In chapter seven, Shubin talks about body plans of animals and specifically on page 131 he talks about the body plan of a sponge. Explain the body plan of the sponge and what role does it play in how/why this organism has been successful? What environmental pressures have led the body plans of organisms to shift more into other forms like that of a jellyfish, then fish, etc? Describe the communication of cells within a sponge, expanding on Shubin's description on the beginning of page 132.
-Michelle Layvant, mlayvan2@students.d125.org
Sponges' body plan allows the animal to eat via filtration. These suspension feeders' bodies consist of a central cavity called the spongocoel, which water is drawn through and then flows out of a large opening called the osculum. Sponges lack true tissues, “groups of similar cells act as a functional unit and are isolated from other tissues by membranous layers” (Campbell 670). Choanocytes, collar cells, form the inner lining of the sponge. They possess flagellum to draw water through the entire body of the sponge, and food particles are trapped in the mucus of its fingerlike projections. The actual body of a “sponge consists of two layers of cells separated by a gelatinous region called the mesohyl. Wandering through the mesohyl are cells called amoebocytes… Amoebocytes have many functions. They take up food from the water and from choanocytes, digest it, and carry nutrients to other cells” (Campbell 670). In a nutshell, the body of a sponge is like a big tube of filter paper that filters out food particles from the water that passes through out.
ReplyDeleteThis very simple body plan contributes to the reason why sponges have been so successful in terms of survival. Sponges are able to live off of very little food because their bodies don’t require a lot of energy to function, and the fact that they don’t have any true body tissue also contributes to a sponge’s energy efficiency. Sponges are also attached to a substrate, so movement is virtually obsolete in a sponge’s life. Being sessile requires a lot less energy than being mobile. The overall simplicity of a sponge’s body has allowed them to survive on Earth for millions and millions of years.
Although being sessile conserves a lot of energy, immobile organisms eventually had to evolve in order to survive. Environmental pressures like nutrient-poor water or no underwater current could have caused sessile animals to evolve into mobile animals that were able to move where food was abundant. A convincing process that supports the fact that sessile organisms evolved into mobile ones is the polyp-medusa cycle in Cnidarians. Polyps are “cylindrical forms that adhere to the substrate by the aboral end of their body (opposite the mouth)” and a medusa “is a flattened, mouth-down version of the polyp. It moves freely in the water by a combination of passive drifting and contractions of its bell-shaped body” (Campbell 670). Medusae are able to move away from their sessile origin and reproduce where conditions are more favorable.
Choanoflagellates communicate between each other via phosphorylation, the addition of a phosphate group to a protein at one or more of its amino acids. Tyrosine (a type of amino acid) is phosphorylated in choanoflagellates, and other animals during cell-cell communications including immune system responses, hormone system stimulation and other crucial functions. There are three other crucial molecular components that make communication possible: Protein tyrosine phosphatases (PTP), Src Homology 2 (SH2), and Tyrosine kinases (TyrK). These kinases “write” message between cells by adding phospho-tyrosine modifications. PTP modify or “erase” these modifications, and (SH2) “read” these modifications so the recipient cell gets the message. These three signaling proteins are found in choanoflagellates in significant amounts, which supports the fact that choanoflagellates communicate with each other in complex ways.
Mikey Ling (mikeyling@ymail.com)
Shubin begins talking about sponges comparing the similarities that they share with humans. Shubin points out that we have many cells, there is a division of labor among parts in our bodies, "various devices that help cells signal to one another" are present, along with many other general similarities in the "basic bodybuilding apparatus" (Shubin 132). Even though sponges are much, much simpler than humans, we share many of the same characteristics, giving us humans an insight on how we evolved into the beings we are today.
ReplyDeleteLike Mikey pointed out, sponges are suspension feeders: they "captures food particles suspended in the water that passes through their body" (Campbell 670). Their body plan is relatively simple. Water enters the sponge through pores that span the body wall, then enters a central cavity called the spongocoel. Then the water flows out of the sponge through a large opening called the osculum. Sponges lack true tissues, which are defined as "groups of similar cells that act as a functional unit and are isolated from other tissues by membranous layers" (Campbell 670). However, the sponge body does contain several different types of cells. The flagellated choanocytes, or collar cells, line the interior of the spongocoel and are able to draw water through their "collar" of fingerlike projections when there is movement of a choanocyte's flagellum. Food parties are "trapped in the mucus coating the projections, engulfed by phagocytosis, and either digested or transferred to amoebocytes" (Campbell 670).
The wall of a sponge consists of two layers of cells spearated by a gelationus region called the mesohyl. Amoebocytes are found in this region and are named for their use of pseudopodia. As Mikey mentioned, amoebocytes have numerous functions: "they take up food from the water and from choanocytes, digest it, and carry nutrients to other cells...they also manufacture tough skeletal fibers within the meshyl" (Campbell 670). The fact that sponges have a relatively simple body plan and use very little energy to survive is what has made them so successful. Since they are sessile and attached to substrates, the food needed for survival is at a minimum, allowing sponges to survive without much food at all.
