10. ELECTRIC ORGANS
381
are commonly called knife fish. The greater part of the length of the
fish contains only muscle, spinal column, and electric organ. The viscera
are located in the anterior end of the body, and the anus and genital
papilla are just behind the chin. The anal fin begins slightly more
posteriorly and runs to near the caudal end of the fish. The electric
organ runs along most or all of the length of the body and is innervated
by spinal nerves. A caudal fin is absent in all but the sternarchids, in
which it is greatly reduced. There is little muscle caudal to the anal
fin, and this region, sometimes called the caudal filament, contains
mostly electric organ.
Gymnotids can regenerate new posterior regions if part is removed
(Ellis, 1913). Often in fish taken from the wild the posterior is regenerated indicating that there were encounters with predators in
which only the front end of the fish escaped. Ordinary swimming movements of the gymnotids are quite different from those of almost all
other fish. The body is held straight, and waves along the anal fin
provide the propulsive force. Most species appear to swim about equally
well forward and backward and often investigate objects by swimming
backward toward them (see Gymnarchus, Section 11, C, 3 ) . Most of
the body musculature is not involved in movement of the anal fin and
is probably used only in emergency movements when the body is rapidly
flexed in the more usual fishlike manner.
The species of gymnotids are poorly described. Keys are available
for a few areas of South America, but it is doubtful that these are
complete. Tropical fish dealers, the usual source of supply of gymnotids,
often bring in fish from other areas that are clearly new species. On
the Rio Negro expedition of the R. V. Alpha Helix, perhaps one-third
of the gymnotid species collected were undescribed taxomically. While
there are difficulties in working on undescribed species, classification
into genera is generally simple, and generic characteristics are apparently
the most important ones from a physiological point of view. Nevertheless, it may be desirable to save specimens of a particular species studied
for subsequent identification when the systematics of gymnotids is
clarified.
Table I1 and Fig. 16 present a key to gymnotids that is modified
from that of Ellis (1913). A second system has been added, one that
is nondestructive and applicable to living specimens. Accurate classification sometimes requires dissection of the specimen, but a reasonable
identification can usually be achieved without it, particularly if an
oscilloscope is available to observe electric organ discharges.
a. The Electric Eel. The electric eel was the first electric fish for which
the cellular mechanisms of the discharge were elucidated (Keynes and
381
are commonly called knife fish. The greater part of the length of the
fish contains only muscle, spinal column, and electric organ. The viscera
are located in the anterior end of the body, and the anus and genital
papilla are just behind the chin. The anal fin begins slightly more
posteriorly and runs to near the caudal end of the fish. The electric
organ runs along most or all of the length of the body and is innervated
by spinal nerves. A caudal fin is absent in all but the sternarchids, in
which it is greatly reduced. There is little muscle caudal to the anal
fin, and this region, sometimes called the caudal filament, contains
mostly electric organ.
Gymnotids can regenerate new posterior regions if part is removed
(Ellis, 1913). Often in fish taken from the wild the posterior is regenerated indicating that there were encounters with predators in
which only the front end of the fish escaped. Ordinary swimming movements of the gymnotids are quite different from those of almost all
other fish. The body is held straight, and waves along the anal fin
provide the propulsive force. Most species appear to swim about equally
well forward and backward and often investigate objects by swimming
backward toward them (see Gymnarchus, Section 11, C, 3 ) . Most of
the body musculature is not involved in movement of the anal fin and
is probably used only in emergency movements when the body is rapidly
flexed in the more usual fishlike manner.
The species of gymnotids are poorly described. Keys are available
for a few areas of South America, but it is doubtful that these are
complete. Tropical fish dealers, the usual source of supply of gymnotids,
often bring in fish from other areas that are clearly new species. On
the Rio Negro expedition of the R. V. Alpha Helix, perhaps one-third
of the gymnotid species collected were undescribed taxomically. While
there are difficulties in working on undescribed species, classification
into genera is generally simple, and generic characteristics are apparently
the most important ones from a physiological point of view. Nevertheless, it may be desirable to save specimens of a particular species studied
for subsequent identification when the systematics of gymnotids is
clarified.
Table I1 and Fig. 16 present a key to gymnotids that is modified
from that of Ellis (1913). A second system has been added, one that
is nondestructive and applicable to living specimens. Accurate classification sometimes requires dissection of the specimen, but a reasonable
identification can usually be achieved without it, particularly if an
oscilloscope is available to observe electric organ discharges.
a. The Electric Eel. The electric eel was the first electric fish for which
the cellular mechanisms of the discharge were elucidated (Keynes and