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The Research of Dr. Harold Zakon

quotes photo

ur lab studies a number of questions using weakly electric fish as our model organism. These fish live in murky waters and are nocturnally active. They generate weak electric fields around themselves from a specialized electric organ and sense these electric fields, with specialized sensory receptors, called electroreceptors. They sense the distortions caused in their own electric fields to locate nearby objects, and the electric fields of other fish as communication signals.

Sternopygus photo
Sternopygus macurus

Weakly Electric Fish

Weakly electric fish have evolved twice. One group, the mormyriformes, live in Africa, the other group, the gymnotiformes, live in South America.
Electric fish of both groups can be classified by their EODs into one of two types: "pulse" fish, which generate brief irregularly-occurring pulses, or "wave" fish, which generate highly regular sinusoidal EODs.
In the Zakon laboratory, we study South American wave fish. The EOD frequency of these wave fish is species specific, sexually dimorphic and individually distinct. The gold-lined knifefish (Sternopygus macrurus) generates EODs from 50-200 Hz, the glass knife (Eigenmannia virescens) has an EOD frequency range of 250-600 Hz; the brown ghost (Apteronotus leptorhynchus) fires at 650-1,100 Hz, and the black ghost (A. albifrons) from 800-1,200 Hz. Males discharge at a lower frequency than females in each of these species, except the brown ghost, where males are higher in frequency. We are able to alter the EOD frequency of individuals by treatment with sex steroids such as testosterone and estrogen.

Eigenmannia photo
Eigenmannia virescens
(Picture by T. Smith)

Electric Organ

Electric organs (EOs) have evolved independently in at least six different groups of fish including two groups of elasmobranchs (Torpedo rays and the skates), and four groups of teleosts (gymnotiforms, mormyriformes, stargazers, catfish). In some groups EOs generate strong discharges (such as the Torpedo ray) and in others, weak discharges (such as the knifefish we study). In all but one case, they derive from muscle (the exception is the family Apteronotidae in which the axons of the EMNs form the electric organ). How the electric organ comes from muscle evolutionarily and developmentally is one question that we have been pursuing in our laboratory. The morphology of the electrocytes varies greatly between species and these variations are intimately bound up in the generation of species-specific electric organ discharge (EOD) waveforms. However, they operate on fundamental features of excitable membranes and current flow. Mainly, electric organs are composed of columns of electrocytes oriented in the same axis and ensheathed in high resistance connective tissue. The connective tissue channels the flow of current along the axis of the organ, out into the water, and back into the other end of the electric organ. Species such as Torpedo rays or electric eels, which have many stacks of flattened electrocytes, are capable of generating discharges of hundreds of volts. Weakly electric fish, such as the ones we study, make modest discharges of only hundreds of millivolts to a few volts.

electric field pic
Schematic drawing of the electric field around a weakly electric fish. An object (blue area) with a higher conductivity than water increases the density of the iso-current density lines on the body surface of the fish, whereas a nonconducting object decreases their density (not shown). The undulating movement of the dorsal fin in gymnotids allows them to move without bending their tail and therefore minimizes the disturbance of the geometry of the electric field. This stabilizes the orientation of the electric organ (light gray area) and hence the orientation of the current density lines towards the electroreceptors, which is important for accurate electroreception (diagram drawn after Heiligenberg (1977)).


Apteronotus photo
Apteronotus leptorhynchus
(Picture by J. Oestreich)