The diagram on page 24 lays out the transition of an aquatic animal, fish, to a land-living animal, tetrapod. There were changes in the presence of neck, head shape, and eye positions. Just like any other animal, fish also need to be aware of their 360 degree surroundings. Why is it that fish had eyes only on their sides and no neck? Wouldn’t it seem logical that a neck would help them move their head to get a better view? What factors/adaptations led these fish to develop a neck and a “flat head [with] eyes on top” (24) and why is this a better advantage than having eyes on the sides? Compare the advantages and their adaptations of animals with eyes on the sides as opposed to eyes on the top of the head.
Chandrika D.
(shiningstar0393@gmail.com)
This comment has been removed by the author.
ReplyDeleteTiktaalik is the missing link between us and our fishy ancestors. All fish before tiktaalik had a set of bones that connect the head of the fish to its shoulder. However, tiktaalik has a neck which sets it head free from its shoulder like us mammals. Also tiktaalik was able to advance from the fish eyes on the side of the head to the normal eyes on top of its head. Over time selective advantage and the necessity of these animals to develop better eyes led the fish and animals to adapt and have eyes on the top of their heads.
ReplyDeleteA fish’s eyes are adapted or modified for underwater vision, but they are not
very different from human eyes. Fish do not have true eyelids. Human eyelids prevent the eyes from becoming dry and also protect against dirt. A fish’s eyes are always covered by water; therefore, they require no lids. The human iris can expand or contract, depending upon light conditions. Because light never attains great intensity underwater, a fish needs no such adaptation. The big difference between a human eye and the eye of a fish occurs in the lens. In humans it is fairly flat or dish-like; in fish, it is spherical or globular. Human eyes are capable of changing the curvature of the lens to focus at varying distances Although the eye of a fish has a rigid lens and its curvature is incapable of change, it can be moved towards and away from the retina to focus like a camera.
According to Campbell, All photoreceptors used for visual perception have pigments that absorb light. Early bilateral animals had genes that evolved the embryonic development of photoreceptors. “The evolution of the complex eye took place through a series of incremental modifications that benefited the eyes’ owners at every stage”. Selective advantage led to the production of a more superior and complex eye.
The benefit of having eyes mounted on the front of the head, facing forward, is that it permits stereoscopic vision and more accurate depth perception (although at the sacrifice of a wide field of view). This arrangement is found mainly in predators ( mammalian carnivores, birds of prey), which need to make fine assessments of the distance to their prey. It is also found in primates, undoubtedly as an adaptation to being able to judge the distance to the next limb when leaping about in trees.
The reason that this arrangement is not found more often in predatory fish may be because of the need for streamlining. Most of these predators need to accelerate rapidly in order to catch their prey. Having eyes mounted in front perpendicular to the direction of movement would necessarily slow them down. “But because most fish have eyes situated on the sides of the heads, their ability to judge distance is not as acute as it is with terrestrial vertebrates”. Humans however, look forward at all moments of time which allows us to judge distance fairly accurately. The advantage of fish having eyes on both sides of their head is that they almost always have a 360 degree view of their surroundings. Giving them an awareness of predators and other possible dangers. As for fish with either broad heads or large tails, they sweep their head from side to side while swimming to make sure they can get a complete view.(http://www.advancedaquarist.com/2007/1/aafeature2/view)
-Adnan jahan
(adnanjahan@gmail.com)
Before speaking about why fish have eyes on top of their head with no neck, in response to Adnan’s point about Tiktaalik being the missing link between us and fish -- I would respectfully disagree. Even Shubin agreed Tiktaalik wasn’t the transitioning point during evolution, he was just a small piece of it. Evolution is a continuous transition. One mutation can lead to it being a selective advantage and because of environmental pressures and other various reasons, natural selection occurs to allow this organism to survive and reproduce, and after many generations, perhaps this one mutation allowed many other organisms to be born with this benefit too. To call Tiktaalik the missing link is assuming too greatly that he was the one and only organism with a neck and limbs that can walk on the ocean floor.
ReplyDeleteThe eyes of fish are in fact very similar to our own and those of other vertebrates. The main differences being: 1) They have no tear ducts, living in water which is constantly washing their eyes they have no need of them; 2) They have no eyelids, although some species do have extensions of the skin that cover part of the eye. Scientists believe that this is mostly to protect the eye during feeding (http://www.earthlife.net/fish/sight.html). While the fish's lens is spherical the eye itself is elliptical, so light entering from some directions has a short journey to the retina than light from other directions. Its eyes are close focused to light from in front, but distanced focused to light from the side, or from behind. Fish constantly need to have 360-degree vision because they are surrounded by predators, and as Shubin explained, evolution was necessary to survive in a fish-eat-fish world. Tiktaalik on the other hand had limbs to move around on the bottom of the ocean. He also had a neck! Adnan touched on the idea why having a neck is so important, but I’d like to expand. Fish have an operculum, but after millions of years, a neck was a greater advantage. With a neck, one could turn the head separate from the body, allowing the ability to look around while staying still. Because fish don’t have necks, they have to turn their entire body, making them more vulnerable. Eyes on the side of their head allowed them to not frequently have to make 180-degree turns to see what is going on behind them.
Of course it would seem logical for a fish to be able to move their head to get a better view, like Tiktaalik was able to do, but at the time, they simply didn’t have the ability. A neck was nonexistant, so the fish had to rely on their eyes to get the job done.
Tiktaalik lacked the series of bones that attach the head to the body in the fish (Shubin 204).
The advantages of having eyes on the side of one’s head, like Adnan already said, is the ability to have a 360-view; however, this advantage is outweighted by the fact that, to get a better view, it is necessary to turn around, and this puts the fish in danger: slowing them down, etc. A flat head with eyes on the top is an advantage because it shows signs of complexity. Nearly all mammals (if not all) have eyes toward the top of their head. “In daylight, you achieve your sharpest vision by looking directly at an object” (Campbell 1102). The only way you can do this is by having both eyes in one area of your head looking straight on. In fish, light came in from the sides, not directly, so their vision wasn’t as sharp.
-Michelle Layvant, michellel94@hotmail.com