Tag Archives: water

Do Hammer-Shaped Heads Help Sharks Swim?

With their sandpaper skin, cartilage skeleton, electroreceptive sensors, and rows of dangerous teeth, sharks fascinate many people. However, even within this distinctive group the hammerhead sharks that make up the Sphyrnidae family have attracted a special attention due to the unusual shapes of their namesake heads, called cephalofoils. Several evolutionary benefits of the cephalofoil have been proposed by researchers. The wide hammer-shaped head may allow the shark to house more sensory receptors in its snout, to bludgeon prey, and to move and maneuver through the water more easily. Here we will address the question posed by the third theory: Does the cephalofoil found on hammerhead sharks provide an advantage in moving and maneuvering underwater?

Great Hammerhead Shark
Image of “Great Hammerhead Shark” by Wendell Reed showing a close-up of the cephalofoil. https://search.creativecommons.org/photos/3fa18a9b-9085-4867-93e1-15a88b01389b

Many advancements in the aviation and nautical industries have been developed from the study of sea creatures. There is certainly some potential that research into the mobility of sharks could someday be used as inspiration to advance locomotion technologies. In addition, a deeper understanding of the physiology and behavior of hammerhead sharks could help us to better preserve their habitat and species from endangerment – a crisis which some of them are already facing.

The theory that the cephalofoil provides advantages in forward swimming to hammerhead sharks relies on it supplying some hydrodynamic lift similar to the wing of an aircraft. Aircraft wings provide lift partly by creating a pressure difference between the top and bottom wing surfaces. An area of higher pressure on the bottom surface of a wing will generate upward lift. One study by Matthew Gaylord, Eric Blades, and Glenn Parsons applied a derivation of the Navier-Stokes equations – a set of partial differential equations for analyzing fluid flow – to water flow around digitized models of the heads of the eight most common species of hammerhead. It was found that in level, forward swimming there was some pressure differential that developed between the dorsal (top) and ventral (bottom) surfaces of the cephalofoil, but for each species it was very small and often in the direction to produce negative lift. The drag coefficients of the cephalofoil of each hammerhead species were then calculated and shown to increase as the size of the cephalofoil increased. The drag created by a cephalofoil was always much greater than the drags caused by the heads of a control group of non-hammerhead Carcharhinidae sharks.

Images of the pressure contours at zero angle of attack on the dorsal (top image) and ventral (bottom image) sides of the cephalofoil. The eight leftmost sharks are the hammerheads. Taken from Gaylord, Blades, Parsons.

However, the same study showed that the pressure difference between either surface of the cephalofoil did significantly increase in some species if the shark raised or lowered its head. This extra hydrodynamic force caused by the pressure differential at nonzero angles of attack would help the shark to turn its head up or down very quickly. This more explosive maneuverability was particularly present in the hammerheads that commonly feed on fish and less present in the species that feed predominantly on slower bottom dwellers. Another study by Stephen Kajiura, Jesica Forni, and Adam Summers theorized that the unbalanced, front-heavy cephalofoil may provide extra stability during tight turns by preventing banking. The unbalanced head would create a moment – or torque – to counteract the force from the tail that causes most sharks to roll into their turns. Not banking around turns could be important to some hammerhead sharks that often swim so close to the seafloor that banking into a turn could cause their head or fins to bump into the floor. It is likely that hammerhead sharks evolved the cephalofoil at least in part to provide more explosive and stable maneuvering.

Featured image “Hammerhead Shark” by bocagrandelasvegas.

Dolphin Magic or Dolphin Muscle?

Because of the film Bee Movie, many people at one point were intrigued by the idea that bumblebees should not physically be able to fly due to their large bodies and tiny wings. But, they fly anyway. Technology is advanced enough to study bee wing movement and determine that they produce enough lift to allow them to fly, disproving the previous notion. Similarly, Gray’s Paradox for a long time inferred that dolphins should not be able to swim nearly as fast as they do. But, they still consistently swim at speeds over twenty miles per hour. It was not until recent history that advancements allowed researchers to determine why they are able to reach such high speeds.

