10. ELECTRIC ORGANS
457
become modified to resemble the fibers of the main electric organ. The
independent evolution of rostra1 accessory organs in other gymnotids
attests to the value of these organs. Is it to be supposed that the sensory
fibers of the primitive accessory organ gradually enlarged over many
generations to repeat the evolutionary sequence followed in the main
organ? It seems much more likely that having evolved the developmental mechanism to make large generating nerve fibers in its main
organ, the fish merely evolved the ability to apply the same mechanism
to the chin fibers. In molecular terms, DNA coded information required
to make the main organ fibers was also present in the DNA of the
sensory neurons; the fish then acquired a way of expressing this information at the different site. In brief, the argument is that there is evolution
of a mechanism of turning-on in a new part of the organism a previously
evolved very complex developmental sequence. This process appears
much more probable than evolution of a very similar complex sequence
all over again.
Evolution of a structure in one part of the body and then the sudden
appearance of a very similar structure in a quite different part of the
body may not be uncommon in phylogenetic history. Another example
from electric fish is Hunter’s organ of the eel, which appears to be
evolved from a different muscle group than the main and Sachs’ organs,
but which has very similar electrocytes (see Section 11, D, 1, a ) . Other
examples are discussed in Bennett ( 1970). In accessory electric organs of
other gymnotids and Narcine the innervation suggests that the organs
developed by migration of electrocytes from the main organ rather than
by development from a different and local muscle group (see Sections
11, C, 2 and 11, D, 1, d).
This concept of preadaptation also raises the possibility that electric
organs may have arisen in elasmobranchs only once. The argument for
separate evolution is based on the muscles of origin being different in
torpedinids and rajids. The organs could have originated in one muscle
type in a common ancestor, then “jumped to a second muscle type, and
finally have been lost at the first site. The rajids seem the more generalized of the two, and a hypothetical electric common ancestor would
probably have been rajid-like. The same argument appears inapplicable
to separate evolution of electric organs in different teleost groups because
of their much wider evolutionary separation.
H. Electrocytes as Experimental Material
Many electrocytes are very large cells that are of low resistance and
easily studied by microelectrode techniques. Their greatest advantage
457
become modified to resemble the fibers of the main electric organ. The
independent evolution of rostra1 accessory organs in other gymnotids
attests to the value of these organs. Is it to be supposed that the sensory
fibers of the primitive accessory organ gradually enlarged over many
generations to repeat the evolutionary sequence followed in the main
organ? It seems much more likely that having evolved the developmental mechanism to make large generating nerve fibers in its main
organ, the fish merely evolved the ability to apply the same mechanism
to the chin fibers. In molecular terms, DNA coded information required
to make the main organ fibers was also present in the DNA of the
sensory neurons; the fish then acquired a way of expressing this information at the different site. In brief, the argument is that there is evolution
of a mechanism of turning-on in a new part of the organism a previously
evolved very complex developmental sequence. This process appears
much more probable than evolution of a very similar complex sequence
all over again.
Evolution of a structure in one part of the body and then the sudden
appearance of a very similar structure in a quite different part of the
body may not be uncommon in phylogenetic history. Another example
from electric fish is Hunter’s organ of the eel, which appears to be
evolved from a different muscle group than the main and Sachs’ organs,
but which has very similar electrocytes (see Section 11, D, 1, a ) . Other
examples are discussed in Bennett ( 1970). In accessory electric organs of
other gymnotids and Narcine the innervation suggests that the organs
developed by migration of electrocytes from the main organ rather than
by development from a different and local muscle group (see Sections
11, C, 2 and 11, D, 1, d).
This concept of preadaptation also raises the possibility that electric
organs may have arisen in elasmobranchs only once. The argument for
separate evolution is based on the muscles of origin being different in
torpedinids and rajids. The organs could have originated in one muscle
type in a common ancestor, then “jumped to a second muscle type, and
finally have been lost at the first site. The rajids seem the more generalized of the two, and a hypothetical electric common ancestor would
probably have been rajid-like. The same argument appears inapplicable
to separate evolution of electric organs in different teleost groups because
of their much wider evolutionary separation.
H. Electrocytes as Experimental Material
Many electrocytes are very large cells that are of low resistance and
easily studied by microelectrode techniques. Their greatest advantage