Fish Eyes
More than Meets the Eye
Written by: Susan Haynes and Dr. Kerstin Fritsches,
Credits: edited by Lisa Ayers Lawrence
Summary
Evaluate fish physiology and ecology using vision research data from Dr. Kerstin Fritsches at the University of Queensland in Brisbane, Australia.
Objectives
- 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.
Vocabulary
Cones, Retina, Lens, Endothermy, Vascular heat exchange, Speed of vision, Endothermy
Introduction
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.)
Fish |
Cone Types |
Lens Diameter
(cm) |
Speed of Vision
(Hz) |
Temp. Range
(°C) |
Diving
Behavior |
Max. Depth
(m) |
Bigeye
Tuna |
2
types of cones,
one dominant |
2.2 |
48 |
|
deep |
250 |
Mahimahi |
3
types of cones |
1.1 |
82 |
|
shallow |
|
Swordfish |
Mainly
1 type of cone |
2.6 |
43 |
|
deep |
|
Striped
Marlin |
2
types of cones,
both abundant |
2.0 |
71 |
12-27 |
shallow |
|
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?)
