# Archer fish use advanced hydrodynamics

Article from:
Creation
36(3):36-37
June 2014

istockphoto.com/ZU_09

Archer fish (also known as spinner fish or archerfish)1 are small fish, usually 5–10 cm (2–4 inches) long, which have an unusual method of hunting. Instead of seeking prey in the water where they live, they hunt prey perched far above the water. The archer fish brings them down by shooting a powerful jet of water at them. This is much harder than it seems.

The jet itself is generated in a narrow channel in the roof of the mouth, formed by pressing the tongue against a groove. Then the fish contracts its gill covers to force water through this channel and out through its lips.

The mature archer fish normally hits its target the first time. Yet it spots the prey from underwater, which has the problem of light refracting (bending) at the water-air interface. At a typical shooting angle of 74° from the horizontal, the refraction causes the archer fish to see it at 78°. It can shoot at shallower angles up to 45°, where the deviation is even greater—58°.2 The archer fish must compensate for this difference. It must also compensate for the fact that the jet will not travel in a straight line, but will curve downwards due to gravity, forming a parabolic path.

All this would be of no use unless the archer fish could then catch its prey after it has fallen into the water. In only a tenth of a second—a reaction time twice as fast as a human’s—it turns its body to be ready to sprint to its prey. Here, it doesn’t turn towards the actual position, but towards the predicted (or calculated) landing point. This means that the archer fish has already calculated its prey’s landing point from its height, speed, and angle at which it is dislodged.3 Creationist biologist Dr David Catchpoole comments:

“This means the Archer Fish’s brain is capable of complex mathematics (trigonometry and calculus). But the programming required for this calculation is very advanced, and would also be useless unless it was fully functional.”4

Furthermore, the archer fish not only knows the direction, but also the distance, and chooses its starting sprint speed to arrive only about a twentieth of a second after impact.5

## Super-jet

The wonders of the archer fish don’t end with accuracy. It also exploits the often unappreciated unique properties of water6 to produce a jet powerful enough to knock off its strongly anchored prey.7 Perching insects are usually firmly held by a force about 10 times their weight, often with ingenious sticking mechanisms using both hydraulics and mechanics.8,9 The main portion of the water jet actually speeds up during flight, and bunches up just before hitting. Because the projectile is water, the collision with the insect is inelastic, transferring as much kinetic energy as possible from the projectile to the target. The result is that the force of impact against the insect is 10 times stronger than the anchoring force holding it.10

The jet is powerful for two reasons. One is the high surface tension of water, a force caused by water molecules attracting each other, which tries to keep the surface area as small as possible. This effect makes a stream of liquid unstable, called the Plateau–Rayleigh instability.11 What happens is that even the smoothest stream still has tiny irregularities: small bumps and necks. The surface tension contracts the necks even further until they are pinched off, leaving the flow in separate drops. But this would seem counterproductive, since the power of the water would be spread out into the separate drops.

So the archer fish uses a second effect called kinematic gathering. It controls the power of the jet so the rear drops are travelling faster than the front ones. Near the target, the fast rear drops catch up to the front ones. They merge and add both their mass and speed to form one big fast drop.

But the kinematic gathering process without the Plateau–Rayleigh instability would also be ineffective, because the drop would spread out. Thus much of it would not hit the prey, so its energy would be wasted. So it’s only the combination of the effects that makes this jet so effective. This can perhaps be yet another example of a functionality threshold—the mechanism would not work unless several sub-mechanisms were present simultaneously. Moreover, this concentration of power, weight for weight, is five times stronger than any vertebrate muscle can produce.7

## Conclusion

Ph.D. physicist Aatish Bhatia summarizes this work:

“The archerfish hunts with a working knowledge of motion, gravity, optics, and fluid dynamics, effortlessly solving problems that might keep a physics student up at night. It uses science to give itself superhuman (or rather, superfish) strength—like the Hawkeye of the animal kingdom, it’s always on target and never runs out of arrows.”12

Furthermore, the archer fish design may well help human designers in the future, as the research team writes:

“This process [kinematic gathering] is similar to that occurring during the generation of droplets in Drop-on-Demand inkjet printing, a technological field still largely driven by empirical developments that can potentially benefit from the biomimicry of archer fish.”7

If it would take ingenuity to design the inkjet copy, how much more ingenuity did it not take to make the original?13 Doesn’t this testify to a master Designer?

