11, ELECTRORFCEPTION
563
well be increased skin resistance, at least in freshwater fish. Further insight into these questions should be gained when membrane properties of
receptor cells of ordinary lateral line organs are determined. Mechanoreceptors in weakly electric fish are 20 (Roth, 1968) to 400 times (Suga,
1967b) less sensitive to electric stimuli than are the electroreceptors. This
difference is considerably less than the difference between electric and
nonelectric fish ( Machin and Lissmann, 1960). Probably the increased
skin resistance in freshwater fish with electroreceptors increases the
electric sensitivity of the mechanoreceptors as well.
Many electric fish live in turbid waters or, if they live in clear waters,
are nocturnal. The electrosensory system which is basically quite short
range would appear to be most useful under these conditions of restricted
use of vision. Some electric fish have quite reduced eyes and the retinae
of the electric eel apparently degenerates with age (see discussion by
Keynes following de Oliveira Castro, 1961). But a number of electric
fish have what appear to be quite good eyes, and many elasmobranchs
at least are diurnal. In addition to the parallel evolution of electric
organs, electroreceptors, and a peculiar mode of swimming, long snouts
have been developed in both mormyrids and gymnotids (and probably
more than once in each group; see Figs. 16 and 60 in Chapter 10). The
snouts are certainly used in poking about muddy bottoms, but are most
likely a later development than the electrosensory systems. It is not clear
that development of electrosensory systems was always in compensation
for poor visual conditions, although it seems likely to have been so a
number of groups (Lissmann, 1958, 1961). In any case, development of
lateral line organs with electric sensitivity has not led to any great
decline of the ordinary lateral line system, which like the electrosensory
systems should be particularly useful under poor visual conditions
(Dijkgraaf, 1963).
It would be useful to know more about electric fields present in the
ordinary fish's environment and the extent to which these stimulate
ordinary lateral line receptors. Muscle potentials up to tens of millivolts
can be produced by synchronously contracting muscles. Examples are
sonic muscles where the fundamental frequency is set by the frequency
of muscle contraction and axial muscles when excited by Mauthner fibers
(unpublished, cf. Lissmann, 1958). These potentials conceivably could
activate mechanoreceptors electrically. Many catfish have sonic muscles
and their innervation is similar to that of the electric organ of the electric
catfish, which lacks a sonic muscle. Perhaps this organ was modified from
a sonic muscle, a suggestion which could be strengthened by further
morphological and embryological study of the catfish group (see Johnels,
1956).
563
well be increased skin resistance, at least in freshwater fish. Further insight into these questions should be gained when membrane properties of
receptor cells of ordinary lateral line organs are determined. Mechanoreceptors in weakly electric fish are 20 (Roth, 1968) to 400 times (Suga,
1967b) less sensitive to electric stimuli than are the electroreceptors. This
difference is considerably less than the difference between electric and
nonelectric fish ( Machin and Lissmann, 1960). Probably the increased
skin resistance in freshwater fish with electroreceptors increases the
electric sensitivity of the mechanoreceptors as well.
Many electric fish live in turbid waters or, if they live in clear waters,
are nocturnal. The electrosensory system which is basically quite short
range would appear to be most useful under these conditions of restricted
use of vision. Some electric fish have quite reduced eyes and the retinae
of the electric eel apparently degenerates with age (see discussion by
Keynes following de Oliveira Castro, 1961). But a number of electric
fish have what appear to be quite good eyes, and many elasmobranchs
at least are diurnal. In addition to the parallel evolution of electric
organs, electroreceptors, and a peculiar mode of swimming, long snouts
have been developed in both mormyrids and gymnotids (and probably
more than once in each group; see Figs. 16 and 60 in Chapter 10). The
snouts are certainly used in poking about muddy bottoms, but are most
likely a later development than the electrosensory systems. It is not clear
that development of electrosensory systems was always in compensation
for poor visual conditions, although it seems likely to have been so a
number of groups (Lissmann, 1958, 1961). In any case, development of
lateral line organs with electric sensitivity has not led to any great
decline of the ordinary lateral line system, which like the electrosensory
systems should be particularly useful under poor visual conditions
(Dijkgraaf, 1963).
It would be useful to know more about electric fields present in the
ordinary fish's environment and the extent to which these stimulate
ordinary lateral line receptors. Muscle potentials up to tens of millivolts
can be produced by synchronously contracting muscles. Examples are
sonic muscles where the fundamental frequency is set by the frequency
of muscle contraction and axial muscles when excited by Mauthner fibers
(unpublished, cf. Lissmann, 1958). These potentials conceivably could
activate mechanoreceptors electrically. Many catfish have sonic muscles
and their innervation is similar to that of the electric organ of the electric
catfish, which lacks a sonic muscle. Perhaps this organ was modified from
a sonic muscle, a suggestion which could be strengthened by further
morphological and embryological study of the catfish group (see Johnels,
1956).