Measuring is believing: quantifying adaptation behaviour of Hydra Teach article

Amazing Hydra: A spineless creature with astounding regenerative properties that can adapt to changing environments.

This module describes an activity on Hydra behaviour that is designed to teach students the scientific method via hands-on experiments and the value of quantitative measurements in biology. Hydra provides a simple model for students to understand organismal interactions with the environment. 

Students will measure changes in Hydra’s body shape in response to mechanical stimuli. The activity thereby explores how structure relates to function, a core concept from introductory biology, as well as how the nervous system senses external stimuli and triggers a behavioural response. This is achieved through hands-on experiments using a low-cost microscope and quantitative analysis.

The experiments are easy to implement and modular, so they can be fitted into various time constraints. Hydra can either be obtained from local streams^[1], from a Hydra research lab or obtained commercially (table 1). Hydra can be maintained in bottled spring water, and the cost and complexity of their care and handling is low, as described in this protocol.^[2]  The activity is aimed at students aged 11–19 years. Instructions for implementation, as well as worksheets and ideas to include independent inquiry and discussions about the ethics of animal research, are also provided.  

Activity: Mimicking river conditions

Hydra contract in response to sudden changes in water movement or current, possibly as an escape mechanism and/or to prevent detachment from the substrate. After prolonged exposure to a new water current intensity, Hydra habituates, ceases contraction and elongates to its resting state.^[6] This habituation behaviour reflects Hydra’s adaptability to changes in its environment. Students create water movement at two different frequencies (every 5 s and every 30 s for 2.5 minutes) through mechanical movement (moving the stage of the microscope) and record when and how often Hydra contracts. They take notes on their observations in worksheets and analyse and interpret their data. 

The activity will take approximately 60 minutes.

Materials (per two students)

• 2 deep petri dishes (Simport®, 100 x 20 mm polystyrene) with lids
• 2 intact Hydra per dish in ~20 ml of bottled spring water 
• 1 microscope with built-in LED screen (we used the Andonstar 5-inch screen, 1080p digital microscope) – set to video recording
• 1 stopwatch
• 32 GB mini-SD card to record data on the microscope
• Hydra info sheet
• Scientific method info sheet
• Worksheet 1
• Worksheet 2
• Reference movies showing Hydra behaviour for 5 s and 30 s stimuli

Pre-class preparation instructions

1. The microscope stage has non-slip feet. Use tape to stick a piece of felt to the front feet of the stage, so it can be moved back and forth by the students. 
2. Put a piece of tape at each side of the microscope to act as limits for the motion. 
3. To transfer the Hydra to the dishes, use a disposable glass or plastic pipette and gently suction the Hydra through the pipette tip.^[2] To avoid distraction, do not initially provide the petri dishes with Hydrato the students. 

Procedure

1. Explain the activity and spend 5–10 minutes reviewing the info sheets (Hydra and scientific method info sheet) as a class. 
2. Distribute the dishes with the Hydra to each pair of students.
3. Show students how to position one petri dish on the microscope (figure 2a).
4. Guide the students through the observation questions on worksheet 1, then let them work on it individually for a few minutes before discussing as a class.
5. Using the Hydra info sheet and students’ responses to the observation activity, create a class hypothesis about Hydra’s ability to respond to its environment. Ask students to predict the time period that activates an adaptation response. Prompt students to identify which step of the scientific method they are on.
6. Demonstrate how to use the stopwatch and how to record a video (see video guide). 
7. Demonstrate how to move the microscope stage to the tape and back at a fixed speed (either centre-left-centre or centre-right-centre) to generate water movement (figure 2b). Optional: Show the excerpt from the supplemental movie.
8. Ask the students to choose one elongated Hydra and position it in the centre so that it is visible in the middle of the LED screen. The Hydra should be attached to the bottom of the dish with its foot. 
9. Students will record two separate trials, each lasting a total of 2.5 minutes. In the first trial, they will move the stage every 5 seconds; in the second trial, they will move the stage every 30 seconds.
10. Always one student will move the stage and the other will use the stopwatch and tell their partner when to move the stage. Encourage students to take turns in their roles after completing one trial. Students need to start the video before they start moving the stage. 

1. After recording the two videos data collection is completed. Show the students how to play back their videos and how to analyse their data by counting the observed contractions and how to document the data in their worksheet 1 (see video guide). 
2. Once all students have completed their data analysis, the results will be discussed as a class.
3. Optional: The instructor could collect students’ results on index cards and enter them in a spreadsheet that could be projected for the in-class discussion after completion of the experiments. 

Answers are provided in the answer sheet 1.

Discussion

The introduction section of the article ‘Aquatic and terrestrial invertebrate welfare’ may be useful reading for students prior to a discussion.^[7]

Project the class data (if available) and discuss the questions below using activity worksheet 2. Sample responses can be found in the answer sheet 2 in the supporting material. 

• What could the time periods of the movement represent in the Hydra’s natural environment?
• How often did the Hydra contract in the 2.5 min of observation time? When did these contractions occur?
• Did the two animals tested for each time period respond in the same way (yes/no)? Explain your answer. How many animals do you think are ideal for this experiment?
• Why did we count the number of contractions to compare the different time period trials?
• Why would the Hydra not want to stay contracted forever?
• Do the results support our class hypothesis? Why or why not?
• What future experiments can you come up with to explore this topic further?
• How does Hydra’s nerve net compare to the nervous system in the human body?
• While experiments on vertebrate animals are regulated by laws in many countries, no laws exist for invertebrates, such as Hydra. Given what you have learned about Hydra’s nervous system and its ability to sense and adapt to its environment, what do you think are important factors to consider when studying Hydra and other invertebrates? 

The last section of worksheet 1 contains a fill-in-the-blank table with steps from the scientific method that could be completed in class or given as a homework assignment. Revisiting the scientific method info sheet, students are encouraged to reflect on how their experimental process fits with the scientific method. 

Extension Activity

Like Hydra, many other invertebrates inhabit the beds of natural freshwater sources and respond to varying conditions in their environment. The teacher can collect water samples with students from a local creek or pond and then use a microscope to study the behaviour and count the invertebrates living in the water. The abundance or absence of invertebrates can be used to assess water quality. References 8 and 9 can guide such an extension activity.^[8,9]

Acknowledgements

The authors thank Vivien Zheng, Magnus Collins, and Nikita Collins for feedback on the activity and worksheet and testing the experiments. This work was partially funded by National Science Foundation Grant 2102916. The funders had no role in the design and conduct of the study; in the collection, analysis, and interpretation of the data; or in the preparation, review, or approval of the manuscript.

---

Supporting materials

Activity worksheet 1

Activity worksheet 2

Hydra info sheet

Info sheet scientific method

Microscope video guide

Answers sheet 1

Answers sheet 2

Reference video 5 s stimuli

Reference video 30 s stimuli

Download

Download this article as a PDF

---

Share this article

• twitter
• facebook
• linkedin

---

Subscribe to our newsletter

Subscribe to our newsletter

Subscribe