F. W. MUNZ
20
group, retinenez; he called their visual pigment “porphyropsin.” Fishes
that migrate between the sea and freshwater have mixtures of the two
visual pigments, with the pigment which corresponds to the salinity of
the spawning habitat predominating in the mixture.
Freshwater fish families are not a homogeneous group but have been
placed by zoogeographers in three divisions, according to their salinity
tolerance and presumed evolutionary history ( Darlington, 1957 ) . Families
in a primary division are restricted to freshwater and “have probably
been confined to fresh water so long that their present distributions are
the result of dispersal through fresh water, even though their remote
ancestors may have lived in the sea” (Darlington, 1957, p. 46). Fishes in
a secondary division have greater salinity tolerance and may have dispersed through the ocean. Peripheral freshwater fishes, “although found
in fresh water, are somehow closely connected with the sea or have been
so recently derived from it that their present distributions may be and
often are largely the result of dispersal through the sea” (Darlington,
1957, p. 47). The physiological salt tolerance of a given species, of course,
may not fit the probable evolutionary history of the family. The primary
division constitutes what should be called freshwater fishes in a strict
sense. Two families (Characidae and Cyprinidae) make up a very large
proportion of this group. “Euryhaline” is a relative term, which is
roughly equivalent to the secondary and peripheral divisions. Many
peripheral freshwater fishes migrate between the sea and freshwater, but
most secondary species do not. These divisions by zoogeographers seem
to provide the most rational background against which to examine the
distribution of visual pigments.
Wald’s simple pattern does not adequately describe the experimental
results obtained by several investigators over the last 20 years; for a competent review, see Schwanzara (1967). The most striking discrepancy
is the common occurrence in primary freshwater fishes of rhodopsin,
either alone or mixed with porphyropsin (Table I ) . This is evident among
the approximately 10 characid and 20 cyprinid species that have been
tested. Freshwater catfishes and centrarchids are the most prominent
groups having porphyropsin alone. Some species ( mostly cyprinids ) even
have rhodopsin alone. Secondary and peripheral freshwater fishes are
considered together for their visual pigments have similar distributions.
Many species have mixtures and many (notably poeciliids) have rhodopsin alone. It is true that more of the strictly freshwater species have
porphyropsin alone and more of the euryhaline species have rhodopsin
alone. Perhaps more significant is the unexpected fact that about half
of the sample, whether euryhaline or not, has mixtures of the two visual
pigments. Marine fishes, at least, seem to be as conservative as predicted
20
group, retinenez; he called their visual pigment “porphyropsin.” Fishes
that migrate between the sea and freshwater have mixtures of the two
visual pigments, with the pigment which corresponds to the salinity of
the spawning habitat predominating in the mixture.
Freshwater fish families are not a homogeneous group but have been
placed by zoogeographers in three divisions, according to their salinity
tolerance and presumed evolutionary history ( Darlington, 1957 ) . Families
in a primary division are restricted to freshwater and “have probably
been confined to fresh water so long that their present distributions are
the result of dispersal through fresh water, even though their remote
ancestors may have lived in the sea” (Darlington, 1957, p. 46). Fishes in
a secondary division have greater salinity tolerance and may have dispersed through the ocean. Peripheral freshwater fishes, “although found
in fresh water, are somehow closely connected with the sea or have been
so recently derived from it that their present distributions may be and
often are largely the result of dispersal through the sea” (Darlington,
1957, p. 47). The physiological salt tolerance of a given species, of course,
may not fit the probable evolutionary history of the family. The primary
division constitutes what should be called freshwater fishes in a strict
sense. Two families (Characidae and Cyprinidae) make up a very large
proportion of this group. “Euryhaline” is a relative term, which is
roughly equivalent to the secondary and peripheral divisions. Many
peripheral freshwater fishes migrate between the sea and freshwater, but
most secondary species do not. These divisions by zoogeographers seem
to provide the most rational background against which to examine the
distribution of visual pigments.
Wald’s simple pattern does not adequately describe the experimental
results obtained by several investigators over the last 20 years; for a competent review, see Schwanzara (1967). The most striking discrepancy
is the common occurrence in primary freshwater fishes of rhodopsin,
either alone or mixed with porphyropsin (Table I ) . This is evident among
the approximately 10 characid and 20 cyprinid species that have been
tested. Freshwater catfishes and centrarchids are the most prominent
groups having porphyropsin alone. Some species ( mostly cyprinids ) even
have rhodopsin alone. Secondary and peripheral freshwater fishes are
considered together for their visual pigments have similar distributions.
Many species have mixtures and many (notably poeciliids) have rhodopsin alone. It is true that more of the strictly freshwater species have
porphyropsin alone and more of the euryhaline species have rhodopsin
alone. Perhaps more significant is the unexpected fact that about half
of the sample, whether euryhaline or not, has mixtures of the two visual
pigments. Marine fishes, at least, seem to be as conservative as predicted