Gray’s Paradox

All the way back in 1936, Sir James Gray observed the high speeds dolphins could reach in the ocean. He calculated an approximation of the amount of power the dolphins would need to produce to sustain these speeds, based on the drag force on the dolphin as it travels through the water. Gray compared this to the amount of power he expected the dolphin to be able to produce. In order to compute this, Gray used muscle power data from oarsmen. When he compared the muscle mass of these oarsmen compared to dolphins, he determined that the power dolphins could produce was only about one seventh what was needed to travel at the high speeds of which they are capable.

Force Diagram, showing that the same forces that the swimming mammal applies to water are applied back on it. Allows observation of max speed to determine these forces.
This diagram shows that the drag force, D, thrust force, T, and net axial force, Fx, must be equal for the swimmer and the fluid. The lateral velocity, u, can be used to determine the resulting drag force, allowing researchers to estimate how much thrust is needed. Credit: [2]

And now we have arrived at Gray’s Paradox. What allows dolphins to move so quickly? To Gray and other researchers for most of a century, this was a mystery. If the assumptions they had made were correct, that would mean dolphins have some way of travelling through water more efficiently than was thought to be possible. This sparked a large amount of speculation into how dolphin skin could reduce the drag force of the water, which was originally believed to be the way Gray’s Paradox would be resolved.

Answering Questions while Creating More

Finally in 2008, Timothy Wei’s research team was able to definitively disprove Gray’s Paradox. He set up an experiment that would allow the force that dolphins exert to be measured. This mainly consisted of having dolphins swim through a curtain of bubbles in a tank. By recording at high resolution the movement of these bubbles as the dolphins swam by, the researchers determined the speed of the water around the dolphin as it traveled. With this information, Wei’s team showed that dolphins are able to produce over 300 pounds of force at one moment, and over longer periods of time 200 pounds of force. This is approximately ten times more force than Gray estimated.

Wei’s findings resolve Gray’s paradox by showing that dolphins have the ability to produce sufficient power from their tail movement to overcome the strong drag force of the water as they move at high speeds. However, this does not explain how dolphins produce so much power with their amount of muscle mass, which is still being examined. One idea is that this is caused by anaerobic muscle fibers that behave in different ways than in humans, and allow more power to be generated than Gray expected.

Future Plans: Investigating Force Generation

Timothy Wei plans to continue examining force generation in the swimming of other marine animals. This has the potential to provide more understanding of how marine animals evolved in their swimming aptitude. On the level of microbiology, this research could improve understanding of how dolphin and other animal muscles can perform such high levels of power generation over sustained periods of time.

Additional Reading and Sources

sticks and stones may break my bones but dirt will wash right off

There you are, sitting in the park eating your spaghetti picnic on your favorite picnic blanket when your pollen allergy acts up. You let out a sneeze powerful enough to compete with Aeolus’ bag of wind, but now your spaghetti is all over your favorite picnic blanket. You immediately go to rinse it off, but your fine Italian sauce has thoroughly soaked in. If only nature had a solution to keep a surface clean. Enter: the lotus leaf.

The lotus leaf is renowned for its ability to stay clean in murky environments. This characteristic of the plant is regularly attributed to its superhydrophobic surface features and chemistry. A superhydrophobic surface is a surface which can maintain a contact angle with water above 150o and is correlated with a low free surface energy—which really means water pools and rolls off rather than soaking into the surface.

Nearly perfectly spherical water droplet on an artificially prepared surface

Modified from Zorba et al. 2008

A key attribute of the superhydrophobic surface is a hierarchical micro- and nanostructure. The microstructure is composed of plant cells grown in little mounds known as a “papillae” with small channels for air flow in between called “stomata.” The nanostructure is composed of hair-like wax crystal towers (epicuticular wax) built on the peaks of the papillae topography. The elevated wax towers combined with the stomata trap air and reduce the contact area of the water with the surface. The epicuticular wax chemistry reduces the adhesion to the towers themselves by being naturally hydrophobic.

Graphic of water drop resting across uneven wax pillars on a lotus leaf

Modified from Zorba et al. 2008

The tips of the wax towers create the largest repelling forces which form larger contact angles, while shorter towers can actually produce adhesive forces that reduce the contact angle. If the air is displaced and filled with water, the contact angle will decrease due to the water-water adhesion which “pulls” the droplet to the surface. Similarly, if the surface is damaged, the wax can be removed and decrease the surface’s hydrophobicity. The wax is naturally soft material and prone to mechanical damage increasing water adhesion and reducing the self-cleaning abilities of the leaf.

