Fiddler Crabs

Whenever you get the chance, you need to go the salt marsh and take a look at the fiddler crabs along the coastal mud flats. It is simply amazing to see all of them scurrying around swinging their big claw in the air. But as soon as you step into view, these little crabs will freeze and then run into burrows, out of sight.

This is amusing as a spectator; but as a scientist, this could prove frustrating if you are trying to get a count of these critters. Biologist must often take a census of a population of organisms in order to determine the range of the population.

In this activity, you will act as a biologist and estimate the number of fiddler crabs in a population by taking a survey of a hypothetical mud flat.

Introduction:

The purpose of this experiment is to practice acting as a biologist and estimate the

number of fiddler crabs in a population by taking a survey of hypothetical mud flat. What I hope to learn from this experiment is first, a better understanding of how scientists estimate the number of crabs in a population. I also hope to learn about scientists estimating the population of different things. I believe that there are 40 crabs on the page.

Methods:

Materials:

calculator

pencil

ruler

index card

- Print one of the crab images listed at the end of this assignment. Cut a one inch square from an index card. Drop the index card onto the crab diagram. Trace around the square.
- Count how many crabs are in the square. Do this 10 times. Take the total and average number of crabs you found. Using the ruler, measure the length and width of the crab diagram. Multiply the length and width to determine total area of diagram in square inches. Multiply the total area by the average number of crabs. The answer to this will give you an estimate of the number of crabs in the diagram.
- This will give you an estimate of the number of crabs represented in the fiddler crab colony.

Analysis Questions

- How accurate do you think your estimation really was?
- What errors may have occurred?
- What other populations would scientists need to estimate rather than actually count?
- Why do you think scientists use quadrant studies to measure populations of organisms

Images to choose from:

From GVL (image links added)