Biological Variation in Eastern Mud Snails 

• Accurately measure shell height and movement in Eastern mud snails

• Learn how to calculate mean and standard deviation in Excel

• Test hypotheses about the relationship between size and movement in Eastern mud snails

• Make a properly labeled scatter plot and column graph in Excel

• Read through the lab.

• Write the Lab Title on a new page of your notebook. *Remember to include the Lab Date!*

• Write the Background, Aim and your Hypothesis for the lab

• Draw the required Data Table (See Procedures, Step 2) *Remember to include the Table number and title!* 

• Make an entry in your Table of Contents

• Model Organism: Eastern mud snail Ilyanassa obsoleta

• Experimental Question: What is the relationship between snail shell height and snail movement? 

• Independent Variable:  Snail shell height (mm)

• Dependent Variable: Snail movement (cm moved in 15 minutes)

I hypothesize that shell height influences snail movement and predict that as snail shell height increases, snail movement will significantly (CHOOSE: increase or decrease) 

OR  

I hypothesize that snail movement is NOT related to snail shell height and predict that there will be no relationship between snail shell height and snail movement.

Why do we study size variation and movement in snails?

The Eastern Mudsnail, Ilyanassa obsoleta, is a small, marine snail (Phylum Mollusca, Class Gastropoda) that is commonly found on mudflats within the intertidal zone of temperate shores. Like any snail, they move around using mucus and their large muscular foot.  Eastern Mudsnails are "grazers" and feed on detritus (dead stuff) and microalgae that it finds on the mud surface. 

Biologists are interested in learning more about how snail size, aggregation with other snails, and the presence of food influence snail movement. This will help us to understand more about the important role that snails play in the food web.  Biologists are also interested in studying the Eastern Mudsnail in particular because it was introduced to the Pacific Coast and is having a negative impact on the native animal life there. 

Measuring Variability in Populations

Individual variation is inherent across living organisms. However, biologists cannot sample every single organism to draw conclusions about a particular group. Therefore, when taking biological measurements from study subjects, we infer population-level characteristics from representative samples of the population.  It is generally accepted that the larger your sample size, the closer you are to representing the whole (or "true") population. Therefore, we often include replicate samples of each experimental group of interest into our experimental designs. 

The mean (or numerical average) is a useful way to communicate that "central tendency" of measurements from a set of replicate samples. 

When reported alone, the mean may not be particularly representative of your set of sample measures, because it can be strongly affected by measurements on the low and high extremes. Therefore, it is useful to also provide an estimate of variability, or how measurements are scattered or spread around a mean. In this course we will calculate standard deviation as our estimate of variability.  Both biological variation and experimenter accuracy are sources of variability in our data that could increase standard deviation. Generally (but not always!), the larger the sample size, the less variability there will be, and  the smaller the standard deviation will be.  

Standard Error (aka Standard Error of the Mean) is another common estimate of variability that you may see in scientific papers. Standard error takes the standard deviation divided by the square root of the sample size.  Therefore, the larger the sample size, the smaller your standard error will be. The smaller the standard error value is, the more accurately our measurements represent the true population. We won't be calculating standard error regularly in this class, but we will explore that impact of sample size on standard error in this lab. 

Example demonstrating the influence of sample size on variability

Using Rulers to measure Snail shell 

1. Marking Snails 

1. Each lab bench group will have a set of snails to measure. You must coordinate with your group to give each snail a unique label (this may have already been done for you).

2. To label a snail, gently pick it up and dry the shell with a paper towel. Brush a small spot of colored nail polish or paint on your snail, and place snail in a bowl to dry. 

*NOTE! You may need to create combinations of two color spots to create unique labels among your group of snails.

*NOTE! Try to put a thin layer of paint on the shell to decrease drying time!

2. Measurement of Shell Height

Each lab bench group  will measure the shell height of their set of snails using rulers and pool their data.

1. Gently pick up a snail. 

2. Place the snail on the ruler to measure the shell height, from the tip of the spire (apex) to the base of the aperture (opening)

3. Work with your benchmates to record the shell heights of all snails (to the nearest mm) in your notebook. Make sure that each group member has the complete data set in their notebook. 

3. Measuring Snail Movement

Lab bench groups will also measure the movement of every snail in their set.

1. Each person in your group will obtain a  glass dish with a grid containing 1cm x 1 cm squares beneath it. (Each line of the grid is 1cm in length).

2.  Add room temperature seawater to your dish. There should be enough water to cover the snail when placed in the dish. 

3. Take one of the snails from your bench population and place it into the large bowl filled with room temperature seawater. NOTE! Before you start, discuss with your partners and instructor what to do if the snails climb up the side of the bowl.

4. Start a timer for 15 min and carefully observe your snail's movement. Record the number of squares moved in 15 minutes in your data table.  

5. Make sure that all of your group members have the complete data set in their notebooks.

6. Record in your notebook your observations about the relationship between shell height and movement. Was your hypothesis supported? 

Calculating Mean & SD in Excel

You will use Excel to calculate the Mean and Standard Deviation for your small bench population and for the larger snail population. You will use  formulas in Excel to calculate Mean and Standard Deviation separately for the two columns of data.

Column Graph to display mean & SD

You will use Excel to create a Column Graph to display mean & SD of shell height snails from your bench and from the larger snail population

Use the mean and SD values calculated in the first tab of your Excel file to create a properly formatted column graph.

Scatter Plot to examine the relationship between two numerical variables

You will use Excel to create a scatter plot to examine the relationship between snail mass and movement in your lab population. Use the values in the 'Scatter Plot' tab of your Excel sheet to create a scatter plot to examine the relationship between shell height and snail movement.