However, Mikey correctly pointed out that in order to respond to environmental pressures such as nutrient-poor water or a lack of an undercurrent to aid the flow of water through the sponge, the species needed to adapt and develop some sort of mobility. Through natural selection over many generations, cnidarians emerged, containing a polyp-medusa cycle. The medusa is able to move freely in the water by a combination of passive drifting and contractions of its bell-shaped body, allowing these animals to move to better locations and conditions for surviving and reproducing.
Choanoflagellates communicate using phosphorylation, like Mikey pointed out, which is the addition of a phosphate group to a protein at one of more of its amino acids. An amino acid called tyrosine is phosphorylated in choanoflagellates, and with the use of PTP, SH2, and TyrK (kinases), cells are able to communicate by adding phospho-tyrosine modifications. TyrK is the "writer" of the messages, PTP is the "eraser" that modifies or erases messages, and SH2 is the "reader" that allows cells to receive the message (http://www.pnas.org/content/105/28/9680.figures-only). These choanoflagellates communicate in the same way as choanocytes in sponges, and this similarity supports evidence suggeseting that animals evolved from a choanoflagellate-like ancestor.
http://www.pnas.org/content/105/28/9680.figures-only
Hannah Kay (hgkay@aol.com)
Like Mikey and Hannah mentioned, organisms in the phlyum Porifera are suspension feeders. They are largely sedentary, and were mistaken for plants for this reason. Like the phylum name “Porifera” (pore-bearer) suggests, sponges have thousands of pores which allow water to flow into their spongocoel, its central cavity. As a suspension feeder, they “capture food particles suspended in the water that passes through their body” (Campbell 670). A sponge has two layers of cells that are divided by a region called the mesohyl, which contains cells known as amoebocytes. Amoebocytes have several functions, and its main function is to gather food from the water that comes in, and to digest it for energy. Sponges have several different cell types. One specific example is flagellated choanocytes, which draw water through the pores and into the central cavity using the movement of its flagellum. The food particles trapped by the choanocytes allow phagocytosis by the surrounding amoebocytes, which transport the nutrients to other parts of the body which require the nutrients.
ReplyDeleteBecause sponges are largely sessile, this simple body plan and mode of nutrition is effective. They can acquire nutrients without moving, allowing them to survive without wasting precious energy on foraging. Recall the optimal foraging model from the Ecology and Behavior unit. The optimal foraging model suggests that the method of which an organism acquires its food is “a compromise between the benefits of nutrition and the costs of obtaining food” (Campbell 1133). Like the Northwestern crow, which must drop a whelk at a certain altitude to acquire the maximum benefits of nutrition, sponges have also developed these methods of suspension feeding to maximize their benefits in its specific environment.
In my opinion, the biggest factor that has allowed simple organisms like sponges to become multi-cellular and complex is time. On page 121, Neil Shubin gives us a timescale of when the first life appeared, and when the first bodies appeared. To emphasize his point, Shubin notes the extremely long period of time in which there were no bodies on earth, only single-celled organisms which lives alone or in colonies. Until about 600 million years ago out of the 4.5 billion years that Earth has existed, creatures with patterns of symmetry and specialized structures did not exist (Shubin, 122). This gives us an idea of how time-consuming and special our body structures are.
(continued)...
ReplyDeleteHowever, as competition among and between different species increased, multi-cellular organisms did not only have to acquire food, but acquire food efficiently. Recall the Invertebrate unit. As we go down the phylogenic tree of animals, we see various differences between species. For example, organisms started to develop true tissues, which sponges previously did not have. The presence of true tissues allow for the development of specific structures, adding on to the complexity of the organism. As we go further down the phylogenic tree, we see cephalization, the evolutionary trend of having the central nervous system develop at the anterior end, occur. A radially symmetrical organism, which does not have a right or left side, has the advantage of acquiring information about its environment from all sides. However, a bilaterally symmetrical organism can rapidly respond to an environmental stimuli because of its body plan. This allows the organism to find and acquire a prey, or to escape from a predator effectively.
However, as competition among aquatic species increased, organisms had to develop a new way to find its food and survive. This led to the development of specific structures which allowed them to live the life on land. In describing his encounter with the "missing link" in evolutionary history, Neil Shubin identifies an extraordinary feature in Tiktaalik. This organism had a wrist bone in its fins, a feature seemingly useless for a fish to have. However, a close look at the structures of various joints revealed that the wrist enabled Tiktaalik to perform push-ups. Neil Shubin theorizes that "[f]ins capable of supporting the body would have been very helpful indeed for a fish that needed to maneuver in [various environments]", allowing them to walk on land (Shubin, 40). However, the transition from water to land was not simple. Because they lacked water, they had to engage in internal fertilization, and develop various structures that were necessary for reproduction. Recall the Vertebrate unit. Birds have an amniotic egg, which keep the eggs that have no access to water from drying. This is a great example of how environmental pressures have led to the development of organisms that we have today.
Works consulted:
Campbell Biology
Your Inner Fish
http://en.wikipedia.org/wiki/Sponge
http://en.wikipedia.org/wiki/Body_plan
http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amniota.html
http://www.pnas.org/content/74/5/2069.full.pdf+html
(Keigo Tanaka; tanakarus3@hotmail.com)