Monday, May 1, 2006 – A practical on electrocomunnication from Dr. Joseph Bastian
Many fish are electroreceptive meaning they can sense electric signals in the environment via specialized receptors. Certain species (e.g. sharks) are highly sensitive to electric signals and preferentially use this information to detect prey. (Electrical signals in fish are produced by the muscular potentials and the gill potentials; this is what the fish cue in on.) Even though many fish can use electric signals of other animals, most have a passive electrosensory system. Electric fish have an active electrosensory system in which they produce an electric field around themselves and sense their own signal. There are two types of electric fish, those that produce pulses of electric charge and those that produce it in a continuous wave. In the latter, the electric receptors are tuned to the frequency of that species. This, in effect, gives each sympatric species, its own “channel” to use to communicate.
Electric fish use this specialized sensory system to navigate through the turbid streams they live in and to communicate with each other. In navigation/orientation, when an object is placed near the fish, it disturbs the electric field and an electric “image” is projected onto the skin of the electric fish. This changed is registered by the electroreceptors and results in an increase in action potential frequency that is processed by the brain. When two electric fish are next to each other, they can both sense the electric organ discharge (EOD) of the other. The two overlapping waves produce a beat frequency between the two fish that is dependent on the difference in the two frequencies. These beats can be used as social signals or one fish may try to “jam” the signal of the other fish by producing a signal of the same frequency. Jamming makes fish have a hard time locating objects nearby and most try to avoid it actively. This behavior is called the jamming avoidance response (JAR). The higher frequency signaler will produce an even higher frequency and the lower signaler will produce a lower frequency to increase the difference in EOD frequencies between them. This separates the two signals so that each can be processed with less interference. One study showed an opposite response. When two fish were placed together, instead of increasing the difference in their frequencies, they decreased it to both produce the same frequency. This is called phase locking and may be used so that each fish can still detect predators and prey in the environment. Besides jamming avoidance and phase locking, electric fish can also use rapid changes in frequency and amplitude of the EOD signal to communicate to male or female conspecifics. A. leptorhynchus males produce two such signals called chirps due to the short high frequency EOD burst they make. Type I chirps are much higher in frequency than the male’s normal wave and are thought to be a sexual signal to females. Type II chirps are slightly higher in frequency and are thought to be an agonistic signal to other males. Females also produce type II chirps.
When we went down to the lab, we were able to see (via an oscilloscope) the EOD waveform that an A. leptorhynchus makes. The fish was placed in a small chamber hooked up to two sets of electrodes where we could not only record the EOD the fish was producing, but we could also simulate another fish by sending an electric current of similar frequency through the chamber. When we set the frequency to the same value as the fish in the chamber, we observed the JAR. We also observed several type I and type II chirps. When Dr. Bastian placed a Sternarchorhyncus sp. fish in the chamber, it almost immediately phase locked onto the signal we were sending. Dr. Bastian said he had never seen anything like that before and we were all impressed. Not much is known about this species and so there are many possible answers as to the function of this behavior. This species, along with most of the weakly electric fish, live in a habitat that makes studying the field behavior logistically difficult. However, if you could find a way around those difficulties, it seems like there are a wealth of questions to be answered about the evolution of electrocommunication in this unique group of fish.