The Effects of Salt on 

Tadpoles

A student science project
What are Tadpoles? 

Frogs have a complex life cycle with several progressions.

Egg  Tadpole  Froglet  Adult

Tadpoles are juvenile frogs that emerge from the egg stage.

Characteristics of this stage:

Live in water – have specialized tail for swimming

Eat underwater plants like algae or periphyton

Breathe through gills like fish

http://commons.wikimedia.org/wiki/File:Litoria_littlejohni_tadpole.jpg http://science.howstuffworks.com/zoology/reptiles-amphibians/frog.htm
Where do tadpoles live? 

In freshwater wetlands around the world

Live in bogs, swamps, lakes, ponds, even roadside ditches! 

In coastal wetlands, salt can contaminate freshwater 

Storm surges, intrusion from aquifers, flooding events

http://www.greglasley.net/nonBirds/cypresswamp.html

http://www.sheepsheadbites.com/tag/marine-park-salt-marsh/

http://en.wikipedia.org/wiki/Hurricane_Isabel
Why should we care about 

saltwater into freshwater 

habitats?? 

When solute concentrations are high OUTSIDE the cells, 

water will passively flow out of the cell, shriveling the cell 

like a raisin.

Why? Because there is less water outside the cell and therefore 

the water will naturally flow from high to low concentrations.

This is a hypertonic solution

When there are more solutes INSIDE the cell, water will 

passively flow into the cell, causing the cell to swell up.

Why? Because there is less water inside the cell and therefore 

the water will naturally flow from high to low concentrations.

This is a hypotonic solution.
More solutes outside the cell

Compared to the inside

More solutes inside the cell 

Compared to the outside

Iso = same

Same concentrations

Saltwater! Freshwater!

Above teaching aide: http://0.tqn.com/d/chemistry/1/0/p/U/1/osmotic-pressure-blood-cells.jpg
Tadpoles in Saltwater

Tadpoles are HYPERtonic to saltwater – and they can’t leave 

the water either!

Cells will shrink, tadpoles will dehydrate, and cannot survive 

for long.

http://ngm.nationalgeographic.com/wallpaper/2013/april-ngm-wallpaper#/11-0413-black-cloud-tadpoles-water-lilies-670.jpg
UNLESS... 

Tadpoles will die quickly in saltwater unless they have an 

adaptation to maintain homeostasis 

Homeostasis: maintaining constant internal conditions

http://upload.wikimedia.org/wikipedia/commons/b/ba/Brown_Tree_Frog_2.jpg
How do tadpoles maintain water 

balance?

Tadpoles breathe through gills

Gills also excrete excess salts

How? Sodium-potassium pumps!

http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%208/chap08.html http://www.erodent.co.uk/gardenpond/march2002.htm
Sodium-Potassium Pumps

These pumps are located in the cells membrane of the gills

They actively (i.e., use energy) pump the extra sodium out of 

the cell, thereby allowing the tadpole to maintain osmotic 

balance

This animation shows how sodium potassium pumps work:

Click here

http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%208/chap08.html
But there’s a catch...

Tadpoles spend their time foraging for food to find energy to 

grow. 

Getting lots of food means that tadpoles can metamorphose 

faster and avoid a drying pond, aquatic predators, and food 

shortages. 

When tadpoles have to divert the energy into energy to run 

the sodium-potassium pumps (rather than using it to grow), 

they grow slower and are smaller at metamorphosis. 
Let’s do a Science Project!
Research Question

Do tadpoles that live in salty water grow more slowly 

compared to tadpoles in freshwater? 

Hypothesis: proposed explanation made on the basis of 

limited knowledge as a starting point for further investigation

Using your knowledge about the tradeoff between energy for 

growth and energy to run the sodium-potassium pumps: 

What is your hypothesis for this question? 

My hypothesis is that __________________________________.
Experimental Design

Experimental Design: process to gather information (data) 

to help answer our question. 

For our experimental design, we need two treatments.

Treatment one: Tadpoles in saltwater

Treatment two: Tadpoles in freshwater

Except for the salinity of the water, we should make every 

effort to make these treatments identical. Why? 
Our Experimental Design: 

Treatment 1: 

1⁄2 Tadpoles in Tank 1

1 tsp. of salt mixed in

Treatment 2: 

1⁄2 Tadpoles in Tank 2

No salt mixed in
Methods

Now that we have the two tanks set up, we can sit and 

observe the tadpoles as they grow and develop!

It is important to feed the tadpoles, but make sure you feed 

the same amount to each tank. 

Record observations periodically. 

We will collect our data once the tadpoles begin to 

metamorphose!
Data Collection

Tadpoles will sprout legs before they metamorphose. 

They will sprout their back legs first, and will only sprout their 

front legs right when they are ready to emerge from the water. 

After sprouting all 4 legs, tadpoles will climb out of the water and 

will cling to the sides of the tank. 

On the data sheet, record how many tadpoles emerge and how 

many days have passed since hatching until they have all left the 

water.

After you have recorded the individuals that have left the water, 

scoop them out and place them in a separate container (lined with 

a moist paper towel – but not fully in water or they will drown!)

http://frogsaregreen.org/tag/froglet/
Results!

To determine if there are any differences in the time to 

metamorphosis, lets average the results from each treatment 

and then plot our data in a graph! 

Days passed until metamorphosis

Treatment 1 Treatment 2 Day 45 Day 46 Day 47 Day 48 Day 49

= Treatment 1

= Treatment 2

Number that metamorphose
Conclusions

What patterns did your data reveal from the experiment? 

Did the results support or refute your original hypothesis? 

What might be some reasons behind the different 

observations between the treatments? 
Lab Report

This lab report will be about this experiment. It is different 

from an essay because it is divided into sections. 

Section 1: Introduction

Use this section to cover the background of the topic. You may 

want to cover frog life cycles, how tadpoles handle salt stress, 

and the tradeoffs associated with osmoregulation.

Include information that you gather from outside sources

Be sure to cite your works! 

Important! Include your hypothesis in this section. 
Lab Report

Section 2: Methods and Materials.

The purpose of this section is to outline our experimental 

design. It should be detailed enough for another student to run 

the same experiment from the information in your report.

Diagrams and drawings are always helpful! 
Lab Report

Section 3: Results! 

This section is where you put your data. Since we made graphs, 

the graphs will go in this section. 

Be sure to label your axes, include a legend, and put a figure 

caption below the graph so that people reading your report 

understand the data. 

Important note: Do no discuss what your results mean in this 

section. All you want in the results section are the data and 

graphics. 
Lab Report

Section 4: Conclusion/Discussion

This is the section where you discuss what your results mean.

Discuss whether your results supported your original hypothesis

Why do you think they supported your hypothesis? Why not? 

What are some explanations for your observed data? 

Can you tie in natural selection into these results? 

How might it be adaptive to tolerate salty environments? 
Congratulations! 

You have just gone through the scientific method just like a 

real life scientist! 

http://science-fair-coach.com/scientific-method/