Electroreception in elasmobranches

As a bioengineer I learn to apply mathematical, chemical, and physical concepts to the analysis of biological systems. In the Writing 405 course that I took with Roberta Kirby-Werner, I was given an opportunity to address research issues in my discipline in a formal project. I chose to analyze electroreception, a sensory modality that enables sharks and other animals to perceive electric fields, and wrote a professional-technical paper in which I reported my findings. I studied electroreception because of the insight it gives into the life of animals that perceive the world in a way that we cannot. It also teaches us about identifying and classifying receptors. My objective for this particular paper was to make some of the technical concepts in my discipline accessible to a more general audience which possesses an interest in science.

The Physical Stimulus for Electroreception

In oceans, electric fields are induced by both biological and geological causes. In the latter case electric fields are induced by water flowing or fish swimming through the earth’s magnetic field by geomagnetic variations4 and by geophysical events5. The animals use these electric fields for navigation and identification of their environment.

Electric fields in the oceans can also be produced by marine animals. The internal and external electrochemical environments of marine animals differ. The difference creates a voltage gradient across the water skin boundary. The potential difference produces current loops which yield a bioelectric field in the surrounding waters. An animal’s behavior can produce additional electric fields. For example, when a fish swims, muscles contract. Muscle contraction takes place when chemically-dependent channels, impermeable to sodium and potassium, open. The movement of such ions across the membrane produces an electric field that travels away from the animal in the conducting medium (salt water).

The number of muscle contractions affects the magnitude of the electric fields. If more muscles contract, the magnitude of the field is greater and vice versa. Furthermore, the intensity of the electric fields changes in the case of a wounded animal. For example, crustaceans can generate a voltage of 50.0 mV measured with a sensing electrode 1 mm away from the surface of the animal. The same crustacean, if wounded, generates a much higher voltage of 1250.0 mV (Kalmijn, 1974). H. S. Burr in 1947 established the presence of these bioelectric fields in the vicinity of marine animals (Kalmijn, 1974). These gradients can be easily detected by certain members of