Salvinia and weed plants in leeches


This biology extended essay is being done with an aim to find out whether Salvinia plant (Salvinia molesta) and water weed plant (Elodea canadensis) can give positive effects on the rate of paternity of the leeches (Hirudo medicinalis). Salvinia plant is a plant that commonly used by the leech breeder to include in with the leeches in the leech’s habitat[1]. Based on the observation at the leech’s farm, the water in the container of the leech’s habitat was a bit cloudy and smelly[2]. This type of condition could give insignificant positive affects onto the leeches growth and survival. This is because leeches prefer more to live in clear water rather than in cloudy water[3]. The water weed plant is a common plant that being put in an aquarium[4]. From the observation at few aquariums that contain water weed plants, the water is clear and fish seemed can survive in a longer period of time, claimed by the owners. So this experiment is basically conducted to investigate to see is there any scientifically relationship between the leeches and with both type of the plants. Several experiments were conducted and there are few variables to be investigated which are the average mass of the leeches, the pH value of the water in each experiments, the rate of paternity of the leeches and the biological oxygen demand (BOD) level of the water. This experiment is conducted for about 12 weeks with three trials had been done. From the data collected, the BOD level shows a significant effect to the leeches. A low BOD level in the water will lower the rate of paternity of the leeches. This experiment show that water weed plant can provide a low level of BOD in the water. This result shows that water weed plant is a better plant to be put in the leech’s habitat.

(298 words)


First and foremost, I would to acknowledge a gratitude to my extended essay advisor and supervisor who is Miss Suhaili who had help me a lot in advising me conducting this experiment. With her help, many difficulties had been overcome easily. Thank you for all your support and helps. I also want to express a gratitude to Mister Sabar who had allowed me to come and do some observation on how the leeches are being breed at his place. He also had helped me in giving a helping hand regarding any information about leeches.

Lastly, I also want to express a word of appreciation to my family who give such kindness in helping me doing this experiment. Nevertheless, I would also want to say a word of thank to all my friends who had me in many ways, helping me finishing this precious experiments.

With that, I thank you.


Medicinal Leeches or Hirudo medicinalis are organisms that more likely a carnivorous or blood sucking worm[5]. In the past 100 years, these organisms are used widely in a medicine purpose for bloodletting. Till now, some part of the world still practicing using leeches for that purpose. Recent years, these organisms had boomed in modern medicine to use not only for bloodletting, but for relieved blood congestion in delicate operations which are less likely to cause infections rather than other technique[6]. Moreover, leeches now being a big impact as leech’s breeding had became a huge market for cosmetic products and modern therapy because of their high nutritional value and functions. Leeches are widespread in marine waters and are also found in fresh water and on land in temperate and tropical regions.

The leeches breeder, often breed the leeches with Sylvania plant for an unknown purposes. Salvinia plants are small, floating aquatics with creeping stems, have many branches and bearing hairs but there are no true roots[7]. It has a small and hair like growths, known as microgametical follicles, are not known to have any productive function, and are currently a biological mystery. It is claimed from that the plant may give an extra nutrition for the benefits of the leeches. Rather than that, because leeches are organisms which only live in clean and cold water, water weed plant is said best to live with the leeches[8]. This is because water weed plant will act as a cleaner to clean the water where it is conducive for the leeches to live.

This experiment is worth for studying because it may give a better knowledge not only to the leech’s breeder, but to the people on why the salvinia plant or water weed plant is best to be put in a water for the leech’s’ habitat.


This research is basically to find out, what are the effect of the two type of plants which are the Salvinia plants (Salvinia molesta) and the water weed plant (Elodea canadensis) to the leeches (Hirudo medicinalis) as both plants have their own uniqueness and specialties which are known to be good for the leeches. In this experiment, I have an aim to see is there any effect of the two plants on the leeches’ growth.

So, I give out a hypothesis; the water weed plant will reduce the rate of paternity of the leeches, because the leeches are seemed to be more preferable in a clean water rather than food as an extra nutrition[9]. So I think that water weed plant can give better places for the leeches to live.