The papillae topography is the key to the robustness of the lotus leaf hydrophobicity. The papillae create natural valleys and creases which—like the tops—are still densely packed with wax hairs. When the surface is impacted, only the top of the papillae are exposed to the mechanical force so the wax tubules in the valleys are left undeformed and maintain their hydrophobic characteristics.

Water beads on rain jacket

Photo by Chase Pellerin via Gear Patrol

Hydrophobic surfaces have many applications in everyday life, for example rain jackets and umbrellas perform their best when they are hydrophobic. Manufacturing processes rely on hydrophobic surfaces to reduce oxidation and stay clean in past-paced environments, and your favorite picnic blanket would be much less prone to spaghetti stains if it were hydrophobic. Nature has solutions to keeping surfaces clean; we just have to recognize them.

Swimming Fast and Slow: What We Know About the Sailfish’s Iconic Fin

Indo-Pacific Sailfish
Indo-Pacific Sailfish. Credit: D. Corson/Shostal Associates

Sailfish, or Istiophorus platypterus, are one of the most recognizable fishes in the ocean due to their large sail-like dorsal fin. But, did you know that they are also iconic because they are one of the fastest swimmers in the ocean? Sailfish top speeds have been recorded to be up to 30 m/s, which roughly translates to 67 miles per hour. When researchers examined the sailfish swimming out in the ocean, they discovered that they have the unique ability to retract and deploy their sail and other fins. Furthermore, they saw that when swimming at top speeds, swordfish retract their sail, and when the fish are hunting prey, they deploy it. How does retracting the sail help them swim fast, and how does deploying the sail help them hunt? 

Sailfish swimming with both the sail retracted (0:24) and extended (0:40). Disregard the silliness of the video. Credit: Pew

Speeding Up

When it comes to swimming fast, the answer may have to do with drag. Due to their hydrodynamic characteristics, sailfish have a very low drag coefficient, which is the quantity used to describe the resistance of an object moving through fluid. Sailfish have a drag coefficient of 0.0075, which is similar to smaller fish such as pike, dogfish, and small trout. Additionally, due to their size, sailfish are able a generate much more force with each swimming motion than their smaller peers. The combination of these two factors allows them to move at such high speeds. The thing that truly sets sailfish apart, though, is this ability to retract their sail and pectoral fins. Studies show that when the sail and other fins were retracted, sailfish are able to reduce their drag by about 18%. With less drag to worry about, the fish can be more efficient in generating thrust from its swimming motion, which allows it to speed ahead of the competition. To get a sense of just how fast these fish move, watch the end of the video above, where they only get up to 40 mph!

Slowing Down

Sailfish usually feed on smaller fish that swim in schools, like sardines. These small fish “exhibit higher performance than large fish in unsteady swimming,” which is to say making quick lateral movements. So, if a sardine senses a sailfish coming at it at full speed, it will likely have no problems avoiding it. With the sail dorsal-fin raised, the increase in drag slows the fish down dramatically and steadies its swimming path.

In the figure below, sub-figures C and D show how much the tail oscillates (in grey) and the direction in which the fish is moving (in black) when the sail is retracted and deployed, respectively. Zero on the Y-axis represents the tail being directly in line with the rest of the body and the fish is moving straight forward. While the sail is up, the tail moves far less aggressively and the fish more consistently moves in a straight line.

Side by Side comparison of Sailfish swimming with and without sail
Affect of Sail on stability while swimming. Credit: Stefano Marras

This allows the sailfish to become more controlled in its movements and help pursue a sardine performing evasive maneuvers. When it gets within striking distance, the sail helps counterbalance the fish’s swift lunge at its prey, which provides much-needed accuracy. Biologists have also theorized that the large size of the sail helps herd the school closer together, but no official research has been done on this.

Take-Aways

The sailfish’s ability to retract and deploy its sail dorsal-fin gives it unique advantages in both long-distance swimming and hunting. Minimizing drag is one of the most important concepts for travel through any fluid (air or water), so understanding how the sailfish is able to reduce drag could provide a new perspective on things. The sail’s ability to provide stability at lower speeds has potential use in any sort of water travel, whether it be with cargo ships (like the Evergreen in the Suez), boats, or submarines.  

If you are interested in another unique feature of the Sailfish, its bill has been a popular topic as of late, especially with its feeding habits. The bill is often used like a sword, either spearing or stunning prey, and also has been thought to have helpful drag-reducing qualities. Click here for more information.

