Robofish: How Color and Tail Wagging Helped Bring a Robot Fish to “Life”

Figure 1

In an age of 3D printing and bionic limbs, distinctions between the manmade and the natural can sometimes blur. Take, for example, the case of the robotic fish depicted above (part A). This little guy is modeled after Notemigonus crysoleucas (image, part B), also known as the golden shiner, and in a recent PLOS ONE study, researchers put it to the test: can a robotic fish influence the behavior of a real fish, and if so, what characteristics enable the robotic fish to do so? According to the researchers at Polytechnic Institute of New York University, answers may depend on the robot fish’s color and the frequency with which it waggles its tail.

To find out more, the authors commissioned the making of two robot fish for this study: one gray and one red. While both physically modeled the golden shiner in many respects, only the gray robot fish was painted to mimic its real-life counterpart. Other than color, the two robots were identical: both consisted of three rigid parts, connected on hinges, and sported silicone tail fins.

pone.0077589 Figure 2

As illustrated above, one robot fish was placed in a water tunnel with a real fish during each trial. The real fish was free to swim in the tunnel while the robot fish “swam,” or waggled its tail fin, in the center of the apparatus. The robotic fish’s tail waggled at various frequencies, ranging from 0 Hz to 4 Hz, as webcams tracked the real fish’s movements. The middle of the tunnel was designated the “focal region” to indicate where fish and robot interaction was likely to occur. The researchers further divided the region behind the fish into four parts, explaining that the robot fish’s tail wagging was likely to affect the water flow, and thus the real fish’s behavior, in this area.

pone.0077589 Figure 4

After reviewing the webcam footage, they found that neither factor (color, tail wagging frequency) working alone had a significant impact on the real fish’s swimming behavior. However, when the gray robot wagged its tail at 3 Hz, the real fish spent a significantly longer time swimming in the center of the tunnel, preferring to spend most of its time swimming right behind the robot. When this happened, the wake created by the robot’s tail wagging could allow the real fish to reduce its energy expenditure while swimming.

What’s so special about wagging your tail fin at 3 Hz, you ask? The researchers ascertained through preliminary research that when golden shiners swim, their tails waggle at 3.32 Hz. In addition, the gray robot’s coloring may have been more attractive to the golden shiner than the red robot’s, as it may have elicited a likeness-related social response in the real shiner. This suggestion is in line with other robot work in comparable fish species.

In other words, the robot fish exerted the most influence—or was the most convincing to the real fish—when its coloring and movements closely corresponded to the coloring and movements of a real fish. Go figure!

If you are interested in learning more, visit our website and see what others had to say.


Citation: Polverino G, Phamduy P, Porfiri M (2013) Fish and Robots Swimming Together in a Water Tunnel: Robot Color and Tail-Beat Frequency Influence Fish Behavior. PLoS ONE 8(10): e77589. doi:10.1371/journal.pone.0077589

Image 1: Figure 1 from the paper

Image 2: Figure 2 from the paper

Image 3: Figure 4 from the paper

Watch Out, Here Comes the Round Goby: How One Fish Changed the Danube River

Round Goby (Neogobius_melanostomus)

As big as a pickle and about as intimidating as one, too, the round goby pictured above doesn’t look like an aggressive invader. But this little fish is naturally invasive and uses more than its meager looks to work its way up waterways, conquering native species, settling new habitat, and creating a completely new ecosystem along the way.

Understanding the invasion process of this fish may help scientists better estimate the ecological impacts that invasive species have on their new environments. The round goby is actively invading the German stretch of the Danube River and the authors of this PLOS ONE study used this opportunity to monitor the characteristics of the invasion to better understand the the process. Specifically, they measured the round goby’s population composition, physical characteristics, and feeding and sexual behavior at ten sites as it invaded a 200 kilometer stretch of the upper Danube River from 2009 to 2011. The sites were divided into ten stages of invasion, ranging from already established populations in the lower portion of the river to the ‘invasion front,’ or where they anticipated the goby would invade next.

