Leech Ingestion          1



Leech Ingestion in Relation to Body Weight

Jennifer H., Hanna R., and Amy S.

Southern Wells High School


The purpose of the lab is to discover how much blood a leech can ingest in relation to its body weight. Three days of observations were made of the leeches throughout the experimental period. The focus of the experiment is to determine how much weight a leech will gain, in relation to body weight, after attachment. The goals are to understand the importance of leeches in our ecosystem, appreciate the complexity of leeches, and comprehend the extreme amount of blood a leech can ingest compared to its size. Tap water and 10 leeches will be used in the experiment, as well as chunks of chicken liver to satisfy the recommended feeding requirements of the leeches. A weight scale will be used to obtain knowledge of any weight changes.


     How much will the leeches ingest in relation to their body weight? Leech is the common name for more than 300 species of annelid worms. Leeches belong to the class Hirudinea that prey on small invertebrates in fresh water or suck blood from vertebrate animals. Leeches tend to feed on fish, turtles, wading birds, or mammals to suck blood. Every leech has muscular sucker at the rear end of the body; many have a second sucker around the mouth. Theses organs are used to assist a leech in the feeding process (Milne, 1987). The posterior sucker is used for holding a leech in place while it is feeding. The anterior sucker is used for feeding where small teeth or jaws may be located, to assist in this process. The posterior sucker is the main organ of attachment to a host. While feeding, the posterior sucker is moved up beside the anterior sucker causing the leech to appear ‘looped’ (Thompson et al, 1985). Other leeches have an extendable proboscis that is forced into the flush of a host (Milne, 1987).

     Most leeches that feed from fish possess a proboscis, but lack jaws. It is rare for a mammal to be bitten because mammalian skin is too tough (Milne, 1987).

     Blood-sucking leeches feed infrequently because their gut is divided into compartments to hold a very large meal. Some only need to feed only one to two times in one year (Thompson et al, 1985). The adult leech can ingest 5-15 ml of blood, which is approximately ten times its own body weight (www.austmus.gov). The medicinal leech can consume over five times it body weight. Then the excess blood is stored for later usage (http://sp.uconn.edu).

     The hypothesis is that the majority of leeches will ingest approximately 5 times their body weights. The hypothesis was formed based on the calculations of multiple sources.



(10) Leeches

(2) 7.1g sections of chicken liver

Large glass bowl

Smaller bowl (to keep the leeches in while the water is changed)

Tap water 

Digital scale

Weigh boat

(2) Q-tips

Computer (to type the lab and gather information)





     Before starting the lab make sure all the required information is gathered and collect all the materials needed. Know and understand how the lab should be conducted correctly.

Part 1: Preparation

In this part of the lab, gather the leeches for weight measurement and transference.

1. Let tap water acclimate for 24 hours to dissipate chlorine and achieve room. temperature.

2. Place enough water for leeches to live comfortably into a glass bowl.

3. Pick up each leech with two Q-tips, place it into a weigh boat, and place the boat onto a digital scale.

4. Record the weight of each leech before placing the leeches into the glass bowl previously prepared.

5. Pull a small chunk of raw chicken liver weighing 7.1 grams from the container.

6. Place the liver into the glass bowl with the leeches. 

Part 2: Observation

1. Remove a leech using two Q-tips carefully from the glass bowl.

2. Place each leech separately into a Petri dish for weighing.

3. Place the Petri dish on a digital scale and record the results.

4. Record the observations.

5. Place each leech into the separate smaller bowl, clean the large, original glass bowl of water.

6. Rinse the original bowl and refill it with the 24 hour acclimated water.

7. Transfer the leeches into the original bowl.

8. Repeat steps 1 through 7 every other day.


     Ten leeches were purchased, 4 lived through approximately 2/3 of the lab, but only 3 survived to the end of the lab. After being placed in the water for approximately 24 hours, 4 leeches remained living. The weights of the leeches averaged 1.85 grams on the first day of observations (see Graph). From the time the chicken liver was placed in the bowl to when it was removed, there was no notice of the leeches attaching.

     On the second day of observation, the 4 leeches were weighed and averaged 1.5 grams (see Graph). As before, no observation of attachment was made. The bowl used for keeping the leeches was cleaned and the water changed before replacing the leeches into it. A new section of chicken liver was weighed and put into the bowl with the leeches.

     On the final day of observation, only three leeches remained alive. Each was weighed and the average weight was recorded as 1.025 grams (see Graph). Still, no observation of any attachment was made. All of the leeches were inactive and only made slight movements when taken from the water to be weighed.


     How much will the leeches ingest in relation to their body weight? The hypothesis is that the majority of leeches will ingest approximately 5 times their body weight. The hypothesis was not supported. The reason the hypothesis was not supported was because the majority of the leeches died, and no observations of the leeches attaching to the chicken liver were made. To improve the lab, use different feeding sources and provide pond water instead of tap water to resemble the natural habitat of a leech. Will providing live goldfish as a feeding source keep the leeches alive longer?



Biology and facts about leeches. (2003). Retrieved Oct. 14, 2004, from http://www. austmus.gov.au/factsheets/leeches.htm


Milne, L. J. (1987). Leech. In Academic American Encyclopedia (Vol. 12, pp. 271). Danbury, Connecticut: Grolier Incorporated.


The digestive-tract symbiosis of aeromonas veronii biovar sobria and hirudo medicinalis, the medicinal leech. (n.d.). Retrieved Oct. 18, 2004, from http://sp.uconn.edu/ ~mcbstaff/graf/AvHm/AvHmmain.htm


Thompson, G., Coldrey, J., & Bernard, G. (1985). The pond. Singapore: Toppan Printing Company.





Figure 1. Average weight changes of all the leeches on the three days of observation.