Sources

Woong Sagong, “Hydrodynamic Characteristics of the Sailfish (Istiophorus platypterus) and Swordfish (Xiphias gladius) in Gliding Postures at Their Cruise Speeds,” Plos One, https://doi.org/10.1371/journal.pone.0081323

Stefano Marras, “Not So Fast: Swimming Behavior of Sailfish during Predator–Prey Interactions using High-Speed Video and Accelerometry,” Integrative and Comparative Biology,” https://doi.org/10.1093/icb/icv017

P. Domenici, “How sailfish use their bills to capture schooling prey,” The Royal Society Publishing, https://doi.org/10.1098/rspb.2014.0444


The Ship of Pearl – Jet Propulsion in the Chambered Nautilus

In the aptly titled poem The Chambered Nautilus, Oliver Wendell Holmes Sr. praises the eponymous cephalopod for its elegant shape and vibrant colors. The ship of pearl, as Wendell calls it, might not be the swiftest vessel; but Thomas R. Neil and Graham N. Askew’s research indicates that the chambered nautilus might be among the most energy efficient ships in the seven seas.

The Nautilus pompilius is constantly moving to depths up to 700 m. At such depths, oxygen concentration decreases significantly: only 30% of that available at the surface. In such harsh environments, the nautilus must find a way to use its oxygen reservoir most efficiently while still being able to carry out metabolic functions. As such, the nautilus has adapted to use jet propulsion in a most efficient manner.

Jet propulsion mechanism of the chambered nautilus
modified from Neil & Askew, Royal Society Open Science (2018)

Jet propulsion in the nautilus is achieved by the simultaneous contraction of the head retractor and funnel muscles (shown in diagram above). The compression of the mantle cavity causes a pressure difference with its surroundings, resulting in a jet of water being expelled from the mantle cavity via the funnel. The nautilus uses its flexible funnel to control its swimming direction as shown in the video. Slower swimming is powered by rhythmic contractions of the funnel flaps that generates a wave that travels along the funnel wings. This produces a unidirectional flow across the gills. Although jet propulsion is less efficient than undulatory swimming (think of the swimming motion of a ray), the Nautilus converts chemical energy into hydrodynamic work for motion more efficiently than fellow cephalopods such as the squid and even salmons (at lower speeds).

 

In their study, Neil and Askew used particle image velocimetry (PIV) to study the wake of the nautilus’s jet. PIV consists of spreading particles of aluminum oxide in a water tank and shinning a laser on them. As the nautilus moves, the particles in the tank move with the jet. A high-speed camera is used to take multiple pictures of the floating particles and the relative position of these between snapshots is used to determine a velocity profile of the nautilus’s water jet.

Nautilus and its jet wake as seen in PIV
Photo by: Simon and Simon Photography, University of Leeds. Taken from Greenwood, The New York Times (2018)

 

 

 

 

 

The results from the study show that the whole cycle propulsive efficiency and thrust generation are related to the swimming orientation of the nautilus. The propulsive efficiency ranged between 30% and 75% for posterior-first swimming and 48% to 76% for anterior-first swimming (orientations defined in diagram above). Moreover, efficiency increases with greater speed in posterior-first orientation but decreases in anterior-first. This might have to do with energy losses associated with the re-orienting of the funnel that turns back on itself to move in the anterior orientation. Moreover, at low speeds, the nautilus spends more time jetting water than refilling, resulting in a lower jet speed but overall more efficient propulsion.

Although not covered in their research, Neil and Askew’s findings about jetting duty cycles and efficient propulsion at low speeds could be potentially applied to the design of more efficient hydrojets to be used in underwater vehicles. Engineering seeks to imitate nature’s most intelligent designs; and as Wendell puts it in his work, the nautilus proves to be a truly awe-inspiring creature worthy of imitation.

Fish in Flight: The Science Behind Great White Breach Attacks on Cape Fur Seals

Great white shark employs vertical attack on prey decoy
Great white shark employs vertical attack on prey decoy – from Sharkcrew via Wikipedia Commons

If you’ve ever turned on Discovery channel during Shark Week, then you’ve probably seen the iconic footage of a 2.5-ton great white shark leaping out of the water to catch its next meal.  If you’re weird like me and you’ve ever tried to mimic one of these epic breaches in a backyard pool, then you realize just how difficult it is to generate enough momentum to jump even partway out of the water and therefore have a real appreciation for what it takes to pull off this incredible feat.