Scientists found that the invading goby populations differed from their established counterparts when they compared sex ratio, age, size, feeding, and sexual behavior. Contrary to the researcher’s predictions, invaders were more likely to be female, even though male gobies are more exploratory. The authors suggest that a female-dominated invasion front may allow the established, primarily male population to better handle their roles in parental care and territorial defense.

What’s more, the invaders were physically larger than the established populations and appeared to use their size to establish populations in the ‘invasion front,’ rather than depending on rapid reproduction, or a “frequency in numbers” approach. The ‘invasion front’ contained a wider range of food sources, and the invading gobies’ ability to shift their diet from insects and crustaceans to mollusks may have contributed to their increased size. Overall, these results suggest that the round goby’s ability to physically and behaviorally adapt to changing habitats may play a role in invasion success.

All in all, it took the round goby two years to invade and establish populations in the Danube River study area—a rapid change, ecologically speaking. Fierce territorial defenders, 73% of the fish population is now goby in the settled study sites, leaving behind only fish and invertebrate populations able to compete with the goby for food and territory.

The impacts of these changes remain relatively unknown, but studying them may help researchers estimate impacts on ecosystems during future invasions. The Danube River is not the only habitat facing invasion, as this native of both the Black and Caspian Seas has also hitched a ride to the Great Lakes.

Citation: Brandner J, Cerwenka AF, Schliewen UK, Geist J (2013) Bigger Is Better: Characteristics of Round Gobies Forming an Invasion Front in the Danube River. PLoS ONE 8(9): e73036. doi:10.1371/journal.pone.0073036

Photo: Round goby fish by Eric Engbretson

Antarctic bacteria float through winter

As the Northern Hemisphere shivers through winter, bacteria in Antarctica are employing an inventive strategy to survive the extreme cold: they use a specialized antifreeze protein to latch onto the ice and stay afloat.

Antifreeze proteins generally protect their hosts from freezing by controlling the growth of destructive ice crystals. They were first found in fish swimming in icy waters (see this paper about the evolution and transfer of these proteins between different fish species), and have also been found in plants and bacteria.

The bacterial case now has an interesting twist, published earlier this winter. The authors of the recent study isolated and characterized the antifreeze protein from Marinomonas primoryensis, found in ice-covered Ace Lake in Antarctica. They determined that the bacteria display the protein on their surface, where it can bind directly to ice crystals and anchor the microorganism to the ice. This behavior is a significant departure from what is known about similar proteins, which act inside cells to protect against internal ice crystallization. The image above shows the results from one of the experiments that confirmed the protein is found on the bacterial surface, rather than the interior.

It may not be immediately obvious how binding to ice benefits the bug, but the researchers suggest that it helps the bacteria stay closer to the water surface, where oxygen and nutrients are more abundant. Instead of requiring protection from freezing, these bacteria take advantage of the ice, essentially turning lemons into lemonade – although that may be a metaphor for a different season.


Graham LA, Lougheed SC, Ewart KV, Davies PL (2008) Lateral Transfer of a Lectin-Like Antifreeze Protein Gene in Fishes. PLoS ONE 3(7): e2616. doi:10.1371/journal.pone.0002616

Guo S, Garnham CP, Whitney JC, Graham LA, Davies PL (2012) Re-Evaluation of a Bacterial Antifreeze Protein as an Adhesin with Ice-Binding Activity. PLoS ONE 7(11): e48805. doi:10.1371/journal.pone.0048805

No sense of direction? Consider consulting some carp

The ability to navigate using the earth’s magnetic field is a skill that is not unique to humans.  Over the last few decades, scientists have discovered that numerous organisms have an ability to tell which way is north.  And the list is growing.

In one study, “Magnetic Alignment in Carps: Evidence from the Czech Christmas Fish Market,” Hart et al. reported that carp tend to align themselves along a north-south axis.  The authors photographed over 14,000 carp swimming in plastic tubs at pre-Christmas fish markets and found that, on average, the fish positioned themselves facing either the North Pole or the South Pole.

