Science Through the Eyes of Fish
In the recesses of the Swenson Science Building on the campus of the University of Minnesota Duluth (UMD), there is a room that could double as the setting for a movie thriller. It’s cold. It’s shadowy with darkened lights and shrouded aquaria, and it hums with the sound of aerators. Within this room, Tom Hrabik, associate professor of biology at UMD, and his colleagues have been studying the eyesight of Lake Superior fish in relation to available light. "The room is lit with a weird blue-green color, like you were about 20 meters down in the lake," said Hrabik.
The research conducted in these bone-chilling conditions has shown that young lake trout can easily see prey at exceedingly low light levels. It has also yielded new methods for studying deepwater fish and a series of results that are clarifying how species get by in a place as dark as the bottom of Lake Superior (see Minnesota Sea Grant’s newly funded projects).
To understand eyes, it’s important to understand the scene for which they are suited. For humans, the medium is air and color vision evolved to ... well, we’re not sure why. (A new theory suggests color vision evolved in certain primates, including humans, so that we could sense the emotions and the condition of those around us; www.scientificamerican.com/article.cfm?id=mind-reviews-the-vision-revolution). For fish, the medium is water, a medium that does tricky things to natural light (see Water, Light, and Lures below).
Despite differences in habitats, humans and fish have similar eyes. Among our similarities, we share lens-covered retinas that house rod and cone cells. Rod cells perceive contrast in low light. Cone cells provide color vision. Less than 10 percent of your cone cells respond to light from the blue end of the visible spectrum. The other two types of cones – red-sensitive and green-sensitive – occur in wildly variable proportions in people that have otherwise normal color vision. In fish, of which there are many species, the three cone types come in even more variable proportions. Plus, many fish have a forth type of cone cell for sensing ultraviolet light, and some can sense polarized light. Then, of course, there are those sightless cave-dwelling fish with virtually no eyes.
The distinct pattern of rods and cones that developed in different species of fish provides evolutionary biologists with insights on how light availability affects retinal development. The results from Hrabik’s cold quarters are adding to the growing volume of knowledge about fish eyes in general, but more importantly, they are shedding light on predator-prey interactions and fish health in the Great Lakes. Studying the eyes of fish has led to a better understanding of thiamine deficiency (a lack of vitamin B1). Thiamine deficiency afflicts fish and humans. Without enough thiamine, young lake trout lose visual acuity, which diminishes their prey capture rates and growth rates. Research suggests that thiamine deficiency brought on by a diet rich in alewives contributed to difficulties with lake trout restoration in Lake Huron. In humans, severe thiamine deficiency is known as beriberi, a disease that can be lethal. Mild thiamine deficiencies can lead to blurred vision or double vision.
In addition to vitamin B1, retinal health in fish and humans has been linked to a particular essential omega-3 fatty acid called docosahexaenoic acid (DHA). DHA makes up a large proportion of the fats in your retina and also in your brain. The same is true for fish. Eyes need DHA to ensure the correct functioning of photoreceptor cells.
Dr. Michael Arts, a research scientist with Environment Canada, calls DHA "a critical and environmentally-sensitive fatty acid." He should know, having devoted much of his career to understanding the function of lipids and fatty acids in aquatic ecosystems. Arts found that the amount of DHA in the retina of lake whitefish, a commercially harvested fish in Lake Superior, is proportional to the amount of DHA in their bodies, which is linked to diet. He also reports that the amount of DHA within a fish is correlated to water temperature. Arts said, "the ideal temperature for an Atlantic salmon is 15.9 °C <60.6 °F>. If you raise water temperature by just a few degrees, there is a significant drop in DHA levels within salmon muscle tissues." Both salmon and lake whitefish belong to the family Salmonidae and are touted as fish rich in omega-3 fatty acids.
It’s no surprise that anglers and commercial fishermen care deeply about the way fish see; their luck and lives depend on it. What might be surprising is how Sea Grant and many scientists care about fish eyes ... and why.
Want to follow up on the way fish perceive the world? Log onto www.seagrant.umn.edu/fisheries/senses and read Fish may be cold-blooded, but they’re not insensitive.
High School Educators! Here’s a lesson plan for you: Fish Eyes, More than Meets the Eye. Find it on The Bridge, an online collection of marine education resources made possible by the National Sea Grant Office, the National Oceanographic Partnership Program, and the National Marine Educators Association. Log onto www2.vims.edu/bridge/DATA.cfm?Bridge_Location=archive0305.html.
By Sea Grant Staff