October 10, 2006 | David F. Coppedge

Archer Fish Shoot Efficiently

Archer fish, the sharpshooters of the underwater world, have another trick in their blowguns: energy efficiency.  Three German scientists were curious how they knew how hard to shoot at targets of differing mass.  Publishing in Current Biology,1 the team first determined that the prey’s ability to cling to its leaf or stem is proportional to its body mass.  Without a calculator, the predatory fish seem to figure out how hard to shoot:

Hence, the maximum adhesive forces an archerfish’s shot must overcome in order to actually dislodge prey increase linearly with prey’s size…. Archerfish force-scaling closely matches this prediction, ensuring a reasonable safety margin: for any given size of prey, the fish apply about ten times the forces the adhesive organs of prey of that size could maximally sustain.

Prior to this study, scientists thought the fish had a one-size-fits-all water bullet.  By weighing the mass of water spit out, the scientists determined that the fish don’t waste big bullets on smaller targets.  Also, they hit hard enough to win on the first shot: “Moreover, because the first shot hits prey unprepared in an average posture, the fish needed not to adjust to the probably much larger forces some prey might exert by clawing to the substrate.”  In non-scientese, this means the bug or lizard is going to grab on harder if the fish fails to dislodge it on the first shot.
The scientists attempted an evolutionary explanation for this prey-matching marksmanship.  Since spitting is costly in energy terms, “Fitting with the costs of shooting, archerfish use the most economic way of tuning their shots,” they said.  The fish seem to know that kinetic energy varies with 1/2 m v2, so the fish vary the mass instead of the velocity: “As the kinetic energy of their shot varies with the square of speed but only linearly with mass, this simple trick enables archerfish to scale their forces in the least costly way and to double force transfer at doubled instead of quadrupled energetic costs.”
But wait a minute; how does all this physics talk count as an evolutionary explanation?  After all, their article was entitled, “Archerfish shots are evolutionarily matched to prey adhesion.”  They explain: “The evolutionary pressures for adjusting the shots at all, instead of firing an all-or-none shot of sufficient maximum force, became evident when we analyzed the mass, speed and kinetic energy of the shots.”  In other words, all they needed to observe was an “evolutionary pressure” related to the phenomenon.  Natural selection, in their view, did the math.