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The study of biomechanics brings together the fields of biology and physics.  In doing so, we can study the functions of a living organism using principles of physics such as force, power, and energy and relate them to the adaptations of the organism to its environment.  In this exercise you will look at the claws of lobsters as simple mechanical levers.  Using the basic concepts of  mechanical advantage and force you will be able to  compare the two types of claws (or chelae) and relate these differences to their function.

This lab has been adapted from the work of Dr. Edwin DeMont (1996, 1997) at St. Francis Xavier  University, Nova Scotia and is used with his permission.
 

The materials needed for this lab are listed below.  Assemble all the equipment and specimens before you start your work.  Be sure to record all procedures and observations in your lab notebook.
 

• Lobster claws, one pair including the pincer chela and the crusher chela (substitute crawfish if necessary)
• Metric ruler
• Metric graph paper, 1 mm or 2 mm grid
• Pencil
• Scissors
• Calculator
• Laboratory Notebook

Part One: Observations.

 Spend approximately five minutes observing the two chelae.  There are some obvious differences between the pincer and the crusher.  Write down three differences you and your lab partners observe in your notebook.  Suggest a hypothesis which would explain these differences.
 
 

Part Two: Dissection.

Observe the muscles in the pincer chela.  Using scissors make two parallel cuts along the length of the chela from the opening at the proximal end to the tip.  Then remove the carapace and observe the muscles inside the claw.  In preserved specimens the muscle will not fill the space inside the chela due to shrinkage.  Open and close the lower part of the chela (the dactyl).  Observe the movement of the muscle and the point of attachment of the dactyl to the body of the chela.  You may have to remove some of the muscle to see this clearly.

 Next remove the dactyl from the chela.  The white "cartilage" or apodeme must remain attached to the dactyl.  To do this, pinch the point of attachment of the dactyl to the chela and move the dactyl back and forth in a sideways direction.  When the dactyl feels loose, carefully pull it out of the chela.  Clean any muscle off the apodeme that are still attached.
 
 

Part Three:  Analysis of the lobster claw as a mechanical lever.

 The lobster claw can be analyzed as a simple mechanical lever with the fulcrum located at the point where the dactyl articulates with the chela.  The effort force is applied by the muscle at the attachment of the apodeme and the resulting force is generated along the length of the dactyl.  Start with the pincer chela.

 The large flexor muscle which closes the claw applies its force at the point where it attaches to the apodeme.  The flexor is a pinnate muscle.  In this type of muscle the individual fibers are shorter than the muscle itself and attach to the apodeme at an angle rather than running parallel to the length of the muscle (Alexander, 1968). This force is directly proportional to the surface area of the apodeme.

 Blot the dactyl dry using paper toweling.  Place the dactyl on a piece of graph paper and outline the shape of the dactyl with a pencil.  To estimate the surface area of the apodeme count the number of squares inside the outline of the apodeme.  Use your judgment to estimate the fractional squares on the graph paper that should be included in the area measurement.  Remember your graph paper is probably marked off in millimeters.  Your estimate of the surface area must be in meters squared.

 The force of the muscle can be calculated using the formula F_m = A ^. K_p, where A is the surface area of the apodeme and K_p is an empirically derived constant.  For the pincer K_p = 1.9 X 105 N/m².  Record F_m on the data chart.

 Use a metric ruler to measure the length from the apodeme attachment to the fulcrum, l_M, and the length from the tip of the dactyl to the fulcrum, l_L.  Calculate the mechanical advantage of the lever using the formula MA = l_M/l_L. The "speed advantage" is equal to the reciprocal of the mechanical advantage. Record these data on the chart.

 The force generated at the tip of the dactyl is calculated using the formula F_L = MA ^. FM.  Record this on your data chart.

Repeat all of these measurements and calculations using the crusher chela and record the results on the chart.
 
 

RESULTS	PINCER CHELA	CRUSHER CHELA
A, apodeme surface area	.	.
K, constant	1.9 X 105 N/m2	2.5 X 10 5 N/m2
FM, muscle force on apodeme	.	.
lM, apodeme to fulcrum length	.	.
lL, dactyl tip to fulcrum length	.	.
MA, mechanical advantage	.	.
1/MA, speed advantage	.	.
FL, generated force at dactyl	.	.

 
 

Include in your lab report answers to the following questions.

1. Compare the two chelae, the pincer and crusher, in terms of the forces and speed they can generate when closing.

2. What is the functional significance of these differences to the lobster?

3. Compare your answers to the first two questions to the hypotheses you made in your initial observations.

4. There is another muscle in the chela, the extensor, which opens the dactyl.  Would you expect this muscle to show a difference between the pincher and crusher chelae?  Explain.

5. What are the SI units of the forces in this system?  What are the units of measurement of the mechanical advantage?
 
 
 

Alexander, R McNeill.  (1968)  Animal Mechanics (University of Washington Press).

DeMont, Edwin. (1996) "Measuring how muscles function in levers", American Biology Teacher 58(8): 490- 492.

DeMont, Edwin and Smith B. (1997) The Lobster Lab. http://juliet.stfx.ca/~edemont/lobster.htm.
 
 

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