452
M. V. L. BENNETT
flattened in the two kinds of strongly electric marine fish and elongated
in the two kinds of strongly electric freshwater fish. This difference is
ascribable to the different resistances of the environment. The low
resistance of seawater requires a high current, low voltage output; the
high resistance of freshwater requires a low current, high voltage output.
Another striking difference between marine and freshwater electric
fish is the absence of spike generating membrane in electrocytes of the
three marine groups and its presence in those of the three freshwater
groups. While it is natural to ascribe this difference to differences in
loading by the environments, it is difficult to do so convincingly
(Bennett, 1961, 1970) since each organ consists of many elements in
series-parallel array and the most energetically efficient kind of membrane would appear to be the best in both environments. Yet there is
almost sure to be some advantage to PSP membrane for the marine
environment because in each case the muscle giving rise to the electric
organ does possess spike generating membrane (Bennett et al., 1961;
Grundfest and Bennett, 1901, and unpublished data).
An interesting convergence is in the development of stalks. These
structures are most remarkable in mormyrids but are also prominent
in catfish electrocytes. Stalks are present but seem of no great importance
in gymnotids. Stalks do allow a smaller PSP current to excite the
electrocytes since the input resistance of the stalks is higher, but the
advantages of this are far from obvious if one considers marine electrocytes where the entire output consists of PSPs. The stalks act in synchronization of Ging of mormyrid electrocytes, but the same output
could be produced by diffusely innervated cells, at least in the species
with longer lasting discharges and the more primitive stalk systems.
Another generalization concerns the presence of a dc component
in the organ discharge. All strongly electric fish emit dc pulses; perhaps
this leads to more effective shocking of prey or predator. A few weakly
electric fish that fire at low frequencies also emit pulses that are dc or
have a large dc component, but in most weakly electric fish including
all high frequency species, the organ discharge has little or no dc component. The absence of a dc component in the organ discharge allows
the fish to have a dual electrosensory system in which one set of receptors is sensitive to low frequency signals arising in the environment
and another set detects distortions in the high frequency field produced
by the electric organ (see Chapter 11, this volume).
In Gymnurchus, the sternarchids, and perhaps Eigenmannia the
absence of a dc component is achieved by modification of one electrocyte face to pass only capacitative current. In Sternopygus the uninnervated face apparently acts as a polarization capacity. In others
M. V. L. BENNETT
flattened in the two kinds of strongly electric marine fish and elongated
in the two kinds of strongly electric freshwater fish. This difference is
ascribable to the different resistances of the environment. The low
resistance of seawater requires a high current, low voltage output; the
high resistance of freshwater requires a low current, high voltage output.
Another striking difference between marine and freshwater electric
fish is the absence of spike generating membrane in electrocytes of the
three marine groups and its presence in those of the three freshwater
groups. While it is natural to ascribe this difference to differences in
loading by the environments, it is difficult to do so convincingly
(Bennett, 1961, 1970) since each organ consists of many elements in
series-parallel array and the most energetically efficient kind of membrane would appear to be the best in both environments. Yet there is
almost sure to be some advantage to PSP membrane for the marine
environment because in each case the muscle giving rise to the electric
organ does possess spike generating membrane (Bennett et al., 1961;
Grundfest and Bennett, 1901, and unpublished data).
An interesting convergence is in the development of stalks. These
structures are most remarkable in mormyrids but are also prominent
in catfish electrocytes. Stalks are present but seem of no great importance
in gymnotids. Stalks do allow a smaller PSP current to excite the
electrocytes since the input resistance of the stalks is higher, but the
advantages of this are far from obvious if one considers marine electrocytes where the entire output consists of PSPs. The stalks act in synchronization of Ging of mormyrid electrocytes, but the same output
could be produced by diffusely innervated cells, at least in the species
with longer lasting discharges and the more primitive stalk systems.
Another generalization concerns the presence of a dc component
in the organ discharge. All strongly electric fish emit dc pulses; perhaps
this leads to more effective shocking of prey or predator. A few weakly
electric fish that fire at low frequencies also emit pulses that are dc or
have a large dc component, but in most weakly electric fish including
all high frequency species, the organ discharge has little or no dc component. The absence of a dc component in the organ discharge allows
the fish to have a dual electrosensory system in which one set of receptors is sensitive to low frequency signals arising in the environment
and another set detects distortions in the high frequency field produced
by the electric organ (see Chapter 11, this volume).
In Gymnurchus, the sternarchids, and perhaps Eigenmannia the
absence of a dc component is achieved by modification of one electrocyte face to pass only capacitative current. In Sternopygus the uninnervated face apparently acts as a polarization capacity. In others