Water snails are predominantly herbivores that feed on the naturally growing algae in water (Galli, 1956). Most of the shallow-water corals are a zooxanthellate, i.e. their tissues contain numerous algae called zooxanthallae. These algae are photosynthetic and produce nutrients, which are utilized by these corals. Presence of herbivorous snail that may feed on these algae therefore may affect the growth of corals by limiting their energy source.
The research conducted will aim at validating or refuting two hypotheses based on the information above.
Experimental hypothesis: Absence of snails affects the growth of corals.
Null hypothesis: Absence of snails has no effect on the growth of corals.
In the experiment conducted, Caulastrea furcata corals were placed in tanks (8 in each tank) containing water with varying growth conditions. The control tank contained 16 Cerinth spp. Snails, while no snails were placed in the experimental tank. The set up was left to run for 4 weeks and different variable (weight, pH, temperature, and nutrient concentration) measured at the end of this time interval.
Corals are marine invertebrates belonging to the class Anthozoa in the phylum Cnidaria (Daly, Fautin, & Coppola, 2003). They range from stony shallow-water corals that build reefs by secreting calcium carbonate to soft deep-water corals that inhabit the dark cold water. Typically all corals live as colonial organisms i.e. a coral consists of a myriad of polyps, which are genetically identical and interconnected through a system of gastrovascular channels, which allows them to share nutrients. Polyps are distinct spineless coelenterate, with only a few millimeters to a couple of centimeters in diameter and length, respectively. Each of this coelenterate has a stomach that opens to a central mouth, surrounded by tentacles. The mouth is used both for ingestion of food as well and egestion of digestion waste products while the tentacles are used for mobilizing their prey (Goldenberg, 2011). Generally, corals breed asexually by gamete fusion but individual heads of the polyps grow by means of sexual reproduction.
While some of the corals feed on small fish and zooplanktons by means of stinging cells, most corals derive their energy and nutrients from photosynthetic algae (zooxanthellae), which are found living within tissues of corals. Wood-Charlson (2008, pp. 95) explains the symbiotic relationship between corals and algae based on specific cell surface recognition. Corals depend on the algae for nutrients, while the algae benefit from carbon (IV) oxide and organic matters (algae are also responsible for the characteristic green or brown color of corals). Therefore sunlight is essential for the growth of corals and thus most of them can be found at waters not deeper than 200 ft. (60 meters). However, some corals inhabit in deeper waters of up to 9,800 ft. i.e. about 3.000 meters (Squires, 1959). These corals do not possess associated algae (azooxanthellate species). Good example of such corals is genus Lophelia found in Darwin Mound, Scotland, coast of the U.S. in Washington DC, and Aleutian Islands in Alaska.
Coral reefs that develop from excretions of hard exoskeleton by the polyps called corallite, is a unique ecosystem that hosts different species of fish, molluscs, and crustacean among many other thalassophiluses. However, man’s activities have posed a great threat to corals by encroaching to their ecosystems through digging of canals and by means of environmental pollution e.g. chemical runoffs and emission of greenhouse gases. The overall impact of pollution is the rise in sea temperatures and sea levels as well as acidification of oceans, which result in death of corals.
This study will embark on the study of Caulastrea furcata, which is a shallow-water zooxanthellate (Goldenberg, 2011, pp. 309). Live colonies are from brown to green in color and have green oral discs (Veron, 2000). Other species found in this genus include C. connate, C. echinulata, and C. curvata. The species obtain up to 70% of their nutrient need from photosynthesis of associated algae and a smaller fraction through feeding on zooplanktons (Veron, 2000). Its reproductive biology is typical to a normal coral. Caulastrea furcata inhabits the waters of Indo-West-Pacific that ranges from southwest and north of Indian Ocean to the West Pacific (IUCN Red list, 2011). This tropical region has partly sandy substrate and water depth with a minimum of 30 meters. The species may also be found in lagoons.
Although the species is under great threat because of environmental changes, this research will concentrate on the effect of herbivorous snails on the growth of Caulastrea furcata.
Materials and Methods
Coral species: Caulastrea furcata
Independent variable: Whether Snails Present/Absent
Control tank: 16 Species of Cerith snails
Experimental tank: 0 Species of Cerith snails
Materials (same for both tanks):
Before starting the experiment it was made sure that all the supplies and equipment were in place.
Water buckets were filled from the tanks, where the corals were initially held. It acted as a temporary housing for corals before they were transferred to the experimental aquariums. To do so, plastic airline tubing and a gang drip valve were set up to pour water from the aquarium holding the corals and the bucket. The use of this water was allowed to slowly siphon from the tank to the buckets, while using the gang air valve to regulate the drip rate. Siphoning was done until the buckets were three quarters full. At this point the temperature, pH and salinity were measured to ensure that the parameters matched with those in the tank containing corals. The buckets were placed close to the table bases, where the tanks would be placed, to await transfer of corals.
