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Ron H. Douglas
approximately around 650 nm (this is a very rough estimate since existing formulae
relating rhodopsin and porphyropsin A max values almost certainly do not apply at such
long wavelengths). This would produce an excellent fit to their longwave
bioluminescence (Fig. 4 ). If such a pigment exists, the most probable reason it has so
far not been seen is that the pigments have always been isolated using dim red
illumination, which is likely to bleach such a pigment.
The third genus of far-red light producing stomiid, Malacosteus, has achieved farred sensitivity in a different way. Its retina contains only two pigments, which are
equivalent to the shortwave pigment pair isolated in Aristostomias and Pachystomias
(A max values ca. 515 nm and 542 nm) (Douglas et al. 1998b, 1999; Fig. 5). It cannot
therefore see its own bioluminescence efficiently using just these two pigments.
However, it has compensated for this apparent reduced sensitivity by utilizing a
photostable, chlorophyll-derived, photosensitizer within its outer segments which
absorbs light at its main absorption peak (672 nm) and in some, as yet undetermined,
way, possibly involving the generation of a triplet state, isomerizing the shorter wavesensitive visual pigments (Bowmaker et al. 1988; Douglas et al. 1998b, 1999; Fig. 5).
Thus the three genera of far-red light producing stomiids have evolved two quite
separate ways of ensuring visibility of their own bioluminescence. This is probably
the best example of visual pigment adaptation to a specific photic stimulus.
Since stomiids are sensitive to the far-red bioluminescence produced by their
suborbital photophores, which other animals in the deepsea cannot see, they have
what could be regarded as a private waveband. This longwave light could therefore
be used for intraspecific signalling, immune from detection by potential predators, or
for the covert illumination of prey (Partridge and Douglas 1995). It would, however,
not be altogether surprising if other, non-red light-producing, organisms had also
evolved similar longwave sensitivity to counteract the advantage enjoyed by the
stomiids.
5 Teleost Cone Visual Pigments
Most fish possess more than one spectral type of cone. While this does not prove that
these animals have colour vision, which also requires the neural machinery to
compare the output of these receptors, (proof of which requires extensive behavioral
experiments which have only been done for a few species, e.g. Neumeyer 1992), it
would be a huge surprise if most fish did not possess colour vision.
Teleost cones are morphologically distinct. There is a broad distinction between
single and double cones (Fig. 2) and in general the smaller a single cone, the shorter
the A max of the pigment it contains (Loew and Lythgoe 1978). While it is relatively
easy to draw correlations between a fish's visual environment and its generally singlerod visual pigment, this is much harder to do with photopic pigments because each
animal possesses up to four types of these receptors. Any correlations are therefore
less clear-cut because different photoreceptors are likely to perform diverse visual
functions, and exceptions to any rule are quite common.
Ron H. Douglas
approximately around 650 nm (this is a very rough estimate since existing formulae
relating rhodopsin and porphyropsin A max values almost certainly do not apply at such
long wavelengths). This would produce an excellent fit to their longwave
bioluminescence (Fig. 4 ). If such a pigment exists, the most probable reason it has so
far not been seen is that the pigments have always been isolated using dim red
illumination, which is likely to bleach such a pigment.
The third genus of far-red light producing stomiid, Malacosteus, has achieved farred sensitivity in a different way. Its retina contains only two pigments, which are
equivalent to the shortwave pigment pair isolated in Aristostomias and Pachystomias
(A max values ca. 515 nm and 542 nm) (Douglas et al. 1998b, 1999; Fig. 5). It cannot
therefore see its own bioluminescence efficiently using just these two pigments.
However, it has compensated for this apparent reduced sensitivity by utilizing a
photostable, chlorophyll-derived, photosensitizer within its outer segments which
absorbs light at its main absorption peak (672 nm) and in some, as yet undetermined,
way, possibly involving the generation of a triplet state, isomerizing the shorter wavesensitive visual pigments (Bowmaker et al. 1988; Douglas et al. 1998b, 1999; Fig. 5).
Thus the three genera of far-red light producing stomiids have evolved two quite
separate ways of ensuring visibility of their own bioluminescence. This is probably
the best example of visual pigment adaptation to a specific photic stimulus.
Since stomiids are sensitive to the far-red bioluminescence produced by their
suborbital photophores, which other animals in the deepsea cannot see, they have
what could be regarded as a private waveband. This longwave light could therefore
be used for intraspecific signalling, immune from detection by potential predators, or
for the covert illumination of prey (Partridge and Douglas 1995). It would, however,
not be altogether surprising if other, non-red light-producing, organisms had also
evolved similar longwave sensitivity to counteract the advantage enjoyed by the
stomiids.
5 Teleost Cone Visual Pigments
Most fish possess more than one spectral type of cone. While this does not prove that
these animals have colour vision, which also requires the neural machinery to
compare the output of these receptors, (proof of which requires extensive behavioral
experiments which have only been done for a few species, e.g. Neumeyer 1992), it
would be a huge surprise if most fish did not possess colour vision.
Teleost cones are morphologically distinct. There is a broad distinction between
single and double cones (Fig. 2) and in general the smaller a single cone, the shorter
the A max of the pigment it contains (Loew and Lythgoe 1978). While it is relatively
easy to draw correlations between a fish's visual environment and its generally singlerod visual pigment, this is much harder to do with photopic pigments because each
animal possesses up to four types of these receptors. Any correlations are therefore
less clear-cut because different photoreceptors are likely to perform diverse visual
functions, and exceptions to any rule are quite common.