11. ELECTRORECEPTION
557
Many gymnotids caught in the field have partially regenerated tails. This
common history of injury could be a result of their peculiar mode of
investigation. It is also true that the tip of the tail is the most salient part
of an electric fish to another fish that is detecting it electrically, and when
electric fish fight in aquaria, the tails are often the primary target of
attack.
Several kinds of unconditional responses in addition to avoidance
of metallic rods have been used in exploration of electric sensitivity. For
example, heart rate in the skate or dogfish may accelerate when a weak
electric stimulus is presented ( Dijkgraaf and Kalmijn, 1966). These fish
will also burrow into sand over electrodes emitting signals similar to the
respiratory potentials of flatfish on which they feed, a dramatic demonstration of a function of passive electrosensory systems. Mormyrids
and variable frequency gymnotids accelerate their organ discharge rate
when weak electric stimuli are given or when the conductance is altered
between a pair of electrodes at head and tail (Bennett, 1965; Bennett and
Grundfest, 1959; Enger and Szabo, 1965; Hagiwara et al., 1965a; Larimer
and MacDonald, 1968; Moller, 1970; Szabo and Fessard, 1965). In the
second, fifth, and seventh of these examples the case for active electroreception is strong because the response to a conductance change occurs
only if an organ discharge occurs during the change; no response is obtained to changes restricted to periods between discharges.
An interesting form of unconditional response can be evoked in
higher frequency fish ( Watanabe and Takeda, 1963; Larimer and MacDonald, 1968; Bullock, 1970). If the animal is presented with a stimulus
close in frequency to its own discharge, it will shift its discharge frequency away from the interfering frequency. The experimenter can
slowly “chase” the fish‘s frequency up or down within a certain range;
beyond this range the fish will fairly rapidly change its frequency to the
other side of that of the interfering signal. The response appears to
represent avoidance of “jamming” of the sensory system by the closely
neighboring frequency.
A related kind of response is evoked in Gymnotus and Gymnurchus.
A weak electric stimulus near the fish‘s own frequency can cause the fish
to cease discharging entirely for a period of a fraction of a second up to
minutes (references in Section 111 of Chapter 10, this volume). The longer
cessations may represent a kind of hiding in these fish which are highly
aggressive toward other electric fish including members of their own
species. Similar cessations can be evoked in mormyrids by weak electric
stimuli over a fairly wide range of frequencies (Moller, 1970). There
are interactions between mormyrids evidenced by other changes in
discharge rate that must be mediated by the electrosensory system
557
Many gymnotids caught in the field have partially regenerated tails. This
common history of injury could be a result of their peculiar mode of
investigation. It is also true that the tip of the tail is the most salient part
of an electric fish to another fish that is detecting it electrically, and when
electric fish fight in aquaria, the tails are often the primary target of
attack.
Several kinds of unconditional responses in addition to avoidance
of metallic rods have been used in exploration of electric sensitivity. For
example, heart rate in the skate or dogfish may accelerate when a weak
electric stimulus is presented ( Dijkgraaf and Kalmijn, 1966). These fish
will also burrow into sand over electrodes emitting signals similar to the
respiratory potentials of flatfish on which they feed, a dramatic demonstration of a function of passive electrosensory systems. Mormyrids
and variable frequency gymnotids accelerate their organ discharge rate
when weak electric stimuli are given or when the conductance is altered
between a pair of electrodes at head and tail (Bennett, 1965; Bennett and
Grundfest, 1959; Enger and Szabo, 1965; Hagiwara et al., 1965a; Larimer
and MacDonald, 1968; Moller, 1970; Szabo and Fessard, 1965). In the
second, fifth, and seventh of these examples the case for active electroreception is strong because the response to a conductance change occurs
only if an organ discharge occurs during the change; no response is obtained to changes restricted to periods between discharges.
An interesting form of unconditional response can be evoked in
higher frequency fish ( Watanabe and Takeda, 1963; Larimer and MacDonald, 1968; Bullock, 1970). If the animal is presented with a stimulus
close in frequency to its own discharge, it will shift its discharge frequency away from the interfering frequency. The experimenter can
slowly “chase” the fish‘s frequency up or down within a certain range;
beyond this range the fish will fairly rapidly change its frequency to the
other side of that of the interfering signal. The response appears to
represent avoidance of “jamming” of the sensory system by the closely
neighboring frequency.
A related kind of response is evoked in Gymnotus and Gymnurchus.
A weak electric stimulus near the fish‘s own frequency can cause the fish
to cease discharging entirely for a period of a fraction of a second up to
minutes (references in Section 111 of Chapter 10, this volume). The longer
cessations may represent a kind of hiding in these fish which are highly
aggressive toward other electric fish including members of their own
species. Similar cessations can be evoked in mormyrids by weak electric
stimuli over a fairly wide range of frequencies (Moller, 1970). There
are interactions between mormyrids evidenced by other changes in
discharge rate that must be mediated by the electrosensory system