Two 30 gallon Aquariums were placed on the individual table bases and were filled with water of the correct pH (8.1) and salinity. A Seachem Brand Submergible Pump – 150 watt Aquarium Heater was placed in each aquarium and connected to the power source. The temperature was to be maintained at 27 0 C. Plastic PVC Grids were then used to divide each tank area into 8 compartments equidistant from each other. Equal amount of filtered Red Sea Brand Protein Skimmer and Aragonite Calcium Carbonate substrate materials from the bowls were placed in each compartment. Two 175 Watt Metal Halide Lamps were set above each tank to provide lighting. Tank 1was labeled as a ‘Control tank’ and Tank two 1was labeled as the ‘Experimental tank’.
16 Corals have been held by the base and transferred from the holding buckets into the two tanks, one for every compartment of each tank and eight in total for every aquarium. To avoid injury and chances of getting poisoned by coral tentacle stings, protective gloves were put on.
It is a variable that remained constant during the experiment and was responsible for changes observed in the dependent variable.
16 Cerinth snails were placed in the ‘Control tank’ (each for every compartment) while none placed in the ‘Experimental tank’. After completing this exercise, the parameters of both tanks were recorded (PH 8.0 and temperature 270C).
It involves variables that change during the experiment depending on the conditions in the tank. They were used to analyze the hypotheses.
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The sets were then left to stand for four weeks and regular test have been done in each tank at a time interval of 1 week and the data obtained were recorded in a sheet.
Water Tests were performed with the use of Seachem Brand test kits and following standardized instructions on the kit. The water temperature was recorded using a thermometer.
Table 1. Change in tanks’ variables from week 1 to week 4
|Week 1||Week 2||Week 3||Week 4||Week 1||Week 2||Week 3||Week 4|
*parts per million
Table 2. Change in mass of corals from Week 1 to Week 4
|Week 1||Week 4||Week 1||Week 4|
Descriptive Statistics: Control, Experimental
Variable N N* Mean SE Mean St Dev. Variance Minimum Q1
Control 8 0 0.03925 0.00356 0.01008 0.00010 0.01880 0.03453
Experimental 8 0 0.0704 0.0101 0.0285 0.0008 0.0546 0.0562
Variable Median Q3 Maximum
Control 0.03930 0.04827 0.04980
Experimental 0.0607 0.0678 0.1400
Mann-Whitney Test and CI: Control, Experimental
Control 8 0.03930
Experimental 8 0.06075
Point estimates for ETA1-ETA2 is -0.02110
95.9 Percent CI for ETA1-ETA2 is (-0.03980, -0.01270)
W = 36.0
Test of ETA1 = ETA2 vs. ETA1 not = ETA2 is significant at 0.0009
P value = 0.03115
The p-value is 0.03115, which is less than 0.05. It therefore means that the experimental hypothesis is correct, i.e. the independent value (absence of snails) affects the growth of corals, in this case, Caulastrea furcata species. Consequently the null hypothesis was rejected. That is to say that the hypothesis that ‘the absence of snails has no effect on the growth of corals’ was incorrect.
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However, the values obtained from measurement of weight using the analytical scale may be incorrect or not accurate due to errors in measuring, equipment error or inaccurate reading of the measurements. The conditions in the aquarium (temperature and pH) may also have fluctuated thus affecting the growth of corals.
The knowledge that absence of snails in the marine water can affect the growth of zooxanthellate, such as Caulastrea furcata, is important in coral conservation. With the number of corals declining over the years due to human activities, it is important to control snail population in order to ensure that there is no further threat to coral existence. Information obtained from this research is useful to the marine conservancy agencies like fishery management, habitat protection, and coral farmers, who practice coral aquaculture. (Horoszowski-Fridman, Izhaki, & Rinkevich, 2011)
For the future:
For the future repetition of this experiment, the researcher would recommend that more variables be included in addition to the changes in the weight of corals. Students or scientists considering the repetition of the experiment should all try to refine experimental environment in order to minimize the movement and time required for the corals to adapt to a new setting. Extensive research using other zooxanthellate and snails’ species needs also to be done in order to ascertain the inference made and ensure that the information obtained is consistent for other species.
From the research conducted, the researcher has gained broad knowledge on the coral ecosystems and the need to conserve them. The investigation had a good emphasis on the need of highly accurate and competitive analysis of scientific data. Raw data obtained in the experiment could not indicate a substantial inference until they were analytically processed. The concept of p-value in particular was very helpful in final analysis and the researcher is motivated to consider it in future scientific work.
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In conclusion, the study was an eye opener and a great inspiration in the researcher’s academic work and future involvement in activities like science projects and workshops.
Acknowledgments (1 Point)
I would like to thank Ren McCormick for his discussion on the feeding of corals and Sue Smith for taking the temperature and pH readings.