While the authors have not yet proven that carp can sense the geomagnetic field, they did rule out other possible orientation cues, including light, wind, temperature, and water flow. What benefit a common orientation may provide the fish remains unknown.  One possible explanation the authors present is that it may help the fish coordinate their movement when they swim in a school.

Some other organisms also have an ability to detect localized magnetic fields.  In a paper titled “Desert Ants Learn Vibration and Magnetic Landmarks,” Buehlmann et al. demonstrated that ants can sense a strong magnetic field created by two small magnets and use this as a landmark to find their nest.

In the absence of any other landmark (such as a vibrational, visual, or olfactory cue), ants who had been trained to associate the magnetic field with the nest entrance spent a lot more time near the magnetic field than ants who were naive to this landmark.  It is unclear how relevant this experiment is to ants in their natural environment, but the study nevertheless highlights the ants’ ability to sense a magnetic field.

While little is known about how carp align with the earth’s magnetic field or how ants sense a localized magnetic field, more is known about how some tiny organisms, aptly named magnetotactic bacteria, orient with a magnetic field.  These bacteria form straight chains of nano-size magnetic particles within the cells. The magnetic chains are attached to intracellular structures, thus allowing the bacteria to align passively with the earth’s magnetic field, like compass needles.

In a paper published earlier this month, Kalirai et al. showed that some magnetotactic bacteria form anomalous magnetic chains, with some sections of the chain oriented north and others south.  This finding contradicts scientists’ previous understanding that all the magnetic particles in a single chain would have the same alignment.  The study raises many questions: Would bacteria with anomalous magnetic chains have a competitive disadvantage in their natural environment?  Is there a single genetic mutation that leads to the anomalous magnetic chains?

All three of these studies raise intriguing questions, and we look forward to future discoveries from these scientists.

Image: Arrows indicate the orientation of carp swimming in a plastic tub (Hart et al. PLOS ONE 2012)


Hart V, Kušta T, N?mec P, Bláhová V, Ježek M, et al. (2012) Magnetic Alignment in Carps: Evidence from the Czech Christmas Fish Market. PLoS ONE 7(12): e51100. doi:10.1371/journal.pone.0051100

Buehlmann C, Hansson BS, Knaden M (2012) Desert Ants Learn Vibration and Magnetic Landmarks. PLoS ONE 7(3): e33117. doi:10.1371/journal.pone.0033117

Kalirai SS, Bazylinski DA, Hitchcock AP (2013) Anomalous Magnetic Orientations of Magnetosome Chains in a Magnetotactic Bacterium: Magnetovibrio blakemorei Strain MV-1. PLoS ONE 8(1): e53368. doi:10.1371/journal.pone.0053368

Prowling Catfish Catch Pigeons on Land

Cats hunt birds, and sea-birds hunt fish.  And in some odd ecological pockets, catfish hunt pigeons.

In a study published today by researchers at the University of Toulouse, France, scientists have investigated this unusual predator-prey relationship between European catfish and pigeons in the Southwest region of France.

European catfish have been reported to capture the pigeons on land and drag them back into the water.  This surprising behavior has not been known to occur in the native range of the species; however this article discovers that in France, where the fish are an invasive species, they have adapted their natural behavior in order to feed on novel prey in their new environment.

The researchers completed this study along the Tarn River in Southwestern France.  European catfish originate from Europe, east of the Rhine River, but were introduced to the Tarn River in 1983.

From a bridge above a gravel island on the river, the researchers watched the fish from June through October 2011. Over that time they saw 54 pigeon hunting incidents, and in 28% of these cases, the catfish successfully captured their prey on land and dragged them back into the water to eat them. These attacks were nearly always triggered by active pigeons, as catfish never attacked motionless pigeons. This evidence suggests that the catfish used water vibrations to hunt their prey rather than visual cues.

