Electric Fields and Electroreception
325
gymnotiforms evolved their electric sense resembled each other, forming similar
evolutionary pressures.
Can South American electric fish also detect capacitive object properties? In
order to answer this question we trained several species of gymnotiforms, which
emit either pulse-type signals (Brachyhypopomus pinnicaudatus; Gymnotus
carapo) or continuous wave type EODs (Eigenmannia sp.), using the same
method as we had used for the mormyrids. The results were clear: all individuals
of each species tested discriminated between a capacitive object and any resistive
objects (Fig. 4B). Even if the impedances of the two objects were identical, they
had no difficulties in discriminating. Thus gymnotiforms, like mormyriforms, can
perform capacitance detection (von der Emde 1998). The similarities in the
behavior of the two groups of electric fish are, however, not reflected by the
sensory mechanisms employed. As in mormyrids, the local signals stimulating an
electrolocating gymnotiform fish are distorted when the fish approaches a
capacitive object. However, the resulting changes in signal timing (the timing of
the positive-negative zero crossing) are much larger than those occurring in
mormyrid EODs. While in the latter the timing changes maximally by 3 f..LS, it
changes by almost 200 f..LS in Eigenmannia (Fig. 5B).
In contrast to mormyrids, gymnotiforms do not possess mormyromasts with Aand B-type receptor cells, but instead have tuberous electroreceptor organs with a
single population of receptor cells. In Eigenmannia, one type of such organ (Ptype organs) responds only to EOD amplitude. The other type, called T-type
receptor organs (t for timing), respond only to EOD timing, independent of
stimulus amplitude (Scheich et al. 1973). When signal timing changes because of
the proximity of a capacitive object, the t-type receptor organs report these
changes to the brain. T-receptors in other skin areas, which lie outside the electric
image of that object, do not experience a change in local signal timing. They
continue to report the original EOD timing to the brain. Neurons in the torus
semicircularis compare signal timing reported by t-receptors from different body
regions (Heiligenberg 1991) and thus respond to the presence of a capacitive
object.
In order to detect changes in signal wave form, the mormyrid brain compares
A- and B-receptor responses from single mormyromast organs located at a single
spot on the skin. In these fishes, changes in signal timing are too small and
therefore of no importance. In contrast, Eigenmannia compares t-receptor
responses originating from different skin locations. In this species, timing
changes are the relevant cues while wave form changes most likely do not play a
role. Thus, very similar-looking sensory capabilities in the two groups of weakly
electric fishes are based on completely different sensory mechanisms (von der
Emde 1998).
325
gymnotiforms evolved their electric sense resembled each other, forming similar
evolutionary pressures.
Can South American electric fish also detect capacitive object properties? In
order to answer this question we trained several species of gymnotiforms, which
emit either pulse-type signals (Brachyhypopomus pinnicaudatus; Gymnotus
carapo) or continuous wave type EODs (Eigenmannia sp.), using the same
method as we had used for the mormyrids. The results were clear: all individuals
of each species tested discriminated between a capacitive object and any resistive
objects (Fig. 4B). Even if the impedances of the two objects were identical, they
had no difficulties in discriminating. Thus gymnotiforms, like mormyriforms, can
perform capacitance detection (von der Emde 1998). The similarities in the
behavior of the two groups of electric fish are, however, not reflected by the
sensory mechanisms employed. As in mormyrids, the local signals stimulating an
electrolocating gymnotiform fish are distorted when the fish approaches a
capacitive object. However, the resulting changes in signal timing (the timing of
the positive-negative zero crossing) are much larger than those occurring in
mormyrid EODs. While in the latter the timing changes maximally by 3 f..LS, it
changes by almost 200 f..LS in Eigenmannia (Fig. 5B).
In contrast to mormyrids, gymnotiforms do not possess mormyromasts with Aand B-type receptor cells, but instead have tuberous electroreceptor organs with a
single population of receptor cells. In Eigenmannia, one type of such organ (Ptype organs) responds only to EOD amplitude. The other type, called T-type
receptor organs (t for timing), respond only to EOD timing, independent of
stimulus amplitude (Scheich et al. 1973). When signal timing changes because of
the proximity of a capacitive object, the t-type receptor organs report these
changes to the brain. T-receptors in other skin areas, which lie outside the electric
image of that object, do not experience a change in local signal timing. They
continue to report the original EOD timing to the brain. Neurons in the torus
semicircularis compare signal timing reported by t-receptors from different body
regions (Heiligenberg 1991) and thus respond to the presence of a capacitive
object.
In order to detect changes in signal wave form, the mormyrid brain compares
A- and B-receptor responses from single mormyromast organs located at a single
spot on the skin. In these fishes, changes in signal timing are too small and
therefore of no importance. In contrast, Eigenmannia compares t-receptor
responses originating from different skin locations. In this species, timing
changes are the relevant cues while wave form changes most likely do not play a
role. Thus, very similar-looking sensory capabilities in the two groups of weakly
electric fishes are based on completely different sensory mechanisms (von der
Emde 1998).