Porcellio scaber are small, land dwelling crustaceans more commonly known as slaters or woodlice. They play an important role in the community in which they live as they feed on decaying matter such as dead leaves and rotting bark, allowing essential nutrients to re enter the food chain faster by depositing them in the soil through their faeces.
P. scaber are most likely to prosper in a habitat that is damp and dark, and are therefore commonly found under bark, fallen logs and leaf litter, although in domesticated areas e.g. gardens, they can also be found underneath pots and firewood stacks. Apart from their thick exoskeleton, which forms armoured plates on their backs, P. scaber has little in the way of defence against predators. Thus they display photo kinesis, an orientation response where the stimulus, light, causes the slater to begin running in a random pattern, ceasing only when it has found a place that is dark. Kinesis is a response where the stimulus determines the rate of movement, but not the direction thus the slater will continue making random turns as it runs, to increase the chance of finding a more suitable environment. In this fashion P. scaber is adapted to avoid predators, as when it is in a dark place, for example the underside of a log, it cannot be seen by birds or small mammals, and is therefore safer than if it was exposed. Because of its preference for low light levels P. scaber is most active at night, when birds and many other predators are inactive, increasing its chances of survival.
P. scaber has also developed a second orientation response, hygro kinesis. With this adaptation P. scaber is better able to find an environment with a favourable humidity. This is important as P. scaber, like most slaters, respires through “gills”, more accurately called pseudo tracheae, which require a certain level of moisture for respiration to occur; otherwise the individual risks desiccation, and resultantly death. Slaters tend to lose moisture very quickly through their cuticle and their pseudotrachea, and as such depend even more highly on the humidity of their environment to keep them damp than other species better adapted to dry environments. Due to the hygro kinesis it is expected that P. scaber will have a higher activity rate in areas with lower % humidity, as they are moving to find more favourable conditions, and thus the respiration rate of individuals in these drier conditions is higher compared to those in damper conditions. This is because cellular respiration is the method whereby energy is produced for functions within the cells, and thus an increased activity rate will require more energy and result in a greater respiration rate, as the individual needs more oxygen for cellular respiration to occur.
- Aim: To determine the effect of different % humidity on the respiration rate of Porcellio scaber.
- Hypothesis: Due to the positive hygro kinesis of Porcellio scaber, the higher the % humidity, the lower the respiration rate of the individual.
- Null Hypothesis: The % humidity of the environment will not affect the respiration rate of Porcellio scaber.
Take the boiling tube and add 1ml of magnesium chloride in the bottom. Ensure no magnesium chloride touches the sides of the boiling tube as this may poison or otherwise affect the Porcellio scaber in the experiment. The magnesium chloride will create a 33% humidity above its surface, thus when the boiling tube is stoppered a uniform % humidity will be created.
Place enough gauze in the bottom of the boiling tube that the magnesium chloride is blocked in the bottom. This also prevents the Porcellio scaber in the experiment from being poisoned. It is helpful to leave a tail of gauze up to the mouth of the boiling tube to make it easier to remove and give the slaters something to climb over and hide beneath.
Take 0.2g of soda lime and wrap it in gauze. This ensures the slaters will not come into contact with the soda lime, but the wrapping is permeable enough to allow gases to enter the packet. The soda lime will absorb the carbon dioxide produced by the respiration of the slaters, ensuring that the change in volume of air is representative of the oxygen used in respiration.
Place 4 adult Porcellio scaber in the boiling tube. These need to be of a similar size so that their respiration rates are similar.
Place the bung with a hole in the centre into the mouth of the boiling tube tightly so that there is no chance of water leaking into the boiling tube and drowning the slaters. Thread a graduated pipette into the hole. This creates an enclosed environment, preventing the slaters from escaping and ensuring the % humidity is kept constant. The pipette is kept steady by the bung, preventing fluctuations in the volume of water in the pipette.
Place the boiling tube sideways in a bath of water, ensuring the tip of the pipette is completely submerged. As the oxygen in the boiling tube is used by the slaters for respiration, and the carbon dioxide produced is absorbed by the soda lime, the volume of water in the pipette will increase as the volume of air decreases, allowing a change in volume to be calculated.
Record the initial volume of water in the pipette. This is most easily achieved by measuring the amount of water in centimetres, and then converting the measurement into millilitres.
Cover the experiment so that the level of light is 0.17 lux or less. This ensures that the activity of the slaters that will determine the rate of respiration will be due only to their hygro kinesis, not both the hygro and photo kinesis combined.
Leave the experiment for 7 hours to allow enough time for the volume of water in the pipette to change due to the respiration of P. scaber.
Record the new volume of water in the pipette. As the water in the pipette is replacing the oxygen, we can conclude that the volume of water is also the volume of oxygen used in the respiration of P. scaber. Place results in a table and calculate the change in volume, and thus the respiration rate using the following formulas
To find the volume of water: (cm÷1.8)x0.1
As 1.8cm is equal to 0.1mL, in the graduated pipettes used in this experiment.