The cause of this unusual predation behavior is still unknown. However, these new findings may bring us closer to understanding the implications of such novel behavior in a new ecosystem.

To view the fascinating catfish behavior described in this article, please see the video below:

Citation: Cucherousset J, Boulêtreau S, Azémar F, Compin A, Guillaume M, et al. (2012) “Freshwater Killer Whales”: Beaching Behavior of an Alien Fish to Hunt Land Birds. PLoS ONE 7(12): e50840. doi:10.1371/journal.pone.0050840

PLOS ONE News and Media Roundup

Lesions found on coral trout.

Last month, the media covered PLOS ONE papers on germs in airports, skin cancer in fish, a potentially life extending pill, and more!

Research by a team at MIT identified New York City’s JFK, Los Angeles’s LAX and Honolulu’s HNL as the nation’s airports most likely to influence the spread of a major pandemic in the first few days of an emerging disease. The team used geographical information, traffic structure and individual mobility patterns to model contagious disease dynamics through the air transportation network. The study was covered by NPR, CNN, and Wired.

A recent study comparing a hunter-gatherer population with a modern Western population found that daily energy expenditure between the two populations is not all that different; challenging the view that obesity in Western society is largely due to a lack of exercise.  This research may encourage shifting the focus of this debate to the importance of calorie consumption and was covered by The Atlantic, Mother Nature Network and the BBC.

Dark patches found on fish in the Great Barrier Reef have been identified as a deadly form of skin cancer, melanoma. “Evidence of Melanoma in Wild Marine Fish Populations” is the first published study of melanoma in a wild fish population but it is unlikely the problem is new. The Great Barrier Reef sits under the largest hole in the ozone, exposing fish populations there to high levels of UV radiation. The image above is Figure 1 of the manuscript. The study was covered by Science, LA Times and Scientific American.

Autistic children may benefit from getting a pet. According to this study by a French research team, children who received a pet around the age of five showed improved social skills, including increased ability to share with and comfort others, compared to autistic children who either grew up with a pet or never had one. US News, Fox and Time all covered this study.

Findings from the study “Randomized Polypill Crossover Trial in People Aged 50 and Over” suggest that people over fifty may benefit from taking a once daily “polypill” comprised of three blood pressure-lowering drugs and a cholesterol-lowering statin. Read more at CBS, Reuters and ABC.

For more in-depth coverage on news and blog articles about PLOS ONE papers, please visit our Media Tracking Project.

PLoS ONE News and Blog Round-Up

Researchers find a possible cure for the common cold and more – in this week’s media digest.

Human Pathogen Shown to Cause Disease in the Threatened Eklhorn Coral Acropora palmata was covered by The NewsHour, The New York Times, NPR, and CNN.

CNET, Hindustan Times, and Okezone covered Automatic Prediction of Facial Trait Judgments: Appearance vs. Structural Models.

The paper, Predator Cat Odors Activate Sexual Arousal Pathways in Brains of Toxoplasma gondii Infected Rats, received coverage from The New York Times, Scientific American, TIME’s Healthland, and The Loom.

Broad-Spectrum Antiviral Therapeutics received media coverage from Voice of America, LA Times, and Forbes.

Large Recovery of Fish Biomass in a No-Take Marine Reserve was covered by National Geographic (nice slideshow too), The Christian Science Monitor, Nature News, and KGTV San Diego. The image above, is taken from Figure 4 of this manuscript.

80 Beats covered the article, Artificial Skin – Culturing of Different Skin Cell Lines for Generating an Artificial Skin Substitute on Cross-Weaved Spider Silk Fibres. So did Treehugger.

The article entitled, Scientists Want More Children received media coverage from The Wall Street Journal, TIME’s Ecocentric, Science Career Blog, and Inside Higher Ed.

Elevated Non-Esterified Fatty Acid Concentrations during Bovine Oocyte Maturation Compromise Early Embryo Physiology was covered by Reuters, The Guardian, and The Press Association.