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Experiment: Neural Recording from Fiddler Crab

Introduction​

This experiment will allow students to explore how neurons in the leg of a fiddler crab encode and transmit information. By recording the electrical activity of these neurons, students will gain insights into the basic principles of neurobiology and electrophysiology.

Background​

The Nervous System​

The nervous system is the command center of the body. It senses the environment, communicates information to different parts of the body, and controls actions. Neurons, the specialized cells that make up the nervous system, use electrical signals to pass information rapidly. This experiment will focus on recording these electrical signals, known as action potentials or "spikes."

Neurons in the Crab Leg​

In fiddler crabs, neurons in the leg are responsible for sensing touch and movement. These sensory neurons help the crab detect changes in its environment, which is crucial for survival. By examining these neurons, we can learn how sensory information is encoded and transmitted to the brain.

Electrophysiology​

Electrophysiology is a method used by scientists to detect and amplify small electrical changes in biological tissues. By recording these electrical signals, we can study the activity of neurons in real-time.

Materials​

  • Fiddler crabs
  • Plastic container with wet sand or marsh mud
  • Saltwater
  • Pellet or flake fish food
  • Ice
  • Plastic cup (or small container)
  • Toothpick or Plastic coffee stirrer
  • Recording device (computer, tablet, or smartphone)
  • Neuron SpikerBox
  • Spike Recorder app

Procedure​

Obtaining and Caring for Fiddler Crabs​

  1. Collecting Crabs: Collect fiddler crabs at low tide in a marsh or purchase them from bait and tackle shops.
  2. Setting Up Habitat: Place wet sand or marsh mud in a plastic container, covering about two-thirds of the bottom, and fill the rest with salt water.
  3. Maintaining Habitat: Keep extra saltwater on hand to replenish as needed to prevent dehydration.
  4. Feeding Crabs: Feed the crabs with pellet or flake fish food twice a week.

Preparing the Specimen​

  1. Anesthetizing the Crab: Place a fiddler crab in a plastic cup filled with ice to anesthetize it. Do Not Leave the Crab on Ice for over ~5 minutes! Crabs can die if kept for too long.
  2. Removing a Leg: After a few minutes, remove the crab from the ice and gently pull off one of its legs from the base of the carapace. Don't keep the crab in ice for too long. Just a few minutes is all you need... until you see the crab is not moving.
  3. Setting Up the Leg: Place the leg on the small corkboard on top of the Spikerbox with part of it hanging off the edge.

Viability of Neurons After Leg Removal​

Fiddler crabs, like other crustaceans, have an open circulatory system where the blood (hemolymph) is not confined exclusively to blood vessels but flows freely through the body cavity. The hemolymph in fiddler crabs is typically blue due to the presence of hemocyanin, a copper-based molecule that carries oxygen, similar to how hemoglobin functions in vertebrates. This blue blood transports nutrients and oxygen to cells and removes waste products.

After the leg is removed, the neurons inside can survive for a limited time by using the residual oxygen in the tissues and the hemolymph remaining within the leg. However, without the active circulation of hemolymph, the supply of oxygen is finite. The neurons will gradually deplete the available oxygen, leading to a decline in their function and eventual death. The duration of viability depends on several factors, including temperature, humidity, and how the leg is handled. Under optimal conditions, such as keeping the leg cool and moist, you can expect to record electrical activity from the neurons for approximately 1h post-removal.

Recording Electrical Activity​

  1. Inserting Electrodes: Insert the electrodes into the leg, one at the top of the femur and the other at the bottom of the femur near the knee joint.
  2. Connecting Equipment: Connect the SpikerBox to your recording device and open the Spike Recorder app.
  3. Recording Activity: Rotate the “ON” switch on the SpikerBox and listen for spontaneous neuronal action potentials. Also, observe the signal trace on the Spike Recorder App to determine if you can see spikes.
  4. Brush the Leg: Use a toothpick or small coffee stirrer to gently poke the leg and observe changes in electrical activity.

Data Analysis​

  1. Recording Spontaneous Activity: Record the spontaneous electrical activity of the neuron.
  2. Recording Stimulated Activity: Record the electrical activity when the leg is stimulated with different stimuli (taps, pulls, air blows).
  3. Analyzing Spikes: Identify and count the action potentials, noting the different phases of the action potential (depolarization, repolarization, hyperpolarization).

Post-Activity Questions​

  1. What differences in electrical activity do you observe between spontaneous and stimulated conditions?
  2. How does the spiking activity compare with other invertebrates you may have recorded?

Conclusion​

By conducting this experiment, students will learn how neurons in the fiddler crab's leg encode sensory information and how electrophysiology can be used to study neural activity. This hands-on activity provides a practical understanding of neurobiology principles and the fascinating mechanisms of neuronal communication.

Credits​

This lab was designed by Lydia Naughton in 2023 at the Marine Biological Laboratories in Woods Hole, MBL. Adopted and modified in July 2024 by Greg Gage.