To find the respiration rate: (change in volume of experiment- change in volume of control) ÷ 28
As the control is an experiment without P. scaber in it to give the value of water moving into the pipette without respiration. Dividing by 7 and 4, for the number of hours and slaters to give the respiration rate in mL per hour for 1 slater.
Repeat steps 1-10 twice more with magnesium chloride, and then once more omitting step 4, so that there are 3 experiment set ups with slaters and 1 control with no slaters.
Repeat steps 1-11, changing the magnesium chloride for magnesium nitrate (52.9% humidity), sodium chloride (76% humidity), potassium chloride (85% humidity), and potassium nitrate (93.5% humidity).
Repeat steps 1-12 at least 3 times to create a fair test environment, meaning the sample size is large enough to be a fair representation of the population involved, and enough results can be obtained to provide accurate conclusions.
Conclusion: From the results it can be concluded that as the % humidity increases, the respiration rate of P. scaber decreases. For example the highest humidity, 93.5% produced an average respiration rate of 0.003017 mL per hour, while the lowest humidity, 33%, produced an average respiration rate of 0.007804mL per hour, which is considerably larger. Thus we can see that the respiration rate of P. scaber is affected by the % humidity of its habitat.
Discussion: Porcellio scaber is a species that is remarkably well adapted to living in dark and damp environments, where other organisms may struggle to survive. Because of these adaptations, its ecological niche is very specific, and thus slaters have developed orientation responses (photo and hygro kinesis) that enable them to detect when the habitat has become less than ideal and discover a new environment more suited to them. However as kineses are non directional responses, the slater will continue to run until it has found a more favourable environment. It was this behaviour that was the basis for my experiment.
In animals, energy is produced by cellular respiration, a process where by ATP (adenosine triphosphate), the universal energy carrier in cells, is “charged” by breaking down chemicals such as proteins, lipids and carbohydrates from food. The energy is then released by breaking one of the bonds within the ATP molecule, forming ADP (adenosine diphosphate), and providing the energy required for all functions in the cells. For cellular respiration to occur however, a gas exchange system is required, as aerobic respiration needs oxygen to occur and releases carbon dioxide. In Porcellio scaber this respiratory structure takes the form of pseudotrachea, which are “Respiratory structure developed in pleopods of some Isopoda for air- breathing; they consist of small ramified tubules inside limb opening outward in slit like apertures and filled with air”1.Gas exchange occurs when air moves through the pseudotrachea through a system of ever smaller tubules that end either adjacent or near to each cell in the body. The ends of these tubules are filled with fluid. This fluid moistens the semi permeable cell membrane, allowing the diffusion of oxygen into the cell from the area of high concentration in the tubules to the area of low concentration in the cells. It also allows the waste product of respiration, carbon dioxide, to diffuse out of the cell into the tubules and move out of the pseudotrachea. Because slaters are unable to close off their respiratory structures as they are formed from a hard substance called chitin, it is very easy for them to lose water during respiration, and if there is no fluid in the ends of the tubules then the semi permeable membrane will not be moist. This means that the gaseous oxygen and carbon dioxide cannot dissolve in to a liquid state, and are thusly unable to diffuse into and out of the cell, preventing cellular respiration from occurring. This means that in order for slaters to survive in their habitat, there needs to be a fairly high % humidity, to ensure that the slaters don’t desiccate and die.
The respiration rate of an organism is directly related to its activity rate. If an individual is more active, then it requires a greater amount of energy, and thus the respiration rate increases. Due to the hygro kinesis of P. scaber, an orientation response in reaction to humidity, the activity of the slaters increased in lower % humidity, as they attempted to move into a more suitable humidity. Thus the lowest % humidity, 33%, produced an average respiration rate of 0.007804 ml of oxygen per hour, the highest respiration rate in the results. Then came 52.9% humidity, producing 0.0071 ml per hour, 76% humidity, producing 0.006507 ml per hour, and 85% humidity, producing 0.00543 ml per hour. Finally 93.5 % humidity, the highest sample, produced the lowest average respiration rate of the experiment, 0.003017 ml per hour. When these results are graphed it becomes apparent that the effect of % humidity on the respiration rate of the slaters forms a non-linear regression trend, showing a direct cause and effect relationship between the two variables, humidity being the cause, while respiration rate shows the effect. Thus we can see that the lower humidities produce respiration rates that are similar in size, which then get progressively smaller and less similar as the humidity increases. From this we can see that the change in respiration rate is not regular, but instead increases readily as the humidity decreases, before slowing and evening out. I suspect that an even lower humidity than 33% would reveal that the rate of respiration eventually reaches a point where it stays the at the same level despite the continued decrease in % humidity, as the slater would reach its maximum respiration rate and thus be unable to perform a greater amount of gas exchange than it already is. Thus the amount of energy produced from cellular respiration would remain at the same rate and so would the activity rate of the individual.
Therefore it can be proven that the rate of respiration of P. scaber decreases as the % humidity of its environment increases.
