0 rganisms have adapted to different environments and evolved sense organs that respond selectively to particular forms of energy. Because we lack specialized receptors, we are “blind” to weak forms of electric and nonphotic electromagnetic energy that surround us constantly. The shocking experience with a faulty electric outlet is not mediated through an electric sense but through direct stimulation of the nervous system. Elephantfish, Cnathonemus petersii. At right, elephantfish swimming in a tank.The underwater environment is packed with electric and electromagnetic events. If we were able to sense this electric environment, a whole new world would reveal itself: weak electric fields emanate from many aquatic organisms, especially from fishes and wounded crustaceans, and to a lesser extent from mollusks, starfish, and sponges. In most cases, we would feel direct current (DC)* voltage gradients that in the ocean range from as low as one hundred millionth of a volt per centimeter to as large as one hundred thousandth of a volt per centimeter. In freshwater, the voltage gradient is about one hundred times greater. We would also detect alternating current (AC)** voltage gradients associated with an organism’s movements, breathing, or locomotor behavior. Soon we would discover that many inanimate objects produce DC fields, their interface with water acting as a battery. Lightning discharges and man-made radio waves are sources of electric noise pollution that certainly would not escape our underwater electric ears or eyes. Blaxter has said (see page 28) that sound “has much to recommend it as a form of sensory stimulus.” So, too, does electric energy, and nature, not surprisingly, has exploited it. A number of aquatic organisms have evolved specialized receptors with which they are able to perceive many aspects of the underwater electrical world. Another group of electric signals in an aquatic environment are supplied by species that have evolved the ability to generate their own electric energy which they use in predatory, orientation, and communication behavior. Electric Signals in Water Let us for a moment contemplate the fate of an electric signal underwater and compare it with other energy forms. The conduction of electric signals in water is almost instantaneous and thus comparable with that of visual ones. Acoustic, mechanical, and chemical stimuli travel considerably slower. Like sound, an electric signal does not persist once it is discontinued; both types of signals, then, differ from chemical stimuli, which can linger for quite some time. Turbid water and darkness do not impede the transmission of electric, acoustic, chemical, and mechanical signals but do restrict visual ones. Dense vegetation, submerged trees, roots, and even small rocks present barriers to visual stimuli, but are not obstacles to electric currents, which can go around them. As animal behaviorists, we are concerned with the biologically meaningful range withili which *An electric current flowing in one direction only and substantially constant in value. **An electric current that reverses its direction at regularly recurring intervals. 46 such a stimulus can affect the sense organs of another organism when the stimulus is no longer clouded by environmental noise, thus serving in social communication and orientation. To assess such a biologically effective range we must look at theamount of emitted or available stimulus energy, the sensitivity of the receptors involved, the spherical spread of the signal, and the attenuating effects of the surrounding medium on the transmission of the stimulus. We have studied .the effective range of electric signals in weak-electric fishes (Table I), which are characterized by their ability to emit and perceive weak electric discharges. In contrast to the admirable long-distance performance of acoustic sensory stimuli (up to several hundred kilometers), the range of the electric sense in mormyrids* is restricted to a humble 100 centimeters for electrocommunication and a mere IO centimeters for electrolocation (Figure 1). The effective ranges of the organism’s electric sense in electrolocation and electrocommunication were found to vary with several factors - including species, body shape, and electrical resistance d the fish’s skin; physical and electrical properties of the object; and, most important, conductivity of the surrounding water, that is, the degree of its dissolved ionic material. The optimal ranges of IO and 100 centimeters are associated with low levels of water conductivitythat conform well with measurements taken in the fish’s natural habitats.** Even over short distances, the organism is supplied with enough electrical information to avoid obstacles, maintain proximity to conspecifics, and compromise with extraneous electric noise. Electroreceptive and Electrogenic Fishes There are two groups of fish species distinguished here (Table I): those which have evolved electroreceptors only (electroreceptive fishes) and those which, in addition, have evolved specialized electric organs capable of generating electric discharges (electroreceptive and electrogenic fishes). Species in the first category sense electric fields that are not self-genierated, but rather, are produced by other sources in the environment (passive electrosensory group). Species in the other category also sense electric fields that they actively generate themselves (active electrosensory group). Both groups have representatives among the *A family of African freshwater fish (members of the order Mormyriformes), many of which are distinguished by their long, tube-like snouts. **Less than or equal to 100 micro-Siemens per centimeter compared with distilled wateir, which is 0 micro-Siemens per centimeter, and New York City tap water,