Great white breaks the ocean surface
Great white breaks the ocean surface – from Alex Steyn via Unsplash

So if a breach attack is so difficult to pull off, how are great white sharks able do it, and why do they do it?  As per usual, some basic physics can help us answer both these questions.

 

Great white shark mid-breach
Great white shark mid-breach – from Alex Steyn via Unsplash

According to a 2011 paper by Martin and Hammerschlag, who spent 13 years studying great white predation in South Africa, breach attacks allow great whites to play to their strengths and maximize stealth.

Millennia of evolution have left great whites with long bodies great for straight-line speed (can reach speeds  >11m/s) but not so great for agility. Additionally, roughly 95% of a great white’s muscle is white muscle, which allows for rapid contraction (e.g. speed bursts) but also results in poor endurance.  Considering these aspects of their physiological makeup, it’s in a great white’s best interest to attack swiftly, avoiding prolonged chases.  Martin and Hammerschlag report that the majority of great white attacks on seals are over within 2 minutes and that the longer an attack drags on, the less likely it is to be successful.

Great white shark chases decoy prey from behind
Great white shark chases decoy prey from behind – from Sharkcrew via Wikipedia Commons

As great whites are less agile than seals, maximizing stealth and minimizing the time seals have to react is imperative.  Having evolved to have a dark grey dorsal (top) surface, great whites are hard to distinguish from the coral on the ocean floor when viewed from above (seal’s perspective).  Additionally, since very little of the light entering the water is reflected back towards the surface, it is estimated that under even the best lighting conditions, a seal could only reliably distinguish a shark a maximum distance of roughly 5m below it, which explains why great whites attack from below rather than behind. Great whites need about 4m to reach top speed, so due to this acceleration distance and seal vision, Martin and Hammerschlag report that great white attacks generally start between 7m and 31m below the ocean surface, with the majority staring closer to 30m.  Looking at data for great white breach attacks ranging from vertical to 45 degree ascents, Martin and Hammerschlag estimate that it typically takes a shark between 2 and 2.5 seconds to go from initial acceleration to surface breach, and that when considering shark speed and average visibility conditions, a seal generally has only about 0.1 seconds to react if it spots the shark before contact is made.  Ultimately, due to the advantages it gives them, great whites are successful in over half their breach attacks when lighting conditions are ideal.

Schematic of geometry and optics of great white shark attacks on cape fur seals from Martin and Hammerschlag - not to scale
Schematic of geometry and optics of great white shark attacks on cape fur seals from Martin and Hammerschlag – not to scale

 

Sources & Further Reading:

Fallows, Chris & Aidan Martin, R & Hammerschlag, Neil. (2012). Predator-Prey Interactions between White Sharks (Carcharodon carcharias) and Cape Fur Seals (Arctocephalus pusillus pusillus) at Seal Island, South Africa and Comparisons with Patterns Observed at Other Sites

Martin, R. Aidan, and Neil Hammerschlag. “Marine Biology Research.” Marine Biology Research, vol. 8, no. 1, 30 Nov. 2011, pp. 90–94., doi:10.1080/17451000.2011.614255.

Egdall, Mark. “New Research Reveals Physics Behind Great White Shark Attacks.” Decoded Science, Decoded Science, 10 Dec. 2011, www.decodedscience.org/new-research-reveals-physics-behind-great-white-shark-attacks/7497.

Sloat, Sarah. “Shark Week: Here Is the Wild Physics of a Great White Leap.” Inverse, Inverse, 25 July 2018, www.inverse.com/article/47437-shark-week-great-white-jumps.

Madrigal, Alexis C. “The Physics of Great White Sharks Leaping Out of the Water to Catch Seals.” The Atlantic, The Atlantic Monthly Group, 9 Dec. 2011, www.theatlantic.com/technology/archive/2011/12/the-physics-of-great-white-sharks-leaping-out-of-the-water-to-catch-seals/249799/.

 

Walk [Under] Water: The Benefits of Underwater Running

Just because you can’t walk on water doesn’t mean you shouldn’t run under it!

Aqua-jogging. Hydro-running. Water-treadmills. Have you ever heard some combination of these terms and wondered what the hype is?