  1. First, six of 20 cm³ glass containers are labeled with label A, B, C, D, E, and F. F is for a constant.
  2. Then, each container is filled with 200 gram of red soil. The soil must put in the beaker compactly so that no leeches can enter the soil. It is necessary because I need the leeches to measure the weight, if there are any leeches enter the soil, this experiment will become more complicated because I have to remove the soil and repeat the experiment.
  3. Thirdly, each of the containers is filled with 750 ml of clear and cold water collected from the well. The water from the well is clear and very cold. It is very suitable for the leeches.
  4. After that, 6 groups of five leeches of Hirudo medicinalis are made and the mass of the leeches are taken. Next, the each group of the leeches is put in the labeled containers prepared before.
  5. 6 equal in size of salvinia plants and 5 equal in size of water weed plants are prepared. The table below show how the salvinia plant and water weed plants are put in each labeled containers.
  6. Notes;

    1. This experiment has two objectives which are to find out what is the effect of the plants to the leeches and what type of plants is the best to be put with the leeches.
    2. The different in number of plants between container A and container B is because I want to see is there any differences if I put different amount of plants in the habitat of the leeches. I put both plant in the container C is because I want to investigate is there any differences between the container that has the salvinia plants only and the water weed plants only. So right there, I can see which type of plants is best to put with the leeches, either Salvinia plants, water weed plants or the combination of this plants are the best.
    3. In container D and container E, the reason of different in number of plant is similar to with the reason stated in (b).
    4. For container F, there are no plants are put in the container. This is because I want to make container F as a constant where I can compare the results for each container that have plants with this container.
  7. After all the containers are prepared, the pH value for each container is collected by using pH meter for the initial value.
  8. Then, all containers are kept in the laboratory room and being watched by daily basis to see if there any leeches dead.
  9. After a week, all the leeches in each container are weighted again with the electrical balance to find if there any changes in mass. Directly after weighted the mass, the pH value of the water in each beaker is collected.
  10. Step 7 and 8 are repeated for about 3 weeks. In the end of third week, 10 cm³ of water is collected in each beaker to investigate the BOD (Biological Oxygen Demand) of the water. This BOD experiment is to find out the oxygen level in the water of each beaker. This investigation is being done by mixing the water with 5 cm³ of metylene blue solution and kept the solution for about a week. The fastest solution that decolorized contained high BOD level. This mean that the there are high level of oxygen demand in the water. This demand is due to many organisms which live in the water besides the leeches. For an example, the microorganisms and bacterium.
  11. Step 1 till step 9 is repeated for 2 more times to increase the accuracy of this experiment.
  12. All the data are collected and tabulated.

Development of methods

Preparation of the plants

All the plants are taken from the surrounding ponds around villages at Kampong Labohan Dagang, Banting. The Salvinia plants and the water weed plant are taken in a huge amount, so it increased the probability to get a similar in size. All the plants are measured the lengths by using a measuring tape. An approximate 10 cm in size of plants are taken to be used in this experiment.

Preparation of the red soil.

Red soil is used in this experiment. The red soil used is only to make sure the container or the place where I put the leeches are similar on how the leech’s breeder breeds the leeches. Why I used red soil is because red soil is classified as B horizon[10] of the soil. B horizon is mainly made up of clay and concentrated with minerals. Rather than that, the B horizon is a bit harder and therefore I would not have any problems regarding leeches went into the soil.

First, I take the red soil from the unused fish pond behind my college. The pond is quiet deep and from there, I can take the red soil which is mainly at the B horizon. Then I used a shovel to take the red soil from the inner part of the pond, collect it and weight for about 2 kg using weight balance.



Overall discussions

This extended essay is mainly to experiment whether the effect of water weed plant and salvinia plant are significant to the rate of paternity of the leeches.

