Fluid Physics of Movement in Marine Snails Could Lead to Novel Bio-Inspired Underwater Vehicles

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Fluid Physics of Movement in Marine Snails Could Lead to Novel Bio-Inspired Underwater Vehicles

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Cuvierina atlantica, a thecosomatous pteropod with an extended shell. Credit: David Murphy and coauthors

Poetry in movement: Engineers evaluate the fluid physics of motion in marine snails.

In the world’s oceans, billions of small marine snails (a kind of plankton) commute daily in between surface area waters, where they feed in the evening, to depths of a number of hundred meters throughout the day to rest while preventing predators. Marine snails play an essential function in geochemical cycles and environment: 12-13% of the international carbonate flux happens when the calcium carbonate shells of dead snails sink to the depths, where they liquify and add to climatic carbon and ocean acidification. But since they are challenging to study and can’t be kept in the lab, the habits of these animals — which bear poetic names such as sea butterflies — is badly understood, particularly for the subtropical and tropical areas where their variety is biggest.

Here, a group of oceanographers and engineers who concentrate on research study at the crossway of fluid physics and biology, movie the motions of tropical marine snails and evaluate these both from a fluid physics and environmental point of view. They reveal that each types has an unique design of swimming and sinking, stunning to enjoy, depending upon the shape of their shell (coiled, lengthened, or round), body size, existence of flapping “wings”, and speed. The tiniest, slowest types have more trouble swimming due to seawater being “stickier” and more thick for them — in technical terms, with a lower “Reynolds number” — which impacts the angle, trajectory, and stability of their motion.


Hyalocylis striata, a thecosomatous pteropod with an extended shell. Credit: David Murphy and coauthors

“We wanted to answer how the swimming behavior of these beautiful animals is affected by their different shell shapes and sizes. We found that species with a shell shaped like an airplane wing swims faster and is more maneuverable than those with ‘snail-like’ coiled shells. Understanding the swimming ability of these animals is helping us better understand their ecological importance and distribution in the ocean. Further, as engineers, we hope to learn from the swimming style of these organisms to design a new generation of bio-inspired underwater vehicles,” states matching author Dr. David Murphy, Assistant Professor at the Department of Mechanical Engineering of the University of South Florida, Tampa, Florida.

Between 2017-2019, the scientists captured several people of 9 types of marine snails (0.9-13.1 mm long) in the evening off Bermuda, consisting of 7 types of thecosomatous pteropods (“sea butterflies”), one types of gymnosomatous pteropods (“sea angels”, which do not have a shell as grownups), and one types of atlantid heteropods. They transferred them to the lab, where they taped their habits in a salt-water fish tank with high-speed stereophotogrammetry, a method that tracks motion in 3D with a set of cams. For each types, they computed the outright and stabilized speed (relative to body length) throughout active swimming and passive sinking, the frequency of wing motion, the angle of descent throughout sinking, the tortuosity of the course of climb throughout swimming, and the Reynolds number.


Heliconoides inflatus, a thecosomatous pteropod with a coiled shell. Credit: David Murphy and coauthors

They reveal that each types has an unique swimming pattern, usually rising in a saw-toothed spiral at 12-114 mm/s, or 1-24 body lengths per second — representing an average-sized human male swimming at as much as 40 m per second. The snails sink at comparable speeds, however in a straight line, at an angle of 4-30° relative to vertical.

“We conclude that the swimming and sinking behavior of these pelagic snails corresponds strongly with shell shape and size. Tiny snails with coiled shells swim more slowly whereas larger snails with bottle-shaped or wing-shaped shells swim faster because their larger sizes allow them to overcome the effects of water viscosity. However, swimming speed does not correlate with how far these animals migrate each day, which suggests that light and temperature levels and the presence of predators and prey also play a role. We also found that the sea butterfly with the wing-shaped shell uses its shell to ‘hang-glide’ downwards in order to slow its sinking,” states Murphy.

To research study each types’ depth choices, Murphy et al. even more tested great deals of snails with a computer-operated web, called a Multiple Opening/Closing Net and Environmental Sensing System, 0-1000 m listed below the surface area. They utilized artificial intelligence (based upon images) and ribosomal DNA barcoding to figure out types. Based on these outcomes, the scientists approximate that these types travel 50-300 m each day, in a day-to-day vertical “commute” that takes an overall of 1-3.7 h each day.

“It’s absolutely mesmerizing to watch these tiny, delicate animals flap their wings in really complex motions in order to essentially fly through the water. We’re lucky to have high speed cameras that can slow down this motion enough for us to see it. And it’s stunning to think that these sea butterflies are using the same fluid dynamics principles to fly through water that insects use to fly through air,” concludes Murphy.

Reference: “Swimming and Sinking Behavior of Warm Water Pelagic Snails” by Ferhat Karakas, Jordan Wingate, Leocadio Blanco-Bercial, Amy E. Maas and David W. Murphy, 7 September 2020, Frontiers in Marine Science.
DOI: 10.3389/fmars.2020.556239