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Designing an experiment on sea stars

Title: How do light and dark rocky surfaces affect the relative fitness of the orange and purple ochre stars? 

Pisaster ochraceus (also known as ‘ochre stars’) is a keystone species and common starfish found in the Pacific Ocean and are very interesting species to research on. They are found mainly in Alaska and Baja California. Their size range from 15 to 36cm in diameter come in different ranges of colours eg: red, yellow, orange and purple. They are mainly found near rocky shores and found under rocks and in crevices in the low and intertidal zones  and they often cluster together. They are simple organisms, they do not have brain or ganglia and around its mouth there is a nerve ring which connects with 5 radial nerves. The population of Pisaster ochraceus that are orange are 6- 28%, whilst majority are purple and researchers have seen that mainly genetic traits cause these species to have different colours whilst they develop. There have also been experiments that examined how colour changes across the geographic range. 







Figure 1: Image of purple and orange ochre stars 


The aim of the experiment would be to see how either light or dark rocky surfaces affect the relative fitness of the orange and purple ochre stars, meaning their offspring. The relative fitness shows how much fitness there is in a genotype compared to the maximum fitness. Before starting this experiment, a risk assessment has to be done to make sure it is safe and increases hazard awareness when the experiment is being done. The likelihood, severity and risk has to be looked into during the assessment and how to reduce the risk.  One example is, doing the experiment by the shores can be risky due to wind waves and tides and so appropriate footwear has to be worn and the weather should be looked into before going to do this experiment. There are going to be control variables such  as: season, quadrat area, number of samples calculated and same equipment being used throughout the whole day so validity would be affected. The uncontrolled variables would be: temperature, pH of seawater and predators that consume Pisaster ochraceus.


In order to see how the Pisaster ochraceus are affected, 10 - 15 sites should be chosen and a quadrat can be used (10 metres by 10 metres) on each site and running parallel by using a tape measure on darker rocky surfaces and then after on lighter rocky surfaces. This will be useful as you can see the distribution. Place 15 quadrats randomly over each area in every site to work out the abundance. Within each quadrat, orange and purple Pisaster ochraceus are counted separately to illustrate the set of results with the different colours and the rocky surfaces on a table of results.

After collecting the results, this should be shown on a set of tables and then placed on a stratified bar graph showing all the sites, the colour of the starfish (on the x- axis) and results of relative fitness(on the y-axis) showing a good visualisation of the experiment. A paired t-test should be done as we want to see the difference between two variables which are the light and dark rocky surfaces for the same sample which is the colour of the starfish through their means. It should then be concluded by seeing which morph has a higher relative fitness and conclude to see if there is an effect. If the p-value is lesser or equal to the significance value, then the hypothesis should be rejected if the p-value is higher than the significance value the hypothesis should be accepted. 



Figure 2: Purple and orange ochre stars on rocky surfaces


Carrying out an experiment in a natural environment is an advantage as this can be reflected on real life therefore having higher ecological validity. However, doing this experiment can have some disadvantages, even though this is cost-effective and done in a natural environment, we do not know how reliable these results will be because the collection of results can have some inaccuracy. Also, It also has to be understood that many other biotic and abiotic factors can affect this experiment. As it is done in the natural environment there will be issues with Pisaster ochraceus being predated by sea otters or even seagulls which can have an effect in results and also making it less generalisable. Air temperature and water temperature can also have an effect on these species as well and it cannot be controlled which can create issues on results. Also, by using a quadrat, it can be prone to human errors (miscounting or overcounting) and having randomly spaced quadrats, can miss out individual species therefore showing under-representative estimates and results in the populations of the Pisaster ochraceus.  More repeats would have to be done throughout the years to collect more accurate results and also be tested by other variables such as temperature, wave exposure and even pH of seawater to see if this also affects relative fitness of Pisaster ochraceus with different colouration. 


It is important to think about the ethical considerations as it is a natural area and these species organisms live there and it should not be damaged before, during and after the experiment. The creatures must be respected as well as the environment they live in. With many equipment being used, it is vital not to interfere with the organisms, create litter or disturb the habitat as it will be unethical. 


In conclusion, this experiment is effective as it is done in a natural environment at different sites but it will be time consuming due to changes in weather and working out the abundance over all the sites for a long period of time. By doing the paired t-test, a difference in the two means can be seen and create smaller effects on error from the samples. 


By Jeevana Thavarajah 





The Biological Bulletin. 2022. Color Polymorphism and Genetic Structure in the Sea Star Pisaster ochraceus | The Biological Bulletin: Vol 211, No 3. [online] Available at: <> [Accessed 18 January 2022].


Animal Diversity Web. 2022. Pisaster ochraceus. [online] Available at: <> [Accessed 18 January 2022]. 2022. SIMoN :: Species Database. [online] Available at: <,to%2036%20cm%20in%20diameter.> [Accessed 18 January 2022]. 2022. Royal Geographical Society - Fieldwork in schools. [online] Available at: <> [Accessed 18 January 2022].

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