### References and notes

1. Family Toxotidae (Cuvier 1816), comprising seven species in genus Toxotes (Cuvier 1816). Return to text.
2. Calculated from Snell’s Law of refraction, nasin(i) = nwsin(r), where n is index of refraction, 1 for air and 1.33 for water; i is the angle of incidence measured relative to the perpendicular or normal (90° – (angle from horizontal)), and r is the angle of refraction. Return to text.
3. Rossel, S., Corlija, J. and Schuster, S., Predicting three-dimensional target motion: how archer fish determine where to catch their dislodged prey, Journal of Experimental Biology 205(21):3321–3326, 1 November 2002. Return to text.
4. Catchpoole, D., Aim, spit and catch, Creation 25(2):43, 2003; creation.com/archer. Return to text.
5. Wöhl, S. and Schuster, S., Hunting archer fish match their take-off speed to distance from the future point of catch, Journal of Experimental Biology 209(1):141–151, 1 January 2006 | doi: 10.1242/​jeb.01981. Return to text.
6. Sarfati, J., The wonders of water, Creation 20(1):44–47, 1997; creation.com/water. Return to text.
7. Vailati, A., Zinnato, L., Cerbino, R., How archer fish achieve a powerful impact: hydrodynamic instability of a pulsed jet in Toxotes jaculatrix, PLoS One 7(1):e47867 | doi:10.1371/journal.pone.0047867. Return to text.
8. Federle, W. et al., Biomechanics of the movable pretarsal adhesive organ in ants and bees, Proceedings of the National Academy of Sciences USA 98(11):6215–6220, 22 May 2001 | doi: 10.1073/pnas.111139298. Return to text.
9. Sarfati, J., Startling stickiness: How ants and bees adhere with amazing machinery, Creation 24(2):37, 2002, creation.com/stickiness. Return to text.
10. Vailati et al., Ref. 7, write, “The force at the impact has an average value of about 200 mN [millinewtons]. The anchoring force of insects such as flies and bugs is typically smaller than 20 mN for specimens with a body mass up to 100 mg [milligrams], with peak values of about 40 mN for beetles.” Return to text.
11. After Belgian physicist Joseph Plateau (1801–1883) and Nobel Prizewinning English physicist and creationist Lord Rayleigh (John William Strutt 1842–1919). Return to text.
12. Bhatia, A., The fluid dynamics of spitting: how archerfish use physics to hunt with their spit, wired.com, 29 November 2013. Return to text.
13. Finally, why would a good designer make something designed for knocking down insects for the purpose of killing and eating them? First, such a system might be useful before the Fall for shooting down plant matter for food, before God cursed the ground and vegetation (Genesis 3:17). Second, God foreknew the fall, and so programmed-infeatures likely to be useful in a fallen world. Third, insects are probably not ‘living creatures’ anyway in the biblical sense of the Hebrew nephesh chayyāh (נפשהיה); Dr Catchpoole has called them ‘God’s robots’ (personal communication). Return to text.
Family Toxotidae (Cuvier 1816), comprising seven species in genus Toxotes (Cuvier 1816).
Calculated from Snell’s Law of refraction, nasin(i) = nwsin(r), where n is index of refraction, 1 for air and 1.33 for water; i is the angle of incidence measured relative to the perpendicular or normal (90° – (angle from horizontal)), and r is the angle of refraction.
Rossel, S., Corlija, J. and Schuster, S., Predicting three-dimensional target motion: how archer fish determine where to catch their dislodged prey, Journal of Experimental Biology 205(21):3321–3326, 1 November 2002.
Catchpoole, D.,
Wöhl, S. and Schuster, S., Hunting archer fish match their take-off speed to distance from the future point of catch, Journal of Experimental Biology 209(1):141–151, 1 January 2006 | doi: 10.1242/​jeb.01981.
Sarfati, J.,
Vailati, A., Zinnato, L., Cerbino, R., How archer fish achieve a powerful impact: hydrodynamic instability of a pulsed jet in Toxotes jaculatrix, PLoS One 7(1):e47867 | doi:10.1371/journal.pone.0047867.
Federle, W. et al., Biomechanics of the movable pretarsal adhesive organ in ants and bees, Proceedings of the National Academy of Sciences USA 98(11):6215–6220, 22 May 2001 | doi: 10.1073/pnas.111139298.
Sarfati, J.,
Vailati et al., Ref. 7, write, “The force at the impact has an average value of about 200 mN [millinewtons]. The anchoring force of insects such as flies and bugs is typically smaller than 20 mN for specimens with a body mass up to 100 mg [milligrams], with peak values of about 40 mN for beetles.”
After Belgian physicist Joseph Plateau (1801–1883) and Nobel Prizewinning English physicist and creationist Lord Rayleigh (John William Strutt 1842–1919).
Bhatia, A., The fluid dynamics of spitting: how archerfish use physics to hunt with their spit, wired.com, 29 November 2013.
Finally, why would a good designer make something designed for knocking down insects for the purpose of killing and eating them? First, such a system might be useful before the Fall for shooting down plant matter for food, before God cursed the ground and vegetation (Genesis 3:17). Second, God foreknew the fall, and so programmed-infeatures likely to be useful in a fallen world. Third, insects are probably not ‘living creatures’ anyway in the biblical sense of the Hebrew nephesh chayyāh (נפשהיה); Dr Catchpoole has called them ‘God’s robots’ (personal communication).