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 such a stimulus can affect the sense organs of electric and electromagnetic events. If we were able another organism when the stimulus is no longer to sense this electric environment, a whole new clouded by environmental noise, thus serving in world would reveal itself: weak electric fields social communication and orientation. To assess emanate from many aquatic organisms, especially such a biologically effective range we must look at from fishes and wounded crustaceans, and to a theamount of emitted or available stimulus energy, lesser extent from mollusks, starfish, and sponges. the sensitivity of the receptors involved, the In most cases, we would feel direct current (DC)* spherical spread of the signal, and the attenuating voltage gradients that in the ocean range from as effects of the surrounding medium on the low as one hundred millionth of a volt per transmission of the stimulus. centimeter to as large as one hundred thousandth We have studied .the effective range of of a volt per centimeter. In freshwater, the voltage electric signals in weak-electric fishes (Table I), gradient is about one hundred times greater. We which are characterized by their ability to emit and would also detect alternating current (AC)** voltage perceive weak electric discharges. In contrast to the gradients associated with an organism’s admirable long-distance performance of acoustic movements, breathing, or locomotor behavior. sensory stimuli (up to several hundred kilometers), Soon we would discover that many inanimate the range of the electric sense in mormyrids* is objects produce DC fields, their interface with restricted to a humble 100 centimeters for water acting as a battery. Lightning discharges and electrocommunication and a mere IO centimeters man-made radio waves are sources of electric noise for electrolocation (Figure 1). pollution that certainly would not escape our The effective ranges of the organism’s underwater electric ears or eyes. Blaxter has said electric sense in electrolocation and (see page 28) that sound “has much to recommend electrocommunication were found to vary with it as a form of sensory stimulus.” So, too, does several factors - including species, body shape, electric energy, and nature, not surprisingly, has and electrical resistance d the fish’s skin; physical exploited it. and electrical properties of the object; and, most A number of aquatic organisms have evolved important, conductivity of the surrounding water, specialized receptors with which they are able to that is, the degree of its dissolved ionic material. perceive many aspects of the underwater electrical The optimal ranges of IO and 100 centimeters are world. Another group of electric signals in an associated with low levels of water conductivitythat aquatic environment are supplied by species that conform well with measurements taken in the fish’s have evolved the ability to generate their own natural habitats.** Even over short distances, the electric energy which they use in predatory, organism is supplied with enough electrical orientation, and communication behavior. information to avoid obstacles, maintain proximity to conspecifics, and compromise with extraneous Electric Signals in Water electric noise. Let us for a moment contemplate the fate of an electric signal underwater and compare it with Electroreceptive and Electrogenic Fishes other energy forms. The conduction of electric There are two groups of fish species distinguished signals in water is almost instantaneous and thus here (Table I): those which have evolved comparable with that of visual ones. Acoustic, electroreceptors only (electroreceptive fishes) and mechanical, and chemical stimuli travel those which, in addition, have evolved specialized considerably slower. Like sound, an electric signal electric organs capable of generating electric does not persist once it is discontinued; both types discharges (electroreceptive and electrogenic of signals, then, differ from chemical stimuli, which fishes). Species in the first category sense electric can linger for quite some time. Turbid water and fields that are not self-genierated, but rather, are darkness do not impede the transmission of produced by other sources in the environment electric, acoustic, chemical, and mechanical signals (passive electrosensory group). Species in the other but do restrict visual ones. Dense vegetation, category also sense electric fields that they actively submerged trees, roots, and