Undergrad researcher helps track methods of movement, propulsion

By Emma Lederer

Deep into my second summer with the Rhode Island NSF EPSCoR Summer Undergraduate Research Fellowship (SURF) program, I am often struck by how the details of my work compare and contrast between last year and this year.

Last year, I looked at fluid movement of invertebrates, siphonophores and jellyfish. This year, I am looking at pectoral fin placement in different classes of invertebrates. Although the topics of the projects are quite different, the nitty-gritty details of what I do in the lab this year are closely related to my experience last summer.

I worked on two SURF 2015 projects — “Predicted jet velocities of the hydromedusae Sarsia tubulosa during bell contraction” and “Coordination of multi-jet propulsion the siphonophore Nanomia bijuga.” I did a lot of work with image analysis and had the opportunity to work with another Providence College student, Jillian O’Melia.

The first project sought to develop a method for measuring how different regions of the semi-circle-shaped bell of a species of jellyfish, Sarsia tubulosa, contributed to overall jet formation during propulsion. Sarsia are a type of small, simple, jellyfish with smaller tentacles and feeding arms than the more complex ones you might find on a lion jellyfish.

I ended up watching a lot of videos of the Sarsia last summer, where they were magnified times 10 on a computer screen. So, sometimes it is easy to forget how truly small and simple they are. I probably wouldn’t notice one if I swam by it in the ocean.

However, a Sarsia’s small size doesn’t lessen its importance. Studying its method of contraction can help reveal more about how jet propulsion works.

We went into our project with the notion that jellyfish jet propulsion may be a little more complex than simply contract and release. Our approach was to divide the bell of the jellyfish into 10 evenly spaced slices and measure the combined contributions of these volumes to the velocity of water as it is pushed out if its bell during contraction. We looked at differences in the diameter of the slices throughout contraction to see which areas of the bell, if any, contributed more to contraction. Our results were inconclusive, but I did learn a lot about the different types of video analysis we used for this project, like particle image velocimetry (PIV).

In my second project I investigated whether there was a pattern in the way N. bijuga contract their individual jets when they move in a linear path. N. bijuga are siphonophores – a gelatinous sea animal made up of multiple jets that are fired at different times in order to move the animal in its desired direction.

One of the most interesting animals I have ever worked with, siphonophores are simple creatures; they are almost transparent, only 5 to 10 mm long, and made of a jelly-like substance. They are so fragile that they almost always disintegrate when divers try to catch them. They have long stinging tentacles you might not notice until you’re stung.

And what’s more, the way siphonophores move is incredibly complex for something so small. They coordinate the firing of sometimes 20 to 30 jets in order to propel themselves. The process happens so quickly, the complexity of what is taking place does not immediately reveal itself. But, breaking it down with a high-speed camera shows that the asynchronous, seemingly sporadic firing of jets can somehow lead to steady forward movement.

Our goal was to understand whether there was some coordination in the contraction of the individual jets. Siphonophores have the ability to maneuver through their environment much more precisely than other jet-propelled animals like jellyfish because of their coordinated multi-jet system. Understanding this coordination could help to build underwater vehicles that can move as precisely as siphonophores by utilizing multiple jets.

As part of my role in the project, I watched high-speed videos of two individual siphonophores and analyzed them by calculating contraction length and frequency of contraction of each individual jet, called a nectophore, during a contraction sequence.

The data showed that the first nectophores of each individual siphonophore were often either inactive, or they were contracting so minimally that this movement could not be recorded within the parameters of the video quality used. This is most likely due to the fact that the back nectophores provide more power for movement, while the front nectophores rotate the colony. These findings suggest that in a colonial jet system, like the one siphonophores utilize, some mode of coordination is essential for efficient movement. With further analysis, these patterns could be more easily tracked and add to the broader understanding of fluid movement in invertebrates.

“I have become so captivated with my work this summer and last because while scientists and engineers understand why animals bend when they turn, nobody has been able to pinpoint, yet, how they accomplish this.”

Last summer, I spent most of my time doing image analysis; I would watch videos of siphonophores swimming and take note of the frames when each nectophore contracted. Later, I looked back at the data and tried to find trends or patterns in the contraction cycles. This summer, my work is not so different. I’m looking at huge numbers of images, measuring them, and then hoping to find trends in the unity or diversity of my measurements.

In this 2016 project, we are looking into the dynamics of turning motion in fluid for vertebrates that move using bending and fins, which contrasts to the jet propulsion models I looked at last year. While there have been plenty of studies about the dynamics of straight, steady-swimming motion, there is less published about turning.

Aquatic animals have to cope with many challenges such as changing currents, predators, and feeding, and as a result they seem spend a lot of time turning and twisting rather than moving straight forward. As a result, studying the turning and twisting motions of aquatic species is critical to understanding how animals move in fluid.

There are multiple factors that affect how animals move in water. By understanding individually how things like the shape and fin placement of an animal as well as energy expenditure affect turning in water, a better picture of how these factors work together to propel an animal may emerge.

I have become so captivated with my work this summer and last because while scientists and engineers understand why animals bend when they turn, nobody has been able to pinpoint, yet, how they accomplish this.

Animals turn in a way that allows for them to maximize the efficiency of that turn. And, since animals are the most efficient vehicles on the planet, understanding how they have been able to develop such an efficient and complex system are concepts that could help engineers develop efficient turning systems for vehicles.

Right now, we are just getting started at tackling such a big goal and complex questions. I have been looking at differences in pectoral fin placement of different classes of animals to see how it could affect fluid turning by studying pectoral fin placement in different aquatic animals.

There is so much work to do and so much left to learn, and I’m excited to see what the rest of the summer has in store both for me and science.

(Last week, Lederer introduced herself and wrote about how she came to be an undergraduate researcher with Rhode Island NSF EPSCoR’s SURF program.)