My hypothesis stated that the water weed plant can give a significant effect on the. Based on the data collected in graph C, the highest number of average paternity which is 2.67 leeches per week is in the container A. Container A is filled with 4 Salvinia plants. As the average number of paternity is high, the average mass of the leeches are fluctuate decreasing as the time goes. Rather than that, the pH value of the water in the container A shows the most acidic. This data show that acidic water is not a suitable place for the leeches’ growth due to high rate of paternity[13]. Besides that, the graph 4 shows that the BOD (Biological Oxygen Demand) level is also high. Higher demand of oxygen shows that there is competition to obtain enough oxygen for respiration in the water. This data could be lead to why the rate of paternity is high in the container A because the leeches cannot sustain enough oxygen needed.

In container B that contains a salvinia plant, the average numbers of paternity of the leeches are low, mainly at 1.33 leeches per 3 weeks. As the rate of paternity is quite low, there is an increasing in the average mass of the leeches. This shows that the leeches are growing in mass when the salvinia plant was being put in there. The pH value of the leeches are showing that the water is slightly acidic, this could be why the rate of paternity is slightly lower than the rate of paternity of leeches in container A. besides that, the time for metylene blue solution to decolorize is also decrease as time goes. This trend shows that the biological oxygen demand is increasing as the time goes which significant with the increasing of the rate of paternity of leeches through the experiment in container B.

In contrary, leeches in container C shows higher average rate of paternity per week rather than rate in container B. this could be reasoned by the decreasing in mass of the leeches as the time goes. In this container, there are water weed plant and salvinia plant. There could be an inter-relationship between both plants that may affect the leeches. This is because both plants are in the different habitat. Salvini plant may live in a clear and fresh water such as river and salvinia plant live in a pond water. We can see the different from the pH of the water where the salvinia plants cause the water in the container A and B turns to slightly acidic while water weed cause the water turns to slightly acidic. From the graph 4, the average numbers of days for methylene blue solution to decrease over the time are falling. This trend show that the BOD is rising as the time goes.

There is only 1 leech that died during the whole experiment in the container D that contains a water weed based in the graph 3. This shows that the water in the container D set a good place for the growth of leeches. Based on the graph 2, the graph of average mass of the leeches are increasing even there are small increasing during the experiment. Rather than that, the pH value of the water is slightly basic, which the best pH condition for the leeches. The graph 4 shows that the time for methylene blue solution to decrease over time is in the lengthiest period of time. This data shows that the BOD level of the water is the lowest, which explain on why the rate of paternity is lowest.

There were slightly different situations with compared to container E with the condition in container D. In container D, there are four water weed plants have been put with the leeches. From the graph 1, the average mass if the leeches show an increasing trend over the time. Even with the increasing patent, the average rate of paternity is slightly higher compared to the container D based on the graph 3. The reason for this could be because of the decreasing patent of average time for the methylene blue solution to decolorize. This suggests that the higher level of BOD will increase the rate of paternity. Instead of that, the pH value shows a consistent trend about slightly basic higher that in the container D. This also could be the reason of few leeches died due to the high alkaline features if the water.

Lastly, for the constant (container F) shows that the leeches also died naturally due to many factors instead due to the effects from the plants. From the data collected, the average mass of the leeches are only slightly changed over the time. Comparing to the other containers, when there are small amount of plant in container B, D, and E the average mass of the leeches are increasing. Besides that, even the pH of the water is only around neutral, it could not prevent from the increasing patent of the average rate of paternity of the leeches. Indifferently with container D and E, the pH water of the containers is slightly basic, but the leeches seemed to be more survive in that nature. But, the significance effects could be seen from the time for the average time for methylene blue solution to decolorize. This is because the container F has a decreasing patent of the graph, which indirectly shows the increasing of BOD level.

Even though the statistical testing shows there are no different in the average rate of paternity of the leeches, the trend shows in all the graph especially in graph 4 shows there are interrelation between the type of plants used and leeches. This is because the plants show a great affect on the BOD level of the water. It seems that the lower BOD level will results in lower rate of paternity of the leeches. A water weed plant will results in lower BOD level, and lowering the rate of paternity of the leeches.

This could be a reason on why water weed plant is an important ecosystem in a lake or fresh water[14]. This plant had been known as provider of good ecosystem for invertebrates such as the leeches, and also many type of fish and amphibians. Instead of giving food to the marine lives[15], water weeds plant also can supply enough oxygen based on this experiment done. As leeches also are organism that needs oxygen for respirations, energy and many chemical reactions in the body, leeches are more preferable to live in sufficient supply of oxygen in their habitat.


