236
THE BIOLOGY O F EUPHAUSIIDS
rods, on the other hand, are the elements which enable vertebrates to
see in conditions of low light intensity, known as scotopic vision. There
are photosensitive or visual pigments in the rods and cones and these
consist of the aldehyde form of vitamin A, called retinene, combined
with protein molecules, to give two pigments absorbing light at different
wavelengths. The pigment associated with the rods in scotopic vision
is called visual purple or rhodopsin and that of the cones in photopic
vision is called iodopsin. Each species of vertebrate has its own specific
rhodopsin and iodopsin, differing from those of other species in the
nature of the protein or opsin part, since they all have retinene as the
chromophore or pigment carrier. Some vertebrates, including many
freshwater fish and certain amphibians, have a different form of vitamin
A, vitamin A,, which has retinene, as its aldehyde. Retinene, combines
with opsins to give porphyropsin instead of rhodopsin and cyanopsin
instead of iodopsin in these animals, but they function in the same way
as the vitamin A, pigments.
To return to euphausiids, their eyes contain, a t least, a rhodopsin
type of visual pigment. It is, indeed, not surprising that a similar
pigment to that found in the rods of vertebrate eyes, adapted to night
vision, is present in the eyes of euphausiids, the majority of which
spend their lives in a dimly illuminated environment. On the other hand,
it is a remarkable fact that the visual pigments of all animals with
image-forming eyes, whether they be those of vertebrates with the
retinal layer beneath the nervous layer, or those of cephalopod molluscs
with the retinal layer nearer the lens than the nervous layer, or those of
insects or crustaceans which are compound structures, all consist of
retinene combined with an opsin type of molecule. The retinene
involved is, of course, the aldehyde of the 11-cis isomer of vitamin A,
as stated in Chapter 7. This molecule has a peculiar shape (Fig. 82))
which fits into the opsin molecule. It is probable that when the
rhodopsin is exposed to light the first thing that happens is that the
light in some way causes the bent retinene molecule to straighten out.
When this happens, the molecule is no longer able to fit on to the opsin
part and so the rhodopsin molecule splits into two parts, retinene and
opsin ; the immediate effect of this rupture of the rhodopsin is its loss
of colour because neither of the components is heavily pigmented and
it is only when combined that they possess the purple coloration
characteristic of rhodopsin. According to Pitt and Morton (1960)) the
probable visual cycle of rhodopsin in the eyes is that illustrated in
Fig. 83 where rhodopsin splits, by the action of light, into opsin and
all-trans retinene. This, by the enzyme retinene isomerase, is converted
to opsin and 11-cis retinene which regenerate rhodopsin. It is not yet
THE BIOLOGY O F EUPHAUSIIDS
rods, on the other hand, are the elements which enable vertebrates to
see in conditions of low light intensity, known as scotopic vision. There
are photosensitive or visual pigments in the rods and cones and these
consist of the aldehyde form of vitamin A, called retinene, combined
with protein molecules, to give two pigments absorbing light at different
wavelengths. The pigment associated with the rods in scotopic vision
is called visual purple or rhodopsin and that of the cones in photopic
vision is called iodopsin. Each species of vertebrate has its own specific
rhodopsin and iodopsin, differing from those of other species in the
nature of the protein or opsin part, since they all have retinene as the
chromophore or pigment carrier. Some vertebrates, including many
freshwater fish and certain amphibians, have a different form of vitamin
A, vitamin A,, which has retinene, as its aldehyde. Retinene, combines
with opsins to give porphyropsin instead of rhodopsin and cyanopsin
instead of iodopsin in these animals, but they function in the same way
as the vitamin A, pigments.
To return to euphausiids, their eyes contain, a t least, a rhodopsin
type of visual pigment. It is, indeed, not surprising that a similar
pigment to that found in the rods of vertebrate eyes, adapted to night
vision, is present in the eyes of euphausiids, the majority of which
spend their lives in a dimly illuminated environment. On the other hand,
it is a remarkable fact that the visual pigments of all animals with
image-forming eyes, whether they be those of vertebrates with the
retinal layer beneath the nervous layer, or those of cephalopod molluscs
with the retinal layer nearer the lens than the nervous layer, or those of
insects or crustaceans which are compound structures, all consist of
retinene combined with an opsin type of molecule. The retinene
involved is, of course, the aldehyde of the 11-cis isomer of vitamin A,
as stated in Chapter 7. This molecule has a peculiar shape (Fig. 82))
which fits into the opsin molecule. It is probable that when the
rhodopsin is exposed to light the first thing that happens is that the
light in some way causes the bent retinene molecule to straighten out.
When this happens, the molecule is no longer able to fit on to the opsin
part and so the rhodopsin molecule splits into two parts, retinene and
opsin ; the immediate effect of this rupture of the rhodopsin is its loss
of colour because neither of the components is heavily pigmented and
it is only when combined that they possess the purple coloration
characteristic of rhodopsin. According to Pitt and Morton (1960)) the
probable visual cycle of rhodopsin in the eyes is that illustrated in
Fig. 83 where rhodopsin splits, by the action of light, into opsin and
all-trans retinene. This, by the enzyme retinene isomerase, is converted
to opsin and 11-cis retinene which regenerate rhodopsin. It is not yet