Running underwater offers benefits for people throughout their fitness journey. Underwater running has proven useful for a variety of focuses, including recovery after injury, cross training, and even improved gait. This article includes a video showing a Runner’s World coach tries out a Hydrotrack and discusses some of the benefits!

So, why does it work?

Three basic water properties: hydrostatic pressure, buoyancy, and viscosity.

Hydrostatic pressure is the force that the water exerts on a submerged point. Hydrostatic pressure acts all around the point. However, since hydrostatic pressure is proportional to the weight of liquid above the point, it increases with increased water depth. This means that your feet would experience greater hydrostatic pressures than your knees. While running, this pressure helps support your body and decrease impact forces. In addition to helping prevent injuries through a decreased risk of falling, it also helps decrease swelling and promote cardiovascular health. This article talks about the specifics of pressure with swelling and the cardiovascular system.

Diagram showing hydrostatic forces. Magnitude of the hydrostatic force is larger as it goes deeper below the surface.
Hydrostatic pressure acts on all sides of a point. The pressure increases with depth. Created in Microsoft PowerPoint.

Buoyancy is the hydrostatic force applied to an object with volume (rather than just a point). Since they are at the same depth, all the horizontal forces cancel out. Since the bottom of the object is deeper than the top, the net buoyant force on the object pushes up. The difference between the buoyant force and the weight of the object submerged determines if the object will rise, sink, or stay in place. Thus, the more submerged a person is, the more of their weight is supported. This research article explains how this support can help make gait analysis more effective to further prevent injury. When water reaches the person’s navel, 50% of their weight is supported. This weight bearing capability of water decreases forces on joints and can even help improve range of motion. This allows physical therapy to begin sooner and, overall, take less time out of the patient’s normal routine. This allows shorter rehabilitation times without sacrificing quality of care or recovery.

 

Diagrams showing how the hydrostatic force varies around the submerged object due to depth. The side forces cancel out at equal depth leaving a net buoyant force acting upward against the downward force of the object weight.
Buoyant forces cancel out on the sides leading to the second image showing the net buoyant force and the weight of the object. Created in Microsoft PowerPoint.

Viscosity is a fluid property that affects the resistance that an object encounters during motion. In the case of underwater running, viscosity explains why you move significantly slower in water than on land. It also can offer resistance up to 15 times the amount of resistance on land. Forcing your limbs through the water strengthens muscles that are not typically used out of the water and even burns more calories!

As noted above, viscosity can help strengthen muscles as shown in this study on deep water running (DWR) in a community of elderly women shows how viscosity affects overall strength training. It showed that the women who participated in DWR increased their muscle strength (measured through power) and performed better in various tests, including ones that involved sitting down and getting up. The study showed that deep water running helped to mitigate some of the negative muscular effects of aging.

Overall, running underwater offers some great benefits. The basic properties of water (hydrostatic pressure, buoyancy, and viscosity) provide scientific background for why hydro-running provides benefits for all.

 

 

 

 

 

 

 

Archerfish: Nature’s Master Marksmen

Picture of an archerfish swimming
Photo by Chrumps on Wikipedia

The name archerfish refers to seven species of freshwater fish that are all members of the Toxotes genus. These fish derive their name from their ability to hunt land-based creatures, ranging from insects to small lizards, using jets of water shot from their mouth with remarkable accuracy. They only grow to a maximum of a foot long, but they’ve been recorded in the wild propelling their water jets distances of up to two meters. A recent study in the Journal of Experimental Biology was conducted by Stephan Schuster to investigate the mechanics behind their unorthodox hunting technique.

Archerfish Learning to Aim

As it turns out, the archerfish has many circumstances working against it. Not only must they account for gravity when hunting, but because their eyes remain underwater during the attack, they must also adjust for the effects of the light refracting through the water. Refraction could cause the fish’s perceived angle of the prey to be off by up to 25˚ from the actual angle. They compensate for these factors by adjusting their spitting angle prior to releasing the jet. The ability to compensate for various environmental factors seems to be a skill developed through experimentation and practice. Archerfish will indiscriminately fire off shots at objects that are new to their habitat. They then judge whether they should continue to attack those objects in the future by remembering if a successful hit rewarded them with food. This was tested in the lab and shown that when the fish wasn’t rewarded, it would ignore the target in the future.