During this experiment, there are few limitations and weaknesses that had affected the overall data and should be improvise in future experiment. This limitations and weaknesses may seem to be simple, but they should not be taken granted. All the limitation should to be overcome in precisely so an effective data could be obtained.

First of all, this experiment should do in a long period of time. This is because there are many conditions that may affect the rate of paternity internally such bacteria and externally such as temperature. A parasitic bacterium may cause a big effect the rate of paternity of the leeches as it may kill them. Leeches also are organisms which more likely to live in cold water, this mean the temperature should be maintained at low temperature. By doing in a long period of time, we could observe the overall trend of the affect that caused the leeches to die. This is because biologist needs many years to observe the condition of any animals that caused them to die.

Besides that, this experiment should be focused more only in one type of plants. For an example my hypothesis is regarding the water weed, so I should focus only that type of plant. Then a different number of the plants are introduced into many experimental containers, so the overall trend how the plant is affecting the rate of paternity can be observed more precisely and accurately. This is because the significance different of the effects of the plant cannot be compared statistically. We do not know how many plants should be considered to run this experiment instead of using only one plant and four plants as I did.

Other limitation that could be observed during this experiment is the age of the leeches. All the leeches used in this experiment were considered around 2 months-aged. This age is not precisely, so different age leeches may have different ability to survive. A more precise age should be considered such as during their early birth to a certain period of time. This could lead in producing more accurate data obtained and the trend observed have less ambiguity.

Lastly, this experiment should be done daily rather than weekly. This is because there could be difference data within the week. For an example, the average mass of the leeches may increase during the first day of experiment, but then decreasing during the next day. This could be tedious but it is much better as the data collected from daily inspection could show more obvious trends in this experiments.


As a conclusion, this experiment proves that a water weed plants will give an effect to the number of average rate of paternity of the leeches. Based on containers D and E, the average rate of paternity is lower compared to the constant and also with other containers. Even this hypothesis does not being supported by using the statistical analysis, but the biological conditions still could be observed.

A Hirudo Medicinalis(the leech),is a small organisms which mainly to live not only based to food as the source of nutrients, but the habitat of the leech. This is because leech can feed once and survive for more than one year. Many leech breeders thought that salvinia plant (Salvinia Molesta) could give an advantage in term of raising the leeches. This is because many people thought the plant could give another source of nutrients for the growth and survival of the leeches. But based on this experiment it seemed that the water weed plants (Elodea Canadensis) would give a positive affects to the growth of the leeches (hirudo medicinalis).

Water weed plant is a plant where people always put in the aquarium with fishes. Fish may somehow ate this plant, but not necessary to the leeches. But based on the data, the average mass of the leeches shows an increasing patent when this type of plant is being put in. this is could be the reason that water weed plant will act as the neutralizer of the water and keep the BOD level of the water at a lower level. So, in a nutshell, I would say that it worth to put water weed plant in with the habitat of the leeches as it give more positive effects onto the rate of paternity of the leeches.


General text

An Introduction to biostatistics; Glover and Mitchell; The McGraw-Hill Companies; New York; 2002

Web sites



Calculating the t -test[16]

For this experiment, a hypothesis testing is being done to see how much the data obtained are significant. The testing is the t-test method testing for the difference between the mean data of the subject (µ 1) and the mean data of the constant subject (µ2) which had been assumed to be normal populations. The statistical test is:

T = x1-x2(µ1-µ2)(s1+s2)(1n1+1n2 )

Where T ~ t (n1+n2 - 2)

This statistical testing is using the t – test because the limitation below;

  1. The sample size that will use is small ( less than 30 )
  2. The mean, µ, of each population are unknown
  3. The variance of each population, s² are unknown
  11. Appendix 1
  12. Appendix 1
  16. All this calculation are referred from the books of; page 118, test hypothesis between two samples, Intodct An Introduction to biostatistics; Glover and Mitchell; The McGraw-Hill Companies; New York; 2002

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