10. ELECTRIC ORGANS
371
would markedly increase the internal resistance of the organ. In spite
of the quantitative discrepancy the principles of series summation and
synchronous activation are well illustrated by this electric organ.
2. TORPEDINIDS
The electric rays are a cosmopolitan group of marine fishes. The
electric organs of the genus Torpedo are diagrammed in Fig. 1. They
are large flattened organs on each side of the head that extend completely through the “disc” from dorsal to ventral surfaces. The Torpedos
are rather slow moving, and the use of the organ discharge in capture
of much faster swimming prey has been studied in the European
species, Torpedo murmorata (Belbenoit, 1970). When a fish comes near
a Torpedo resting on the bottom, it swims forward and upward and
then emits pulses. Small fishes can be stunned. The Torpedo drops
back to the bottom over any immobilized prey. If it has been successful,
it consumes the prey while continuing to emit pulses at a low frequency.
The delay between movement and discharge indicates that the organ
does not function in prey detection. The detection of prey appears to
be mechanical or perhaps electrical. The effectiveness of the organs in
predation is confirmed for the American species, T. nobilianu, by the
presence of large and rapidly swimming fish in the stomach contents
(Bennett et al., 1961).
Each electric organ in T. nobiliuna is made up of some 500-1000
closely packed and roughly circular columns of electrocytes that run
from top to bottom of the organ (Fig. 8). The electrocytes are thin
(10-30 p ) discs and have the same diameter as the columns; about
1000 are stacked one above another as in a role of coins. The columns
can be quite large in diameter, 5-7 mm in a large Torpedo (1 meter
across ) .
The cells are profusely innervated on their ventral surfaces. Nerves
run in the interstices between columns and 5-7 nerve fibers enter the
space between cells at roughly equal intervals around the periphery;
each fiber innervates a sector of the cell (Fig. 9). The nerves arise
from the electromotor lobe in the medulla (perhaps including parts of
the seventh, ninth and tenth cranial nerves, Fig. 8). Electron microthe periphery. The fibers branch profusely, but each innervates a separate segment
of the surface. The inset shows schematically the columnar arrangement of the
electrocytes. (Lower) A region from the lower right of the isolated cell is shown
at higher magnification. Nuclei of the electrocytes are stained as well as many fine
nerve branches. Modified from Bennett and Grundfest (1961a) and Grundfest
(1957).
371
would markedly increase the internal resistance of the organ. In spite
of the quantitative discrepancy the principles of series summation and
synchronous activation are well illustrated by this electric organ.
2. TORPEDINIDS
The electric rays are a cosmopolitan group of marine fishes. The
electric organs of the genus Torpedo are diagrammed in Fig. 1. They
are large flattened organs on each side of the head that extend completely through the “disc” from dorsal to ventral surfaces. The Torpedos
are rather slow moving, and the use of the organ discharge in capture
of much faster swimming prey has been studied in the European
species, Torpedo murmorata (Belbenoit, 1970). When a fish comes near
a Torpedo resting on the bottom, it swims forward and upward and
then emits pulses. Small fishes can be stunned. The Torpedo drops
back to the bottom over any immobilized prey. If it has been successful,
it consumes the prey while continuing to emit pulses at a low frequency.
The delay between movement and discharge indicates that the organ
does not function in prey detection. The detection of prey appears to
be mechanical or perhaps electrical. The effectiveness of the organs in
predation is confirmed for the American species, T. nobilianu, by the
presence of large and rapidly swimming fish in the stomach contents
(Bennett et al., 1961).
Each electric organ in T. nobiliuna is made up of some 500-1000
closely packed and roughly circular columns of electrocytes that run
from top to bottom of the organ (Fig. 8). The electrocytes are thin
(10-30 p ) discs and have the same diameter as the columns; about
1000 are stacked one above another as in a role of coins. The columns
can be quite large in diameter, 5-7 mm in a large Torpedo (1 meter
across ) .
The cells are profusely innervated on their ventral surfaces. Nerves
run in the interstices between columns and 5-7 nerve fibers enter the
space between cells at roughly equal intervals around the periphery;
each fiber innervates a sector of the cell (Fig. 9). The nerves arise
from the electromotor lobe in the medulla (perhaps including parts of
the seventh, ninth and tenth cranial nerves, Fig. 8). Electron microthe periphery. The fibers branch profusely, but each innervates a separate segment
of the surface. The inset shows schematically the columnar arrangement of the
electrocytes. (Lower) A region from the lower right of the isolated cell is shown
at higher magnification. Nuclei of the electrocytes are stained as well as many fine
nerve branches. Modified from Bennett and Grundfest (1961a) and Grundfest
(1957).