INVESTIGATING THE ECOLOGICAL NICHE OF THE CRAB USING A FAIR TEST INTRODUCTION: The ecological niche of the crab Hemigrapsus edwardsi. The crab is a member of the Crustacea phylum and is in the family Grapsidae. This crab species is found only in New Zealand on rocky shores. The rocky shore, where the crabs studied in this investigation were found is quite exposed. There is a large rock platform that provides small crevices and small rocks which help to protect them from wave action and predators. There are also sea lettuce, and other algae growing on parts of the rocks.
The crab has many adaptations that allow it to live on the rocky shore including: • grey/black colour for camouflage • food detection structures (antennae on its head and hairs on the mouth parts to sense chemicals in the sea water) • 4 pairs of legs with muscles that allow it to move sideways, as well as forwards and backwards • behavioural adaptations such as scuttling under rocks when the tide goes out or to avoid predators • freezing when being attacked (we noticed this when we touched some of them on their backs). This might confuse predators. • gills for gas exchange.
The rock pools provide a micro-climate where the temperature and salinity of the water will change, depending on the weather. If it was a really hot day, the rock pools will get warmer, more water will evaporate and the salinity will increase. The crab would have to be adapted to cope with these changes in salinity, otherwise, as the concentration of salt in the water around it changes, it will gain or loose mass due to osmosis. Through the process of osmoregulation, the crab is able to maintain a constant water balance in its body, but to do so requires energy and this could be measured by an increase in the respiration rate.

This is what I am going to investigate. Aim: To determine whether the respiratory rate of the crab changes in different salinities. Hypothesis: The respiratory rate of the crab will increase as the salinity changes away from “normal” salinity. METHOD Thirty crabs of similar size, were collected from the rocky shore. The salinity was varied by diluting the 200% conc. seawater provided in to five different concentrations. The volume of the solutions was 200mL each. The concentrations were 150%, 125%, 100%, 75%, 50% conc. Water with 100% concentration is equivalent to the concentration of normal seawater.
Sixty-five mL of the 150% solution was poured into a petri dish. The petri dish had a thin layer of stones in the base to recreate the natural environment of the crabs. One crab was put into this petri dish and left for five minutes to allow them to adjust to the conditions. The petri dishes were floated on a water bath which was set at 180C. The water bath was placed in the room where the light intensity was the same for all petri dishes. An indirect method of estimating respiration was used. This was to count the number of currents seen on the surface of the water.
I assumed that a higher number of currents indicated a higher respiration rate. A little bit of carmine red was sprinkled onto the solution in the petri dish in order to see the movement of water more clearly. The crabs were left in the petri dishes for five minutes before counting the respiratory currents. The respiratory currents were counted for 30 seconds. This amount was doubled to give the rate per minute. 5 more trials with different crabs were done at each concentration. Each time a fresh 65 mL of solution was added. The results were recorded and the rates per minute were averaged.
This process was repeated for the 4 remaining concentrations and the results for these were averaged. Results: Average respiratory rate of the crab in different concentrations of seawater. | |% concentration of the water (100% = normal sea water) | | |50 |75 |100 |125 |150 | |Average number of respiratory | | | | | | |currents per minute |7. 2 |6. 75 |6. 00 |7. 50 |12. 42 | Average respiratory rate of the crab in different seawater concentrations. Statistical analysis of results: The graph of the results suggests that there is a significant relationship between the concentration of the seawater and the respiratory rate of the crabs. This is confirmed by the r2 value of 0. 9642, which indicates that 96% of the variation in the results is explained by the change in seawater concentration. Conclusion:
The respiratory rate of the crab, as measured by respiratory currents, increased when the concentration of the seawater either increased or decreased from the 100% conc. (normal seawater). Discussion: This investigation was carried out to determine whether respiratory rates of the crab increased as the salinity was changed from normal levels. The crab is a euryhaline organism that lives on the rocky shore, often in rock pools. Because of the tidal movements of water, the salinity of the crab’s environment fluctuates. To maintain homeostasis, the crab osmoregulates.
This means it actively controls the salinity levels inside its body. As the seawater concentration increased or decreased from the concentration of normal seawater (100% conc. ) the respiration rate increased. This is because the crab is most comfortable at the concentration of normal seawater. As osmoregulation involves the active transport of ions, it requires energy to adjust to higher or lower seawater concentrations than 100% seawater. As the salinity of the crab’s environment increases or decreases from the salinity of normal seawater it requires more energy in order to osmoregulate.
So in order to obtain that energy, it needs more oxygen and its respiration rate increases. This is necessary for the crab as it must deal with a range of salinity as the tidal pools dry out. If the concentration of the water is greater than that of the crab’s internal environment, it will go through osmosis. Although the crab has an exoskeleton, water can still leave its body through the space around its joints and gills. Evaluation: Initially I found that there were several problems with my investigation.
The most significant one was that the crabs were out of their natural environment which made them more fidgety. Some of mine even attempted to escape from the petri dishes during the trials. This clearly shows that they were unsettled and perhaps not responding as they usually would. If they decided to stay inside the petri dish, they stayed close to the edge and moved away from the movement of people around them. I moved my investigation to a more secluded position and screened them off with a cover so that were not able to see what was around them.
I also lined the petri dish with a thin layer of stones to attempt to somewhat recreate the natural environment. This change made the crabs calmer and produced a more natural response. The counting of actual respiration itself was difficult as it was a humid day and the carmine red seemed to be congealing before being placed in the water. Perhaps in humid conditions chalk dust or very fine sand would have been more suitable but these were not available where I was carrying out the investigation.
Where the crabs were sampled from may have biased the outcome of the results as the position in the tidal zone might influence the levels of salinity that they were accustomed to. Most of my animals appeared to be female, this may also have biased my results. Further investigations would need to be carried out to determine if sex or original location influences crab respiration rate. Despite these limitations my results show that the increase in respiratory rates can be explained by the changing concentration of the seawater. ———————–
INTRODUCTION ECOLOGICAL NICHE AIM HYPOTHESIS Explains aspects of the ecological niche that are related to the investigation. Hypothesis linked to ecological niche. VARIABLE INDEPENDENT VARIABLE CONTROLLED VARIABLE DEPENDENT VARIABLE Controlled variables A valid range of the Independent variable Controlled variables Dependent variables Sufficient data PROCESSED DATA [pic] STATISTICAL ANALYSIS CONCLUSION Sufficient data that is appropriately and accurately processed to show relevant pattern. (raw data is in log book) Workable method outlined
Sufficient data that is appropriately and accurately processed to show relevant pattern. (raw data is in log book) Statistical analysis of the validity of the conclusion A valid conclusion that is justified by the data and relates to the hypothesis DISCUSSION EVALUATION Critical evaluation through analysis of the validity of the investigation Critical evaluation – limitations discussed and changes made to the method justified in relation to validity of investigation Discussion of results related to niche Significance of findings in relation to the ecological niche

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