even small rocks generate themselves (active electrosensory group). present barriers to visual stimuli, but are not Both groups have representatives among the obstacles to electric currents, which can go around them. As animal behaviorists, we are concerned *A family of African freshwater fish (members of the order with the biologically meaningful range withili which Mormyriformes), many of which are distinguished by their long, tube-like snouts. *An electric current flowing in one direction only and **Less than or equal to 100 micro-Siemens per centimeter substantially constant in value. compared with distilled wateir, which is 0 micro-Siemens **An electric current that reverses its direction at regularly per centimeter, and New York City tap water, which is 70 recurring intervals. micro-Siemens per centimeter. 46 EODs of fish A stimulate electroreceptors in fish 6 (electrocommunication 1 Figure I. Ranges of electrocommunication and electrolocation in mormyrid weak-electric fishes. The “bubble” around the fish indicates \ \ I the extension of the EODs of fish B stimulate electroreceptors in fish A (electrocommunication) electrolocation field to about 10 centimeters. The presence of an object ‘i- [black dot in upper figure) J’ or the fishes themselves (lower figure) will be electrically “‘felt” by the fish emitting the electric discharge. The electrocommunication range is almost IO.*imes larger than the electrolocation range. cartilaginous and bony fishes and can be found in both marine and freshwater environments. A 13 C t Electroreceptors An electroreceptor is a sense organ that responds selectively to natural electric fields. We use both bm. electrophysiological recordings and behavioral tests to determine whether a particular sense organ functions as an electroreceptor. Most known MORMYRIDS electroreceptors are related to the lateral-line f organs (see page 27) (which are water vibration receptors characteristic of all fishes) and are of two fypes - ampullary or tuberous organs (Figure2). The ampullary receptor consists of a b.m. flask-shaped ampulla embedded beneath the surface of the skin. The bottom of the ampulla contains the sensory cells which are in contact with GYMNOTOIDS n the outside through a jelly-filled canal. In sharks, t rays, skates, and marine catfish, in which the - ampullary organs are called ampullae of Lorenzini (after the man who first described them in the 17th century), these canals may be as long as one-third of b.m. the organism’s body (see Figure4, p. 58). In contrast, in all electroreceptive freshwater fishes these canals ‘i are short, often microscopically so. GYMNARCHUS Ampullary organs are the more sensitive electroreceptors. Sharks and sk+ates respond to Figure 2. Typical electroreceptors in weak-electric fishes. stimuli as low as 0.01 microvolts per centimeter, The ampullary organs (A) respondpredominantly to DC which would c&respond to a voltage gradient set electric fields and to low-frequency stimuli emanating from many aquatic organisms, but also to the fish’s own up between the poles of a flashlight cell placed electric discharge. The tuberous mormyromasts (B) serve more than 1,500 kilometers apart. Sharks within in electrolocation and the tuberous knollenorgans (C) in close range of prey are particularly sensitive to the electrocommunication. Shaded areas represent jelly-like electric fields around,aquatic animals. In some material; SC, sensory ce!l; bm, basement membrane; n, electrogenic mormyrid species, the ampullary nerve. Arrow indicates fish’s body surface. (From Szabo, -____*. I I. .I I-*, , I *. “nrr jl_ ~~ n -IT.,_ -I- .I. ~~. __ .L__~ _r.,-.- ” If-_ ,..-, ELECTRORECEPTIVE FISHES (without electric organs) - - - ,Str . Astrosco~us y-graacum (Stargazer) j ELASMOBRANCHII (AL) SQUALIFORMES (sharks) (AL) RAJIFORMES (all ordinary rays ’ (AL and skates) Torj HOLOCEPHALI j ‘(ele (AL) Chimoeridae, Hydrolagus colliei (rat fish) I (AL) SILURIFORMES Plotosidae i IN) P/otosus anguillaris (marine catfish) I Ast ~jsta - i (AL) RAJ I FORM ES (Myliobatoidei) :; Potamotrygonidae, Pofomofrygon circuloris 1 (S-American freshwater stingray) (AR) ACIPENSERI FORMES Polyodon tidae a: (chondrostean paddlefish) ,I ‘. ;: (AR) SILURIFORMES /’Mal Gymnotus csrapo Ictaluridae; Siluridae, Kryptopterus 1,Ma. ( Knifefish) (catfish) (Af (AR) DIPTERIFORMES (Dipnoi) !- Profoplerus dolloi (lungfish) (AR) BRACHIOPTERYGI I (reedfish and bichirs) Gnathonemus petersii (Elephantfish) Figure 3. Electric organs have evolved independently in severalgroups of marine