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    Image Courtesy of Ramon Llaneza

From: Ramon Llaneza
Bahamas, July 5, 2008

Sharks' Visual Sense

For the past few days, the weather conditions have been favorable, and the absence of wind allowed the water to clear so that it now has a crystalline transparency. These are the ideal conditions for the visual sense test. I have often noticed that the clearer the water is the more active the life of the reef becomes. The predators (sharks) had climbed up from the deeper water and were in a constant search for food. They are always ready to pounce on a wounded animal or to profit from any mistake in judgment on the part of a potential victim.

The experiment we were planning took place at a depth of 50 feet; we were going to test sharks in their visual sense. We set out to determine if the reef sharks (Carcharhinus perezii) can see in color.

We used some visual stimuli, and we observed their reactions toward different colors such as yellow, silver, black and red. To do this, we used fabrics on frames and we observed sharks behavior toward these colors. We noticed that the color yellow attracted the sharks more than the other colors; they swim closer and faster to the frame. One of the sharks touched the frame with its fins and another sharks brushed its head against the yellow frame. This behavior suggests that when the sharks are hunting for food, they taste objects simply by touching them.

For the color silver we used aluminum foil in the same frame; it seemed to attract the sharks second to yellow. The effect of light on the silver aluminum and the glint of light reflecting off metal are similar to the light off the scales of fish, which are common in the diets of most sharks.

The sharks didn't show any interest in black or red.

After diving we all agreed that when diving or swimming we should avoid wearing bright dive gear. I personally prefer to use dark fins, tanks, masks, and wetsuits while diving. We tried to eliminate wearing any bright colors and things that reflect light that could attract sharks.

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Postcards from the Field: Shark Watching

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