8. VISION AND BIOLUMINESCENCE
255
Meganyctiphunes norvegica producing a continuous glow of luminescence
of low light intensity for periods as long as 36 min. He experimented
with the animals in the “ plankton wheel ”, described by Hardy and
Bainbridge (1954), but could find no correlation between glowing and
either speed or direction of swimming nor between glowing and time of
day, size or sex of the animals. These animals were examined experimentally under confined conditions so that the negative correlations
obtained may not be significant. Korte did not estimate the intensity
of this glow but it is considerably less intense than the flashing observed
in response to the photoflash. If glowing is a natural form of luminescent
behaviour of euphausiids then further support is lent to Clarke’s (1963)
theory of the function of luminescence. He thinks that the photophores
are used to break up the silhouette because they are directed ventrally
and have dense reflecting pigments dorsal to them thus confining light
emissions to the region ventral to the animal. Predators presumably
attack their prey seen against the higher intensities of ambient light
above them and so production of light by prey organisms, if of the
correct intensity, would effectively make a relatively transparent
organism, such as an euphausiid, invisible. The eye photophore, which
has no lens and so probably produces a more diffuse light than the other
photophores, may be used by the animal as a reference light to enable
it to gauge the intensity of the ambient light around it. The eye photophore lights up before the more posterior photophores and this linking
in series may be indicative of a control mechanism with the eye photophore acting, in conjunction with the eyes, as a receptor, the information obtained through it being used to control the intensity of luminescence from the other photophores. As Clarke (1963) points out, the
ability to rotate the body photophores could be required to maintain
the light emissions in a dorsal/ventral plane when the animal’s body is
not in the horizontal plane. All ideas on the functional aspects of
bioluminescence are, as yet, conjectural because of the difficulty of
designing laboratory experiments whose techniques are above criticism.
Again, if the luminescent behaviour of these animals could be watched
in the sea then it might be possible to discern specific functions but it is
dangerous to translate their behaviour under laboratory conditions to
that in the sea.
The detailed processes by which the eyes of euphausiids can form
and interpret a mosaic image are unknown. There is some question as
to how compound eyes of insects interpret a mosaic image. Burtt and
Catton (1962a,b) have put forward a diffraction theory of insect vision
and since the general processes of insect vision are probably similar to
those of crustacean compound eyes, further work on euphausiid eyes is
9.
255
Meganyctiphunes norvegica producing a continuous glow of luminescence
of low light intensity for periods as long as 36 min. He experimented
with the animals in the “ plankton wheel ”, described by Hardy and
Bainbridge (1954), but could find no correlation between glowing and
either speed or direction of swimming nor between glowing and time of
day, size or sex of the animals. These animals were examined experimentally under confined conditions so that the negative correlations
obtained may not be significant. Korte did not estimate the intensity
of this glow but it is considerably less intense than the flashing observed
in response to the photoflash. If glowing is a natural form of luminescent
behaviour of euphausiids then further support is lent to Clarke’s (1963)
theory of the function of luminescence. He thinks that the photophores
are used to break up the silhouette because they are directed ventrally
and have dense reflecting pigments dorsal to them thus confining light
emissions to the region ventral to the animal. Predators presumably
attack their prey seen against the higher intensities of ambient light
above them and so production of light by prey organisms, if of the
correct intensity, would effectively make a relatively transparent
organism, such as an euphausiid, invisible. The eye photophore, which
has no lens and so probably produces a more diffuse light than the other
photophores, may be used by the animal as a reference light to enable
it to gauge the intensity of the ambient light around it. The eye photophore lights up before the more posterior photophores and this linking
in series may be indicative of a control mechanism with the eye photophore acting, in conjunction with the eyes, as a receptor, the information obtained through it being used to control the intensity of luminescence from the other photophores. As Clarke (1963) points out, the
ability to rotate the body photophores could be required to maintain
the light emissions in a dorsal/ventral plane when the animal’s body is
not in the horizontal plane. All ideas on the functional aspects of
bioluminescence are, as yet, conjectural because of the difficulty of
designing laboratory experiments whose techniques are above criticism.
Again, if the luminescent behaviour of these animals could be watched
in the sea then it might be possible to discern specific functions but it is
dangerous to translate their behaviour under laboratory conditions to
that in the sea.
The detailed processes by which the eyes of euphausiids can form
and interpret a mosaic image are unknown. There is some question as
to how compound eyes of insects interpret a mosaic image. Burtt and
Catton (1962a,b) have put forward a diffraction theory of insect vision
and since the general processes of insect vision are probably similar to
those of crustacean compound eyes, further work on euphausiid eyes is
9.