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ArcherFish: Changing Caliber to Fit Prey

by  Brad Harrub, Ph.D.

archer fishA fish learning the laws of optics is an amazing feat. But to combine those optical laws with the precision shooting of a sniper and then calculating the force of the shot according to the size of the prey (in order to obtain food), defies evolutionary explanation. The archerfish has long been a favorite subject for researchers because of its unique method for obtaining food. As Thomas Schlegel and his colleagues noted: “Archerfish are renowned for their unique hunting technique: with a simple blow tube they fire precisely aimed jets of water at distant aerial prey to catch their dislodged victims on the water surface” (2006, 16:R836). These unique abilities beg the question: How was this creature able to “evolve” these distinctive abilities, and furthermore, why go through all the trouble? Why not just eat aquatic animals like other fish?

Before discounting this as an adaptive “trick” that this creature utilizes while fulfilling its “survival of the fittest” role, consider what must actually occur in order for this special species of fish to eat. Anyone who has opened his eyes underwater and looked at the surface recognizes the distortion that occurs because of the difference in light refraction in air versus water. That distortion must be compensated for as the archerfish aims at aerial prey. Yet, these fish possess the remarkable ability to hit very small creatures (e.g., flies, insects, lizards, etc.) resting on branches above the surface of the water. Additionally, the archerfish also must be able to see well in the water, once the prey falls into the water. So the eyes and focal points must be able to adjust according to the specific environment. Two years ago Stefan Schuster and his colleagues demonstrated that these fish are able not only to compensate for the distortion, but that they can even correct the differences to determine the absolute size of the target they are shooting (2004). Heidi Hardman remarked:

In a series of experiments, the researchers showed that the fish do not learn this by remembering which combinations of spatial configurations and the corresponding images were rewarding in the past. Rather, the fish extracted the underlying law that connects spatial configuration and apparent size. This remarkable cognitive ability allows the fish to readily judge a target’s objective size from underwater views they have never encountered before (2004).

In the conclusion of their research summary, Schuster, et al., observed: “Moreover, we demonstrate that archerfish solve the problem not by interpolating within a set of stored views and distances but by learning the laws that connect apparent size with the fish’s relative position to the target. This enables the fish to readily judge the absolute sizes of objects from completely novel views” (2004, 14:1565, emp. added). They continued:

This ability is remarkable in several respects. First, the optical effects require rather precise knowledge of spatial configuration. The question of how the fish’s visual system is able to provide this information is presently wide open.... Second, the fish apparently is able to combine such spatial knowledge in a yet-unknown way with apparent size (or apparent locomotion-induced image transformations) to deduce a concept of objective size. Whatever sensory representation it uses, the fish evidently is able to form a concept of size that is tailored to the complex optics at the water-air interface. Because this situation poses particularly rigorous requirements on the relation the animal must make between target localization and the apparent image, the fish is an attractive model to explore how animals learn to form concepts to bring order into their sensory experiences (14:1566-1567, parenthetical item in orig.).

The question should be raised as to how many archerfish died from lack of food as they were learning these optical laws. Yet, learning the laws of optics and compensating for optical distortion is just the first step in obtaining food.

In addition, the archerfish must be able to shoot with pinpoint accuracy. As Charles Choi observed “Archerfish, the snipers of the animal world, never waste a shot” (2006). They are hardly ever fooled, and their slim profile and black and white markings allow them to remain camouflaged while assuming a shooting position. The jet spray of water comes from a tube they form in their mouths. Pressing their tongue against the roof of their mouth, they powerfully force water through their gills through this “tube,” which they are able to aim with deadly precision. But any hunter will tell you that shooting is only part of the equation—you must also know what type of gun and ammunition to use. For instance, a small-caliber handgun does little damage to a large animal such as an elk or bear. Thus shooters must also be able to match the force of their shots to the size of their prey. Recent research reveals that archerfish also possess the ability to fine-tune the force they use to dislodge prey.

Schlegel and his colleagues demonstrated that archerfish are able to vary the force of the stream of water according to the size of the prey. It was originally believed that archerfish fired an “all or none” shot to take down prey. Schlegel and his colleagues discovered that the archerfish will actually alter the force according to the adhesive forces and size of the prey it is shooting. Commenting on this research, Choi observed:

To measure the force of each blast, animal physiologist Stefan Schuster and his colleagues at the University of Erlangen-Nürnberg in Germany recorded how fast archerfish shots were, using high-speed video, capturing images at 5,000 frames per second. Normally, televisions and movies display pictures at 24 frames per second. At the same time, the researchers measured how much mass each blast expelled by having the fish fire into bowls filled with an absorptive material known as viscose, making sure no droplet was reflected. Based on the mass and speed at which water was fired over time, Schuster and his colleagues could determine the force of each shot (2006).

The researchers noted: “Hence, the maximum adhesive forces an archerfish’s shot must overcome in order to actually dislodge prey increase linearly with prey’s size (i.e. with its linear dimensions or m1/3). 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” (2006, 16:R836, parenthetical item in orig.). This behavior sure sounds like it is purposeful and deliberate—not the by-product of chance. As Hardman noted: “The researchers showed that for any given size of prey, the archerfish tune their attacks such that prey are hit with about ten times the force that adhesive organs of animals of that size could sustain” (2006).

The abilities of this special fish are nothing less than remarkable. What type of evolutionary pressure can convert an aquatic animal into an expert marksman? And why wouldn’t those “evolutionary pressures” cause other fish—living in the same environment—to follow similar patterns? Evolution by natural selection cannot explain the archerfish, because one would expect other fish to “select” this survival method, if indeed it was so beneficial. Furthermore, genetic mutations are not a sufficient cause to explain the existence of the archerfish, as we know today mutations do not add new genetic material. Evolutionary science has not demonstrated an intermediate or transitional form for this amazing creature, and it is illogical to assume that such has taken place. It is far more rational to conclude that the archerfish was designed by the ultimate Designer—God.

REFERENCES

Choi, Charles Q. (2006), “Fish Shoot Prey with Precise Water Guns,” LiveScience, October 9, [On-line], URL: http://www.livescience.com/animalworld/061009_archerfish.html.

Hardman, Heidi (2004), “In a World of Distortion, Archer Fish Learn to Judge Absolute Size of Aerial Prey,” EurekaAlert, September 6, [On-line], URL: http://www.eurekalert.org/pub_releases/2004-09/cp-iaw083104.php.

Hardman, Heidi (2006), “Archerfish Tune Their Shots to Universal Properties of Prey Adhesion,” EurekaAlert, October 9, [On-line], URL: http://www.eurekalert.org/pub_releases/2006-10/cp-att100506.php.

Schlegel, Thomas, Christine J. Schmid and Stefan Schuster (2006), “Archerfish Shots are Evolutionarily Matched to Prey Adhesion,” Current Biology, 16:R836-R837, October 10.

Schuster, Stefan, Samuel Rossel, Annette Schmidtmann, Ilonka Jäger, and Julia Poralla (2004), “Archer Fish Learn to Compensate for Complex Optical Distortions to Determine the Absolute Size of Their Prey,” Current Biology, 14:1565-1568, September 7.




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