1. Moore R.C. and McCormick L., 1969, General Features of Crustacea, as cited in Definitions- Pseudotrachea, http://crustacea.nhm.org/glossary/define.html?term=Pseudotrachea
Evaluation: There were several controlled variables used in this test to ensure that the results obtained were valid and could be both analysed and discussed. Firstly, in terms of equipment, all the experiments were done using the same sized boiling tubes, bungs and graduated pipettes. This ensured that one sample of slaters did not have a larger volume of air in their set up than another sample, and thus all slaters had the same amount of oxygen available to them to perform respiration. It also meant that no discrepancies in measuring the change in volume occurred due to some pipettes having a different volume to others, as all pipettes had a ratio of 1.8cm to 0.1ml. The pipettes were fitted tightly into the bungs so that they were held steady, preventing the pipette from moving which would have caused fluctuations in the level of water in the pipette, which would have affected the validity of the results. All set ups were also left for the same amount of time before measuring the change in volume, thus when the change in volume in the experiment was measured the variation in results was due only to the respiration of the slaters and not to some experiments being left running longer than others. The amount of all saturated solutions (magnesium chloride, magnesium nitrate, sodium chloride, potassium chloride, potassium nitrate) used in the experiments was 1ml. This ensures that there were enough of the solutions to produce the desired % humidity, but not enough that there is excess chemicals in the boiling tube to poison or otherwise adversely affect the slaters in the experiment. It also means that no set up had more of the chemical than another, thus all had the same environment during the experiment except for the differences in % humidity. All set ups also had the same type and amount of gauze. This meant that the wrapping around the soda lime was the same thickness for all set ups, and thus the same amount of carbon dioxide is able to move through the gauze to be absorbed by the soda lime in all experiments, so that each experiments change in volume was not affected by less carbon dioxide being absorbed by the soda lime than in other set ups. Using the same amount of gauze also ensures that the saturated solution’s % humidity is able to filter through to the rest of the boiling tube, as too much gauze would form a barrier that would keep the air in the area containing slaters at a different humidity to what was required for the experiment, and too little would allow the chemicals to leak through and harm the slaters. Thus having all experiments with the same amount of gauze means that the experiment is consistent across all % humidities. The amount of soda lime in each set up was 0.2g. The consistent level of soda lime ensures that the amount was not large enough in any of the boiling tubes to absorb enough moisture from the air to change the % humidity thus ensuring that the % humidity was consistent across all set ups. The slaters that were used in the experiment were selected randomly to ensure that the samples were an accurate representation of the population. They were all adults, and of a similar size, because juveniles are much smaller than adults and as such would require less oxygen for respiration, thus to make the results valid I made sure that all slaters used in the experiment were close to each other in terms of size, and thus age, so that their respiration rates would be similar. The water bath that the experiment set up was put in was always at room temperature. This was because colder water would cause the air molecules to lose energy and the air to become denser, thus the volume of water in the pipette would increase, compromising the validity of the results by making it seem like more oxygen had been used for respiration than actually was. The opposite effect would occur in warmer water, as the air would expand, giving a reading that was less than it should have been, thus keeping the water at room temperature helps ensure the validity of the results. In each experiment there are 3 set ups, enabling an average to be found. The experiment is repeated 3 times after that to give a final average for all % humidities. This creates a fair test environment and provides a large enough sample range of slaters to be considered representational of the population.
Before doing the experiment I measured the % humidity and lux levels of the slaters ecological niche. This meant that I was able to firstly determine at what level of light the slaters did not respond with photo kinesis. This turned out to be 0.17 lux, and I was therefore able to incorporate this into my method by covering the experiment. Thus I ensured that the activity that would result in an increase in respiration rate was due only to the slater hygro kinesis, without photo kinesis being a factor. I found that the % humidity in their natural habitat was an average of 87.3%, and thus when choosing my range of values for % humidity in my experiment I decided on 33%, 52.9%, 76%, 85%, and 93.5%. I decided not to use a saturated solution of lithium chloride, as I decided the % humidity it caused, 12.5 %, was too low for long term exposure to slaters as the difference from their habitat was large enough that it would almost certainly case desiccation and death.
When I did statistical analysis of my results I first put the final averages into a scatter graph as I was trying to determine if there was a relationship between % humidity and the respiration rates. I then did the regression test, beginning by adding a trend line. However a linear trend line produced an R2 value of 0.6349, which was very low. However after looking at my points again I attempted a non-linear regression. This fitted my points much better, giving an R2 value of 0.9162. The R2 value is an indication of how well the pints on the graph fit the line, in the case of my second graph, as shown earlier in the report; the points fit the line quite well, as the line accounts for 91.62% of the spread of the data. Non-linear regression means that the change in the independent variable (% humidity) causes a change in the dependant variable (respiration rate). However the size of the changes are not corresponding, in this case: in the higher humidities the differences between respiration rates was greater than in the lower % humidities. Thus the statistical analysis shows that my results were not likely to be due to chance, and therefore shows that the respiration rate of Porcellio scaber is affected by the % humidity of the environment.
Thus my method and results were both valid, due to my controlled variables, investigation into the ecological niche and adaptations of the slaters, the number of repeats of the experiment done and the statistical analysis if the results.
Natural History Museum of Los Angeles County, Definitions- Pseudotrachea, 11/4/10, found at http://crustacea.nhm.org/glossary/define.html?term=Pseudotrachea