The Amazing Archerfish

I know…I know…I used “amazing” in my previous post. However, just as “amazing” is the best adjective to describe bowerbirds, it is also the best adjective to describe this fish. Let’s start with why it’s called the archerfish. It likes to eat insects, but rather than waiting for insects to fall into the water, it knocks them into the water by shooting them with a stream of water. You can see the amazing archerfish in action by watching the following video:

As the video tells you, the archerfish understands that gravity will cause the stream of water to follow a curved path, so it adjusts its aim accordingly. In addition, it compensates for the fact that light bends when it travels from air into water. This causes the target to appear at a different place than where it actually is. Nevertheless, the archerfish isn’t fooled by the optical illusion. It aims its jet of water where the insect actually is, not where it appears to be. If all of that isn’t amazing enough, wait until you read what some recent research has uncovered.

In 2006, Thomas Schlegel, Christine J. Schmid, and Stefan Schuster studied the actual stream of water that the archerfish uses to shoot its prey. They found that rather than using the same amount of water to shoot down all prey, an archerfish actually estimates the size of the target and adjusts the amount of water accordingly. For small insects like flies, the archerfish uses a smaller stream of water than it does for larger prey, such as small lizards.1 This, of course, is a smart thing to do, as shooting more water requires more energy. Thus, the archerfish conserves its energy by using smaller amounts of water to shoot smaller prey.

The most interesting aspect of the study is explained by the authors:

Strikingly, forces were strictly tuned to target-size even in fish that had grown up in an artificial situation in which we removed all advantages of adjusting force.

In other words, the authors would use targets that varied in size but contained the same amount of food for the archerfish. Despite that, the archerfish continued to adjust the amount of water to the size of the target. Thus, this is something that seems to be programmed into the fish.

So we see that the archerfish is able to aim at its prey, correcting both for the effect of gravity and the effect of light bending as it travels from air into water. In addition, it judges the size of its prey and shoots an amount of water proportional to that size. However, none of those things are what caused me to write about the archerfish today. Instead, my motivation for writing about this amazing fish is an article by Shelby Temple and colleagues that was recently published in the Proceedings of the Royal Society B.

In this study, the researchers were interested in learning whether or not the archerfish used the visual background when aiming at its prey. They presented the fish with targets that were either aligned with the background or perpendicular to the background. In the end, the fish had much better aim when the target was perpendicular to the background. To the researchers, this meant that the fish uses orientation saliency, which means it uses the background as a filter against which to view objects. If the objects are not aligned with the background, the fish sees them much more clearly.2

This was a huge surprise to the researchers. Why? Well, until this study was done, it was thought that only mammals had the ability to use orientation saliency. Interestingly enough, we think we understand a lot about orientation saliency in mammals, and as far as we can tell, there is a section of the mammal brain, called the visual cortex, that contains the nerve cells that allow for its use. Fish don’t have a visual cortex, however, so either our understanding of this ability is a bit off base, or archerfish do it in a completely different way.

As is the case with most data, this fact presents a bit of a problem for evolutionists. After all, it was thought that orientation saliency evolved with the mammals, which was rather late in the evolutionary timescale. Mammals can use it because the common ancestor of mammals evolved the ability, and it was passed down the evolutionary line. Now that we know of at least one fish that can use it, evolutionists have to postulate that this amazing ability didn’t evolve just once. Instead, it evolved independently at least twice, along completely different evolutionary lines.

Of course, believing that complex systems can evolve multiple times in independent lines isn’t a problem for evolutionists. Indeed, in order to understand bioluminescence, evolutionists are forced to believe it evolved independently 40 different times. In order to understand the eye, evolutionists are forced to believe that it evolved independently in six to 60 different evolutionary lines! I am sure glad that I follow the data rather than blindly believing in evolution. I simply don’t have that kind of faith!

REFERENCES

1. Thomas Schlegel, Christine J. Schmid and Stefan Schuster, “Archerfish shots are evolutionarily matched to prey adhesion,” Current Biology 16:R836-R837, 2006 available online
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2. Shelby Temple, et al., “A spitting image: specializations in archerfish eyes for vision at the interface between air and water,” Proceedings of the Royal Society B 277:2607-2615, 2010 Abstract available online
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