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  • Spectrograph Takes the Fish-Eye View

Photonics Spectra
Feb 2001
Daniel C. McCarthy

Like many creatures, the male stickleback fish, found in both fresh and saltwater, usually appears much like its surroundings: dull and greenish. However, this changes when one stickleback encounters an unfamiliar member of its species. On such occasions, its throat turns a more intense red, its eye a sharper blue and its back a brighter shade of blue-green.

It doesn't take an ichthyologist to theorize that this behavior is related to mating and aggression. However, it is not so easy to prove that this display has a practical function.

Spectrometry helped quantify the color alterations that male stickleback fish display in the presence of other unfamiliar sticklebacks. The enhanced colors correspond with the species' visual range, indicating that courting or defending males make themselves more visible by changing shades.

A group of researchers at the University of Kentucky in Lexington set out to learn if these color changes enhanced color contrast -- or, in other words, visibility -- from the perspective of sticklebacks. The study required some means to examine the fish within an aquatic environment. If removed from the water for very long, sticklebacks turn a dull green.

Victor Rush, now a scientist with Tucker-Davis Technologies, participated in the study as a research assistant at the university. He designed a system incorporating an Instaspec spectrograph from Thermo Oriel to perform reflectance measurements on swimming fish.

Rush selected the Instaspec for its CCD, which was more sensitive to the ultraviolet and short-wavelength visible end of the spectrum. Also, its 1-MHz data capture board provided sampling rates easily encompassing the 10 to 30 fps the experiment required. "We could have used a little more quantum efficiency," Rush added. "We got what we needed at 30-fps capture rates, but it was a little noisy. [The Instaspec] could do it, but sometimes we went down to 10 fps to do a little better."

The system's computer can perform fast Fourier analysis for data collection on the fly, said Rush. "We could synchronize what part of the body we were getting reflection off of and what the animal was doing at the time."

The Instaspec's multichannel detector captured the spectral range of each color alteration. Rush then entered the data into a computer and modeled how the colors would appear to another fish. The researcher's results indicated that they corresponded to the peak color sensitivities in the sticklebacks' visual gear.

"In a sense, these fish are wearing what you or I would wear to attract maximum attention -- a blue shirt and red pants," Rush said.

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