Ancient mystery finally solved! Hays finds tremendous comfort hugging a stone sphere in Costa Rica
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First it is necessary to understand the origins of symbiosis in regard to the beginning of eukaryotic life. Mitochondria and chloroplasts are examples of endosymbionts that have become obligate for their hosts. An obligate relationship means that neither partner could survive without the aid of the other. Mutualistic relationships range from causal to facultative to obligate. The first type may benefit either species but is unnecessary for survival. This is in contrast to the other extreme where the speciesÕ have practically become one because the partnership between them is so vital for sustaining life.
In their evolutionary history it is clear that mitochondria have evolved from a form of symbiotic bacteria, but the nature of the initial symbiosis is still being debated. Evidence of this ancestry is seen by examining mitochondrial DNA and protein synthetic machinery which are unique and of a bacterial type distinct from that in the nucleocytoplasm. Also, their protein and rRNA sequences are most similar to bacterial and as a result of gene transfer, genes for many mitochondrial proteins are found in eukaryotic nuclei. These branching sequences indicate that mitochondria diverged from the intracellular parasite Rickettsia (Searcy 2003). Mitochondria act as endosymbionts in eukaryotic cells by providing the cells with a supply of ATP. However, they can also be problematic because malfunction or mutation of this system can lead to diseases such as ParkinsonÕs, AlzheimerÕs, and a number of other ones as well.
Benefits of mutualism include a lower death rate by the advantages created through the partnership, a higher birth rate because of the increase in food abundance between the two, and/or increasing habitat availability by means of providing shelter to one another. While these are not mutually exclusive, they do play a key role in the dynamics of mutualistic populations. Immigration and emigration play an important role in maintaining mutualist populations. Models show that populations experiencing immigration show greater stability. Immigration can actually prevent population collapse and can stabilize these populations by increasing reproduction and reducing mortality through the increase in the number of mutualistic relationships. Another noteworthy concept, directly related to topics we will be discussing on this trip, is that in organisms such as coral or mangroves, where additional nutrients are provided by a mutualist, an increase in vegetative growth and local biomass gives an increased advantage for space competition. Therefore, a mutualism that leads to an increase in production also leads to an increase in space coverage of the mutualist (Thompson et. al., 2006). In regards to the findings previously stated, it is crucial to further studies by gaining a deeper understanding of the rates of immigration because of the key role it plays in stabilizing population dynamics.
However this dynamic equilibrium is not achieved through positive feedback alone. Instead it is the interplay between the negative and positive feedback within a mutualism that results in the dynamic that contributes to the maintenance of diversity within the population (Bever 1999). It can be found when examining a population containing mutualists and non-mutualists that the mutualist organisms are able to occur in the harshest conditions and the non-mutualists in the favorable ones. As a result, a sort-of dynamic boundary is created separating the mutualists from the non-mutualists. This creates an area with a net interaction of approximately zero, because of the balance between positive and negative interactions, which without would most likely lead to greater rates of community turnover (Travis et. al., 2005). Therefore, if it wasnÕt for the mutualistic interactions permitting survival in these harsher environments, thereby increasing their realized niche, mutualists would be forced to preside in already occupied niches creating competition that could possibly eliminate species from the mix and lead to a decrease in biodiversity.
While there are a vast number of organisms exhibiting mutualism in coral reefs, there are several common interactions we will likely encounter while exploring the reef to be mentioned. Some of these relationships include the mutualism seen between coral and zooxanthellae, cleaner organisms, mangroves and sponges, and goby fish and snapping shrimp, just to name a few.
The relationship between coral and its partner algae, zooxanthellae, is one of the most critical mutualistic relationships found on the reef. Without this relationship, coral reefs as we know today may not exist. Zooxanthellae are a microscopic form of algae that resides inside of hard corals. Zooxanthellae photosynthesize organic compounds from the sun and then pass along these nutrients to their coral hosts in return for protection from the numerous herbivores found grazing in the reef and the coralÕs waste excretions, including nitrogen and carbon dioxide. It is actually this algae that gives coral its characteristic colors. This relationship is an example of an obligate one. Without the algae, the corals starve to death, which is what is seen today in a process known as coral bleaching.
Corals and zooxanthellae have a different metabolic rate, which means active homeostasis is required to limit algal productions and maintain the symbiosis. However, with environmental stress, such as increasing water temperatures due to global warming, the control processes for maintaining homeostasis is compromised. When homeostasis is compromised, a metabolic imbalance is created and the coral is known to expel the algae creating the visual whitening of the coral, known as bleaching. Bleaching is merely a control mechanism for the coral in order to cope with the stress; however it is leading to massive coral death (Obura 2009).
Cleaning mutualisms generally exist between smaller fish or shrimp that remove ectoparasites and/or other materials off of larger marine organisms such as other marine fish, sharks, rays, etc. In doing so, the cleaner fish attains a meal, without being eaten by their usual predators and the larger fish receives the service of having harmful parasites removed. It is believed that the ecological relationships that exist between cleaner fish such as the bluehead wrasse, Thalassoma bifasciatum, the sharknose goby, Gobiosoma genie, and the Spanish hogfish, Bodianus rufus, and their various hosts are crucial to the overall functioning and survival of the host due to the increasing awareness of the role parasitism is playing in the mortality rates of reef fishes (Panek 2005).
