On page 151, Neil Shubin provides a diagram of visual clarity between a limpet, a nautilus, a scallop, and a human. As the eye structure becomes more complex, the visual perception of items achieves a new strength in clarity. Explain why certain organisms do not require as much clarity to be considered fit in their environment. Why is visual clarity more important on land than in water?
Talk about light refraction and how it effects the retina in air and in water.
Yekaterina Khavkhalyuk (kittykatx93x@yahoo.com)
Many organisms, although having bad eye sight, are considered fit in their environment. Animals such as Scallops, Nautilus, and Limpets contain an outer shell layer for protection. With this outer shell layer, the need for sight isn’t the most important characteristic needed. Because these 3 organisms contain shells, they have better defense against predators. Animals without good defense would need a way to recognize they are in danger and to “run away”. Because scallops, nautilus, and limpets have a way to protect themselves, eyesight is not necessary. Humans on the other hand have no outer shell to rely on for protection. It is a selective adaptation that to have good eyesight in order to view the outer world and any threats/danger.
ReplyDeleteAlso, humans have a complex nervous system. With the brain processing the signals sent through the optic nerve, humans are able to see better and in more definition because our brains are well-developed. Also, in water, light tends to diffract into different directions and cause very bright water higher up towards sea level. Thus, marine animals do not require as well eye sight. Instead, they must possess better light sensibility to protect their eyes from the blinding light which diffracts through the water. On land, vision clarity is more important because organisms that live on land tend to have a more developed brain and eyes. Thus, to survive in such a world, it is beneficial to have good vision clarity. Also, light does not diffract on land. Thus, the need for good clarity is useful because eyes wouldn’t be blinded by light.
Sources:
http://www.oceanlink.info/biodiversity/scallops.html
Shubin (152-153)
Shreeraj Patel
shreeraj.patel1@gmail.com
In addition to the defenses Shreeraj has mentioned, some organisms can navigate through means other than vision. These organisms might not even rely on vision at all as their primary sense for navigation.
ReplyDeleteOne example of this is echolocation, as found in bats and dolphins. This mechanism works when the organisms "send out pulses of sound and steer by the echoes that bounce back from obstacles or prey" (http://www.time.com/time/magazine/article/0,9171,812332,00.html). By using a modification of sound to navigate, these animals are able to navigate at night, where vision is less effective, and under water, where visual range is often limited. These environments create pressures that select for non-visual senses for movement, whereas a diurnal land animal's environment does not.
An even more interesting alternative to visual navigation is the mechanism used by electric fish. These organisms have receptors that can detect electric signals from their environment or even from other organisms, which allows them to navigate without any need for light or visual clarity (http://people.virginia.edu/~mk3u/mk_lab/electric_fish_E.htm). This is, of course, a strong benefit in habitats where, for example, the water is too muddy to see effectively in. Some electric fishes are weakly electric, which means they use electrolocation by generating their own electric field that alerts them of an object in their environment when it is disrupted by, for instance, a rock (ACT!).
These mechanisms provide an alternative to Shreeraj's examples of animals with effective defenses, such as scallops with outer shells. Whereas those organisms do not require as much clarity as others to succeed in their environment because of their defenses that enable them to not need to navigate, others can simply navigate by other means, such as echolocation and electrolocation.
- Vincent Fiorentini
(vincent@panatechcomputer.com)
Less developed organisms such as bivalves or echinoderms will have less developed eyes as well. Being mainly marine animals, the waters make vision more difficult because of the lack of clarity. Like Shreeraj said, bivalves have a tough outer shell while echinoderms have spikes for protection. Because of these features, they are well protected from potential predators, and do not require the advantage of spotting danger in advance. The irises that contract by muscles help these less developed species detect light and slight movement in their surroundings (Shubin 150). Building off Shreeraj's statement that many species with less developed eyes have more protection against predators, they can also easily detect light from movement of prey without actually seeing the prey. Bivalves will eat pieces of seaweed or smaller organisms such as phytoplankton that is floating in the water (http://animals.jrank.org/pages/1908/Bivalves-Bivalvia.html). To capture their food doesn't require much energy or movement, so it wouldn't benefit these marine creatures to have extremely developed sight.
ReplyDeleteBeing able to see clearly on land proves to be a huge advantage to species with developed eyesight, like humans. Because humans are a cephalized species, the light detected by the iris is carried by the optic nerve to the brain immediately translated as an image (http://www.tedmontgomery.com/the_eye/index.html). This instantaneous result allows human to have faster reaction times to whatever danger lies in our path. Light refraction in water is another reason developed eyesight works better on land. When light passes through water, the light bends and the image becomes distorted. This effect does not work on land because the air does not refract light. Being able to see distorted images does not benefit a species in distinguishing predator from prey. This is why species with eyespots detect shadows and not specific images. On land, animals have the advantage of developed sight at the cost of their own protection against predators by not having a hard shell.
(http://library.thinkquest.org/27066/lightrays/nlrefraction.html)
Claire Yao (claire.yao521@gmail.com)
In addition to the points made in three of previous posts regarding eye sight and the defense of the organism, many deep sea creatures often have eyes that looks upwards rather than forward. Absurd? Not really. From evolutionary perspective and their habitat, it makes complete sense. Because in deep parts of the ocean, lights enter from the surface thus, rather than looking out for predator or prey's body, these fish looks out for the shadows made on the dim surface above. (http://oceanexplorer.noaa.gov/explorations/04deepscope/logs/aug16/aug16.html) This is possible through natural selection because initially the gene pool included those with eyes that look upwards rather than forward had selective advantage over those fish that looked straight forward and were either able to catch prey more easily or avoid its predator. Over long period of time, those eyes that looked upward thrive while those that looked forward were extinct leading to emergence of new specie.
ReplyDeleteWith ways to detect preys and predators also comes with the abilty to hide from them as well. Thus we can see the case of coevolution. Because of this search for shadows, some fish over long period of selective advantage from random mutation had evolved to be able to create counter illuminating to hide its own shadows that other fish tries to detect. It does this by absorbing certain spectrum of color while reflecting the others through its lens. In all, aquatic animal's eyesight has been specifically selected to favor those that can recognize simply patterns(as opposed to those that can distinguish specific features) with efficiency providing protection and increasing the likelyhood of survival.
In non apquatic environment however, strong eyesight often times becomes a neccessity in its fight to survive and reproduce. For example, when looking at the Bald Eagle, notorious for its good eyesight, has eyesight that specifically fits their purpose. Bald eagles must be able to distinguish fish under the water from high in the sky which gives reason for those with good eyesight to be advantageous. In human eyes, a depression in the retina fovea allows the maximum concentration of light by allowing maximum concentration of sensory allows greatest optical resolution. Bald eagle's eyes has two fovea which allows even greater concentration of sensory cells within the eye. Also, the bald eagles retina is made up of cones (picks up color) and rods (distinguish light). (http://www.bluepage.org/bald-eagles/bald-eagle-eyesight.html) Unlike aquatic animals, land based animals are selectively advantaged in having an outstanding eyesight which not only shows exact location of its target but also its features as well since many of land based organisms mate based on its physical characteristics.
Kevin Jeon
Bboybyung@gmail.com