A diagram showing archerfish potential targets. Once the archerfish was only getting consistently rewarded for a single kind of target, insects, that's the only target it would shoot at
Modified from Schuster, Journal of Experimental Biology

In the past, studies were conducted that underestimated the accuracy of archerfish because of the variation in test specimens. Studies now show that a well-practiced archerfish can achieve a 100% hit rate on targets within distance of 65 cm. Accuracy will begin to drop off with increased height and distances beyond 65 cm.

Hunting Mechanics

While most animals who utilize their ability to spit depend on the spit’s glue-like properties or venom, archerfish hunting relies on the force at which the jet contacts the target. Depending on the size of their prey, archerfish can modulate their jets between the range of 40-500 mN. To exert strong enough forces to dislodge targets from leaves and branches, the fish expelling the jet acts like a vessel under pressure. The water starts out slow and increases in velocity as the fish’s mouth stays open, like turning a valve. This means that the jet’s tail catches up to its tip as the water travels along its trajectory, causing a more focused impact. As the water travels, the stream will break into individual blobs of water due to Plateau-Rayleigh instability, creating multiple jets. Since water ejected later is moving faster, these jets will join over time and create a single, larger jet which will impact the target.

Race to the Food

While utilizing their ability to create forceful jets of water can be an effective method of hunting, it does come with one disadvantage. Archerfish have evolved to react to plummeting objects much quicker than other fish, regardless of what caused the object to fall. While this means that an archerfish having its prey stolen by another type of fish is very rare, its prey will often be stolen by other archerfish which happen to be closer to where the prey impacts the water.

Additional information on archerfish hunting can be found at wired.com and in the video from below. There are also some beautiful photos of archerfish in action at stevegettle.com.

 

How the Mantis Shrimp Packs its Punch

The mantis shrimp, a six inch long crustacean residing in the warm waters of the Pacific and Indian oceans, may look harmless with its rainbow shell, but it is able punch its prey with the same acceleration as a 0.22 caliber bullet, providing around 1500 newtons of force with each blow. The mantis shrimp can shatter the glass of aquariums, catch and kill their prey with minimal effort, and punches so fast that cavitation bubbles form behind their hammer-like clubs. Cavitation bubbles are pockets of low pressure air that form when a liquid is moved faster than it can react, and collapse with tremendous heat and force—enough to crack the shells of other crustaceans or even a glass bottle.

Rainbow colored mantis shrimp on ocean floor, with two white clubs visible.
Mantis shrimp on the ocean floor, with its two white dactyl clubs visible. (Source: Wikimedia Commons)

The mantis shrimp gets the power for its punches from elastic energy storage—that is, it stores energy in its muscles as they are compressed when cocking its dactyl club back into the locked position. A four bar mechanism within the club and body of the shrimp is used to hold the club back in place until it is ready to punch and a latch is released, transferring the stored energy into rapid motion of the club.

Diagram of how the mantis shrimp locks his club back, storing energy, then releases it and swings it forward to attack prey.
Mechanics of the mantis shrimp’s club swing. (Source: Maryam Tadayon/Nanyang Technological University)

The material composition of the mantis shrimp’s shell enables it to hit so hard without damaging itself. The two layers of the shell on the club allow it to withstand large stresses in both tension and compression, which is uncommon of most shells since they are ceramic materials, which are very brittle. Both layers are made out of hydroxyapatite (HA), a highly crystalline and hard calcium phosphate that is found in human bone, and chitin, a hard biopolymer fiber found in outer shells of most crustaceans and insects. Differing structures and amounts of each material provide different functions in the layers of shell. The outer layer is arranged in a wave-like pattern of HA and chitin that effectively redistributes stresses evenly over the whole surface, greatly decreasing the chance of cracking. The inner layer is less stiff than the outer layer, and is composed of HA and chitin in helical patterns, which allows for small cracks to form in a spiral shape instead of cracking straight through the shell. This greatly increases the lifetime of the club’s shell.

Ceramics are typically not used in robotics due to their brittle nature, but, if modeled after the shell of the mantis shrimp, this could be overcome and provide a stiffer and lighter material to be used in the robotics industry. Additionally, the material composition of their clubs could be used to develop resilient protective body armor for the military and police forces, or helmets for bike riders or football players.

Additional information on the mantis shrimp can be found at Wired.com and theoatmeal.com, and in the following video.