Sponges are important creatures due to the immense number of collaborative associations in which they participate. One such association exists between sponges and mangrove roots. The relation between the red mangrove, Rhizophora mangle, and the root-fouling sponge, Tedania ignis, provides the sponge with organic carbon produced by the mangrove and the mangrove exhibits enhanced growth due to the uptake of excretory nitrogen from the sponge (Davy et. al., 2002). One of the spongesÕ primary predators in seagrass beds includes the Oreaster species of starfish. Tedania ignis is also benefited by the relationship with the mangrove because it enables the sponge to resist predation by Oreaster (Wulff 2008). Studies also showed that when sponges were transported from mangrove forests to the reef they were quickly eaten by parrotfish so the protection provided by the mangrove environment is critical for the survival of many species of sponge (Dunlap et. al., 1998).
The interaction between the goby, Nes longus and Ctenogobius saepepallens, and the snapping shrimp, Alpheus floridanus, at first appeared, to many scientists, to be one in which the shrimp made burrows while the goby fish did nothing. Now that the relationship is known, we understand that the shrimp will excavate burrows and the goby fish will reside at the entrance. When the snapping shrimp exits the burrow, the shrimp will actually remain in contact with the fish with its antennae and depending on the species of goby the fish will either give a signal of approaching danger by darting head first into the burrow or by rapidly fluttering its caudal fin indicating the approach of a predator (Randall et al., 2005). Through this mutualism the goby fish receives a free place to hide from potential predators and in return the shrimp receives a look-out so that it may safely hunt for food.
In conclusion, mutualistic relationships in coral reefs play a more vital role than many realize. Without the dynamic interplay between these microbial associates, many inferior creatures, such as coral and algae, that provide the reef with a habitat complexity upon which thousands of species depend, may not exist. It can even be said that this mutualism seen is underappreciated by most, because without this mutualistic partnership these sessile organisms would be outcompeted and would not play the role they do in structuring marine communities (Stachowicz et. al., 1999).
References
Bever, J. D. (2002, June 18). Dynamics within mutualism and the maintenance of diversity: inference from a model of interguild frequency dependence. Ecology Letters, 2(1), 52-61. Retrieved May 11, 2009. doi:10.1046/j.1461-0248.1999.21050.x
Davy, S. K., Trautman, D. A., Borowitzka, M. A., & Hinde, R. (2002). Ammonium excretion by a symbiotic sponge supplies the nitrogen requirements of its rhodophyte partner. The Journal of Experimental Biology, 205, 3505-3511. Retrieved May 10, 2009, from http://jeb.biologists.org/cgi/content/abstract/205/22/3505#otherarticles
Dunlap, M., & Pawlik, J. R. (2008, May 13). Spongivory by Parrotfish in Florida Mangrove and Reef Habitats. Marine Ecology, 19(4), 325-337. Retrieved May 11, 2009. doi:10.1111/j.1439-0485.1998.tb00471.x
Institut de Recherche Pour le Developpement. (2007, September 13). Coral Reef Fish Harbor An Unexpectedly High Biodiversity of Parasites. ScienceDaily. Retrieved May 10, 2009, from http://www.sciencedaily.com/releases/2007/09/070905123839.htm
Obura, D. O. (2009, February). Reef corals bleach to resist stress. Marine Pollution Bulletin, 58(2), 206-212. Retrieved May 11, 2009. doi:10.1016/j.marpolbul.2008.10.002
Panek, F. M. (2005, February). Epizootics and Disease of Coral Reef Fish in the Tropical Western Atlantic and Gulf of Mexico. Reviews in Fisheries Science, 13(1), 1-21. Retrieved May 11, 2009. doi:10.1080/10641260590885852
Randall, J. E., Lobel, P. S., & Kennedy, C. W. (2005, October). Comparative Ecology of the Gobies Nes longus and Ctenogobius saepepallens, Both Symbiotic with the Snapping Shrimp Alpheus floridanus. Environmental Biology of Fishes, 74(2), 119-127. Retrieved May 11, 2005. doi:10.1007/s10641-005-2138-3
Searcy, D. G. (2003). Metabolic integration during the evolutionary origin of mitochondria. Cell Research, 13, 229-238. Retrieved May 11, 2009. doi:10.1038/sj.cr.7290168
Stachowicz, J., & Hay, M. (1999, September). Mutualism and coral persistence: the role of herbivore resistance to algal chemical defense. Ecology, 80(6), 2085-2101. Retrieved May 10, 2009. doi:10.1890/0012-9658(1999)080[2085:MACPTR]2.0.CO;2
Thompson, A. R., Nisbet, R. M., & Schmitt, R. J. (2006, September 26). Dynamics of mutualist populations that are demographically open. Journal of Animal Ecology, 75(6), 1239-1251. Retrieved May 10, 2009. doi:10.1111/j.1365-2656.2006.01145.x
Travis, J. M. J., Brooker, R. W., & Dytham, C. (2005, March 22). The interplay of positive and negative species interactions across an environmental gradient: insights from an individual-based simulation model. Biology Letters, 1, 5-8. Retrieved May 11, 2009. doi:10.1098/rsbl.2004.0236
Wulff, J. L. (2008, April 14). Collaboration among sponge species increases sponge diversity and abundance in a seagrass meadow. Marine Ecology, 29(2), 193-204. Retrieved May 11, 2009. doi:10.1111/j.1439-0485.2008.00224.x
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