Lately Underwater On Planet EarthMarine Biology

Thoughts On Dappling

Pacificklaus
Dappling, or water caustics, discussed with the aid of some some underwater scenes seen on Negros Island in the Philippines, in the video below. How can fish visual systems not be completely thrown off by those crazy lines in the sand formed by light reflected from the waves on the water surface? I argue that the dappling does not conform to Gestalt criteria, and that the combination of fast temporal frequency and long (relative to the body lengths of most fishes) spatial frequencies does not excite visual systems very well.

Fish Behavior and Dappling

And this is the well-done video by Hakai Magazine published a few months ago. Triggerfish are actually thrown off by dappling a little bit.

What to Make of This?

Dappling as seen in my video above will only occur in shallow water (< 5 meters), on a sunny day with a calm ocean, in clear water. These are still relevant conditions during large parts of the year for fishes in tropical coral reefs. So, will fishes have developed special filters to get rid of dappling? Possibly, the complete un-Gestalt like properties of dappling (no temporally and spacialy co-varying shapes which indicate the presence of an object) will have made this unnecessary. The minor performance deficits seen in the trigger fish described above could be not serious enough to warrant a specialized system to get rid of the dappling background-signal in the fish visual system.

Conversely, did fishes develop visual system tricks to take advantage of the information conveyed by dappling, such as my “dappling shadow” in the video above? This is possibly the case, and would make an interesting research topic.