THE COLOR OF THE AMBLYOPSIDÆ.--The three species of Chologaster are
colored, with varying intensity, from C. cornutus, which is darkest, to
C. Agassizii, in which the color is faintest. The color cells are in
all cases arranged in a definite pattern. These are determined by the
underlying muscles. The pattern consists of three longitudinal bands on
the sides, following the line where the muscle segments are angularly
bent, and cross-stripes along the line separating successive segments.
The general color of Typhlichthys is cream and pink. It is abundantly
pigmented. In younger specimens the pigment is arranged in more
definite areas about the head. In the old it is more uniformly
distributed, being, however, specially abundant about the brain. The
pigment pattern of the body is precisely as in Chologaster, except that
the individual pigment cells are minute and their aggregate not evident
except under the lens.
The retention of the color pattern of Chologaster in Typhlichthys
is not less interesting than the retention of similar habits. It is
perhaps due to different causes. The color pattern in Chologaster is
determined by the underlying muscular structure, and the retention
of a similar pattern in Typhlichthys is due to the same underlying
structure, rather than to the direct hereditary transmission of the
color pattern.
Amblyopsis is flesh-colored, ranging to purple in the gill region,
where the blood of the gills shows through the overlying structures,
and over the liver, which can be seen through the translucent sides
and ventral wall. About the head and bases of the fins the color
is yellowish, resembling diluted blood. The surface of the body is
slightly iridescent, and the surface of the head has a velvety,
peach-bloom appearance.
The general pink color of Amblyopsis is due to the blood. It is not
due to any abnormal development of blood-vessels in the dermis. In the
fins, where the blood-vessels are near the surface, the general effect
is a yellowish color. The surface vessels of the dermis also appear
yellowish. It is only on account of the translucent condition of all
the tissues, permitting the deeper vessels to show through a certain
thickness, that the pink effect is produced. Amblyopsis has always been
spoken of as white. The term “white aquatic ghosts” of Cope is very
apt, for they do appear white in the caves, and their gliding motion
has an uncanny effect. All alcoholic specimens are white.
The pigment cells can not be made to show themselves, even by a
prolonged stay in the light. The old, if kept in the light, will not
become darker, and a young one reared in the light until ten months old
not only showed no increase in the pigmentation but lost the pigment it
had at birth, taking on the exact pigmentless coloration of the adult.
Pigment cells are late in appearing in Amblyopsis. When the young
are two months old pigment is abundant. This pigmented condition is
evidently a hereditarily transmitted condition. It disappears with age.
Primarily this disappearance was probably individual. But, as in the
flounder, the depigmentation has also become hereditarily transmitted,
for even those individuals reared in the light lose the color. Numerous
facts and experiments show that while pigment may be, and is, developed
in total darkness, the amount of color in an individual animal depends,
other things equal, directly on the amount of light to which it is
habitually exposed.
[Illustration: FIG. 1.--_Chologaster Agassizii_ from Cedar Sinks Cave,
Kentucky.]
The lower and upper surfaces of the flounder, the one protected and
the other exposed to the light, give the most striking example, and
the argument is clinched here by the fact, noted by Cunningham, that
a flounder whose lower side is for long periods exposed to the light
takes on color. Loeb has shown that in the yolk sacs of Fundulus
embryos more pigment cells are developed if the embryos are kept in the
light than when they are kept in the dark. However, in the body, and
especially in the eye, the pigmentation was not affected by the absence
of light.
The general absence of color in cave animals is conceded. Packard
states, “As regards change of color, we do not recall an exception to
the general rule that all cave animals are either colorless or nearly
white, or, as in the case of Arachnida and insects, much paler than
their out-of-door relatives.” Chilton has made the same observation on
the underground animals of New Zealand. Similar observations have been
recorded by Lönnberg, Carpenter, Schmeil, and Viré. Hamann enumerates a
number of species living both in caves and above ground. In such cases
the underground individuals are paler than the others. This confirms
similar observations by Packard.
[Illustration: FIG. 2.--_Chologaster papilliferus_ from Illinois.]
Poulton has mentioned that Proteus becomes darker when exposed to the
light. This has been verified by others. Typhlotriton larvæ living at
the entrance of a cave are dark, while the adult living farther in the
cave are much lighter, but with many chromatophores containing a small
amount of color. Epigæan fishes found in caves are always lighter in
color than their _confrères_ outside.
[Illustration: FIG. 3.--_Typhlichthys subterraneus._]
We have thus numerous examples of colored epigæan animals bleaching in
caves, and also bleached cave animals turning dark when exposed to the
light. We have also animals in which the side habitually turned to the
dark is colorless, while the side habitually turned to the light is
colored. Finally, we have cave animals that are permanently bleached.
Natural selection can not have affected the coloration of the cave
forms, for it can be absolutely of no consequence whether a cave
species is white or black. It could affect the coloration only
indirectly in one of two ways: First, as a matter of economy, but
since the _individual_ is in part bleached by the direct effect of the
darkness there is no reason why natural selection should come into
play at all in reducing the pigment as a matter of economy; second,
Romanes has supposed that the color decreases through the selection of
correlated structures--a supposition he found scarcely conceivable when
the variety of animals showing the bleached condition is considered.
[Illustration: Fig. 4 _a._]
[Illustration: Fig. 4 _b._]
[Illustration: Fig. 4 _c._