Fish Eyes

More than Meets the Eye

Evaluate fish physiology and ecology using vision research data from Dr. Kerstin Fritsches at the University of Queensland in Brisbane, Australia.

Pelagic fishes inhabit the open waters of the world's oceans Pelagic fishes inhabit the open waters of the world's oceans. However, within this habitat different species have very different lifestyles. Some, such as the mahimahi (dolphinfish), remain close to the water's surface while others, such as the swordfish, travel to great depths in search of prey. The visual environment for these fishes is dramatically different. At depths of several hundred meters there is little light, no color, and very cold temperatures — all factors that can affect a fish's ability to see. The structure of the eye reflects the lifestyle of the fish and the depth to which it hunts. The information below describes various physical adaptations that can affect an organism's visual ability. In this activity, you will use this information along with life history information and research data to make comparisons between the visual abilities of four pelagic fish species.

Cone types
The presence of different types of cones in the retina of the eye determines an animal's ability to see color. The more types of cones an animal has, the better its color vision. Remember that as the light penetrates deeper into the water, "colors" are absorbed and eventually disappear. So, color vision becomes less useful as depth increases.

Lens size
The size of a fish lens is proportional to the size of the eye and is a good indicator of light sensitivity. The larger the lens, the more light will reach the retina. A very important adaptation for dim light conditions! For example, a swordfish has a significantly large lens, even in the fish world. An adult swordfish has an eyeball approximately 9-10 cm (3.5 - 4 inches) in diameter and its lens is approximately 2.6 cm (7/8 inch) in diameter. The human eye is only about 2.5 cm (1 inch) in diameter. The larger swordfish eye lets more light in than a human's eye, allowing the fish to see better than humans in darker conditions.

Endothermy (Ability to warm body temperature above outside water temperature)
Swordfish frequent water depths down to 800 meters (2600 feet). At this depth, the water temperatures can be as low as 5 °C. A drop in temperature drastically affects the performance of the nervous system, including the eyes and brain. Tunas and some sharks (such as the mako ) have developed ways of keeping their body, or parts of it, warm. These fishes have a vascular (blood vessel) heat exchange system (a network of veins and arteries in close contact with each other) which allows them to maintain a core body temperature above that of the surrounding water. Warm blood leaving the muscles via veins heats the cooler, oxygenated blood in the arteries returning from the gills and heading toward the muscle. In contrast, billfishes (swordfish and marlins)and some shark species (such as thresher sharks) have evolved the ability to keep just their eyes and brain consistently warm for deep dives into cold water. These fishes have a smaller version of the tunas' vascular heat exchange system found only close to the eyes and brain. In billfishes the heat is produced by a highly modified eye muscle which has evolved to become a specialized heating organ located close to the eye and brain. These adaptations keep the eye and brain warm during dives into cold water.

Speed of Vision
Speed of vision can best be explained using TV as an example. As you may know, TV images are actually individual pictures shown at a speed so rapid that human eyes cannot detect the pauses between images. So to humans, TV images appear to be in motion. Birds and flies have a much faster speed of vision so they would see the black pause between individual TV images. (That's why it is so hard to swat a fly — they see you coming much more quickly!) A fast speed of vision allows an animal to pick up fast movements, but only if there is enough light in the environment. Under dimly lit conditions, the speed of vision needs to be slower so that the eye can register more light as it attempts to detect the image. This is the same principle that applies when you alter the shutter speed of a camera based on the brightness of the surrounding light. A slow shutter speed is used for low light conditions and a higher shutter speed is used for bright conditions. In the chart below the speed of vision is given in Hertz (Hz), which is equivalent to the number of flashes per second the animal can see.

Data Activity
Data Activity

Diving Behavior and Vision
The fish species outlined in the table below fall in two categories: shallow divers and deep divers. The shallow divers remain in the surface layers of the ocean day and night, feeding on prey animals in these surface layers. While the diving range of these shallow divers can extend to several hundred meters, most of their time is spent closer to the surface. The deep divers, on the other hand, remain close to the water surface during the night but at sunrise commence their steep descent to considerable depths. Usually they remain in the deep water for most of the day before ascending to the surface at sunset. The deep divers feed on prey at greater depth and usually have adaptations to deal with the colder water and lower light levels there.

The table below contains data from fish vision research conducted by Dr. Kerstin Fritsches at the University of Queensland in Brisbane, Australia. Fill in the missing ecological data (highlighted yellow cells) using information from the FishBase database for the following four species. (Click here for a printable version of the table.)


Cone Types

Lens Diameter

Speed of Vision

Temp. Range


Max. Depth

Bigeye Tuna

2 types of cones,
one dominant







3 types of cones







Mainly 1 type of cone






Striped Marlin

2 types of cones,
both abundant






Use the information from the completed table to graph the four species' eye capabilities versus their maximum depth. To do so, first print out the following graphs:

On these graphs, maximum depth is on the y-axis, with 0 m (or the surface) at the top descending down to a depth of 1,000 m. For each species, plot a point on the graph at its correct depth and lens diameter or speed of vision. For the cone type graph, insert the cone type symbol at the correct depth for each species. After completing the graphs, compare your answers to the Bridge's answers (Microsoft Excel file).

Discussion Questions

  • What is the general relationship between a species' maximum diving depth and its lens diameter?
  • What is the general relationship between a species' maximum diving depth and its speed of vision?
  • What is the general relationship between a species' maximum diving depth and its cone types?
  • Taking into account each species' lifestyle, how do these vision adaptations help the fishes?

Click on this link and study the chart of visible light in water.

  • Based on the visible light chart and what you learned earlier about the vision of bigeye tuna, mahi mahi, swordfish, and striped marlin, which species are likely to have color vision?
  • How would color vision be useful for their lifestyle?
  • How could color be used for communication between members of the same species? (i.e. Are their bodies colorful? Do they hunt in groups? Do they have a social structure? Do the males and females look different?)

Print-friendly View

Read Me

Susan Haynes and Dr. Kerstin Fritsches,


Grade Level



Lesson Time

1.5 hr



  • Recognize structural and physiological factors in fish vision.
  • Differentiate among open water fish physiological and ecological traits.
  • Predict habitat and ecology of open water fish.


Cones, Retina, Lens, Endothermy, Vascular heat exchange, Speed of vision, Endothermy


Materials Required

eye data table, Eyes Excel spreadsheet, lens diameter, speed of vision, and cone type vs. maximum depth graphs, chart of visible light in water


Natl. Science Standards

IK-1 IK-2 PH5-3 PH9-6 L5-1 L5-3 L5-4 L5-5 L9-3 L9-6


edited by Lisa Ayers Lawrence


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