it certainly does impact– have a positive influence on CO2 sequestration. Because the carbon in the organic materials that these animals are filtering in and eating is derived from CO2 that’s absorbed by phytoplankton at the surface. And they incorporate the carbon from CO2 into organic material. And then the larvaceans package it up and send it to the deep sea floor. So in essence, they are taking CO2, rendering it into organic material, and sequestering it on the deep sea floor, far from the atmosphere.
FLORA LICHTMAN: I’m Flora Lichtman. This is Science Friday from PRI, Public Radio International. So Bruce, should we just be stocking the oceans full of larvaceans to deal with climate change?
BRUCE ROBISON: I think we shouldn’t mess with Mother Nature that way. Ask the people in Hawaii whether or not it was a good idea to import mongooses to take all of the rats that were in the cane fields, or the gardeners who brought in kudzu from Japan in order to be an ornamental plant.
FLORA LICHTMAN: Fair point. That’s totally fair. I mean, are they threatened in any way, though? Like, do we have to worry about their role in the ecosystem?
BRUCE ROBISON: So far as we can tell, there appears to be no negative impact on them that we can see. There are some indications that as the ocean gets warmer and thus has less oxygen in it, they may be squeezed closer and closer to the surface, in which circumstances they build smaller houses which can’t load up and carry as much carbon away from the surface layers. But that’s just that an indication. We’ve really got no way to test it yet.
FLORA LICHTMAN: Are there any other outstanding questions about the giant larvacean and its mucus house?
KAKANI KATIJA: Wow, where do we start, Bruce?
BRUCE ROBISON: Yeah.
KAKANI KATIJA: I can say from an engineering standpoint, what really interests me with these organisms is their house. Like, how do they build these homes? You know, these are very intricate structures, and also very complex. There’s also differences between these structures as you look at different species of larvaceans. You know, we’re working with giant larvaceans, but the smaller larvaceans, the Oikopleura, also have a lot of variation.
And so obvious questions, too, if you think about it in terms of bio-inspired design, or bioinspiration, you know, these might be models for really efficient or effective filtration systems, and maybe that’s something that we can learn from. And so developing these tools to investigate these complex structures in the ocean is going to be really useful and important for us to address some of these questions.
FLORA LICHTMAN: Very cool. Bruce, you? Are there any questions that you’re still hoping to answer about these creatures?
BRUCE ROBISON: Well, Kakani put her finger on it. The big question is, how do they do it? We can see that basically, it appears that the animal extrudes a glob of mucus and then pumps it up. But that means that a complex structure within that filter has to be already built into the mucus blob that the animal inflates. How do they do that?
FLORA LICHTMAN: I can’t wait to find out.
BRUCE ROBISON: (LAUGHING) Neither can we.
FLORA LICHTMAN: I want to thank you both for taking time to be with us today. Kakani Katija is a principal engineer at the Monterey Bay Aquarium Research Institute in Moss Landing, California. Bruce Robison is a senior scientist and midwater ecologist there. Thank you both.
BRUCE ROBISON: Thank you, Flora.
KAKANI KATIJA: Thank you.