Claim CB921.1:
What use is half an eye?
Source:
Response:
- Half an eye is useful for vision. Many organisms have eyes that lack
some features of human eyes. Examples include the following:
- Dinoflagellates are single cells, but they have eyespots that allow
them to orient toward light sources (Kreimer 1999).
- Starfish and flatworms have eyecups; clustering light-sensitive
cells in a depression allows animals to more accurately detect the
direction from which the light is coming from.
- Most mammals have only two kinds of color photoreceptors, allowing
less color discrimination than most humans have. Some deep-sea fish
can see only black and white.
Visual prosthetics (bionic eyes) with as few as 16 pixels are found
to be very useful by people who had become blind (Wickelgren 2006,
Fildes 2007).
- Humans themselves have far from perfect vision:
- Humans see in only three colors. Some fish see five. (A very few
women are tetrachromats; they have four types of color receptors;
Zorpette 2000.)
- Humans cannot see into the ultraviolet, like bees.
- Humans cannot see infrared, like pit vipers and some fish.
- Humans cannot easily detect the polarization of light, like ants and
bees.
- Humans can see only in front of themselves. Many other animals have
far greater fields of view; examples are sandpipers and dragonflies.
- Human vision is poor in the dark; the vision of owls is 50 to 100
times more sensitive in darkness. Some deep-sea shrimp can detect
light hundreds of times fainter still (Zimmer 1996).
- The range of distances on which one may focus is measured in
diopters. A human's range is about fourteen diopters as children,
dropping to about one diopter in old age. Some diving birds have a
fifty-diopter range.
- The resolution of human vision is not as good as that of hawks. A
hawk's vision is about 20/5; they can see an object from about four
times the distance of a human with 20/20 vision.
- Humans have a blind spot caused by the wiring of their retinas;
octopuses do not.
- The Four-eyed Fish (Anableps microlepis) has eyes divided in half
horizontally, each eye with two separate optical systems for seeing
in and out of the water simultaneously. Whirligig beetles
(family Gyrinidae) also have divided compound eyes, so one pair of
eyes sees underwater and a separate pair sees above.
- The vision of most humans is poor underwater. The penguin has a
flat cornea, allowing it to see clearly underwater. Interestingly,
the Moken (sea gypsies) from Southeast Asia have better underwater
vision than other people (Gislén et al. 2003).
- Humans close their eyes to blink, unlike some snakes.
- Chameleons and seahorses can move each eye independent of the
other.
If you want to know what use is half an eye, ask yourself how you
survive with much less than half of what eyes are capable of.
Links:
Bahar, Sonya, 2002. Evolution of the eye: Lessons from freshman
physics and Richard Dawkins. The Biological Physicist 2(2): 2-5.
http://www.aps.org/units/dbp/newsletter/jun02.pdf
References:
- Fildes, Jonathan. 2007. Trials for 'bionic' eye implants. BBC
News, 2/16/2007. http://news.bbc.co.uk/1/hi/sci/tech/6368089.stm
- Gislén, A. et al., 2003. Superior underwater vision in a human
population of sea gypsies. Current Biology 13: 833-836.
http://www.current-biology.com/content/article/abstract?uid=PIIS0960982203002902
See also Pilcher, Helen R., 2003. How to see shells on the sea floor,
http://www.nature.com/nsu/030512/030512-14.html
- Kreimer, Georg, 1999. Reflective properties of different eyespot types
in dinoflagellates. Protist 150: 311-323.
http://www.urbanfischer.de/journals/protist/content/issue3/Pro0021.pdf
- Wickelgren, Ingrid. 2006. A vision for the blind. Science 312:
1124-1126.
- Zimmer, Carl, 1996. The light at the bottom of the sea. Discover
(Nov.): 62-66,71-73.
- Zorpette, Glenn, 2000. Looking for Madam Tetrachromat. Red
Herring (4 Dec.),
http://www.redherring.com/mag/issue86/mag-mutant-86.html
(registration
required)
created 2003-8-13, modified 2007-2-18